sc-jwt

SC: JWT Implementation Flaws

Purpose

Detects JWT (JSON Web Token) implementation vulnerabilities including algorithm confusion attacks (alg:none, RS256→HS256), weak signing secrets, missing expiration/audience/issuer validation, insecure client-side storage, JWK injection, and key ID (kid) manipulation.

Activation

Called by sc-orchestrator during Phase 2 when JWT usage is detected.

Phase 1: Discovery

Keyword Patterns to Search

"jwt", "JWT", "jsonwebtoken", "jose", "PyJWT", "java-jwt",
"sign(", "verify(", "decode(", "encode(",
"alg", "HS256", "RS256", "ES256", "none",
"expiresIn", "exp", "iat", "aud", "iss", "sub",
"localStorage.*token", "sessionStorage.*token",
"Bearer", "Authorization"

Vulnerability Patterns

1. Algorithm Confusion (alg:none):

// VULNERABLE: Not specifying allowed algorithms
const payload = jwt.verify(token, secret);  // Accepts alg:none!

// SAFE: Specify algorithms explicitly
const payload = jwt.verify(token, secret, { algorithms: ['HS256'] });

2. Weak Signing Secret:

// VULNERABLE: Short/predictable secret
const token = jwt.sign(payload, 'secret');
const token = jwt.sign(payload, 'password123');
const token = jwt.sign(payload, process.env.JWT_SECRET || 'default');

// SAFE: Strong random secret (256+ bits)
const token = jwt.sign(payload, process.env.JWT_SECRET);
// Where JWT_SECRET is a 64+ character random string

3. Missing Expiration:

# VULNERABLE: No expiration
token = jwt.encode({"user_id": user.id}, SECRET_KEY, algorithm="HS256")

# SAFE: With expiration
token = jwt.encode({
    "user_id": user.id,
    "exp": datetime.utcnow() + timedelta(hours=1)
}, SECRET_KEY, algorithm="HS256")

4. JWT in localStorage (XSS Theft):

// VULNERABLE: XSS can steal token
localStorage.setItem('token', jwtToken);

// SAFE: HttpOnly cookie
res.cookie('token', jwtToken, {
  httpOnly: true,
  secure: true,
  sameSite: 'strict'
});

5. Missing Audience/Issuer Validation:

// VULNERABLE: No audience/issuer check
const payload = jwt.verify(token, secret, { algorithms: ['HS256'] });

// SAFE: Validate audience and issuer
const payload = jwt.verify(token, secret, {
  algorithms: ['HS256'],
  audience: 'https://api.example.com',
  issuer: 'https://auth.example.com'
});