OpenStack Compute (Nova) Grizzly, Folsom (2012.2), and Essex (2012.1) allows remote authenticated users to gain access to a VM in opportunistic circumstances by using the VNC token for a deleted VM that was bound to the same VNC port. Per http://www.ubuntu.com/usn/USN-1771-1/
"A security issue affects these releases of Ubuntu and its derivatives:
Ubuntu 12.10
Ubuntu 12.04 LTS
Ubuntu 11.10"
Publication date: Sat, 23 Mar 2013 02:55:00 +0000