This security update resolves a privately reported vulnerability in Windows Server 2008 Hyper-V and Windows Server 2008 R2 Hyper-V. The vulnerability could allow denial of service if a malformed sequence of machine instructions is run by an authenticated user in one of the guest virtual machines hosted by the Hyper-V server. An attacker must have valid logon credentials and be able to log on locally into a guest virtual machine to exploit this vulnerability. The vulnerability could not be exploited remotely or by anonymous users.