Lemur's JWT verifier in versions before 1.9.2 reads the 'alg' field from the JWT header without verifying it, passing this attacker-controlled value to the token decoding function. While PyJWT 2.x rejects 'alg=none' with the configured key, this behavior creates a defense-in-depth gap that could be exploited after migrating to asymmetric signing due to algorithm confusion. It also undermines algorithm-based anomaly detection since the token specifies the algorithm used. The vulnerability is addressed in Lemur 1.9.2 by enforcing a server-controlled allowlist of JWT algorithms, defaulting to HS256.

The vulnerability allows an attacker to influence the JWT verification algorithm, weakening security checks and potentially enabling account takeover (ATO) if combined with other conditions such as disclosure of the LEMUR_TOKEN_SECRET. The flaw itself does not directly bypass authentication in the default configuration but reduces the effectiveness of anomaly detection and could become exploitable after asymmetric-signing migration.

Upgrade Lemur to version 1.9.2 or later, which introduces a server-controlled allowlist for JWT algorithms (LEMUR_TOKEN_ALGORITHMS) defaulting to HS256. This fix prevents attacker-supplied algorithms from being honored during token verification. No other mitigations are necessary if the upgrade is applied.