The qemuAgentCommand function in libvirt before 1.1.1, when a guest agent is not configured, allows remote attackers to cause a denial of service (NULL pointer dereference and crash) via vectors related to "agent based cpu (un)plug," as demonstrated by the "virsh vcpucount foobar --guest" command.
https://bugzilla.redhat.com/show_bug.cgi?id=986386
http://openwall.com/lists/oss-security/2013/07/19/12
http://libvirt.org/git/?p=libvirt.git%3Ba=commitdiff%3Bh=96518d4316b711c72205117f8d5c967d5127bbb6