Can I store sensitive data in a JWT?

No. The payload is base64url-encoded, not encrypted, so anyone holding the token reads it in one line of code. Keep claims to identifiers, expiry, audience, and a coarse role. If you truly need encrypted claims, JWE exists, but at that point a plain opaque session ID plus a server-side lookup is usually simpler and safer.

How do I log a user out if JWTs can't be revoked?

Use short-lived access tokens, 5 to 15 minutes, paired with a server-side refresh token, and delete the refresh token on logout. The user's current access token dies within minutes on its own. If a specific route can't tolerate even that window, check a small denylist of revoked token IDs in Redis for that route only.

Should I use HS256 or RS256?

One service signing and verifying its own tokens can use HS256 and keep the secret in one place. The moment multiple services verify the same tokens, switch to RS256 or ES256, because with HS256 every verifier can also forge. Asymmetric signing plus a JWKS endpoint also makes key rotation routine instead of a coordinated fleet-wide event.

Securitychapter 2 of 4 · 4 lessons

JWT (JSON Web Token)

Stateless, self-contained tokens that carry signed claims, no server-side session lookup needed.

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A request arrives at one of your twelve services carrying a token. That service has to decide, in under a millisecond, whether the person is who they claim and whether they are an admin.

It could ask a central auth service, which means a network call on every request to every service, and an outage there takes all twelve down.

Or the token could carry the answer itself, signed, so any service checks it with local arithmetic and no network at all.

That is a JWT: a small signed document saying this is user 4812 and they are an admin. It removes the shared bottleneck, and it introduces a problem the industry is still arguing about, which is what happens when you need to take that admin badge away before the token expires.

Lessons

4 in this chapter

Anatomy of a JWTThree base64url segments: a header, a claims payload anyone can read, and a signature.2 min
Signing and VerificationHS256 shares one secret with every verifier; RS256 keeps the forging power in one place.3 min
The Revocation ProblemA signature can't be un-signed, so a JWT stays valid until exp no matter what happens.2 min
JWT Mistakes That Get Exploitedalg none, key confusion, localStorage theft: the exploits are verification steps someone skipped.3 min