The networkReloadIptablesRules function in network/bridge_driver.c in libvirt before 0.9.9 does not properly handle firewall rules on bridge networks when libvirtd is restarted, which might allow remote attackers to bypass intended access restrictions via a (1) DNS or (2) DHCP query.
https://bugzilla.redhat.com/show_bug.cgi?id=760442
http://www.ubuntu.com/usn/USN-2867-1
http://libvirt.org/news-2012.html
http://libvirt.org/git/?p=libvirt.git%3Ba=commitdiff%3Bh=ae1232b298323dd7bef909426e2ebafa6bca9157