Server names containing shell metacharacters (e.g., &) can cause command injection on Windows when passed to fastmcp install claude-code or fastmcp install gemini-cli. These install paths use subprocess.run() with a list argument, but on Windows the target CLIs often resolve to .cmd wrappers that are executed through cmd.exe, which interprets metacharacters in the flattened command string.

PoC:

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)]

fastmcp install claude-code server.py  # or: fastmcp install gemini-cli server.py

On Windows, this opens Calculator via the &calc in the server name.

Impact: Arbitrary command execution with the privileges of the user running fastmcp install. Affects Windows hosts where the target CLI (one of claude, gemini) is installed as a .cmd wrapper. Does not affect macOS/Linux, and does not affect config-file-based install targets (cursor, goose, mcp-json).

Patched in #3522 by validating server names to reject shell metacharacters.