php-hmac-chall.py

#!/usr/bin/env python3
import hmac
import os

SECRET = b'secret'


def test(host, in_hmac, nonce=None):
    secret = SECRET

    if nonce is not None:
        secret = hmac.new(secret, msg=nonce, digestmod='sha256').hexdigest()
        secret = secret.encode('ascii')

    real_hmac = hmac.new(secret, msg=host, digestmod='sha256').hexdigest()

    if real_hmac == in_hmac:
        print('passed')
        os.system(f'host {host.decode("ascii")}')
        return True
    else:
        print('failed')
        return False


# Known good HMAC/host pair
known_host = b'twitter.com'
known_hmac = hmac.new(SECRET, msg=known_host, digestmod='sha256').hexdigest()

print(f'known host/hmac: {known_host} - {known_hmac}')

test(known_host, known_hmac)

# Forged HMAC using known host/HMAC as nonce-secret
bad_msg = b'bla;cat /etc/passwd'
# use known HMAC as HMAC secret here
bad_hmac = hmac.new(known_hmac.encode('ascii'), msg=bad_msg, digestmod='sha256').hexdigest()
print(f'evil hmac {bad_hmac}')

# use known host as nonce here
test(bad_msg,
     bad_hmac,
     nonce=known_host)