What does an API need to validate before trusting a JWT?

Validate JWTs and trust claims carefully

Explains JWT validation, trusted issuers, audiences, expiration, signing keys, claims, and server-side authorization boundaries.

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TL;DR

The API should validate the JWT signature, issuer, audience, expiration, and signing key. It should also use the expected token type and algorithm. Only after validation should the API read claims like user id, roles, tenant, or scopes. A token string from the client is just input until those checks pass.

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Issuer, audience, signature, expiry, signing keys, token type, claims trust, clock skew, and authorization use.

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Why this question matters
A JWT is untrusted input until its signature and intended use have been validated. Issuer, audience, lifetime, algorithm, key, and token type define whether its claims can participate in an authorization decision.
The API should validate the JWT signature, issuer, audience, expiration, and signing key. It should also use the expected token type and algorithm. Only after validation should the API read claims like user id, roles, tenant, or scopes. A token string from the client is just input until those checks pass.
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I would be careful about what claims mean. A role or tenant claim may come from the identity provider, but resource ownership can still depend on application data. The API should not accept caller-supplied claims outside the signed token, and it should not trust a token intended for another audience. I would keep token lifetime short enough to reduce risk but long enough for normal user experience.

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Decoding a JWT without verifying its signature.
Ignoring issuer or audience.
Trusting tenant or role values sent outside the token.
Using long-lived access tokens for convenience.

What does an API need to validate before trusting a JWT?

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