Critical Exchange Flaw Causes Denial of Service

German researchers uncovered a critical vulnerability in Exchange 2000 that makes the messaging server vulnerable to a denial of service attack.

Microsoft on Wednesday issued a patch, which can be found in the security bulletin about the issue (

The problem results from a flaw in the way Exchange processes messages that come from SMTP servers. A specially malformed message can cause the Exchange Store service to use 100 percent of available CPU until the message is processed. Rebooting the server or restarting the server wouldn't help, Microsoft says.

The patch Microsoft issued causes Exchange to throw out malformed messages without trying to process them.

Russ Cooper, moderator of the NTBugTraq security mailing list, says the vulnerability doesn't look like it would cause long Exchange outages from individual messages. "Maybe if the message had many, many, many attachments, "To"s or "CC"s, or came in HTML it might take more time," Cooper said. A queue full of the messages would be another story.

Microsoft rates the vulnerability as a critical risk for Internet servers and intranet servers.

The warning came from researchers at the Computing Center at Johannes Gutenberg University in Germany.

About the Author

Scott Bekker is editor in chief of Redmond Channel Partner magazine.