A network covert channel is the use

A network covert channel is the use of a shared resource,
namely a network communications channel, to transfer in-
formation in a way which it was not initially designed for.
Network covert channels are used to exltrate data from a
secure location to a non-secure location without being de-
tected or prevented by traditional information security pro-
tection techniques such as rewalls, encryption, or intrusion
detection systems.