VU#762428: Authlib library contains a signature‑verification bypass vulnerability

Authlib (versions up to and including 1.7.2) contain a signature‑verification bypass in the JSON Web Signature (JWS) general JSON serialization handling. The JsonWebSignature.deserialize_json() function accepts a JWS object with an empty "signatures" array and treats the payload as successfully verified, allowing attackers to supply arbitrary forged content without possessing any key material.

Authlib is a Python library that provides tools for implementing OAuth, OpenID Connect, JWT/JWS/JWE (JSON Web Token / JSON Web Signature / JSON Web Encryption), and other modern authentication and authorization standards. It’s widely used in web applications and microservices to handle token creation, cryptographic validation, and secure communication.

As discussed in CVE-2026-96760, a security flaw in Authlib’s handling of JSON Web Signatures (JWS) makes it possible for an attacker to skip signature verification completely. Normally, a JWS should include at least one valid signature to prove the data hasn’t been tampered with. However, Authlib’s deserialize_json() function mistakenly accepts JWS objects even when the "signatures" section is an empty list. Because the function starts by assuming the signatures are valid and never performs any checks when the list is empty, it ends up treating unsigned data as if it were properly signed. This means an attacker could provide a JWS with no signatures, and Authlib would still treat it as trusted. Both ways of loading a JWS in Authlib are affected:

jws.deserialize_json({"payload":"...", "signatures":[]}, key=None)
jws.deserialize('{"payload":"...","signatures":[]}', key=None)