What is a backdoor?

A backdoor, in computing, is a method of bypassing authentication in a piece of software or computer system which can be used for accessing the software without being detected.

Typically this is an undocumented portal which may have been built in to the system in order to allow an administrator to enter and troubleshoot the software or to do general upkeep.

However, a backdoor can in some cases also be accessed by hackers and intelligence agencies in order to gain illicit access.

The existence of the backdoor is often unknown to the system owner, often due to it being created by the software developer.

How can hackers use backdoors?