Understanding JSON Web Tokens: A Developer Guide

JSON Web Tokens (JWTs) are a compact, URL-safe way to represent claims between two parties. They are widely used for authentication and authorization in modern web applications.

JWT Structure

A JWT consists of three parts separated by dots: header.payload.signature. Each part is Base64URL-encoded JSON.

• Header — Contains the token type and signing algorithm
• Payload — Contains claims (registered, public, and private)
• Signature — Verifies the token has not been tampered with

Security Considerations

• Never store sensitive data in the payload — JWTs are encoded, not encrypted
• Always validate the signature — Accepting unsigned tokens is a critical vulnerability
• Set short expiration times — Use refresh tokens for long-lived sessions
• Use strong signing keys — At least 256 bits for HMAC algorithms

Inspecting JWTs Safely

Use a client-side JWT inspector to decode and examine tokens. Server-side decoders may log your tokens, which is especially dangerous for production credentials.