To support the exchange of mail with heterogeneous systems, Exchange messages use the attributes of SMTP mail messages that are specified by RFC's 821 and 822. There is a flaw in the way Exchange 2000 handles certain malformed RFC message attributes on received mail. Upon receiving a message containing such a malformation, the flaw causes the Store service to consume 100% of the available CPU in processing the message.
A security vulnerability results because it is possible for an attacker to seek to exploit this flaw and mount a denial of service attack. An attacker could attempt to levy an attack by connecting directly to the Exchange server and passing a raw, hand-crafted mail message with a specially malformed attribute. When the message was received and processed by the Store service, the CPU would spike to 100%. The effects of the attack would last as long as it took for the Exchange Store service to process the message. Neither restarting the service nor rebooting the server would remedy the denial of service.