Several security issues were fixed in the X.Org X server.
It was discovered that the X.Org X server incorrectly handled certain lengths. An attacker able to connect to an X server, either locally or remotely, could use these issues to crash the server, or possibly execute arbitrary code.
17 October 2017
A security issue affects these releases of Ubuntu and its derivatives:
Several security issues were fixed in the X.Org X server.
It was discovered that the X.Org X server incorrectly handled certain lengths. An attacker able to connect to an X server, either locally or remotely, could use these issues to crash the server, or possibly execute arbitrary code.
The problem can be corrected by updating your system to the following package versions:
To update your system, please follow these instructions: https://wiki.ubuntu.com/Security/Upgrades.
After a standard system update you need to reboot your computer to make all the necessary changes.