There is a privilege-escalation vulnerability in several versions of Ubuntu that results from the fact that the operating system fails to check permissions when users are creating files in some specific circumstances. The vulnerability lies in the overlayfs component of Ubuntu, a file system that is designed to be a writeable file system in cases where an underlying one is read-only. Security researcher Philip Pettersson has developed and published a proof-of-concept exploit for the vulnerability that gives the user a root shell.
Source: https://threatpost.com/ubuntu-patches-privilege-escalation-bug/113404/