Matthieu Herrb of X.Org reports:
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.