Microsoft IIS 5.1 and 6 allows remote attackers to spoof the SERVER_NAME variable to bypass security checks and conduct various attacks via a GET request with an http://localhost URI, which makes it appear as if the request is coming from localhost.
http://www.vupen.com/english/advisories/2005/1503
http://secunia.com/advisories/16548
http://marc.info/?l=bugtraq&m=112474727903399&w=2
http://ingehenriksen.blogspot.com/2005/08/remote-iis-5x-and-iis-60-server-name.html