Citirix XenServer is a hypervisor platform that enables the creation and management of virtualized server infrastructure.Since modern defense machanism can effectively protected cyber attack. However if the threat actor re-engineering their attack method integrate with rare system bug. The overall suituation will become worst. Hypervisor locate in the middle in between VM and hardware. VM relies on this isolation to avoid ring-0 attack. But now solution released so I am not going to say anymore.
Be quick to read below url if you are the Citrix XenServer end user.
Citrix XenServer Multiple Security Updates