JSON Web Tokens: what can go wrong and RFC 8725's answers

What it is

A JWT carries a header (algorithm), a payload of claims (iss, sub, aud, exp, iat, jti) and a signature. RFC 8725 collects best current practices: pin the accepted algorithms per application, validate all cryptographic operations before using any claim, require and check aud and iss, use exp with short lifetimes, and use a key identifier with keys served from a trusted location.

Why it matters

Historic vulnerabilities came from libraries that trusted the token's own alg header (including none or confusing RSA with HMAC), from missing audience checks that let a token for one service be replayed against another, and from tokens that lived for days without revocation.

How to apply

• Configure the verifier with an explicit algorithm and key; reject anything else.
• Verify iss, aud, exp and nbf on every request; treat clock skew with a small tolerance, not a large one.
• Keep access tokens short-lived (minutes) and use a refresh mechanism with server-side state for revocation.
• Do not put secrets or personal data beyond what the recipient needs in the payload; it is only base64url-encoded.
• Prefer opaque session identifiers for first-party browser sessions; JWTs shine for cross-service delegation.

Pitfalls

Storing JWTs in localStorage, exposed to script injection. Using the same key for signing and other purposes. Accepting unsigned tokens in "development mode" that ships to production.

What it is

A JWT carries a header (algorithm), a payload of claims (iss, sub, aud, exp, iat, jti) and a signature. RFC 8725 collects best current practices: pin the accepted algorithms per application, validate all cryptographic operations before using any claim, require and check aud and iss, use exp with short lifetimes, and use a key identifier with keys served from a trusted location.

Why it matters

Historic vulnerabilities came from libraries that trusted the token's own alg header (including none or confusing RSA with HMAC), from missing audience checks that let a token for one service be replayed against another, and from tokens that lived for days without revocation.

How to apply

• Configure the verifier with an explicit algorithm and key; reject anything else.
• Verify iss, aud, exp and nbf on every request; treat clock skew with a small tolerance, not a large one.
• Keep access tokens short-lived (minutes) and use a refresh mechanism with server-side state for revocation.
• Do not put secrets or personal data beyond what the recipient needs in the payload; it is only base64url-encoded.
• Prefer opaque session identifiers for first-party browser sessions; JWTs shine for cross-service delegation.

Pitfalls

Storing JWTs in localStorage, exposed to script injection. Using the same key for signing and other purposes. Accepting unsigned tokens in "development mode" that ships to production.

When not to use JWTs

For first-party browser sessions, an opaque server-side session identifier gives revocation, rotation and small cookies with none of the pitfalls above. JWTs earn their complexity when a token must be verified by a service that does not share the session store: cross-service delegation, machine-to-machine access, short-lived capability tokens.