The On-Screen Keyboard application of Microsoft Windows is prone to a privilege escalation vulnerability when handling mouse input originated from a process running with Low Integrity Level. This vulnerability allows an agent running with Low Integrity Level to escalate privileges in order to install a new agent that will run with Medium Integrity Level, by sending mouse input to the On-Screen Keyboard when its input mode is set to "Hover over keys".
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.