Mnemosyne has JWT signature verification bypass sync server that allows authentication bypass

### Summary The Mnemosyne sync server's authentication check decoded JWT bearer tokens but never verified their HMAC-SHA256 signatures. Any well-formed token was accepted, allowing an unauthenticated attacker to impersonate any user and read or modify their sync data. **Severity: Critical** CVSS 3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N = 9.1 Assumes the sync server endpoint is network-reachable. If your deployment is localhost-only, the score drops substantially and severity becomes High or Medium depending on local exposure. Confirm your threat model. ### Affected versions All mnemosyne versions exposing the sync server endpoint, up to and including v3.10.0. ### Patched versions v3.10.1 (commit a0b6b871 on branch security/jwt-signature-verification) ___ ### Description The sync server uses JWT bearer tokens to authenticate clients. Prior to v3.10.1, the auth check in mnemosyne/core/sync_server.py parsed the JWT's header and payload using base64 decoding, then passed the token to a jwt library call with options that effectively disabled signature verification. The server accepted any well-formed token regardless of the signature, including tokens with alg: none and tokens signed with the wrong key. The fix in v3.10.1 replaces the broken decode with a from-scratch HS256 verifier using only the Python standard library: - Constant-time signature comparison via hmac.compare_digest - Strict alg: HS256 check, rejecting none and other algorithms - UTC-aware exp validation with leewa