The GNTTABOP_swap_grant_ref sub-operation in the grant table hypercall in Xen 4.2 and Citrix XenServer 6.0.2 allows local guest kernels or administrators to cause a denial of service (host crash) and possibly gain privileges via a crafted grant reference that triggers a write to an arbitrary hypervisor memory location.
http://www.securityfocus.com/bid/55411
http://www.openwall.com/lists/oss-security/2012/09/05/11
http://support.citrix.com/article/CTX134708
http://secunia.com/advisories/50530
http://secunia.com/advisories/50472
http://lists.opensuse.org/opensuse-security-announce/2012-09/msg00004.html