keystone/middleware/auth_token.py in OpenStack Nova Folsom, Grizzly, and Havana uses an insecure temporary directory for storing signing certificates, which allows local users to spoof servers by pre-creating this directory, which is reused by Nova, as demonstrated using /tmp/keystone-signing-nova on Fedora.
https://bugzilla.redhat.com/show_bug.cgi?id=958285
https://bugs.launchpad.net/nova/+bug/1174608
http://www.openwall.com/lists/oss-security/2013/05/09/2
http://lists.openstack.org/pipermail/openstack-announce/2013-May/000098.html
http://lists.fedoraproject.org/pipermail/package-announce/2013-May/105916.html