A backdoor is a deliberate configuration or specific program designed to allow access to a system without requiring the usual security checks. This doesnt necessarily mean its solely for a hackers benefit. A system administrator may indeed leave a backdoor into all client PCs that he installs for users just in case they inadvertently lock themselves out. However, the type of backdoor we are concerned with here is the malicious kind.
Malicious backdoors range from simple to complex and feature-laden. One of the simpler backdoors is NetBus, a Trojan (see section 1.5) that allows complete remote access to any system its installed on. It provides a remote-control type desktop, much like PCAnywhere or Microsoft Terminal Services, giving an attacker complete control over a machine as if they were physically in front of it. NetBus is relatively unsophisticated though, and easy to detect.
A far more complete backdoor package is BackOrifice. BackOrifice even provides a configuration screen where you select the modules you wish to include and the overall configuration of listener ports, behavior, etc. When youve set your configuration, BackOrifice builds a custom executable for you to distribute as you wish.
There are a wide range of modules available for BackOrifice, including port mappers, remote control, key capturing and application binding (where your BackOrifice executable attaches itself to system files like a virus).
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