The libxl device-handling in Xen 4.6.x and earlier allows local OS guest administrators to cause a denial of service (resource consumption or management facility confusion) or gain host OS privileges by manipulating information in guest controlled areas of xenstore.
http://www.debian.org/security/2016/dsa-3633
http://www.oracle.com/technetwork/topics/security/ovmbulletinjul2016-3090546.html