Netflix Lemur up to 1.9.1 JWT Verifier lemur/auth/service.py fetch_token_header alg certificate validation

Summaryinfo

A vulnerability identified as problematic has been detected in Netflix Lemur up to 1.9.1. The affected element is the function fetch_token_header of the file lemur/auth/service.py of the component JWT Verifier. The manipulation of the argument alg leads to certificate validation. This vulnerability is referenced as CVE-2026-55165. Remote exploitation of the attack is possible. No exploit is available.

Detailsinfo

A vulnerability, which was classified as problematic, has been found in Netflix Lemur up to 1.9.1. This issue affects the function fetch_token_header of the file lemur/auth/service.py of the component JWT Verifier. The manipulation of the argument alg with an unknown input leads to a certificate validation vulnerability. Using CWE to declare the problem leads to CWE-295. The product does not validate, or incorrectly validates, a certificate. Impacted is integrity. The summary by CVE is:

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.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-55165 since 06/16/2026. The exploitation is known to be difficult. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details are known, but no exploit is available. MITRE ATT&CK project uses the attack technique T1587.003 for this issue.

Upgrading to version 1.9.2 eliminates this vulnerability.

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Productinfo

Vendor

• Netflix

Name

• Lemur

Version

• 1.9.0
• 1.9.1

Website

• Product: https://github.com/Netflix/lemur/

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