BREW-FASTMCP-CVE-2025-62801 (GHSA-RJ5C-58RQ-J5G5)

Vulnerability from osv_homebrew – Published: 2026-08-13 16:46 – Updated: 2026-09-25 10:26 – Source website
VLAI
Summary
FastMCP vulnerable to windows command injection in FastMCP Cursor installer via server_name
Details

Summary

A command-injection vulnerability lets any attacker who can influence the server_name field of an MCP execute arbitrary OS commands on Windows hosts that run fastmcp install cursor

Details

  1. generate_cursor_deeplink(server_name, …) embeds server_name verbatim in a cursor://…?name= query string.
  2. open_deeplink() is invoked with shell=True only on Windows. That calls cmd.exe /c start .
  3. Any cmd metacharacter inside server_name (&, |, >, ^, …) escapes the start command and spawns an attacker-chosen process.

PoC

server.py 


import random
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)]

if __name__ == "__main__":
    mcp.run()

then run in the terminal: fastmcp install cursor server.py

Impact

OS Command / Shell Injection (CWE-78) Every Windows host that runs fastmcp install cursor is at risk. Developers on their local workstations, CI/CD agents and corporate build machines alike.

Severity
5.4 (Medium)
CVSS:4.0/AV:L/AC:L/AT:P/PR:L/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
References
To clipboard 

{
  "affected": [
    {
      "ecosystem_specific": {
        "fix": "bump",
        "range_state": "fixed",
        "upstream_fixed_in": "2.13.0"
      },
      "package": {
        "ecosystem": "Homebrew",
        "name": "fastmcp",
        "purl": "pkg:brew/fastmcp"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.12.3"
            },
            {
              "fixed": "2.13.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "database_specific": {
    "confidence": "high",
    "source": "matched",
    "strategy": "registry",
    "upstream_evidence": [
      {
        "ecosystem": "PyPI",
        "key": "pkg:pypi/fastmcp@4.0.8",
        "name": "fastmcp",
        "strategy": "registry",
        "subject_version": "4.0.8"
      }
    ]
  },
  "details": "### Summary\nA command-injection vulnerability lets any attacker who can influence the server_name field of an MCP execute arbitrary OS commands on Windows hosts that run fastmcp install cursor\n\n### Details\n1. generate_cursor_deeplink(server_name, \u2026) embeds server_name verbatim in a cursor://\u2026?name= query string.\n2. open_deeplink() is invoked with shell=True only on Windows. That calls cmd.exe /c start \u003cdeeplink\u003e.\n3. Any cmd metacharacter inside server_name (\u0026, |, \u003e, ^, \u2026) escapes the start command and spawns an attacker-chosen process.\n\n### PoC\nserver.py \n```\n\nimport random\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\nif __name__ == \"__main__\":\n    mcp.run()\n```\n\nthen run in the terminal:\n`fastmcp install cursor server.py`\n\n### Impact\nOS Command / Shell Injection (CWE-78)\nEvery Windows host that runs fastmcp install cursor is at risk. Developers on their local workstations, CI/CD agents and corporate build machines alike.",
  "id": "BREW-fastmcp-CVE-2025-62801",
  "modified": "2026-09-25T10:26:46Z",
  "published": "2026-08-13T16:46:31Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/jlowin/fastmcp/security/advisories/GHSA-rj5c-58rq-j5g5"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-62801"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/jlowin/fastmcp"
    }
  ],
  "schema_version": "1.7.3",
  "severity": [
    {
      "score": "CVSS:4.0/AV:L/AC:L/AT:P/PR:L/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "FastMCP vulnerable to windows command injection in FastMCP Cursor installer via server_name",
  "upstream": [
    "GHSA-rj5c-58rq-j5g5",
    "CVE-2025-62801",
    "PYSEC-2026-1365"
  ]
}



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