Don’t use shell=True in mcp dev subprocess on Windows (command injection risk)

[0x-Professor]

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

What’s happening:

When you run the mcp dev command on Windows, it starts another program using a method (subprocess.run with shell=True) that lets the Windows command prompt (cmd.exe) handle the command. This is risky because if any part of the command includes special characters (like &, |, %, etc.), Windows might run something you didn’t expect — even another program, if the file path or arguments are weirdly named or crafted.

Why this is a real problem:

• This isn’t just a theory — it’s a well-known risk with shell=True in Python. If anyone (or any script) can control part of the file path or arguments, they might be able to run extra commands on your computer.
• The Python documentation says to avoid shell=True when possible for exactly this reason.
• The fix is easy: use shell=False and make sure the right Windows executable is picked (like npx.cmd).
• This keeps things safe and works the same on all systems.

What should happen instead:

• The command should be run without shell=True on Windows, just like it is on Linux/Mac.
• File paths and arguments should always be passed as a list, not a single string.

How this could be abused:

• If someone manages to sneak a file or argument with a shell special character into your project, running mcp dev could run extra commands (for example, opening Calculator if the file had &calc in its name).

Please fix:

• Remove shell=True from the subprocess.run call in src/mcp/cli/cli.py (Windows part).
• Make sure the command and its arguments are always passed as a list.
• Make sure it works on Windows by using the right executable (like npx.cmd).

Thanks!

Example Code

# Example of risky situation on Windows:
# If a file is named "server&calc.py" and you run:
#   mcp dev path\to\server&calc.py
# Windows might run Calculator because of the &

# Please see src/mcp/cli/cli.py (mcp dev command) for the subprocess.run([npx_cmd, ...], shell=True, ...)

Python & MCP Python SDK

Python 3.11, Windows 11, latest MCP Python SDK (main branch, August 2025)