UFO: Alien Invasion versions up to and including 2.2.1 contain a buffer overflow vulnerability in its built-in IRC client component. When the client connects to an IRC server and receives a crafted numeric reply (specifically a 001 message), the application fails to properly validate the length of the response string. This results in a stack-based buffer overflow, which may corrupt control flow structures and allow arbitrary code execution. The vulnerability is triggered during automatic IRC connection handling and does not require user interaction beyond launching the game.
A stack-based buffer overflow condition is a condition where the buffer being overwritten is allocated on the stack (i.e., is a local variable or, rarely, a parameter to a function).