jwt-security

What this skill does

# 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

```json
{
  "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

```json
{
  "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

```javascript
// 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

```javascript
// 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)

```javascript
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

```javascript
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

```javascript
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

```javascript
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

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

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

### 2. None Algorithm Attack

```javascript
// 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)

```javascript
// 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

```javascript
// 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

```javascript
// 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

```javascript
// 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

```javascript
// 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

```javascript
// 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 && isTo