jwt-security

SKILL.md

JWT Security

You are an expert in JSON Web Token (JWT) security implementation. Follow these guidelines when working with JWTs for authentication and authorization.

Core Principles

JWTs are not inherently secure - security depends on implementation

Always validate tokens server-side, even for internal services

Use asymmetric signing (RS256, ES256) when possible

Keep tokens short-lived and implement proper refresh mechanisms

Never store sensitive data in JWT payloads

Token Structure

A JWT consists of three parts: Header, Payload, and Signature.

header.payload.signature

Header Best Practices

{
  "alg": "RS256",
  "typ": "JWT",
  "kid": "key-identifier-for-rotation"
}

Always include kid (key ID) for key rotation support

Use typ: "JWT" explicitly

Never accept alg: "none"

Payload Best Practices

{
  "iss": "https://auth.example.com",
  "sub": "user-uuid-here",
  "aud": "https://api.example.com",
  "exp": 1704067200,
  "iat": 1704063600,
  "nbf": 1704063600,
  "jti": "unique-token-id"
}

Required claims:

iss (issuer): Who created the token

sub (subject): Who the token represents

aud (audience): Who the token is intended for

exp (expiration): When the token expires

iat (issued at): When the token was created

Recommended claims:

nbf (not before): Token not valid before this time

jti (JWT ID): Unique identifier for token revocation

Signing Algorithm Selection

Recommended: Asymmetric Algorithms

// RS256 - RSA with SHA-256 (most widely supported)
// ES256 - ECDSA with P-256 and SHA-256 (smaller keys)
// EdDSA - Edwards-curve Digital Signature Algorithm (most secure)

const ALLOWED_ALGORITHMS = ['RS256', 'ES256', 'EdDSA'];

When Symmetric is Required

// HS256 - HMAC with SHA-256
// Only use with a strong secret (minimum 256 bits / 32 bytes)
const secret = crypto.randomBytes(64).toString('hex');

Token Creation

Using RS256 (Recommended)

const jwt = require('jsonwebtoken');
const fs = require('fs');

const privateKey = fs.readFileSync('private.pem');

function createToken(userId, roles) {
  const payload = {
    sub: userId,
    roles: roles,
    // Keep custom claims minimal
  };

  const options = {
    algorithm: 'RS256',
    expiresIn: '15m', // Short-lived access tokens
    issuer: 'https://auth.example.com',
    audience: 'https://api.example.com',
    keyid: 'current-key-id',
  };

  return jwt.sign(payload, privateKey, options);
}

Token Lifetime Guidelines

const TOKEN_LIFETIMES = {
  accessToken: '15m',      // 15 minutes max
  refreshToken: '7d',      // 7 days with rotation
  idToken: '1h',           // 1 hour
  passwordReset: '15m',    // 15 minutes
  emailVerification: '24h', // 24 hours
};

Token Validation

Complete Validation Example

const jwt = require('jsonwebtoken');
const jwksClient = require('jwks-rsa');

// JWKS client for fetching public keys
const client = jwksClient({
  jwksUri: 'https://auth.example.com/.well-known/jwks.json',
  cache: true,
  cacheMaxAge: 600000, // 10 minutes
  rateLimit: true,
  jwksRequestsPerMinute: 10,
});

async function validateToken(token) {
  // 1. Decode header without verification to get kid
  const decoded = jwt.decode(token, { complete: true });

  if (!decoded) {
    throw new Error('Invalid token format');
  }

  // 2. Validate algorithm against whitelist
  if (!ALLOWED_ALGORITHMS.includes(decoded.header.alg)) {
    throw new Error(`Algorithm ${decoded.header.alg} not allowed`);
  }

  // 3. Get signing key
  const key = await client.getSigningKey(decoded.header.kid);
  const publicKey = key.getPublicKey();

  // 4. Verify signature and claims
  const verified = jwt.verify(token, publicKey, {
    algorithms: ALLOWED_ALGORITHMS, // Whitelist algorithms
    issuer: 'https://auth.example.com',
    audience: 'https://api.example.com',
    clockTolerance: 30, // 30 seconds clock skew tolerance
  });

  return verified;
}

Validation Checklist

function validateTokenClaims(decoded) {
  const now = Math.floor(Date.now() / 1000);

  // 1. Check expiration
  if (decoded.exp && decoded.exp < now) {
    throw new Error('Token expired');
  }

  // 2. Check not before
  if (decoded.nbf && decoded.nbf > now) {
    throw new Error('Token not yet valid');
  }

  // 3. Check issuer
  if (decoded.iss !== EXPECTED_ISSUER) {
    throw new Error('Invalid issuer');
  }

  // 4. Check audience
  const audiences = Array.isArray(decoded.aud) ? decoded.aud : [decoded.aud];
  if (!audiences.includes(EXPECTED_AUDIENCE)) {
    throw new Error('Invalid audience');
  }

  // 5. Check required claims exist
  if (!decoded.sub) {
    throw new Error('Missing subject claim');
  }

  return true;
}

Security Vulnerabilities to Prevent

1. Algorithm Confusion Attack

// WRONG: Accepting any algorithm
jwt.verify(token, secret); // Vulnerable!

