Two programmers write a new secure application for the human resources department to store personal identifiable information. The programmers make the application available to themselves using an…

A rootkit is malware designed to hide the presence of other malicious software on a system by subverting the OS; it does not describe a hidden authentication bypass built into an application.

Spyware is malicious software that secretly collects user data and transmits it to a third party; it does not describe a secret access mechanism built into an application by its developers.

A logic bomb is malicious code that executes when a specific condition or trigger is met; it is not a persistent hidden access mechanism.

A backdoor is a covert access mechanism deliberately inserted into software or a system that allows unauthorized entry without going through normal authentication channels. The programmers' use of an obscure port and secret credentials unknown to others is a textbook backdoor, providing them persistent hidden access to the application.