libxl in Xen 4.1.x through 4.6.x does not properly handle the readonly flag on disks when using the qemu-xen device model, which allows local guest users to write to a read-only disk image.
http://lists.fedoraproject.org/pipermail/package-announce/2015-September/167077.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-September/167792.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-September/167820.html
http://lists.opensuse.org/opensuse-updates/2015-12/msg00053.html
https://bugzilla.redhat.com/show_bug.cgi?id=1257893
https://security.gentoo.org/glsa/201604-03
http://www.debian.org/security/2015/dsa-3414