// CORRECT: Whitelist allowed algorithms
jwt.verify(token, key, { algorithms: ['RS256'] });

2. None Algorithm Attack

// Always reject 'none' algorithm
if (decoded.header.alg === 'none' || decoded.header.alg.toLowerCase() === 'none') {
  throw new Error('Algorithm none is not allowed');
}

3. Key Confusion (RS256 vs HS256)

// When using asymmetric keys, never allow symmetric algorithms
const ASYMMETRIC_ONLY = ['RS256', 'RS384', 'RS512', 'ES256', 'ES384', 'ES512', 'EdDSA'];

jwt.verify(token, publicKey, { algorithms: ASYMMETRIC_ONLY });

4. Weak HMAC Secrets

// Minimum 256-bit (32 byte) secret for HS256
// Minimum 384-bit (48 byte) secret for HS384
// Minimum 512-bit (64 byte) secret for HS512

function generateHmacSecret(algorithm) {
  const bits = parseInt(algorithm.slice(2)); // HS256 -> 256
  const bytes = bits / 8;
  return crypto.randomBytes(Math.max(bytes, 32)).toString('hex');
}

Token Storage

Browser Storage Security

// Best: HttpOnly cookie (requires backend support)
// Server sets:
res.cookie('access_token', token, {
  httpOnly: true,
  secure: true,
  sameSite: 'strict',
  maxAge: 900000, // 15 minutes
});

// Acceptable: In-memory (lost on refresh)
let accessToken = null;
function setToken(token) {
  accessToken = token;
}

// Avoid: localStorage (vulnerable to XSS)
// Avoid: sessionStorage for sensitive tokens

Token Transmission

// Always use Authorization header
fetch('/api/resource', {
  headers: {
    Authorization: `Bearer ${accessToken}`,
  },
});

// Never put tokens in URLs (logged, cached, visible in history)
// WRONG: /api/resource?token=eyJ...

Refresh Token Implementation

// Refresh tokens should be:
// 1. Stored securely (httpOnly cookie or secure server-side storage)
// 2. Rotated on each use
// 3. Bound to the client (if possible)

async function refreshAccessToken(refreshToken) {
  // Validate refresh token
  const decoded = await validateRefreshToken(refreshToken);

  // Check if token has been revoked
  const isRevoked = await checkTokenRevocation(decoded.jti);
  if (isRevoked) {
    throw new Error('Refresh token has been revoked');
  }

  // Generate new tokens
  const newAccessToken = createAccessToken(decoded.sub);
  const newRefreshToken = createRefreshToken(decoded.sub);

  // Revoke old refresh token (rotation)
  await revokeToken(decoded.jti);

  return { accessToken: newAccessToken, refreshToken: newRefreshToken };
}

Token Revocation

// Maintain a revocation list for early token invalidation
const revokedTokens = new Set(); // Use Redis in production

function revokeToken(jti) {
  revokedTokens.add(jti);
}

function isTokenRevoked(jti) {
  return revokedTokens.has(jti);
}

// Include revocation check in validation
async function validateToken(token) {
  const decoded = jwt.verify(token, key, options);

  if (decoded.jti && isTokenRevoked(decoded.jti)) {
    throw new Error('Token has been revoked');
  }

  return decoded;
}

Key Rotation

// Support multiple keys during rotation
const keyStore = {
  'key-2024-01': { /* current key */ },
  'key-2023-12': { /* previous key, still valid */ },
};

// JWKS endpoint should expose all valid public keys
app.get('/.well-known/jwks.json', (req, res) => {
  const keys = Object.entries(keyStore).map(([kid, key]) => ({
    kid,
    kty: 'RSA',
    use: 'sig',
    alg: 'RS256',
    n: key.publicKey.n,
    e: key.publicKey.e,
  }));

  res.json({ keys });
});

Express Middleware Example

const expressJwt = require('express-jwt');
const jwksRsa = require('jwks-rsa');

const jwtMiddleware = expressJwt({
  secret: jwksRsa.expressJwtSecret({
    cache: true,
    rateLimit: true,
    jwksRequestsPerMinute: 5,
    jwksUri: 'https://auth.example.com/.well-known/jwks.json',
  }),
  audience: 'https://api.example.com',
  issuer: 'https://auth.example.com',
  algorithms: ['RS256'],
});

// Protected route
app.get('/api/protected', jwtMiddleware, (req, res) => {
  // req.auth contains the decoded token
  res.json({ user: req.auth.sub });
});

Testing

describe('JWT Validation', () => {
  it('should reject expired tokens', async () => {
    const expiredToken = createToken({ exp: Math.floor(Date.now() / 1000) - 3600 });
    await expect(validateToken(expiredToken)).rejects.toThrow('expired');
  });

  it('should reject tokens with wrong issuer', async () => {
    const wrongIssuer = createToken({ iss: 'https://evil.com' });
    await expect(validateToken(wrongIssuer)).rejects.toThrow('issuer');
  });

  it('should reject none algorithm', async () => {
    const noneAlg = 'eyJhbGciOiJub25lIiwidHlwIjoiSldUIn0.eyJzdWIiOiIxMjM0NTY3ODkwIn0.';
    await expect(validateToken(noneAlg)).rejects.toThrow('algorithm');
  });
});

Common Anti-Patterns to Avoid

Using JWTs for session management (prefer server-side sessions for web apps)

Storing sensitive data in JWT payload (it's only encoded, not encrypted)

Not validating all claims

Using weak or hardcoded secrets

Not implementing token expiration

Trusting the algorithm header without validation

Not implementing refresh token rotation

Logging full tokens