OpenStack Compute (Nova) before 2014.2.4 (juno) and 2015.1.x before 2015.1.2 (kilo) do not properly apply security group changes, which allows remote attackers to bypass intended restriction by leveraging an instance that was running when the change was made.
https://security.openstack.org/ossa/OSSA-2015-021.html
https://bugs.launchpad.net/nova/+bug/1492961
https://bugs.launchpad.net/nova/+bug/1491307
https://access.redhat.com/errata/RHSA-2015:2673