BREW-FASTMCP-CVE-2025-64340 (GHSA-M8X7-R2RG-VH5G)

Vulnerability from osv_homebrew – Published: 2026-08-13 16:46 – Updated: 2026-09-28 18:19 – Source website
VLAI
Summary
FastMCP has a Command Injection vulnerability - Gemini CLI
Details

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.

Severity
6.7 (Medium)
CVSS:3.1/AV:L/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H
References
To clipboard 

{
  "affected": [
    {
      "ecosystem_specific": {
        "fix": "bump",
        "range_state": "fixed",
        "upstream_fixed_in": "3.2.0"
      },
      "package": {
        "ecosystem": "Homebrew",
        "name": "fastmcp",
        "purl": "pkg:brew/fastmcp"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.12.3"
            },
            {
              "fixed": "3.2.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "database_specific": {
    "confidence": "high",
    "source": "matched",
    "strategy": "registry",
    "upstream_evidence": [
      {
        "ecosystem": "PyPI",
        "key": "pkg:pypi/fastmcp@4.0.10",
        "name": "fastmcp",
        "strategy": "registry",
        "subject_version": "4.0.10"
      }
    ]
  },
  "details": "Server names containing shell metacharacters (e.g., `\u0026`) 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.\n\nPoC:\n```python\nfrom fastmcp import FastMCP\n\nmcp = FastMCP(name=\"test\u0026calc\")\n\n@mcp.tool\ndef roll_dice(n_dice: int) -\u003e list[int]:\n    \"\"\"Roll `n_dice` 6-sided dice and return the results.\"\"\"\n    return [random.randint(1, 6) for _ in range(n_dice)]\n```\n\n```\nfastmcp install claude-code server.py   # or: fastmcp install gemini-cli server.py\n```\n\nOn Windows, this opens Calculator via the `\u0026calc` in the server name.\n\nImpact:\nArbitrary 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).\n\nPatched in #3522 by validating server names to reject shell metacharacters.",
  "id": "BREW-fastmcp-CVE-2025-64340",
  "modified": "2026-09-28T18:19:41Z",
  "published": "2026-08-13T16:46:31Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-m8x7-r2rg-vh5g"
    },
    {
      "type": "WEB",
      "url": "https://github.com/jlowin/fastmcp/security/advisories/GHSA-m8x7-r2rg-vh5g"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-64340"
    },
    {
      "type": "WEB",
      "url": "https://github.com/PrefectHQ/fastmcp/pull/3522"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/PrefectHQ/fastmcp"
    }
  ],
  "schema_version": "1.7.3",
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "FastMCP has a Command Injection vulnerability - Gemini CLI",
  "upstream": [
    "GHSA-m8x7-r2rg-vh5g",
    "CVE-2025-64340",
    "PYSEC-2026-2475"
  ]
}



Log in or create an account to share your comment.




Tags
Taxonomy of the tags.





Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.