The joserfc Python library, which implements JSON Object Signing and Encryption (JOSE) standards, has an improper authentication vulnerability (CWE-287) in versions before 1.6.8. Specifically, the jwt.decode function accepts forged HMAC-signed tokens if the caller supplies an empty string or None as the verification key. This occurs because the HMACAlgorithm.sign and verify methods pass the output of OctKey.get_op_key(...) to hmac.new(...) without rejecting zero-length keys. OctKey.import_key only emits a SecurityWarning for keys shorter than 14 bytes but does not reject zero-length keys, allowing token forgery. The issue is resolved in joserfc version 1.6.8.

An attacker can forge HMAC-signed JSON Web Tokens (JWTs) that the library will accept as valid if the verification key is empty or None. This can lead to bypassing authentication controls relying on token verification, potentially allowing unauthorized access or privilege escalation in applications using vulnerable versions of joserfc.

Upgrade to joserfc version 1.6.8 or later, where this vulnerability is fixed by rejecting zero-length keys during HMAC verification. No other mitigations are indicated by the vendor advisory.