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--Introduction to Viruses

Welcome! Viruses can seem mysterious but computer viruses are actually quite easy to understand. Our web site is dedicated to demystifying how viruses and anti-virus products work.

I'll give you the information you need know to make sure that your PC is safe from viruses and all the other threats that may damage your programs and data. In these pages I'll explain exactly what viruses are, how they work, and how to protect against them.

Viruses are actually very simple. Once you understand exactly what they can and cannot do, it's much easier to take appropriate precautions. While we'll be spending most of our time talking about viruses, I'll also cover the threats that are much more likely than viruses to damage your programs and data. Although I'll occasionally touch on some rather esoteric or complex topics, you won't need to be a "techy" to understand this text or to find it useful in your day-to-day use of your computer. I will go one step at a time and I will explain all the concepts and jargon clearly before I use the terms. I'll also focus on practical information that will help you protect your PC. Everyone should benefit from reading these pages; those of you that are experts will be able to skip the background information, yet I will still explain everything clearly for those of you that are new to PCs.

Let me quickly introduce myself. I am Wolfgang Stiller, the primary developer of Integrity Master, a leading anti-virus and data integrity package for the IBM PC. Most of my comments therefore are specific to the IBM PC but don't worry if you are on another platform, the general virus principals hold true for any computer.

You may be wondering why you should bother to read this text. You already have anti-virus software on your PC, so why should you need to actually understand any of this stuff? One reason is that your anti-virus software may not be giving you the protection you think it is. You'll learn how to determine what your software can and cannot do. Another reason is that viruses are but one threat to your programs and data; I'll explain how to protect yourself fully.

You may even be wondering if viruses are really worth worrying about at all. Do you think you're safe because you rarely download software or buy only from a trusted retailer? Are viruses really a serious threat to your PC or are viruses mostly hype? Let me begin by quickly putting this issue into perspective. Viruses and anti-virus programs are not really the mysterious, complex, and hard to understand software that many people consider them to be. Not only can these programs be understood by anyone, but these days, it's critical that we all fully grasp how they work so as to to protect ourselves.

--What Do Viruses Do?

I'm going to present an easy to understand but detailed explanation of viruses and other types of malicious software. For now, it's enough to understand that viruses are potentially destructive software that spreads from program to program or from disk to disk. Computer viruses, like biological viruses, need a host to infect; in the case of computer viruses this host is an innocent program. If such a program is transferred to your PC, other programs on your PC will become infected. (I'll shortly explain in more detail how this happens.) Even though some viruses do not intentionally damage your data, I consider all viruses to be malicious software since they modify your programs without your permission with occasional disastrous results.

The bottom line is that if you have a virus, you are no longer in control of your PC. Every time you boot your PC or execute a program the virus may also be executing and spreading its infection. While most viruses haven't been written to be destructive, almost all viruses can cause damage to your files--mostly because the viruses themselves are very poorly written programs. If viruses destroy nothing else, they destroy your trust in your PC--something that is quite valuable.

Are Viruses Mostly Hype?

Unfortunately not! There is some confusion about this issue because some extreme claims have been made regarding numbers of viruses and how likely you are to become infected. During the Michelangelo media extravaganza in early 1991, some exaggerated figures were presented in the media which led some people to suspect that all viruses were nothing but hype. One company was quoted in Information Week that based on their reports, one out of four PCs was infected every month! (I won't speculate on the motivation for these type of claims.) You may also hear reports of there being from ten to thirty thousand different PC viruses with the number expected to double in six to nine months. So, are we faced with impending doom? No, not quite. The truth is viruses are very wide-spread but a relatively small number (about one-hundred) account for ninety percent of all infections. Most of the twenty thousand viruses in our collection are so poorly written that they will not spread in the real world. Many of these viruses are created by kids that can't even program. They use automated viruses creation programs that produce very poor quality viruses. These viruses are so obvious that they rarely spread in the wild. Still, viruses are a real threat that we can't afford to ignore. Viruses have been found on brand-new PCs, direct from the manufacturer, and on shrink-wrapped software, direct from the publisher. Viruses are not merely hype and no one is safe from potentially being infected. If you value your data and programs, you have to take some precautions.

