What is a Trojan Horse?
How does a Trojan Horse infect a computer?
How does a Trojan Horse replicate?
Examples of Trojan Horses

What is a Trojan Horse?

Much like its real-life counterpart, a Trojan horse isn't what it appears to be. A Trojan horse is a malicious computer program that is disguised as one thing (a game, for example) but does something else (takes over control of the computer, for example). A Trojan horse consists of two parts: a server, and a client. The server is the viral code that resides on the infected machine, waiting for a network connection on the port it is listening on. The client is the interface for the malicious user to remotely control the infected computer. Whereas there is no graphical interface for the server, since it is running covertly, there is a complete graphic user interface running on the client.

How does a Trojan Horse infect a computer?

A Trojan horse cannot automatically infect a computer; it requires the computer user to run the program. Once the program is run, the Trojan horse will write some lines of code to the startup routine of the computer to make sure it is loaded whenever the computer is booted up. Once loaded, a malicious user can remotely take control of the infected computer. The remote user can view any files on the infected computer's hard drive, cause message boxes to display, cause the CD-ROM drive to open and shut, and many other things.

How does a Trojan Horse replicate?

A Trojan horse is unable to replicate on its own, however some Trojan horses have file transfer capabilities, so it would be possible for a malicious user to upload other viruses to the infected computer.

Examples of Trojan Horses

Two infamous examples of Trojan horses are Sub7 and BackOrifice, both of which allow malicious users to connect remotely to an infected computer, and have complete control over that computer. Sub7 was originally created for Windows, but recently a Macintosh version of both the server and client were released. The Macintosh client can control bother Macintosh and Windows versions of the server, but the Windows client can only control the Windows version of the server. BackOrifice is Windows-only as far as the server goes, but there is a Macintosh client for the Trojan. Neither of these Trojan horses run on Linux in any form.