Reject JWT tokens with alg:none to prevent signature bypass

[emrazik]

Finding

NetSPI Finding #1048219779 — JWT Weak Signature Validation | High

The JWT library accepted tokens whose header declared "alg": "none". When a token has no algorithm, the signature field is ignored and the library treats the payload as trusted without any cryptographic check. An unauthenticated attacker can craft a token with any email address, set alg to none (or case variants like None, nOnE), omit the signature, and be accepted as that user by every protected endpoint.

What changed

lib/insecurity.ts — isAuthorized()

The function previously returned expressJwt(...) directly. It now wraps that middleware: before delegating to expressJwt, it decodes the token header with jws.decode() and returns 401 immediately if the alg field is missing or equals none (case-insensitive). expressJwt only receives tokens with a real algorithm.

lib/insecurity.ts — updateAuthenticatedUsers()

Before calling jwt.verify() to cache the decoded user, the middleware now checks the token header via jws.decode() and skips verification entirely for alg:none tokens, so they are never stored as authenticated sessions.

Test plan

• Send a forged alg:none token (e.g. header.payload.) to GET /rest/basket/1 — expect 401
• Log in normally (RS256 token) and verify protected endpoints still work
• Confirm alg:none tokens with a forged admin email are rejected

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