Backdoor

A backdoor is a hidden method of bypassing normal authentication or security controls to gain unauthorized access to a system. Backdoors can be intentionally installed by attackers who have already compromised a system (to maintain persistent access) or they can be hidden in software by malicious developers.

Once a backdoor is installed, attackers can return to the compromised system at any time, often without triggering security alerts. Backdoors are commonly installed by APT actors to maintain long-term access to target networks — they compromise the system, install a backdoor, and then use it for ongoing data theft.