Verifying tokens properly

Never trust the token about itself

The token is attacker-controlled, so nothing in it — least of all how to verify it — can be trusted:

Tip: Pin the accepted algorithm in your verification call (never read it from the token), and reject none outright. Use a long random secret for HMAC or proper keys for RSA/ECDSA, verify exp/nbf/aud, and keep anything truly sensitive server-side rather than trusting claims in a token.