On March 20, Canonical published in a security notice details about an OpenStack Nova vulnerability for its Ubuntu 12.10, Ubuntu 12.04 LTS, and Ubuntu 11.10 operating systems.According to Canonical, a couple of security problems were fixed in Nova.
For example, it was discovered that Nova did not properly validate VNC tokens, after an instance was deleted. An authenticated attacker could have exploited this problem in order to gain access to other virtual machines, under certain circumstances.
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.