Trojan Horses

A Trojan horse is a program or code fragment that hides inside a program and performs a disguised function. This type of threat gets its name from Greek mythology and the story of the siege of Troy. The story tells of how Odysseus and his men conquered Troy by hiding within a giant wooden horse. A Trojan horse program hides within another program or disguises itself as a legitimate program. This can be accomplished by modifying the existing program or by simply replacing the existing program with a new one. The Trojan horse program functions much the same way as the legitimate program, but usually it also performs some other function, such as recording sensitive information or providing a trap door.

An example would be a password grabber program. A password grabber is a program designed to look and function like the normal login prompt that a user sees when first accessing a system. For example, the user has entered the username john and the correct password. However, the system tells the user that the login is incorrect. When the user tries again it works and he or she is able to log on.

In this example a Trojan horse designed to steal passwords is actually controlling the interaction. The standard login.exe has been replaced with a Trojan horse program. It looks like the standard login prompt, but what is actually occurring is that the first login prompt is the Trojan horse. When the username and password is entered that information is recorded and stored. Then the Trojan horse program displays the "login incorrect" message and passes the user off to the real login program, so that he or she can actually log on to the system. The user simply assumes that he or she mistyped the password the first time never knowing that her or his username and password have just been stolen.