Back Note: This is only a draft. Some errors may still be in here and this is only as information about firewalls itself.

Viruses

Computer viruses are mysterious and grab our attention. 

Every time a new virus hits, it makes the news if it spreads quickly.
On the one hand, viruses show us how unknowingly vulnerable we are. A properly engineered virus can have an amazing effect on the world-wide Internet. On the other hand, they show how sophisticated and interconnected human beings have become. For example, the "Melissa" virus -- which became a world-wide phenomenon in March of 1999 -- 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. That's pretty impressive when you consider how simple the Melissa and ILOVEYOU viruses are!


We will discuss viruses -- both "traditional" viruses and the newer e-mail viruses -- so that you can learn how they work and also understand how to protect yourself. Viruses in general are on the wane, but occasionally a person finds a new way to create one and that's when they make the news!

Viruses -

A virus is a small piece of software that piggy-backs on real programs. For example, a virus might attach itself to a program like 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.

Email viruses -

An email virus moves around in email messages, and usually replicates itself by automatically mailing itself to dozens of people in the victim's email address book.

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.

Trojan Horses -

A Trojan horse is simply a normal computer program. The program claims to do one thing (e.g. - it claims to be a game) but instead does damage when you run it (e.g. - it erases your hard disk). Trojan horses have no way to replicate automatically.


What's a "Virus"?

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. At a deeper level there are similarities as well. 

A biological virus is not a living thing. A virus is a fragment of DNA inside a protective jacket. Unlike a cell, a virus has no way to do anything or to reproduce by itself -- it is not alive. 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 get executed. Once it is running, it is then able to 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.

Virus History

Traditional computer viruses were first widely seen in the late 1980s and they came about because of several factors. The first factor was the spread of personal computers (PCs). Prior to the 1980s, home computers were non-existent or they were toys. Real computers were rare and "experts" locked them away for use. During the 1980s, real computers started to spread to businesses and homes because of the popularity of the IBM PC (released in 1982) and the Apple Macintosh (released in 1984). By the late 1980s, PCs were widespread in businesses, homes and college campuses.


The second factor was the use of computer "bulletin boards." People could dial up a bulletin board with a modem and download programs of all types. Games were extremely popular, and so were simple word processors, spreadsheets, etc. Bulletin boards led to the precursor of the virus known as the Trojan Horse. A Trojan horse is a program that sounds really cool when you read about it. So you download it. When you run the program, however, it does something uncool like erasing your disk. So you think you are getting a neat game but it wipes out your system. Trojan horses only hit a small number of people because they are discovered quickly. Either the bulletin board owner would erase the file from the system or people would send out messages to warn one another.


The third factor that led to the creation of viruses was the floppy disk. In the 1980s, programs were small and you could fit the operating system, a word processor (plus several other programs) and some documents onto a floppy disk or two. Many computers did not have hard disks, so you would turn on your machine and it would load the operating system and everything else off of the floppy disk.
Viruses took advantage of these three facts to create the first self-replicating programs!

Follow the Trail

Early viruses were pieces of code attached to a common program like a popular game or a popular word processor. A person might download an infected game from a bulletin board and run it. A virus like this is a small piece of code embedded in a larger, legitimate program. Any virus is designed so it runs first when the legitimate program gets executed. The virus loads itself into memory and looks around to see if it can find any other programs on the disk. If it can find one, it modifies it to add the virus's code to the unsuspecting program. Then the virus launches the "real program." The user really has no way to know that the virus ever ran. Unfortunately, the virus has now reproduced itself, so two programs are infected. The next time either of those programs gets executed, they infect other programs, and the cycle continues.


If one of the infected programs is given to another person on a floppy disk, or if it is uploaded to a bulletin board, then other programs get infected. This is how the virus spreads.
The spreading part is the "infection" phase of the virus. Viruses wouldn't be so violently despised if all they did were replicate themselves. Unfortunately, most viruses also have some sort of destructive "attack" phase where they do some damage. Some sort of trigger will activate the attack phase, and the virus will then "do something" -- anything from printing a silly message on the screen to erasing all of your data. The trigger might be a specific date, or the number of times the virus has been replicated, or something similar.


As virus creators got more sophisticated, they learned new tricks. One important trick was the ability to load viruses into memory so they could keep running in the background as long as the computer remained on. This gave viruses a much more effective way to replicate themselves. Another trick was the ability to infect the boot sector on floppy disks and hard disks. The boot sector is a small program that is the first part of the operating system that the computer loads. The boot sector contains a tiny program that tells the computer how to load the rest of the operating system. By putting its code in the boot sector, a virus can guarantee it gets executed. It can load itself into memory immediately and it is able to run whenever the computer is on. Boot sector viruses can infect the boot sector of any floppy disk inserted in the machine, and on college campuses where lots of people share machines they spread like wildfire.


