A covert channel is an unintended communication path within a system, therefore it is not protected by the system’s normal security mechanisms. Covert channels are a secret way to convey information.
Covert channels are addressed from TCSEC level B2.
The following are incorrect answers:
A trusted path is the protected channel that allows a user to access the Trusted Computing Base (TCB) without being compromised by other processes or users.
A protection domain consists of the execution and memory space assigned to each process.
A maintenance hook is a hardware or software mechanism that was installed to permit system maintenance and to bypass the system’s security protections.
Reference used for this question: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 6: Operations Security (page 219).