Backdoor

A backdoor is a piece of code in an access authorization program, the purpose of which is to allow the attacker to enter the protected system without passing though the normal authentication process (i.e., by bypassing the "front door"). Backdoors are inserted either by an attacker who has already gained unauthorized access by other means (e.g., an exploit), so that he can re-enter the system easily later, or by the developers in order to provided them with an easy and hassle-free access to the system, should the normal authentication process fail for one reason or another. The latter is extremely dangerous practice, since the existence of such a backdoor could be discovered and exploited by illegitimate parties.