Hyper-V in Microsoft Windows Server 2008 SP2 and R2 SP1, Windows 8, Windows 8.1, and Windows Server 2012 Gold and R2 does not properly initialize guest OS system data structures, which allows guest OS users to execute arbitrary code on the host OS by leveraging guest OS privileges, aka "Hyper-V System Data Structure Vulnerability."
https://docs.microsoft.com/en-us/security-updates/securitybulletins/2015/ms15-068