# 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.

- **Severity:** Medium 4.8 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 Aug 2026
- **Updated:** 18 Aug 2026
- **Also known as:** GHSA-r9gp-7f88-9r54, PYSEC-2026-2589

## 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 (Improper Verification of Cryptographic Signature)

## Which versions are affected by CVE-2026-55165?

3 open source packages ship an affected version, including lemur.


| Package | Ecosystem | 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.

## Is CVE-2026-55165 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55165 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-55165 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55165 exists.

Affected packages and fixed versions:
- lemur (PyPI): affected all versions; fixed in 1.9.2
- lemur (PyPI): affected all versions; fixed in 1.9.2
- lemur (PyPI): affected all versions; fixed in 1.9.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-55165/ (Markdown: https://insights.sbomify.com/CVE-2026-55165/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-55165
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-55165
- OSV: https://osv.dev/vulnerability/CVE-2026-55165

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-55165/
Summaries are generated and may contain mistakes; the official description above is the record.