GHSA-7xgj-8392-5q4r: fastmcp

Package	Ecosystem	Affected	Fixed in
fastmcp	PyPI	>=3.0.0, <3.4.10 \|\| >=4.0.0, <4.1.2	3.4.10, 4.1.2
fastmcp-slim	PyPI	>=3.0.0, <3.4.10 \|\| >=4.0.0, <4.1.2	3.4.10, 4.1.2

Machine-written summary

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

FastMCP and fastmcp-slim versions 3.0.0 to before 3.4.10, and 4.0.0 to before 4.1.2, are affected. When a server transform changes a component's metadata, direct lookups return the transformed component without rechecking its authorization policy, so a client that knows the component identifier can invoke a tool or retrieve a resource or prompt that the policy denies. Upgrade to fastmcp or fastmcp-slim 3.4.10 or 4.1.2. Until then, avoid overlapping proxy templates for protected resources and enforce required authorization in the handler or at the server boundary.

Written 1 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

Score	Rating	CVSS	Vector	Source
6.8	Medium	3.1	CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N	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

Sources

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.