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(message, signature):
    expected_signature = hashlib.sha256(SECRET_KEY + message.encode()).hexdigest()
    return signature == expected_signature

# 1. Legitimate URL generated:
#    Message: "user=alice&action=view"
#    URL: /api/action?user=alice&action=view&signature=...

# 2. Attacker intercepts URL. Knows signature and message.
#    Doesn't know SECRET_KEY but can guess length (16 bytes)

# 3. Using `hashpump`, attacker computes new valid signature for extended message
#    Original: "user=alice&action=view"
#    Extended: "user=alice&action=view" + padding + "&action=delete&target=bob"
#    Tool generates new signature and message with padding

# 4. Server receives forged request, recomputes hash of `SECRET_KEY + extended_message`,
#    finds it matches attacker's signature. Delete action processed

GOOD Code Example

# SECURE: Use HMAC (Hash-based Message Authentication Code)
import hmac
import hashlib

SECRET_KEY = b"my_super_secret_key_16b"

def get_signed_url_secure(message):
    # HMAC designed to prevent length extension attacks
    # Two-step hashing: hash(key XOR opad, hash(key XOR ipad, message))
    signature = hmac.new(SECRET_KEY, message.encode(), hashlib.sha256).hexdigest()
    return f"/api/action?{message}&signature={signature}"

def verify_request_secure(message, signature):
    expected_signature = hmac.new(SECRET_KEY, message.encode(), hashlib.sha256).hexdigest()
    # Use hmac.compare_digest for constant-time comparison, prevents timing attacks
    return hmac.compare_digest(signature, expected_signature)

# Attacker cannot extend HMAC-signed message without secret key
# Inner hash `hash(key XOR ipad, message)` prevents continuing hash chain

Language-Specific Examples

JavaScript/Node.js:

// VULNERABLE: hash(secret + message) construction
const crypto = require('crypto');

const SECRET = 'my_secret_key';

function signMessage(message) {
    // Vulnerable to length extension!
    const signature = crypto.createHash('sha256')
        .update(SECRET + message)
        .digest('hex');
    return signature;
}

// SECURE: Use HMAC
const crypto = require('crypto');

const SECRET = 'my_secret_key';

function signMessageSecure(message) {
    const signature = crypto.createHmac('sha256', SECRET)
        .update(message)
        .digest('hex');
    return signature;
}

// Verify with constant-time comparison
function verifySignature(message, signature) {
    const expected = signMessageSecure(message);
    return crypto.timingSafeEqual(
        Buffer.from(signature),
        Buffer.from(expected)
    );
}

Java:

// VULNERABLE: Manual hash(secret + message)
import java.security.MessageDigest;

public class InsecureSigning {
    private static final String SECRET = "my_secret_key";

    public static String signMessage(String message) throws Exception {
        MessageDigest digest = MessageDigest.getInstance("SHA-256");
        // Vulnerable to length extension!
        String combined = SECRET + message;
        byte[] hash = digest.digest(combined.getBytes());
        return bytesToHex(hash);
    }
}

// SECURE: Use HMAC
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import java.security.MessageDigest;

public class SecureSigning {
    private static final String SECRET = "my_secret_key";

    public static String signMessage(String message) throws Exception {
        Mac hmac = Mac.getInstance("HmacSHA256");
        SecretKeySpec secretKey = new SecretKeySpec(
            SECRET.getBytes(), "HmacSHA256");
        hmac.init(secretKey);
        byte[] hash = hmac.doFinal(message.getBytes());
        return bytesToHex(hash);
    }

    // Constant-time comparison
    public static boolean verifySignature(String message, String signature)
            throws Exception {
        String expected = signMessage(message);
        return MessageDigest.isEqual(
            signature.getBytes(),
            expected.getBytes()
        );
    }
}

Detection

Find hash(secret + message) patterns: Grep for concatenation before hashing:
- rg 'hashlib\.(md5|sha1|sha256)\(.*\+' --type py
- rg 'crypto\.createHash.*update.*\+' --type js
- rg 'MessageDigest.*update.*\+' --type java
- Look for hash(key + data) or hash(data + key) patterns
Identify vulnerable hash functions for MACs: Search for signing without HMAC:
- rg 'hashlib\.(md5|sha1|sha256)' --type py | rg -v 'hmac'
- rg 'crypto\.createHash\(' --type js | rg -v 'createHmac'
- rg 'MessageDigest\.getInstance.*MD5|SHA-1|SHA-256' --type java | rg -v 'Mac\.getInstance'
Audit signature generation: Find custom MAC implementations:
- rg 'signature.*=.*hash|mac.*=.*hash' -i
- Check API signatures, token generation, cookie signing
Use static analysis: Run tools to detect weak crypto:
- Semgrep: python.lang.security.audit.hashlib-weak-hash
- Bandit: B303 (MD5/SHA1 usage)

Prevention

Use HMAC: Always use HMAC for message authentication codes. Industry standard, immune to length extension attacks, available in standard libraries
Choose secure hash functions: Use HMAC with SHA-256 or SHA-3
Never roll your own crypto: Avoid custom schemes like hash(message + secret) or hash(secret + message + secret). Use HMAC
Alternative if HMAC unavailable: Use SHA-3 or BLAKE2 (not vulnerable to length extension). HMAC still preferred

Related Security Patterns & Anti-Patterns

Weak Encryption Anti-Pattern: Part of broader cryptographic failures category
Timing Attacks Anti-Pattern: Use constant-time comparison for signature verification to prevent timing leaks

length-extension-attacks-anti-pattern