What is back door vulnerability?

Back door is a vulnerability in a system or application that enables intruders to gain access to the machine by bypassing security.

Attackers use various techniques to either detect existing back doors or to install malicious software that would open new back door that they can use to gain access and total control of the affected system.

Some back doors are installed purposely by programmers or administrators so that they can login even when they forgot the administrator password.