covert channel

An unintended or unauthorized intra-system channel that enables two cooperating entities to transfer information in a way that violates the system's security policy but does not exceed the entities' access authorizations. (See: covert storage channel, covert timing channel, out-of-band, tunnel.)

"A communications channel that allows two cooperating processes to transfer information in a manner that violates the system's security policy." [NCS04]

Tutorial: The cooperating entities can be either two insiders or an insider and an outsider. Of course, an outsider has no access authorization at all. A covert channel is a system feature that the system architects neither designed nor intended for information transfer.