# FastMCP MCP client wrappers construct M2M OAuth providers without issuer binding — client secret redirectable to attacker-advertised authorization server

Advisory record · `GHSA-vqqp-x5qm-5qp4`

- **Identifiers:** `GHSA-vqqp-x5qm-5qp4`
- **State:** Confirmed
- **Severity:** CVSS 5.9 Medium
- **Critical:** No
- **Weakness (CWE):** [CWE-287](https://cwe.mitre.org/data/definitions/287.html) · [CWE-522](https://cwe.mitre.org/data/definitions/522.html)
- **Tags:** [Authentication bypass](https://agentprime.io/advisories/tag/auth-bypass.md) · [Hard-coded credentials](https://agentprime.io/advisories/tag/hard-coded-credentials.md)
- **Published:** 23 Oct 2026
- **Updated:** 23 Oct 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 23 Oct 2026, 21:31 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `fastmcp` (PyPI): affected `>=4.0.0a1, <4.1.2`; fixed in `4.1.2`
- `fastmcp-slim` (PyPI): affected `>=4.0.0a1, <4.1.2`; fixed in `4.1.2`

## 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

- 5.9 Medium, CVSS 3.1: `CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N`, from GitHub Advisory Database

- **CISA KEV:** Not listed.
- **EPSS:** No score.

## Timeline

- 31 Oct 2026, 14:08 UTC: First seen by agentprime
- 31 Oct 2026, 14:08 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-vqqp-x5qm-5qp4)

## Sources

- [GitHub Advisory Database: GHSA-vqqp-x5qm-5qp4](https://github.com/advisories/GHSA-vqqp-x5qm-5qp4)
- [OSV: GHSA-vqqp-x5qm-5qp4](https://osv.dev/vulnerability/GHSA-vqqp-x5qm-5qp4)
- <https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-vqqp-x5qm-5qp4>
- <https://github.com/PrefectHQ/fastmcp/pull/5933>
- <https://github.com/PrefectHQ/fastmcp/commit/85df42fbf2aa631cc81e0f986718b314128e91df>
- <https://github.com/PrefectHQ/fastmcp>
- <https://github.com/PrefectHQ/fastmcp/releases/tag/v4.1.2>

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-vqqp-x5qm-5qp4>, 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-vqqp-x5qm-5qp4/>. Everything for agents: <https://agentprime.io/llms.txt>.