An error in the way that the Windows kernel handles string atoms when registering a new window class allows unprivileged users to re-register atoms of privileged applications. This vulnerability can be exploited by local unprivileged users to execute arbitrary code with SYSTEM privileges. This exploit will lock the machine screen (similar to pressing Ctrl+Alt+Del and then clicking on 'Lock this computer'), and the windows theming will be disabled until the machine is restarted.