MyBB (aka MyBulletinBoard) 1.4.2 does not properly handle an uploaded file with a nonstandard file type that contains HTML sequences, which allows remote attackers to cause that file to be processed as HTML by Internet Explorer's content inspection, aka "Incomplete protection against MIME-sniffing." NOTE: this could be leveraged for XSS and other attacks.
http://www.openwall.com/lists/oss-security/2008/11/01/2
http://archives.neohapsis.com/archives/fulldisclosure/2008-10/0472.html
http://archives.neohapsis.com/archives/bugtraq/2008-10/0203.html