Qemu Flaw Lets the Guest Escape
Xen project announces a privilege escalation problem for Qemu host systems
The Xen project has announced a bug (CVE-2015-5154) that allows a process running inside a Qemu virtual machine to escalate its privileges to the privilege level of the Qemu process. This exploit basically lets the guest process escape to the privileges of the host.
The attack uses the Qemu emulated IDE CDROM device. According to the Xen project, “All Xen systems running x86 HVM guests without stubdomains that have been configured with an emulated CD-ROM driver model are vulnerable.”
See the security page for your Linux vendor for more on how to fix the problem. The best advice is to avoid using an emulated CD-ROM device with Qemu until you have taken the necessary steps and installed the patch.