JWT verification — the five ways apps get it wrong

What it is

A JSON Web Token carries a payload and a signature. Verification checks that the signature matches the payload under an expected algorithm and key, and that the declared claims (issuer, audience, expiration) match what your app expects.

Vulnerable example

// Vulnerable: decode without verify
import jwt from "jsonwebtoken";
const payload = jwt.decode(token) as { sub: string };
// Attacker forges ANY payload. jwt.decode does not verify signatures.

// Vulnerable: verify without checking alg
jwt.verify(token, publicKey);
// Vulnerable to alg: 'none' and HS256/RS256 confusion attacks.

Fixed example

// Fixed: verify with explicit algorithm and claim checks
import jwt from "jsonwebtoken";

const payload = jwt.verify(token, publicKey, {
  algorithms: ["RS256"],           // pin the algorithm
  issuer: "https://yourapp.com",   // check issuer
  audience: "api.yourapp.com",     // check audience
});
// expiration is checked by default by jwt.verify

How Securie catches it

// Fixed: verify with explicit algorithm and claim checks
import jwt from "jsonwebtoken";

const payload = jwt.verify(token, publicKey, {
  algorithms: ["RS256"],           // pin the algorithm
  issuer: "https://yourapp.com",   // check issuer
  audience: "api.yourapp.com",     // check audience
});
// expiration is checked by default by jwt.verify

Checklist

• Algorithms whitelist is explicit (never default or empty)
• Issuer claim is verified
• Audience claim is verified
• Expiration is checked (default in most libs — do not disable)
• JWT is rejected if the key is not a known JWK ID
• Public keys rotated at least annually

FAQ

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