CVE-2026-55165 Lemur: JWT verifier honors attacker-supplied alg, enabling ATO
Attackers can bypass JWT authentication in Lemur by manipulating the algorithm header, potentially leading to account takeover if combined with secret disclosure or algorithm confusion.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-55165?
CVE-2026-55165 is a medium-severity authentication bypass vulnerability in Netflix lemur, published on 18 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Weakness: CWE-347
Which versions are affected by CVE-2026-55165?
3 open source packages ship an affected version, including lemur.
| Package | Affected | Fixed in |
|---|---|---|
lemur PyPI | All versions | 1.9.2 |
lemur PyPI | All versions | 1.9.2 |
lemur PyPI | All versions | 1.9.2 |
How do I fix CVE-2026-55165?
Upgrade lemur to 1.9.2 or later.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-55165 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55165 exists.
Sources
Also known as GHSA-r9gp-7f88-9r54, PYSEC-2026-2589.