A remote escalation of privilege vulnerability exists in implementations of Kerberos Key Distribution Center (KDC) in Microsoft Windows which could allow a remote attacker to take control of a vulnerable system. 
The Microsoft Windows Kerberos KDC fails to properly check service tickets for valid signatures, which can allow aspects of the service ticket to be forged. The improper check allows an attacker to escalate valid domain user account privileges to those of a domain administrator account, which renders the entire domain vulnerable to compromise.
At the time this release was issued, Microsoft was aware of limited, targeted attacks attempting to exploit this vulnerability.
A valid domain user can pass invalid domain administrator credentials, gain access and compromise any system on the domain, including the domain controller.