The Xen hypervisor block backend driver for Linux kernel 2.6.18, when running on a 64-bit host with a 32-bit paravirtualized guest, allows local privileged users in the guest OS to cause a denial of service (host OS crash) via a request that specifies a large number of blocks.
Ubuntu Security Notice USN-679-1 - The Linux 2.6 kernel has had various security vulnerabilities addressed. These range from bypass issues to denial of service and improper validation.