A backdoor is to do something, such as getting a job, through having some unfair advantage. See also guanxi
A backdoor in a computer system is a method of bypassing normal authentication or obtaining remote access to a computer, while intended to remain hidden to casual inspection. The backdoor may take the form of an installed program (e.g., Back Orifice) or could be a modification to a legitimate program.
A backdoor in a login system could take the form of a hard-coded user and password combination which gives access to the system.
An attempt to plant a backdoor in the Linux kernel exposed in November 2003 showed how subtle such a code change could be. In this case a two-line change took the form of an apparent software error, which in practice gave the caller to the sys_wait4 function root access to the machine (see external link).
The prevalence of backdoors in proprietary software systems (those supplied without source code that can be inspected) is a topic of speculation, and are occasionally exposed in practice. Programmers have succeeded in secretly installing even large amounts of code as easter eggss in programs without detection.
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