Vulnerability · Published Aug 18, 2026 · Assigned by GitHub_M
CVE-2026-55165Lemur : JWT verifier trusts attacker-supplied alg from token header — defense-in-depth gap; chain-dependent ATO with secret disclosure
Medium 4.8
In plain English
A medium-severity security flaw in lemur from Netflix. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 5% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| PyPI | lemur | 1.9.2 | GHSA-r9gp-7f88-9r54 |
| PyPI | lemur | 1.9.2 | PYSEC-2026-2589 |
Affected products (as reported)
Weakness: CWE-347 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
References
- https://github.com/Netflix/lemur/security/advisories/GHSA-r9gp-7f88-9r54 x_refsource_CONFIRM
- https://github.com/Netflix/lemur/commit/89898e665beed044cf7bd5927dfaa484e55bf242 x_refsource_MISC
- https://github.com/Netflix/lemur/releases/tag/v1.9.2 x_refsource_MISC