Prototype Pollution with Jwt Tokens

Prototype pollution in JWT tokens occurs when an attacker manipulates the token's payload to exploit JavaScript's prototype chain. This vulnerability typically arises when JWT libraries or middleware improperly handle token decoding, allowing malicious actors to inject properties into object prototypes.

The most common Jwt Tokens-specific attack pattern involves manipulating the alg header field. When a JWT token is processed, the alg field determines the signature verification algorithm. Attackers can set alg to none or manipulate it to bypass signature verification entirely.

const tamperedToken = jwt.sign({ 
  data: { admin: true }, 
  iat: Math.floor(Date.now() / 1000) - 30
}, 
'secret', 
{ algorithm: 'none' });

This creates an unsigned token that some libraries will still accept as valid. The prototype pollution occurs because the decoded token object inherits from Object.prototype, allowing attackers to add properties that affect all objects.

Another Jwt Tokens-specific manifestation involves the kid (key ID) header field. When libraries use kid to dynamically select verification keys, attackers can manipulate this field to cause prototype pollution:

const maliciousKid = '__proto__.admin=true.' + 'A'.repeat(100);
const token = jwt.sign({ data: { userId: 1 } }, 'secret', { header: { kid: maliciousKid } });

This payload, when decoded, can pollute the prototype chain if the library doesn't properly sanitize header fields. The attack becomes particularly dangerous when combined with Node.js's require() function, as attackers might attempt to load arbitrary modules through crafted kid values.

Property enumeration attacks represent another Jwt Tokens-specific vector. When JWT libraries expose token properties through enumeration (e.g., for...in loops), prototype pollution can cause sensitive data exposure:

const token = jwt.sign({ data: { userId: 1 } }, 'secret');
const decoded = jwt.decode(token);

// If prototype pollution occurred:
for (let key in decoded) {
  console.log(key); // Might expose prototype properties
}

The danger escalates when JWT tokens are used in object destructuring or spread operations, where polluted properties can silently propagate through the application.

Detecting prototype pollution in JWT tokens requires both static analysis and runtime scanning. middleBrick's black-box scanning approach is particularly effective for this vulnerability because it tests the unauthenticated attack surface without requiring source code access.

middleBrick scans for Jwt Tokens-specific prototype pollution by sending crafted JWT tokens with manipulated header fields and analyzing the server's response behavior. The scanner tests for alg: none bypass attempts, malformed kid fields, and oversized header values that might trigger prototype pollution.

Key detection patterns include:

• Testing if alg: none tokens are accepted without verification
• Checking for prototype pollution via __proto__ manipulation in header fields
• Analyzing response timing differences that might indicate prototype chain traversal
• Verifying that signature verification cannot be bypassed through header manipulation
• Testing for information disclosure through prototype pollution

Manual detection techniques complement automated scanning. Developers should implement JWT middleware that validates tokens against a strict schema:

const jwtMiddleware = (req, res, next) => {
  const authHeader = req.headers.authorization;
  if (!authHeader) return res.status(401).json({ error: 'Missing token' });
  
  const token = authHeader.replace('Bearer ', '');
  try {
    const decoded = jwt.verify(token, process.env.JWT_SECRET, {
      algorithms: ['HS256', 'RS256'], // Whitelist only
      complete: true
    });
    
    // Validate token structure
    if (typeof decoded.header !== 'object' || !decoded.header.alg) {
      throw new Error('Invalid token structure');
    }
    
    req.user = decoded.payload;
    next();
  } catch (err) {
    console.error('JWT verification failed:', err.message);
    res.status(401).json({ error: 'Invalid token' });
  }
};

middleBrick's scanning specifically tests whether your implementation rejects alg: none tokens and properly validates the kid field against a whitelist of allowed keys.

Remediating prototype pollution in JWT tokens requires a defense-in-depth approach that addresses both token validation and library configuration. The most critical step is ensuring your JWT library is up-to-date and configured with strict validation rules.

Start by implementing strict algorithm whitelisting in your JWT verification:

const verifyToken = (token) => {
  try {
    return jwt.verify(token, process.env.JWT_SECRET, {
      algorithms: ['HS256'], // Only allow expected algorithms
      issuer: 'your-domain.com',
      audience: 'your-api',
      ignoreExpiration: false,
      clockTolerance: 30
    });
  } catch (err) {
    // Log and handle specific error types
    if (err.name === 'JsonWebTokenError') {
      console.warn('Invalid JWT structure');
    } else if (err.name === 'TokenExpiredError') {
      console.warn('JWT expired');
    }
    throw err;
  }
};

For kid field validation, implement a strict whitelist approach:

const allowedKeys = {
  'key1': fs.readFileSync('./keys/key1.pem'),
 'key2': fs.readFileSync('./keys/key2.pem')
};

const verifyWithKid = (token) => {
  const decoded = jwt.decode(token, { complete: true });
  if (!decoded || !decoded.header || !decoded.header.kid) {
    throw new Error('Missing kid header');
  }
  
  const key = allowedKeys[decoded.header.kid];
  if (!key) {
    throw new Error('Invalid kid value');
  }
  
  return jwt.verify(token, key, { algorithms: ['RS256'] });
};

Implement input sanitization to prevent prototype pollution:

const sanitizePayload = (payload) => {
  const sanitized = {};
  Object.keys(payload).forEach(key => {
    if (key.includes('__proto__') || key.includes('constructor')) {
      throw new Error('Invalid payload key');
    }
    sanitized[key] = payload[key];
  });
  return sanitized;
};

For applications using multiple JWT libraries or custom implementations, ensure consistent validation across all code paths. middleBrick's continuous monitoring (Pro plan) can help verify that these protections remain effective as your codebase evolves.

Consider implementing token introspection for additional security:

const introspectToken = async (token) => {
  const decoded = jwt.decode(token);
  if (!decoded || !decoded.jti) {
    throw new Error('Token missing jti');
  }
  
  // Check token against database or cache
  const tokenRecord = await TokenModel.findOne({ jti: decoded.jti });
  if (!tokenRecord || tokenRecord.revoked) {
    throw new Error('Token invalid or revoked');
  }
};

This approach adds a layer of protection by verifying token status against a server-side store, making prototype pollution attacks significantly more difficult to execute successfully.