How Serious Are viruses?

Viruses are a problem but they are not the main thing you should be concerned about. There are many other threats to your programs and data that are much more likely to harm you than viruses. Problems such as hardware glitches, software conflicts, software bugs, and even typos are much more likely to cause undetected damage to your data than viruses. A well known anti-virus researcher once said that you have more to fear from a spilled cup of coffee than from viruses. While the growth in number of viruses now puts this statement into question, it's still clear that there are many more occurrences of data corruption from other causes than from viruses. So, does this mean that viruses are nothing to worry about? Emphatically, no! It just means that we need to address the other threats to our data as well as viruses. Because viruses have been deliberately written to invade and possibly damage your PC, they are the most difficult threat to guard against. It's pretty easy to understand the threat that disk failure represents and what to do about it, but the threat of viruses is much more difficult to deal with.

--Quick Virus Guidelines

It's important to keep viruses in perspective. They are but one threat to your data and programs. They need not be regarded as mysterious and they are quite easy to understand. Here are a few tips to keep in mind when considering viruses:

--Software attacks against your computer:

Viruses are one specific type of program written deliberately to cause harm to someone's computer or to use that computer in an unauthorized way. There are many forms of malicious software; sometimes the media calls all malicious software viruses, but it's important to understand the distinction between the various types. Let's examine the different types of malicious software:
Logic Bombs
Just like a real bomb, a logic bomb will lie dormant until triggered by some event. The trigger can be a specific date, the number of times executed, a random number, or even a specific event such as deletion of an employee's payroll record. When the logic bomb is triggered it will usually do something unpleasant. This can range from changing a random byte of data somewhere on your disk to making the entire disk unreadable. The changing of random data on disk may be the most insidious attack since it would do a lot of damage before it would be detected.
These are named after the Trojan horse which delivered soldiers into the city of Troy. Likewise, a trojan program is a delivery vehicle for some destructive code (such as a logic bomb or a virus) onto a computer. The trojan program appears to be a useful program, but when a certain event occurs, it will attack your PC in some way.
A worm is a self-reproducing program which does not infect other programs as a virus will, but instead creates copies of itself, which create even more copies. These are usually seen on networks and on multi-processing operating systems, where the worm will create copies of itself which are also executed. Each new copy will create more copies quickly clogging the system. The so called Morris ARPANET/INTERNET "virus" was actually a worm. It created copies of itself through the ARPA network, eventually bringing the network to its knees. It did not infect other programs as a virus would, but simply kept creating copies of itself which would then execute and try to spread to other machines.
Here's our definition:

A virus is a program which reproduces its own code by attaching itself to other programs in such a way that the virus code is executed when the infected program is executed.

You could also say that the virus must do this without the permission or knowledge of the user.

What Viruses Do:

Our virus definition is very general and covers all viruses. Let's consider specifically how this works. Viruses are programs just like any other on your PC. They consist of instructions for (what I like to call "code") that your computer executes. What makes viruses special is that they do their "job" by placing self-replicating code in other programs, so that when those other programs are executed, even more programs are "infected" with the self-replicating code. "Self-replicating code" is simply a program that copies itself to other programs. This self-replicating code, when triggered by some event, may do a potentially harmful act to your computer--but this is strictly optional. Only a minority of viruses contain deliberately destructive code. You could say that viruses are distributed in the form of a trojan. In other words, the virus code has been planted in some useful program. Since the virus infects other useful programs, absolutely any piece of executable code can suddenly become a trojan delivery vehicle for the virus.

Another way of looking at viruses is simply to consider them to be a program which can create copies of itself. These copies are inserted in other programs (infecting these programs). When one of these other programs is executed, the virus code (which was inserted in that program) executes, and places copies of itself in even more programs.

