OpenStack Compute (Nova) before 2015.1.3 (kilo) and 12.0.x before 12.0.1 (liberty), when using libvirt to spawn instances and use_cow_images is set to false, allow remote authenticated users to read arbitrary files by overwriting an instance disk with a crafted image and requesting a snapshot.
https://security.openstack.org/ossa/OSSA-2016-001.html