Several vulnerabilities have been identified in server code
of the X window system caused by lack of proper input validation
on user controlled data in various parts of the software,
causing various kinds of overflows.
Exploiting these overflows will crash the X server or,
under certain circumstances allow the execution of arbitray
machine code.
When the X server is running with root privileges (which is the
case for the Xorg server and for most kdrive based servers),
these vulnerabilities can thus also be used to raise
privileges.
All these vulnerabilities, to be exploited succesfully, require
either an already established connection to a running X server
(and normally running X servers are only accepting authenticated
connections), or a shell access with a valid user on the machine
where the vulnerable server is installed.