CVE-2025-64340

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.

PoC:

from fastmcp import FastMCP

mcp = FastMCP(name="test&calc")

@mcp.tool
def roll_dice(n_dice: int) -> list[int]:
    """Roll `n_dice` 6-sided dice and return the results."""
    return [random.randint(1, 6) for _ in range(n_dice)]

fastmcp install claude-code server.py   # or: fastmcp install gemini-cli server.py

On Windows, this opens Calculator via the &calc in the server name.

Impact:
Arbitrary command execution with the privileges of the user running fastmcp install. Affects Windows hosts where the target CLI (one of claude, gemini) is installed as a .cmd wrapper. Does not affect macOS/Linux, and does not affect config-file-based install targets (cursor, goose, mcp-json).

Patched in #3522 by validating server names to reject shell metacharacters.

Untrusted input reaches a shell command, allowing arbitrary commands to run on the host. Typical impact: code execution in the application's environment.

CVE-2025-64340 has a CVSS score of 6.7 (Medium). The vector is requires local access, low privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.2.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2025-64340? CVE-2025-64340 is a medium-severity OS command injection vulnerability in fastmcp (pip), affecting versions < 3.2.0. It is fixed in 3.2.0. Untrusted input reaches a shell command, allowing arbitrary commands to run on the host.
2. How severe is CVE-2025-64340? CVE-2025-64340 has a CVSS score of 6.7 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of fastmcp are affected by CVE-2025-64340? fastmcp (pip) versions < 3.2.0 is affected.
4. Is there a fix for CVE-2025-64340? Yes. CVE-2025-64340 is fixed in 3.2.0. Upgrade to this version or later.
5. Is CVE-2025-64340 exploitable, and should I be worried? Whether CVE-2025-64340 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-64340 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-64340? Upgrade fastmcp to 3.2.0 or later.