CVE-2026-55165

Lemur : JWT verifier trusts attacker-supplied alg from token header — defense-in-depth gap; chain-dependent ATO with secret disclosure

Published Aug 18, 2026 CVSS 3.1 · 4.8 Medium EPSS < 1% KEV · Not listed
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.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Github GHSA Github GHSA GHSA-r9gp-7f88-9r54 Lemur: JWT verifier honors attacker-supplied alg, enabling ATO
History
Type Values Removed Values Added
First Time appeared Netflix
Netflix lemur
Vendors & Products Netflix
Netflix lemur
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'poc', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
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.
Title Lemur : JWT verifier trusts attacker-supplied alg from token header — defense-in-depth gap; chain-dependent ATO with secret disclosure
Weaknesses CWE-347
References
Metrics cvssV3_1

{'score': 4.8, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
GitHub_M
Published
Updated
Aug 18, 2026, 7:35 PM
Reserved
Jun 16, 2026, 3:13 PM
Status
Deferred
Published
Aug 18, 2026, 7:16 PM
Modified
Sep 8, 2026, 9:02 PM