Canonical published in a security notice details about an OpenStack Nova vulnerability for its Ubuntu 13.04, Ubuntu 12.10, and Ubuntu 12.04 LTS operating systems.
According to Canonical, Nova could be made to crash the system, if instances used a specially crafted image.
It was discovered that Nova did not verify the size of QCOW2 instance storage. An authenticated attacker could exploit this to cause a denial of service by creating an image with a large virtual size with little data, then filling the virtual disk..
The security flaws can be fixed if you upgrade your system(s) to the latest python-nova packages, specific to each distribution. To apply the update, run the Update Manager application.
In general, a standard system update will make all the necessary changes. A system restart won't be necessary to implement the changes, but users have to restart the virtual machine.
To update your system, please follow these instructions: https://wiki.ubuntu.com/Security/Upgrades.