Ubuntu Security Notice 527-1 – Joris van Rantwijk discovered that the Xen host did not correctly validate the contents of a Xen guests’s grug.conf file. Xen guest root users could exploit this to run arbitrary commands on the host when the guest system was rebooted.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/59954/USN-527-1.txt
Source: https://packetstormsecurity.com/files/59954/Ubuntu-Security-Notice-527-1.html