How to Fix a JWT Verification Bug (verify_signature: False)

What verify_signature: False actually does

PyJWT lets you decode without verifying for debugging purposes. When you pass:

jwt.decode(token, options={"verify_signature": False})

PyJWT skips all signature checks and returns the payload for any token - one signed with a random key, a token from a different service, and critically the alg=none forgery where no signature exists at all. If this is in your verify_jwt() function, authentication is completely bypassed.

The fix: always decode with the secret and algorithm list

Replace the unsafe call with a real decode that enforces the signature and pins the accepted algorithm to HS256:

import jwt

SECRET = "your-signing-key"  # in production: load from env or KMS

def verify_jwt(token: str):
    try:
        return jwt.decode(token, SECRET, algorithms=["HS256"])
    except jwt.PyJWTError:
        return None

Three things are doing the work here:

• SECRET as the second argument - PyJWT verifies the HMAC signature against this key; a forged token signed with any other key fails immediately.
• algorithms=["HS256"] - the allowlist blocks the alg=none attack, where an attacker strips the signature and sets the header to none. Without this list PyJWT may accept it.
• except jwt.PyJWTError - expired tokens, malformed tokens, bad signatures, and wrong algorithms all raise a subclass of PyJWTError. Catching the base class returns None instead of letting the exception propagate and 500 the endpoint.

Why alg=none is dangerous

The alg=none exploit lets an attacker craft a token with a valid-looking payload, set the header to {"alg": "none"}, and send it with an empty signature. Libraries that do not enforce an algorithm allowlist will accept it as valid. This is a well-documented class of vulnerability that has affected production systems - the only defense is an explicit algorithms= list.

What production JWT validation also checks

The snippet above handles signature verification. A production verify_jwt should also validate:

• exp (expiry) - PyJWT checks this automatically; make sure your tokens include it.
• iss / aud (issuer / audience) - pass them as kwargs: jwt.decode(token, SECRET, algorithms=["HS256"], audience="my-api", issuer="auth.example.com").
• Asymmetric keys (RS256 / ES256) - share the public key, not a symmetric secret, so the signing key never leaves the auth service. Load from a JWKS endpoint.
• Key rotation - rotating the signing key invalidates all outstanding tokens; plan for a short-lived transition window.