The Nova scheduler in OpenStack Compute (Nova) Folsom (2012.2) and Essex (2012.1), when DifferentHostFilter or SameHostFilter is enabled, allows remote authenticated users to cause a denial of service (excessive database lookup calls and server hang) via a request with many repeated IDs in the os:scheduler_hints section.
https://lists.launchpad.net/openstack/msg14452.html
http://www.ubuntu.com/usn/USN-1501-1