Microsoft Active Directory is prone to an authentication-bypass vulnerability. An attacker can exploit this issue to bypass certain security restrictions by using a revoked certificate.
Block external access at the network boundary, unless external parties require service.
If global access isn't needed, filter access to the affected computer at the network boundary. Restricting access to only trusted computers and networks might greatly reduce the likelihood of successful exploits.
Deploy network intrusion detection systems to monitor network traffic for malicious activity.
Deploy NIDS to monitor network traffic for signs of anomalous or suspicious activity. This may indicate exploit attempts or activity that results from successful exploits.
The vendor has released an advisory and fixes. Please see the references for details.