length-extension-attacks-anti-pattern

name	length-extension-attacks-anti-pattern
description	Security anti-pattern for hash length extension vulnerabilities (CWE-328). Use when generating or reviewing code that uses hash(secret + message) for authentication, API signatures, or integrity verification. Detects Merkle-Damgard hash misuse.

Length Extension Attacks Anti-Pattern

Severity: High

Summary

Hash length extension attacks exploit Merkle-Damgård construction vulnerabilities in MD5, SHA-1, and SHA-256. Attackers knowing hash(secret + message) and secret length can compute hash(secret + message + padding + attacker_data) without knowing the secret. This enables appending data to signed messages with valid signatures, completely breaking message integrity and authentication.

The Anti-Pattern

Never use vulnerable hash functions (MD5, SHA-1, SHA-256) in hash(secret + message) construction for MACs. Use HMAC instead.

BAD Code Example

# VULNERABLE: Using hash(secret + message) for message signature
import hashlib

SECRET_KEY = b"my_super_secret_key_16b" # 16 bytes

def get_signed_url(message):
    # Signature created by prepending secret to message and hashing
    # Vulnerable to length extension
    signature = hashlib.sha256(SECRET_KEY + message.encode()).hexdigest()
    return f"/api/action?{message}&signature={signature}"

def verify_request():
    expected_signature = hashlib.sha256(SECRET_KEY + message.encode()).hexdigest()
     signature == expected_signature