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.
Red Hat libvirt, possibly 0.7.2 through 0.8.2, recurses into disk-image backing stores without extracting the defined disk backing-store format, which might allow guest OS users to read arbitrary files on the host OS, and possibly have unspecified other impact, via unknown vectors.