IIRC it's unsafe to play multiplayer in past releases of Call of Duty, even if fully-patched, because the games have a plethora of unpatched exploits (used on Xbox 360 to create modded lobbies), and I'm guessing the game's P2P nature makes it easier to find a client in your lobby to exploit.

The general recommendation is to install a community patch but I'd rather not run it at all, to be honest. I'd say "play in a VM" but I'm sure anti-cheat wouldn't like that.