According to the CEH Sniffing and Network Protocol Attacks module, covert channels represent one of the most sophisticated and difficult-to-detect data interception techniques. These channels hide malicious communication within legitimate protocol behavior, making them extremely challenging for traditional IDS/IPS and packet inspection tools to identify.

Industrial and legacy protocols such as Modbus, widely used in OT and legacy enterprise environments, lack encryption and authentication by design. CEH documentation highlights that attackers can manipulate unused or poorly validated Modbus fields to covertly transmit or intercept data while appearing as normal control traffic.

Option D is correct because covert channels over trusted legacy protocols blend seamlessly with legitimate traffic and bypass many security controls.

Option A is not a sniffing technique but a data-hiding method.

Option B describes exploitation, not sniffing.

Option C is a theoretical evasion method but is more detectable through reassembly.

CEH emphasizes covert channels as one of the most formidable sniffing challenges.