Flaw in Microsoft Exchange can cause denial of service

A "critical" flaw in Microsoft's Exchange 2000 e-mail server could allow an attacker to send a message that could consume all of the server's processing resources, according to a new security alert issued Wednesday by Microsoft. The company has released a patch to fix the issue.

The flaw comes as a result of a problem in the way Exchange handles received mail that contains malformed attributes in a number of standard e-mail areas, including the "To" and "From" lines, Microsoft said in its alert. The mishandling of malformed e-mail means that rather than immediately discarding a malformed message, Exchange moves it to the Exchange 2000 Store Service and attempts to process the e-mail, the company said.

A specially crafted e-mail can be made that will force the server to use 100 percent of its processing resources to handle the message using this service, thereby denying service to all other messages and actions running on the server, Microsoft said. Neither restarting the service nor rebooting the server will alleviate the problem, Microsoft said. Rather, users will have to wait for the e-mail to be processed before the server will become available again, said the company, based in Redmond, Washington.

Though the vulnerability is listed as critical by the company, Microsoft also said that the issue is mitigated because an attack e-mail message would have to be sent using a direct connection to the server and a hand-crafted e-mail. An attack could not be launched from an e-mail created from a standard e-mail client, Microsoft said. Additionally, the attack could not cause any code execution or privilege elevation, the company said.

More information on the vulnerability and the patch to fix it at available at http://www.microsoft.com/technet/security/bulletin/ms02-025.asp.

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