One way to defeat a multi-level security solution is to leak data via

A Covert channel is a simple yet very effective mechanism for sending and receiving information data between machines without alerting any firewalls and IDS’s on the network. The technique derives its stealthy nature by virtue of the fact that it sends traffic through ports that most firewalls will permit through. In addition the technique can bypass IDS by appearing to be innocuous packet carrying ordinary information when in fact it is concealing its actual data in one of the several control fields in the TCP and IP headers.