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. 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 <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.