PYSEC-2026-2475

Description

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: ```python 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.