A backdoor in computing is a covert method of bypassing authorized access channels like normal authentication or encryption in a computing environment. They are most often used for securing remote access to a computer in order to execute a far greater payload or malicious intent such as to gain access to privileged information like passwords, emails, or files, corrupt or delete data on hard drives or transfer information out of the network.
A backdoor may be in the form of a Trojan disguised as a legitimate file that the user is tricked into downloading or opening. It could also be code deliberately written into the firmware of the hardware, parts of an operating system, or propriety software.
There are legitimate use cases for backdoors such as providing the manufacturer of hardware devices with a way to restore user passwords, administrators to reset customer’ systems, etc.