You'll notice that I used the word "attach" in our definition of a virus. This is because viruses can "attach" themselves to a program without directly modifying that program. This might seem hard to believe at this point, but I'll explain later exactly how they accomplish this trick.

When you consider our definition of viruses, it's important to understand that "programs" may exist in places that you don't expect. For example, all diskettes contain boot sectors which are "programs" that are executed when you boot your PC and Microsoft Office files (such as MS Word Documents and Excel Spread Sheets) can contain macros which are "programs" that can be executed when you open these files.

--General Virus Behavior

Viruses come in a great many different forms, but they all potentially have two phases to their execution, the infection phase and the attack phase:
  1. When the virus executes it will infect other programs. What is often not clearly understood is precisely when it will infect the other programs. Some viruses infect other programs each time they are executed, other viruses infect only upon a certain trigger. This trigger could by anything; it could be a day or time, an external event on your PC, a counter within the virus etc. Some viruses are very selective about when they infect programs; this is vital to the virus's survival. If the virus infects too often, it is more likely to be discovered before it can spread far. Virus writers want their programs to spread as far as possible before anyone detects them. This brings up an important point which bears repeating:

    It is a serious mistake to execute a program a few times -- find nothing infected and presume there are no viruses in the program. You can never be sure that the virus simply hasn't triggered its infection phase!

    Many viruses go resident in the memory of your PC just as a terminate and stay resident (TSR) program such as Sidekick(R) does. This means the virus can wait for some external event such as inserting a diskette, copying a file, or executing a program to actually infect another program. This makes these viruses very dangerous since it's hard to guess what trigger condition they use for their infection. Resident viruses frequently corrupt the system software on the PC to hide their existence.

  2. The second phase is the attack phase. Many viruses do unpleasant things such as deleting files or changing random data on your disk, simulating typos or merely slowing your PC down; some viruses do less harmful things such as playing music or creating messages or animation on your screen. Just as the virus's infection phase can be triggered by some event, the attack phase also has its own trigger. Viruses usually delay revealing their presence by launching their attack only after they have had ample opportunity to spread. This means that the attack may be delayed for years after the initial infection. The attack phase is optional, many viruses simply reproduce and have no trigger for an attack phase. Does this mean that these are "good" viruses? No, unfortunately not! Anything that writes itself to your disk without your permission is stealing storage and CPU cycles. This is made worse since viruses which "just infect", with no attack phase, damage the programs or disks they infect. This is not intentional on the part of the virus, but simply a result of the fact that many viruses contain extremely poor quality code. One of the most common viruses, the STONED virus is not intentionally harmful. Unfortunately the author did not anticipate other than 360K floppy disks, with the result that the virus will try to hide its own code in an area on 1.2mb diskettes which causes corruption of the entire diskette.
Now that we've examined general virus behavior, let's take a closer look at the two major categories of viruses and how they operate.

System Sector Viruses (AKA Boot Sector Viruses)

These are viruses which plant themselves in your system sectors. System sectors are special areas on your disk containing programs that are executed when you boot your PC. Sectors are not files but simply small areas on your disk that your hardware reads in single chunks. Under DOS, sectors are most commonly 512 bytes in length. These sectors are invisible to normal programs but are vital for correct operation of your PC. They are a common target for viruses. There are two types of system sectors found on DOS PCs, DOS boot sectors and partition sectors (also known as Master Boot Records or MBRs). If the term boot sector is new to you, then please read the page on system sectors for more details on why system sectors are important and how they work.

