An SMTP service installs by default as part of Windows 2000 server products and as part of the Internet Mail Connector (IMC) for Microsoft Exchange Server 5.5. (The IMC, also known as the Microsoft Exchange Internet Mail Service, provides access and message exchange to and from any system that uses SMTP). A vulnerability results in both services because of a flaw in the way they handle a valid response from the NTLM authentication layer of the underlying operating system.
By design, the Windows 2000 SMTP service and the Exchange Server 5.5 IMC, upon receiving notification from the NTLM authentication layer that a user has been authenticated, should perform additional checks before granting the user access to the service. The vulnerability results because the affected services don't perform this additional checking correctly. In some cases, this could result in the SMTP service granting access to a user solely on the basis of their ability to successfully authenticate to the server.
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