Details for this vulnerability have not been published by NIST at this point. Descriptions from software vendor advisories for this issue are provided below.
USN-4656-1 fixed several vulnerabilities in X.Org. This update provides the corresponding update for Ubuntu 14.04 ESM.
Original advisory details:
Jan-Niklas Sohn discovered that the X.Org X Server XKB extension incorrectly handled certain inputs. A local attacker could possibly use this issue to escalate privileges.