Introduction to How Computer Viruses Work
Strange as it may sound, the computer virus is something of an Information Age marvel. On one hand, viruses show us how vulnerable we are -- a properly engineered virus can have a devastating effect, disrupting productivity and doing billions of dollars in damages. On the other hand, they show us how sophisticated and interconnected human beings have become.
For example, experts estimate that the Mydoom worm infected approximately a quarter-million computers in a single day in January 2004. Back in March 1999, the Melissa virus was so powerful that it forced Microsoft and a number of other very large companies to completely turn off their e-mail systems until the virus could be contained. The ILOVEYOU virus in 2000 had a similarly devastating effect. In January 2007, a worm called Storm appeared -- by October, experts believed up to 50 million computers were infected. That's pretty impressive when you consider that many viruses are incredibly simple.
When you listen to the news, you hear about many different forms of electronic infection. The most common are:
Viruses - A virus is a small piece of software that piggybacks on real programs. For example, a virus might attach itself to a program such as a spreadsheet program. Each time the spreadsheet program runs, the virus runs, too, and it has the chance to reproduce (by attaching to other programs) or wreak havoc.
E-mail viruses - An e-mail virus travels as an attachment to e-mail messages, and usually replicates itself by automatically mailing itself to dozens of people in the victim's e-mail address book. Some e-mail viruses don't even require a double-click -- they launch when you view the infected message in the preview pane of your e-mail software [source: Johnson].
Trojan horses - A Trojan horse is simply a computer program. The program claims to do one thing (it may claim to be a game) but instead does damage when you run it (it may erase your hard disk). Trojan horses have no way to replicate automatically.
Worms - A worm is a small piece of software that uses computer networks and security holes to replicate itself. A copy of the worm scans the network for another machine that has a specific security hole. It copies itself to the new machine using the security hole, and then starts replicating from there, as well.
In this article, we will discuss viruses -- both "traditional" viruses and e-mail viruses -- so that you can learn how they work and understand how to protect yourself.
Virus Origins
Computer viruses are called viruses because they share some of the traits of biological viruses. A computer virus passes from computer to computer like a biological virus passes from person to person.
Unlike a cell, a virus has no way to reproduce by itself. Instead, a biological virus must inject its DNA into a cell. The viral DNA then uses the cell's existing machinery to reproduce itself. In some cases, the cell fills with new viral particles until it bursts, releasing the virus. In other cases, the new virus particles bud off the cell one at a time, and the cell remains alive.
A computer virus shares some of these traits. A computer virus must piggyback on top of some other program or document in order to launch. Once it is running, it can infect other programs or documents. Obviously, the analogy between computer and biological viruses stretches things a bit, but there are enough similarities that the name sticks.
People write computer viruses. A person has to write the code, test it to make sure it spreads properly and then release it. A person also designs the virus's attack phase, whether it's a silly message or the destruction of a hard disk. Why do they do it?
There are at least three reasons. The first is the same psychology that drives vandals and arsonists. Why would someone want to break a window on someone's car, paint signs on buildings or burn down a beautiful forest? For some people, that seems to be a thrill. If that sort of person knows computer programming, then he or she may funnel energy into the creation of destructive viruses.
The second reason has to do with the thrill of watching things blow up. Some people have a fascination with things like explosions and car wrecks. When you were growing up, there might have been a kid in your neighborhood who learned how to make gunpowder. And that kid probably built bigger and bigger bombs until he either got bored or did some serious damage to himself. Creating a virus is a little like that -- it creates a bomb inside a computer, and the more computers that get infected the more "fun" the explosion.
The third reason involves bragging rights, or the thrill of doing it. Sort of like Mount Everest -- the mountain is there, so someone is compelled to climb it. If you are a certain type of programmer who sees a security hole that could be exploited, you might simply be compelled to exploit the hole yourself before someone else beats you to it.
Of course, most virus creators seem to miss the point that they cause real damage to real people with their creations. Destroying everything on a person's hard disk is real damage. Forcing a large company to waste thousands of hours cleaning up after a virus is real damage. Even a silly message is real damage because someone has to waste time getting rid of it. For this reason, the legal system is getting much harsher in punishing the people who create viruses.
Patch TuesdayOn the second Tuesday of every month, Microsoft releases a list of known vulnerabilities in the Windows operating system. The company issues patches for those security holes at the same time, which is why the day is known as "Patch Tuesday." Viruses written and launched on Patch Tuesday to hit unpatched systems are known as "zero-day" attacks. Thankfully, the major anti-virus vendors work with Microsoft to identify holes ahead of time, so if you keep your software up to date and patch your system promptly, you shouldn't have to worry about zero-day problems.
For more information I highly recommend that you go see this page at HowStuffWorks. It's called How Computer Viruses Work.
See: http://computer.howstuffworks.com/virus.htm.
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