Hello, this is CVE-2015-1328 which allows a local root privilege escalation in the default configuration on all currently supported versions of Ubuntu.
The overlayfs filesystem does not correctly check file permissions when
creating new files in the upper filesystem directory. This can be exploited
by an unprivileged process in kernels with CONFIG_USER_NS=y and where
overlayfs has the FS_USERNS_MOUNT flag, which allows the mounting of overlayfs
inside unprivileged mount namespaces. This is the default configuration of
Ubuntu 12.04, 14.04, 14.10, and 15.04 .
If you don't want to update your kernel and you don't use overlayfs, a viable
workaround is to just remove or blacklist overlayfs.ko / overlay.ko.