Description
FastMCP is the standard framework for building MCP applications. Prior to version 3.2.0, server names containing shell metacharacters (e.g., &) can cause command injection on Windows when passed to fastmcp install claude-code or fastmcp install gemini-cli. These install paths use subprocess.run() with a list argument, but on Windows the target CLIs often resolve to .cmd wrappers that are executed through cmd.exe, which interprets metacharacters in the flattened command string. This issue has been patched in version 3.2.0.
Affected products
jlowin: fastmcp
References3
- github.com/PrefectHQ/fastmcp/security/advisories/GHSA-m8x7-r2rg… ExploitPatchVendor Advisory
- github.com/PrefectHQ/fastmcp/security/advisories/GHSA-m8x7-r2rg… ExploitPatchVendor Advisory
- github.com/PrefectHQ/fastmcp/pull/3522 Issue TrackingPatch
Metrics
- CVSS 3.1
-
7.8
HIGH
Score source: NVD
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H - EPSS — exploit probability
- 1% percentile: 55 · as of 12 November 2026
- CWE
- CWE-78
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.