Microsoft Internet Explorer supports MHTML, a simple container format that uses MIME encapsulation to combine several documents into a single file. A variant of this issue first appeared in 2004, and has been independently re-discovered several times in that timeframe. The affected sites generally have very little recourse to stop the attack: it is very difficult to block the offending input patterns perfectly. A highly experimental server-side workaround devised by Robert Swiecki may involve returning HTTP code 201 Created rather than 200 OK when encountering vulnerable User-Agent strings.”]
Source: http://lcamtuf.blogspot.com/2011/03/note-on-mhtml-vulnerability.html