Network Security, Firewalls, and VPNs, 3rd Edition

Covert Channels

Covert channels are hidden, unknown, unique, atypical pathways of information transfer. The channel is covert because it is unknown and unseen. Hackers use covert channels for secretive communications, often to leak data (transport it) out of a secured environment, such as an air-gapped system. Covert channels are insecure pathways of transmission. If the pathway were known, it would be an overt channel and likely blocked, filtered, or otherwise secured.

Two main forms of covert channels exist: timing and storage. A timing channel conveys information through timed and synchronized activities. A few potential examples of timing covert channels include:

• Blinking lights to distribute information in Morse code
• Manipulating a fan’s ...