# Lemur: JWT verifier honors attacker-supplied alg, enabling ATO

Advisory record · `GHSA-r9gp-7f88-9r54`

- **Identifiers:** `GHSA-r9gp-7f88-9r54` · `CVE-2026-55165` · `PYSEC-2026-2589`
- **State:** Confirmed
- **Severity:** CVSS 4.8 Medium
- **Critical:** No
- **Weakness (CWE):** [CWE-347](https://cwe.mitre.org/data/definitions/347.html)
- **Tags:** [Weak cryptography or TLS](https://agentprime.io/advisories/tag/weak-crypto.md)
- **Published:** 25 Jun 2026
- **Updated:** 13 Jul 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 25 Jun 2026, 22:05 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `lemur` (PyPI): affected `<1.9.2`; fixed in `1.9.2`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> The Lemur package (PyPI) before 1.9.2 reads the signing algorithm from the unverified JWT header instead of pinning it on the server. With PyJWT 2.x, single-request forgery is blocked by the library, so exploitation requires a separate flaw that discloses the LEMUR_TOKEN_SECRET or a future move to asymmetric signing; the source gives few details on the exact impact. Upgrade to version 1.9.2 or later.

Written 6 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 4.8 Medium, CVSS 3.1: `CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N`, from CNA (GitHub_M)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0015, the 4th percentile, on 12 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 15:02 UTC: First seen by agentprime
- 31 Oct 2026, 15:02 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-r9gp-7f88-9r54)

## Sources

- [GitHub Advisory Database: GHSA-r9gp-7f88-9r54](https://github.com/advisories/GHSA-r9gp-7f88-9r54)
- [OSV: GHSA-r9gp-7f88-9r54](https://osv.dev/vulnerability/GHSA-r9gp-7f88-9r54)
- <https://github.com/Netflix/lemur/security/advisories/GHSA-r9gp-7f88-9r54>
- <https://github.com/Netflix/lemur>
- <https://github.com/Netflix/lemur/releases/tag/v1.9.2>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-r9gp-7f88-9r54>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-r9gp-7f88-9r54/>. Everything for agents: <https://agentprime.io/llms.txt>.