Alien vs Predator (Feb 17 patch) is vulnerable to Format String attacks.
Posting the following in Chat (either in game or in the lobby) will crash your game, I am not sure if it will crash other users, I haven't got anybody to test on.
%s%s%s%s%s%s%s%s%s%s
or
%n%n%n%n%n%n%n%n%n%n
Setting your Name to: '%n' will stop you from ever joining a game, you just get an error reporting your unable to connect.
Setting your Name to '%i' will set your Name to random numbers which changes as you play.
I tried contacting Steam/Rebellion/Sega, but so far had no response.