JSON Web Tokens (JWT) have become the backbone of modern OAuth 2.0 and OpenID Connect (OIDC) authentication, carrying identity and authorization claims securely between parties. Proper decoding and validation of JWTs are critical to maintaining the security of your applications.

What is a JWT?

A JWT is a compact, URL-safe token consisting of three parts:

  • Header: Specifies the token type and signing algorithm.
  • Payload: Contains claims about the user or system (e.g., user ID, roles).
  • Signature: Verifies token integrity and authenticity.

Example JWT:

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.
eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkhvdXBpbmciLCJpYXQiOjE1MTYyMzkwMjJ9.
SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

How to Decode a JWT

You can decode JWTs client-side or server-side. The payload is base64-url encoded and can be decoded without a secret key to inspect claims.

Example (Node.js):

const jwt = require('jsonwebtoken');
const decoded = jwt.decode(token);
console.log(decoded);

Validating JWTs

Decoding alone is insufficient. Proper validation steps include:

  • Verify Signature: Ensure the token was signed by a trusted issuer.
  • Check Expiration (exp claim): Reject tokens past expiry.
  • Validate Audience (aud claim): Confirm token is intended for your app.
  • Validate Issuer (iss claim): Confirm token issuer matches expected authority.
  • Check Other Claims: Such as nonce, scopes, or custom claims.

Common Pitfalls

  • Failing to validate the signature, allowing forged tokens.
  • Ignoring expiration leading to replay attacks.
  • Not validating issuer or audience, risking token misuse.

Code Example: JWT Verification in Node.js

const jwt = require('jsonwebtoken');
const publicKey = "-----BEGIN PUBLIC KEY-----\n...your public key...\n-----END PUBLIC KEY-----";

jwt.verify(token, publicKey, { algorithms: ['RS256'], audience: 'your_audience', issuer: 'https://your-issuer.com' }, (err, decoded) => {
  if (err) {
    console.error('Token validation failed:', err);
  } else {
    console.log('Valid token:', decoded);
  }
});

Real-World Use Cases

  • Validating ID tokens returned by OpenID Connect providers.
  • Ensuring access tokens have not been tampered with before granting API access.
  • Enforcing scopes and user roles embedded within JWT claims.

Reflective Questions

  • How does your current system validate JWT tokens?
  • Are your public keys and JWKS endpoints securely managed?
  • Have you implemented token revocation or blacklisting strategies?

Conclusion

Secure decoding and validation of JWTs is fundamental to protecting OAuth 2.0 based systems from token misuse. Properly implementing these steps ensures trusted authentication and authorization flows.

👉 Related: Understanding the Authorization Code Flow in OAuth 2.0
👉 Related: How PKCE Enhances Security in Authorization Code Flow