A backdoor in an Operating System or a complex application is a method of bypassing normal authentication and gain access. During the development of an Operating System or application, programmers add back doors for different purposes. The backdoors are removed when the product is ready for shipping or production. When a backdoor is detected, which is not removed, the vendor releases a maintenance upgrade or patch to close the back door.
Another type of back door can be an installed program or could be a modification to an existing program. The installed program may allow a user log on to the computer without a password with administrative privileges. Many programs ara available on internet to create back door attacks on systems. One of the more popular tools is Back Orifice which is also available for free download on internet.