The qemu guest agent in Qemu 1.4.1 and earlier, as used by Xen, when started in daemon mode, uses weak permissions for certain files, which allows local users to read and write to these files.
https://exchange.xforce.ibmcloud.com/vulnerabilities/84047
https://bugzilla.redhat.com/show_bug.cgi?id=956082
http://www.securitytracker.com/id/1028521
http://www.securityfocus.com/bid/59675
http://www.openwall.com/lists/oss-security/2013/05/06/5
http://secunia.com/advisories/53325
http://rhn.redhat.com/errata/RHSA-2013-0896.html
http://rhn.redhat.com/errata/RHSA-2013-0791.html
http://lists.opensuse.org/opensuse-updates/2013-07/msg00057.html
http://git.qemu.org/?p=qemu.git%3Ba=commit%3Bh=c689b4f1bac352dcfd6ecb9a1d45337de0f1de67