System sector viruses (also commonly referred to as boot sector viruses) modify the program in either the DOS boot sector or the partition sector. Since there isn't much room in the system sector (only 512 bytes), these viruses often have to hide their code somewhere else on the disk. These viruses sometimes cause problems when this spot already contains data which is then overwritten. Some viruses, such as the Pakistani BRAIN virus mark the spot where they hide their code as having bad sectors. This is one reason to be alarmed if CHKDSK or Scandisk suddenly reports additional bad sectors on your disk. These viruses usually go resident in memory on your PC, and infect any floppy disk which you access. Simply doing a DIR on a floppy disk may cause it to be infected. Some viruses will infect your diskette as soon as you close the drive door. Since they are active in memory (resident), they can hide their presence. If BRAIN is active on your PC, and you use a sector editor to look at the boot sector of an infected diskette, the virus will intercept the attempt to read the infected boot sector and return instead a saved image of the original boot sector. You will see the normal boot sector instead of the infected version. Viruses which do this are known as stealth viruses. In addition to infecting diskettes, some system sector viruses spread by also infecting files.

File Viruses

In terms of sheer number of viruses, these are the most common kind. The simplest file viruses work by locating a type of file that they know how to infect (usually a file name ending in ".COM" or ".EXE") and overwriting part of the program they are infecting. When this program is executed, the virus code executes and infects more files. These overwriting viruses do not tend to be very successful since the overwritten program rarely continues to function correctly and the virus is almost immediately discovered. The more sophisticated file viruses modify the program so that the original instructions are saved and executed after the virus finishes. Just as system sector viruses can remain resident in memory and use "stealth" techniques to hide their presence, file viruses can hide this way also. If you do a directory listing, you will not see any increase in the length of the file and if you attempt to read the file, the virus will intercept the request and return your original uninfected program to you. This can sometimes be used to your advantage. If you have a "stealth" virus (such as 4096 or Dir-2), you can copy your program files (*.EXE and *.COM files) to files with other extensions and allow the virus to automatically disinfect your files! If you "COPY *.COM *.CON", and then cold boot your PC from a known good copy of DOS and "REN *.CON *.COM", this will disinfect the renamed files.

Be aware that many file viruses (such as 4096 which is also known as Frodo) also infect overlay files as well as the more usual *.COM and *.EXE files. Overlay files have various extensions, but ".OVR" and ".OVL" are common examples.

Macro Viruses

There is particular type of file virus that that many people don't understand. These are the files from the MicroSoft Office applications (e.g, MS Word, MS Excel, MS Access, etc.). These programs all have their own macro languages (a BASIC like language) built in. The associated files (MS Word documents or templates and MS Excel spreadsheet files) are usually thought of only as data files so many people are surprised that they can be infected. But these files can contain programs (the macro language) that are executed when you load one of these files into the associated product. The program inside of these files is interpreted by the MS Office application. What is now a language originally began as a very simple macro language that the user could use to combine keystrokes to automate some routine function. The macro language in these products has since grown substantially and now is a fully capable language based on Visual Basic (VBA). Since anything that contains a program can potentially be infected by a virus, these files can harbor viruses.

Read about the the threat of MS Word macro viruses (e.g., Concept) or MS Excel Macro Viruses.

What gives these viruses a chance to execute is the fact that Microsoft has defined special macros that will automatically execute. The mere act of opening an infected MS Word document or an infected MS Excel spread sheet can allow the virus macros to be executed. (One simple prevention for this type of virus is to use the freely available (from Microsoft) viewer programs to rather than MS Word or MS Excel to view these type of files. Even MS Access database files (*.mdb files) can contain macro viruses. Read about: MS Access Macro Viruses.

Macro viruses have been very successful because most people regarded spreadsheets and documents as data, not as programs (and because many anti-virus programs were very slow to address this threat). If you use a mail reader or Web browser, it is very important to use a viewer rather than the full MS Office program (i.e, MS Word or MS Excel) if you want to automatically open downloaded MS Word documents or MS Excel spreadsheets.

By now you should have a pretty good idea of how viruses work and what they are likely to do to your PC. You are now ready to continue and read about:
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Copyright 1994-1999 Stiller Research. Document Last Modified May 25, 2001