Red Hat libvirt, possibly 0.6.1 through 0.8.2, looks up disk backing stores without referring to the user-defined main disk format, which might allow guest OS users to read arbitrary files on the host OS, and possibly have unspecified other impact, via unknown vectors.
libvirt.org/news.html
lists.fedoraproject.org/pipermail/package-announce/2010-July/044520.html
lists.fedoraproject.org/pipermail/package-announce/2010-July/044579.html
lists.opensuse.org/opensuse-security-announce/2010-09/msg00006.html
ubuntu.com/usn/usn-1008-1
ubuntu.com/usn/usn-1008-2
ubuntu.com/usn/usn-1008-3
www.vupen.com/english/advisories/2010/2763
bugzilla.redhat.com/show_bug.cgi?id=607810