A JWT library accepts `alg: none`. What is the risk?

1Tokens expire too quickly
2Anyone can forge a valid token by setting alg to none✓ correct
3The payload is encrypted and unreadable
4Nothing — none is a secure algorithm
A JWT's header includes an alg field specifying the signing algorithm. When a library accepts alg: none, it treats the token as unsigned — meaning anyone can create a JWT with arbitrary claims (e.g. {"sub": "admin"}) by simply stripping the signature and setting alg to none. The server validates 'successfully' because it checks no signature. Fix: always specify the expected algorithm server-side (e.g. { algorithms: ['RS256'] }) and reject tokens that use any other algorithm, including none.

An attacker can forge tokens because no signature is verified.

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