Which of the following describes a backdoor in a system?

A backdoor in a system refers to a hidden entry point that allows unauthorized access to a computer system or network. This entry point is often created to bypass normal authentication procedures, enabling an attacker or unauthorized user to gain control without being detected. Backdoors can be intentionally built into software by developers for legitimate reasons, such as debugging, or they can be introduced by malicious actors.

The other options do not accurately describe a backdoor. A primary means of user authorization pertains to standard authentication methods that legitimize users to access a system, which is counter to the very nature of a backdoor. A method to enhance system security implies protective measures intended to safeguard information systems, while a backdoor typically undermines security. Lastly, a protocol for regular users refers to standard operating procedures or communication methods intended for use by authorized users, which contrasts with the secretive nature of a backdoor.