Software Engineering Institute

The Simple Mail Transfer Protocol (SMTP) is the standard protocol used to transport mail across the Internet. Microsoft Windows 2000 Server contains an SMTP server that requires authentication before users are permitted to send mail. A flaw in the way the server handles authentication could permit an intruder to use the service to send mail without providing genuine credentials.

For more information, see Microsoft security bulletin MS01-037.

It should be noted that even in the best of circumstances, SMTP is a very difficult to authenticate without end-to-end cryptographic solutions. Quoting from RFC 2821:

SMTP mail is inherently insecure in that it is feasible for even fairly casual users to negotiate directly with receiving and relaying SMTP servers and create messages that will trick a naive recipient into believing that they came from somewhere else. Constructing such a message so that the "spoofed" behavior cannot be detected by an expert is somewhat more difficult, but not sufficiently so as to be a deterrent to someone who is determined and knowledgeable. Consequently, as knowledge of Internet mail increases, so does the knowledge that SMTP mail inherently cannot be authenticated, or integrity checks provided, at the transport level. Real mail security lies only in end-to-end methods involving the message bodies, such as those which use digital signatures...

Intruders may be able to send mail through a vulnerable server in violation of local security policies.

The CERT/CC is currently unaware of a general purpose solution to this problem without strong digital signatures on all mail messages. To address the specific problem in the Microsoft SMTP server, apply a patch as described in MS01-037.