What is covert channel?

A covert channel is a type of communication channel that allows information to be transferred in a way that violates a system's security policy. It's a way to communicate without using the usual, legitimate communication paths, often by manipulating system resources or characteristics in a way that's not intended for communication. Covert channels are often used in security contexts to exfiltrate data or bypass security controls. Covert channels are generally categorized as either timing channels (information is conveyed by modulating the timing of events) or storage channels (information is conveyed by modifying shared system resources).