CVE-2026-55165
MediumCVSS 4.8Exploitation Probability (EPSS)
Low risk3th percentile - higher than 3% of all known CVEs
Summary
Lemur before version 1.9.2 has a vulnerability in the JWT verifier that uses an unverified algorithm from the token header. In the default configuration, PyJWT rejects alg=none, so it is not a direct authentication bypass, but it is a defense-in-depth gap. After migration to asymmetric signing, algorithm confusion may occur, and algorithm-based anomaly detection is weakened.
Risk Assessment
The vulnerability could be exploited after migration to asymmetric signing to bypass authentication, and it weakens anomaly detection, increasing the risk of unauthorized access.
Recommendation
Upgrade Lemur to version 1.9.2 or later, which introduces the LEMUR_TOKEN_ALGORITHMS allowlist defaulting to HS256.
Original NVD description (English source)
Lemur manages TLS certificate creation. Prior to 1.9.2, the JWT verifier in lemur/auth/service.py:130-137 used fetch_token_header to read header_data["alg"] from an unverified token and passed that attacker-controlled value to decode_with_multiple_secrets. PyJWT 2.x rejects alg=none with the configured key, so the flaw is a defense-in-depth gap rather than a direct authentication bypass in the shipped configuration. The unpinned algorithm can become exploitable after an asymmetric-signing migration through algorithm confusion, and it weakens algorithm-based anomaly detection because the token chooses the recorded value. A separate disclosure of LEMUR_TOKEN_SECRET would also permit forged HS256 tokens, although that disclosure is an independent prerequisite. The fix introduces the server-controlled LEMUR_TOKEN_ALGORITHMS allowlist and defaults it to HS256. This issue is fixed in version 1.9.2.