JWT Tokens Explained: What Developers Actually Need to Know
How JWT tokens work, how to decode them, common security mistakes, and when to use them vs sessions.
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A JWT (JSON Web Token) is a signed JSON object that proves someone is who they claim to be. It's the most common authentication method for APIs and single-page apps. Here's how it actually works.
The Three Parts
A JWT looks like this: xxxxx.yyyyy.zzzzz (three base64 strings separated by dots).
Header: Algorithm and token type. Usually {"alg": "HS256", "typ": "JWT"}
Payload: The actual data. User ID, email, roles, expiration time. This is NOT encrypted. Anyone can decode it.
Signature: HMAC-SHA256 of the header + payload using a secret key. This proves the token hasn't been tampered with.
Decode Any JWT
Paste a JWT into the JWT Decoder to see its header, payload, and expiration time. This is essential for debugging auth issues. You can see exactly what claims are in the token without writing any code.
Common JWT Mistakes
Storing sensitive data in the payload. The payload is base64 encoded, not encrypted. Anyone with the token can read it. Don't put passwords, SSNs, or credit card numbers in there.
Not setting expiration. JWTs should expire. 15 minutes for access tokens, 7 days for refresh tokens. An eternal JWT is a security nightmare.
Storing JWTs in localStorage. Vulnerable to XSS attacks. Use httpOnly cookies instead.
Using "none" algorithm. Some libraries accept alg: "none" which means no signature verification. Always validate the algorithm.
JWT vs Sessions
JWTs are stateless. The server doesn't need to store anything. Sessions are stateful. The server keeps track of who's logged in. JWTs scale better but can't be instantly revoked. Sessions are easier to manage but need a database or Redis. For most apps, either works fine. Pick whichever your framework supports better.