The ISAPI extension in BadBlue 1.7 through 2.2, and possibly earlier versions, modifies the first two letters of a filename extension after performing a security check, which allows remote attackers to bypass authentication via a filename with a .ats extension instead of a .hts extension.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2003-0327
http://marc.info/?l=bugtraq&m=105346382524169&w=2
http://archives.neohapsis.com/archives/vulnwatch/2003-q2/0075.html