mt-send-entry
movabletype.org: News: Movable Type Spam Vulnerability—"The 'Email this to a friend' functionality in the mt-send-entry.cgi script is vulnerable to being used by spammers to send spam messages. In principle, all "email this to a friend" programs are vulnerable to being used by spammers, because they allow the user to specify a To: address and a message body. But in practice, MT’s implementation of this is not as robust as it should be, and a new version is available..." for more discussion, check out this thread.