Trojan Horses

A Trojan horse is a program that masquerades as another program or, in addition to doing what the genuine program does, performs some other unintended action. Often a Trojan horse masquerades as a commonly used program, such as ls. When a Trojan horse runs, it may send files to the intruder or simply change or erase files.

An example of a Trojan horse has been provided by Morris and Gramp in their article listed at the end of this chapter. Their example is a Trojan horse that masquerades as the su command. The shell script for the Trojan horse is placed in the file su in a directory in the path of the user. The shell script for this Trojan horse is given here: