Microsoft has announced that Exchange Server is affected by a remotely exploitable buffer overflow condition. The overflow can be triggered remotely by unauthenticated SMTP clients. The source of the issue appears to be in how the XEXCH50 verb is handled by the server. Microsoft has stated that remote code execution is possible on hosts running Exchange 2000 Server. Servers running Exchange Server 5.0 and 5.5 are vulnerable to a denial of service attack.
Block external access at the network boundary, unless external parties require service.
Access to internal SMTP servers should be blocked at the network border.
Disallow anonymous access to services. Permit access for trusted individuals only.
Implement SMTP authentication to prevent unauthenticated clients from attempting to use the service.
Deploy network intrusion detection systems to monitor network traffic for malicious activity.
Network intrusion detection systems may detect suspicious SMTP activity.
Patches are available. Microsoft has released an update to thir advisory MS03-046 reporting the Exchange Server and related fixes. Please see the referenced advisory for more information and details on obtaining fixes.