In general, both executable and boot sector viruses are not very threatening any more. The first reason for the decline has been the huge size of today's programs. Nearly every program you buy today comes on a compact disc (CD). CD’s cannot be modified, and that makes viral infection of a CD impossible. The programs are so big that the only easy way to move them around is to buy the CD. People certainly can't carry applications around on a floppy disk like they did in the 1980s, when floppies full of programs were traded like baseball cards. Boot sector viruses have also declined because operating systems now protect the boot sector.
Both boot sector viruses and executable viruses are still possible, but they are a lot harder now and they don't spread nearly as fast as they once could. Call it "shrinking habitat," if you want to use a biological analogy. The environment of floppy disks, small programs and weak operating systems made viruses possible in the 1980s, but that environmental niche has been largely eliminated by huge executables, unchangeable CDs and better operating system safeguards.

E-mail Viruses

The latest thing is the e-mail virus, and the Melissa virus in March of 1999 was spectacular. Melissa spread in Microsoft Word documents sent via e-mail, and it worked like this. Someone created the virus as a Word document uploaded to an Internet newsgroup. Anyone who downloaded the document and opened it would trigger the virus. The virus would then send the document (and therefore itself) in an e-mail message to the first 50 people in the person's address book. The e-mail message contained a friendly note that included the person's name, so the recipient would open the document thinking it was harmless. The virus would then create 50 new messages from the recipient's machine. As a result, the Melissa virus was the fastest-spreading virus ever seen! As mentioned earlier, it forced a number of large companies to shut down their e-mail systems.


The ILOVEYOU virus, which appeared on May 4, 2000, was even simpler. It contained a piece of code as an attachment. People who double clicked on the attachment allowed the code to execute. The code sent copies of itself to everyone in the victim's address book and then started corrupting files on the victim's machine. This is as simple as a virus can get. It is really more of a trojan horse distributed by e-mail than it is a virus.


The Melissa virus took advantage of the programming language built into Microsoft Word called VBA, or Visual Basic for Applications. It is a complete programming language and it can be programmed to do things like modify files and send e-mail messages. It also has a useful but dangerous auto-execute feature. A programmer can insert a program into a document that runs instantly whenever the document is opened. This is how the Melissa virus was programmed. Anyone who opened a document infected with Melissa would immediately activate the virus. It would send the 50 e-mails, and then infect a central file called NORMAL.DOT so that any file saved later would also contain the virus! It created a huge mess.


Microsoft applications have a feature called Macro Virus Protection built in to them to prevent this sort of thing. If you turn Macro Virus Protection on, then the auto-execute feature is disabled. By default the option is ON. So when a document tries to auto-execute viral code, a dialog pops up warning the user. Unfortunately, many people don't know what macros or macro viruses are, and when they see the dialog they ignore it. So the virus runs anyway. Many other people turn off the protection mechanism. So the Melissa virus spread despite the safeguards in place to prevent it.


In the case of the ILOVEYOU virus, the whole thing was human-powered. If a person double-clicked on the program that came as an attachment, then the program ran and did its thing. What fueled this virus was the human willingness to double-click on the executable.

Origins

People create viruses. A person has to write the code, test it to make sure it spreads properly and then release the virus. A person also designs the virus's attack phase, whether its a silly message or destruction of a hard disk. So why do people do it?
There are probably at least three reasons. The first is the same psychology that drives vandals and arsonists. Why would someone want to bust the window on someone else's car, or spray paint signs on buildings or burn down a beautiful forest? For some people that seems to be a thrill. If that sort of person happens to know 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. Many people have a fascination with things like explosions and car wrecks. When you were a kid there was probably a boy in your neighborhood who learned how to make gunpowder and who then built bigger and bigger bombs until he either got bored or did some serious damage to himself. Creating a virus that spreads quickly 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 probably involves bragging rights, or the thrill of doing it. Sort of like Mount. Everest. The mountain is there and no one has climbed it, so someone is compelled to do it. If you are a certain type of programmer and you see a security hole that could be exploited, you might simply be compelled to exploit the hole yourself before someone else beats you to it. "Sure, I could TELL someone about the hole. But wouldn't it be better to SHOW them the hole???" That sort of logic leads to many viruses.


Of course, all of the virus creators 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 the people inside a large company to waste thousands of hours cleaning up after a virus is real damage. Even a silly message is real damage because a person then has to waste the time getting rid of it. For this reason, the legal system is getting much harsher in punishing the people who create viruses.

An Ounce of Prevention

You can protect yourself against viruses with a few simple steps: