/CBC-cipher-block-chain-

Cipher block chain is vulnerable to a bit flip

CBC-cipher-block-chain-

#Cipher block chain is vulnerable to a bit flip #Here is a python3 code to exploit such a vulnerability

import requests from base64 import b64decode, b64encode from tqdm import tqdm

Bit flip code based on https://crypto.stackexchange.com/a/66086.

we need to decode from base64 twice because the cookie was encoded twice.

def bit_flip(pos, bit, data): raw = b64decode(b64decode(data).decode())

list1 = bytearray(raw)
list1[pos] = list1[pos] ^ bit
raw = bytes(list1)
return b64encode(b64encode(raw)).decode()

cookie = "VldrSlNTMHNtQnRBeEVCMFh6bXpMYkZQR3Bmd2lhMTJldXJIcHVub1JTMUtFTnBYdjhuME1uVWtpMEppZnh5Y2loYnhwWU5jUGtJZjIvbjVnR2dadEZVTDcwSWRPY1RtNTVCRHJqMjhuaERWdno0dUxabFAza1dEdCtXdVlTenM="

for position_idx in tqdm(range(10), desc="Bruteforcing Position"): # The 96 really should be 128 to test every bit, but 96 worked for me. for bit_idx in tqdm(range(96), desc="Bruteforcing Bit"): auth_cookie = bit_flip(position_idx, bit_idx, cookie) cookies = {'auth_name': auth_cookie} r = requests.get('http://mercury.picoctf.net:10868/', cookies=cookies) if "picoCTF{" in r.text: # The flag is between <code> and </code> print("Flag: " + r.text.split("")[1].split("")[0]) break