Prototype Pollution with Hmac Signatures

Prototype pollution attacks exploit JavaScript's prototype chain to inject malicious properties into objects. When this vulnerability intersects with Hmac Signatures, attackers can manipulate cryptographic verification processes in subtle but devastating ways.

The core issue arises when Hmac Signatures libraries process request parameters without proper sanitization. Consider a Node.js Express application using Hmac Signatures for API authentication:

app.post('/api/data', (req, res) => {
  const signature = req.body.signature;
  const payload = req.body.payload;
  
  // Vulnerable: Object.create(null) not used
  if (verifyHmacSignature(payload, signature)) {
    // Process request
  }
});

An attacker can exploit prototype pollution by sending a request with a __proto__ property:

POST /api/data HTTP/1.1
Content-Type: application/json

{
  "__proto__": {
    "isAdmin": true
  },
  "payload": "malicious data",
  "signature": "valid-signature-here"
}

This attack works because JavaScript objects inherit from Object.prototype. When the Hmac Signatures library processes the payload, it may inadvertently modify the prototype chain, affecting all objects that inherit from it.

A more sophisticated attack targets the Hmac Signatures verification algorithm itself. Many implementations use object properties to store intermediate verification states:

function verifyHmacSignature(payload, signature) {
  const verificationState = {};
  
  // Vulnerable: Object properties can be polluted
  verificationState.payloadHash = crypto.createHash('sha256').update(payload).digest();
  verificationState.expectedSignature = computeExpectedSignature(verificationState);
  
  return crypto.timingSafeEqual(
    Buffer.from(verificationState.expectedSignature),
    Buffer.from(signature)
  );
}

If an attacker pollutes the prototype with a payloadHash property, they can manipulate the verification state:

Object.prototype.payloadHash = function() {
  return crypto.createHash('sha256').update('attacker-controlled').digest();
};

This causes the Hmac Signatures verification to use attacker-controlled data, potentially allowing signature bypass.

Detecting prototype pollution in Hmac Signatures implementations requires both static analysis and runtime monitoring. Here are specific detection patterns:

Property Enumeration Checks

Implement runtime checks that monitor object property creation patterns:

const originalDefineProperty = Object.defineProperty;
Object.defineProperty = function(target, prop, descriptor) {
  if (prop === '__proto__' || prop === 'constructor') {
    console.warn('Prototype pollution attempt detected:', prop);
    throw new Error('Prototype pollution attempt blocked');
  }
  return originalDefineProperty.apply(this, arguments);
};

Input Sanitization for Hmac Signatures

Validate all request parameters before Hmac Signatures processing:

function sanitizePayload(payload) {
  if (typeof payload !== 'object' || payload === null) {
    return payload;
  }
  
  const sanitized = {};
  for (const [key, value] of Object.entries(payload)) {
    if (key.includes('__proto__') || key.includes('constructor')) {
      throw new Error('Invalid property detected');
    }
    sanitized[key] = sanitizePayload(value);
  }
  return sanitized;
}

// Usage in Hmac Signatures verification
const cleanPayload = sanitizePayload(req.body.payload);
const isValid = verifyHmacSignature(cleanPayload, req.body.signature);

middleBrick Scanning for Prototype Pollution

middleBrick's black-box scanning can detect prototype pollution vulnerabilities in Hmac Signatures implementations by:

• Testing for __proto__ property injection in API endpoints
• Analyzing response patterns that indicate prototype manipulation
• Checking for unsafe object creation patterns in the runtime environment
• Scanning for exposed prototype properties that could be exploited

The scanner sends specifically crafted payloads to test for prototype pollution resistance:

POST /api/verify-signature HTTP/1.1
Content-Type: application/json

{
  "__proto__": {
    "bypassVerification": true
  },
  "data": "test",
  "signature": "AAAA..."
}

middleBrick analyzes the response to determine if the prototype pollution attempt succeeded, providing a risk score and specific remediation guidance.

Remediating prototype pollution in Hmac Signatures implementations requires a defense-in-depth approach. Here are specific fixes:

Object.create(null) for Safe Objects

Always create objects without prototypes when processing Hmac Signatures data:

function createSafeObject() {
  return Object.create(null);
}

function verifyHmacSignature(payload, signature) {
  const verificationState = createSafeObject();
  
  // Safe: no prototype chain to pollute
  verificationState.payloadHash = crypto.createHash('sha256').update(payload).digest();
  verificationState.expectedSignature = computeExpectedSignature(verificationState);
  
  return crypto.timingSafeEqual(
    Buffer.from(verificationState.expectedSignature),
    Buffer.from(signature)
  );
}

Property Validation in Hmac Signatures Libraries

Implement strict property validation in your Hmac Signatures verification code:

function validateProperties(obj) {
  const forbiddenProps = ['__proto__', 'constructor', 'prototype'];
  
  for (const key of Object.keys(obj)) {
    if (forbiddenProps.includes(key)) {
      throw new Error(`Forbidden property: ${key}`);
    }
    
    if (typeof obj[key] === 'object' && obj[key] !== null) {
      validateProperties(obj[key]);
    }
  }
}

function secureVerifyHmac(payload, signature) {
  validateProperties(payload);
  
  // Continue with verification
  const payloadString = JSON.stringify(payload);
  const expectedSignature = crypto.createHmac('sha256', SECRET_KEY)
    .update(payloadString)
    .digest('hex');
  
  return crypto.timingSafeEqual(
    Buffer.from(expectedSignature),
    Buffer.from(signature)
  );
}

Using ES2022 #private features

Modern JavaScript provides private class fields that cannot be accessed via prototype pollution:

class HmacVerifier {
  #secretKey;
  
  constructor(secretKey) {
    this.#secretKey = secretKey;
  }
  
  verify(payload, signature) {
    const payloadString = JSON.stringify(payload);
    const expected = crypto.createHmac('sha256', this.#secretKey)
      .update(payloadString)
      .digest('hex');
    
    return crypto.timingSafeEqual(
      Buffer.from(expected),
      Buffer.from(signature)
    );
  }
}

// Usage
const verifier = new HmacVerifier(process.env.SECRET_KEY);
const isValid = verifier.verify(sanitizedPayload, req.body.signature);

middleBrick Integration for Continuous Monitoring

Integrate middleBrick into your development workflow to continuously scan for prototype pollution:

# Install middleBrick CLI
npm install -g middlebrick

# Scan your API endpoints
middlebrick scan https://api.yourdomain.com/verify-signature --frequency=daily

# Add to GitHub Actions for CI/CD
name: API Security Scan

on: [push, pull_request]

jobs:
  security-scan:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - name: Run middleBrick Scan
        run: |
          npm install -g middlebrick
          middlebrick scan ${{ secrets.API_URL }} --fail-on-severity=high

This setup ensures that any prototype pollution vulnerabilities in your Hmac Signatures implementation are caught before deployment, with middleBrick providing specific findings and remediation guidance.