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This chapter is from the book

Beware of Email Bearing Gifts

Much of the focus on malicious code is on viruses and worms, but another type of malicious code can take over your computer—Trojan horses. Just like the story from the Iliad, a Trojan horse appears to be one thing on the outside, but contains a destructive force inside. In computer terms, a Trojan horse is a program that masquerades as a game or tool, but includes malicious code that can perform many of the same attacks as a virus payload. This section discusses Trojan horses and what steps you can take to avoid becoming their next victim.

Case Study 3-8

Angie logged on to her computer to be faced with a laser-wielding three-headed alien. "David and Alex!" she called out at the top of her lungs. No matter how many times she told them that her computer was for work, her two sons always managed to get access to her computer to play games.

Angie logged in to the network at work and checked her email. "This telecommuting thing is pretty good," she thought as she sipped a cup of coffee. About an hour into the work day, her connection to the network dropped. She tried several times to get connected, with no luck.

Angie called the network support team. They asked her to bring her computer in so that they could check it out. Angie needed to stop by the office that afternoon, so she took the computer in then. Before she left work to pick up her computer, her boss called her into his office.

The company had a strict policy against using corporate computers for personal use. The game the boys had installed had actually been a Trojan horse. When Angie had logged in to the corporate network, the Trojan horse had gained access to the company's internal network. Angie realized that her ability to keep telecommuting was probably in jeopardy.

Case Study 3-9

Ted had received a little cartoon movie in a email from a friend. The movie required installing a small video player software application, which Ted was able to do quickly. The cartoon was funny, so Ted forwarded the email on to some friends.

A few weeks later, Ted received an official-looking envelope in the mail. It was a notice from his cable company that his cable modem was being taken away. Ted called the number at the bottom of the letter to find out what was happening.

The lady who answered informed Ted that the cable company had received numerous complaints from major Web sites around the country about his computer launching attacks against their sites. Ted tried to explain that he hadn't hacked anyone, but the cable company had lots of proof that he or someone with access to his computer was doing just that.

How the Attack Works

You've most likely heard the phrase "If it's too good to be true, it probably is." This certainly applies to the category of programs known as Trojan horses. Of the many resources available on the Internet, software is a key component. Whether it's commercial software, try-before-you-buy deals, shareware, or freeware, being able to download software and have it available immediately is a major draw for a lot of people.

There are Web sites listing thousands of programs available for download that range from games to productivity software to utilities. If you can imagine it, chances are good there's a piece of software out there that does it. Having this huge repository of software is invaluable when you need that one special tool at 3:00 a.m.

However, with all this opportunity comes a great deal of risk. Besides the issue of checking software you download for viruses, there's the added concern of a hidden agenda. Trojan horses aren't viruses, in that they don't replicate themselves. The only way they can move from machine to machine is if someone copies them. To get people to copy the software to their computer, Trojan horses offer two faces: one useful and one malicious.

The first face is a useful software package that people would want to use. It might be a game, a utility, or other software tool that people might download and install. The user downloads the software and installs it. Like any legitimate software, the Trojan horse executes the program and runs the game or utility as the user intended.

The malicious part of a Trojan horse is what happens behind the scene with the software package's second face. A Trojan horse's hidden side takes some other action, such as exposing your system to hackers, deleting files, or intercepting passwords. Although the program that was downloaded and installed seems useful, this hidden agenda can be devastating.

In Angie's case, a Trojan horse contained in a game her boys downloaded from the Internet was able to wreak havoc on the corporate network. Although this Trojan horse probably wasn't designed specifically for Angie's company, some Trojan horse programs are designed to look for certain types of files or data within files. A common technique of Trojan horses is simply to allow a hacker future access. For example, in Angie's case, either the Trojan horse would connect to the attacker to let him know the site had been compromised, or the attacker would just scan blocks of computers looking for ones that had his Trojan horse program running.

This technique of scanning blocks of computers is known as port scanning. Many of the hits on a firewall are this type of attack, which is similar to walking down a street and trying all the doors on all homes to see whether any are unlocked. If one is, that home is added to a list to come back and enter later. An attacker tries to connect to his program on a large number of computers. Any computer that responds is added to a list of systems that give the attacker easy access whenever he chooses.

Many people's first response to the possibility of being hacked is that they don't have any sensitive information a hacker would want, so they don't consider themselves a target. What many people don't realize is that the information on their computers might not be the target at all. The growing trend is for attackers to gain access to large blocks of computers for launching future attacks. A Trojan horse gives an attacker control over your home PC as well as thousands of other PCs. When he wants to launch an attack or send spam, instead of using his own servers and risking detection or being blocked, he uses this block of PCs to do the dirty work. If he's detected, it is the owners of these PCs who will be blamed. If an ISP blocks traffic from the attacker, it is these PCs that will be blocked, not the actual attacker's computer. These blocks of PCs have been used to launch many denial-of-service attacks against large Web sites over the past few years.

An Ounce of Prevention

The first way to prevent Trojan horses from taking control of your computer is to avoid downloading and installing software. A lot of good software is available on the Internet, and you might want to avail yourself of this resource, but by using caution first, you'll limit your exposure.

Also, the other rules for preventing virus infection apply here as well. Most virus protection tools detect common Trojan horses, and a good backup can be helpful if the Trojan horse performs malicious actions on your file system. If you follow these basic rules, many of the problems that Trojan horses cause can be limited or eliminated.

Another step that works particularly well for Trojan horses is installing a firewall, which is typically a small piece of software that restricts access to your computer. This is a good piece of software for all computers that connect to the Internet, but especially for those that use a broadband connection. With dial-up connections, you might get a new IP address every time you call in. With broadband connections, you might get a permanent address or at least keep your temporary address for long periods.

A firewall is the first step toward keeping hackers out of your computer and network. They also are helpful in preventing Trojan horses from establishing themselves on your computer. If you restrict access to the few services you use, such as Web access and email, a program that starts opening new connections to the Internet will be blocked. You can use firewalls not only to protect you, but also to act as a warning system that a Trojan horse might be at work (see Figure 3.1).

Figure 3.1Figure 3.1 Firewall log showing Trojan horse attack.

You can also install a hardware firewall, which is a computer with the sole purpose of protecting your network. The operating system hardware firewalls run and the software installed on them are all configured for the purpose of blocking unwanted traffic from entering your network. Some devices for granting wireless or broadband access can also double as a hardware firewall. Whether you choose a hardware or software solution, the important thing is to protect your network from outside attack and from being used as a launch platform for attacking others.

A Pound of Cure

If you believe you already have a Trojan horse installed, the same steps mentioned previously for prevention can help you fix the problem. Even if an attacker has already gained access to your computer, adding a firewall can block any further communication. Also, if your virus protection software detects a well-known Trojan horse, it can assist in removing the offending program from the computer to prevent further use by attackers.

Again, don't be afraid to use outside help if you believe your system has been compromised. This is a serious issue that needs to be resolved as soon as possible. By dealing with it quickly and correctly, you can limit your exposure and minimize the damage.


  • Avoid downloading software from unknown sources.

  • Install virus protection because most virus protection packages can detect common Trojan horses.

  • Install and configure a firewall.

  • Bring in outside help if needed. Trojan horses are a problem you don't want to mess around with.

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