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.
Exploitability
AV:NAC:HAu:NImpact
C:CI:CA:C7.6/AV:N/AC:H/Au:N/C:C/I:C/A:C