source: http://www.securityfocus.com/bid/9840/info

A format string vulnerability has been reported to exists in the Unreal Tournament server engine. This issue is due to a failure of the server application to properly sanitize user supplied network data.

Ultimately this vulnerability could allow for execution of arbitrary code on the system implementing the affected server software, which would occur in the security context of the server process.

Example:

From:
Class=Engine.Pawn

To:
Class=%n%nEngine.Pawn

If the game is vulnerable it will crash when launched.