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CVE-2025-64340 jlowin CVE debrief

CVE-2025-64340 is a command injection vulnerability in FastMCP, a framework for building MCP applications. Prior to version 3.2.0, server names containing shell metacharacters can cause command injection on Windows when passed to certain install commands. This issue has been patched in version 3.2.0. Affected users, especially those using Windows, should be aware of this vulnerability and take steps to mitigate it by upgrading to the patched version or applying compensating controls.

Vendor
jlowin
Product
fastmcp
CVSS
MEDIUM 6.7
CISA KEV
Not listed in stored evidence
Original CVE published
2026-04-03
Original CVE updated
2026-07-24
Advisory published
2026-04-03
Advisory updated
2026-07-24

Who should care

Users of FastMCP versions prior to 3.2.0, especially those using Windows, should be aware of this vulnerability and take steps to mitigate it. This includes upgrading to the patched version, validating and sanitizing server names, and monitoring for suspicious activity related to FastMCP install commands. Security teams and operators responsible for MCP applications using FastMCP should review the vulnerability details and apply necessary mitigations.

Technical summary

The vulnerability exists in the install commands of FastMCP, specifically when handling server names with shell metacharacters on Windows. The commands use subprocess.run() with a list argument, but Windows resolves the target CLIs to .cmd wrappers executed through cmd.exe, which interprets metacharacters in the flattened command string. This allows attackers to inject malicious commands, potentially leading to arbitrary code execution. Users should upgrade to FastMCP version 3.2.0 or later to mitigate this vulnerability.

Defensive priority

Medium priority due to the specific conditions required for exploitation and the availability of a patch.

Recommended defensive actions

  • Upgrade FastMCP to version 3.2.0 or later
  • Validate and sanitize server names to prevent shell metacharacters from being injected
  • Monitor for suspicious activity related to FastMCP install commands
  • Review compensating controls for exposed systems while remediation is scheduled and verified
  • Check relevant monitoring, detection, and logs for exposed assets that need extra review
  • Track exceptions, retest remediated assets, and close the item only after evidence is documented
  • Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record was published on 2026-04-03T16:16:23.010Z and last modified on 2026-07-24T21:10:00.143Z. The NVD entry is currently Analyzed. Evidence is limited to public sources and may not reflect the full scope or impact of the vulnerability. Defenders should verify affected systems and apply patches or mitigations as needed.

Official resources

AI-assisted PatchSiren debrief based on the supplied source corpus. The CVE record was published on 2026-04-03T16:16:23.010Z and has not been modified since then. The NVD entry is currently Analyzed.