IIS 4.0 allows remote attackers to obtain the internal IP address of the server via an HTTP 1.0 request for a web page which is protected by basic authentication and has no realm defined.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2000-0645
http://www.securityfocus.com/bid/1499
http://archives.neohapsis.com/archives/ntbugtraq/2000-q3/0025.html