Microsoft Exchange is prone to a remote denial-of-service vulnerability because it fails to properly handle unexpected iCal message content. Successfully exploiting this issue allows remote attackers to cause targeted Exchange servers to stop responding to further requests for sending, receiving, or accessing email. As a result, denial-of-service conditions occur for legitimate users of affected servers. A denial-of-service condition will persist until an administrator restarts the Microsoft Exchange Information Store service.
Block external access at the network boundary, unless external parties require service.
Ensure that only trusted hosts and networks can connect to vulnerable servers.
Disallow anonymous access to services. Permit access for trusted individuals only.
Disable unauthenticated access to servers to limit the potential for anonymous remote attackers to exploit this issue.
Deploy network intrusion detection systems to monitor network traffic for malicious activity.
Deploy NIDS to detect and block attacks and anomalous activity such as requests containing suspicious email content. Since the application may log such requests, review its logs regularly.
The vendor has released an advisory along with updates to address this issue. Please see the referenced advisory for more information.