DnA 2-10: What You Should Know About Computer Viruses

eZine lover (@eZine)

Published in 

DnA Electronic Magazine

 · 1 year ago

What You Should Know About
Computer Viruses

by BlenderHead

Q: Why should I learn about viruses??

When people talk about virii (a subject dear to my heart) it is common for people to treat the virus, the trojan horse, the logic bomb, etc. as if they were one and the same. Now, personally, I find the idea insulting and I am sure that many virus writers would feel the same way. Time and time again, I have seen the worthy name of VIRUS heaped upon the ranks of such undeserving pranks as the common TROJAN horse.

To think that the two are one and the same is fine, if you are the common lamer that so often finds himself behind the computer screen. To be unable to differenciate between a virus and a trojan is perfectly acceptable for many. If you are entirely satisified with knowing just enough to be able to start your computer and run your application, then for heaven's sake don't read this article. In fact, why don't you go buy a MacIntosh?

As for the rest of us, we realize that there IS a difference. And in order to prevent ourselves from looking like clueless idiots, we strive to learn the differences between the virus and the trojan horse and what each one is and is not capable of.

What advantage is gained by learning of such things as a computer virus? The person who is well-informed in such matters gains many advantages over one who is not.

For one, he will quickly notice when his system shows signs of virus activity and he will catch it before it has had time to do significant damage to his system. Since he will have taken the proper precautions in advance he will be able to quickly restore his system system while suffering minimal loss.

Since he knows what a virus can and can't do, he won't believe every quirk in his hardware or software is actually the result of some devious virus. He will not be lulled into the false sense of security provided by such worthless products as CPAV or NAV. He will have the wisdom to look a trojan horse 'in the mouth'.

When it comes to virii, people are inclined to believe alot of stupid shit. Let's face it, people are inclined to believe alot of stupid shit period, but when it comes to virii, they tend to get even stupider

Types of Viruses

Q: What is a virus?

a VIRUS is a small, executable program with the ability to replicate itself by adding its code to that of a host program and/or the system area of a hard or floppy disk. The user is generally unaware of the actions of a virus as it replicates and usually only becomes aware of its presence when the virus 'activates', which it does according to a given set of conditions and at which time it is often too late.

However, once the user knows what signs to look for, it can be very obvious when viral activity occurs. More on the signs in a little bit. Let's discuss the difference between viruses.

Every virus has its own personality. Viruses differ in many ways, each having its own unique properties that make it different. Here are some ways that viruses differ from each other:

• SIZE - A virus can be as small as 66 bytes or less, or as large as 4096 bytes or more. Compared to most computer programs a virus must be very small.
• METHOD OF INFECTION - A virus can infect the host program in different ways. Below are three methods commonly used. They are by no means the only ways, but they are the most common. It is possible for a virus to use one or more of these methods.
• OVERWRITING - When a virus infects using this method, it will simply write a copy of itself over the begining of the host program. This is a very simple method and is used by more primitive viruses. An infected file has been destroyed and must be restored from a backup disk. Overwriting tends to make the user suspicious becuase the host program no longer functions. This method of infection causes no change in the size of an infected program.
• APPENDING - This method is a bit more complex. The virus appends itself onto the end of the host program and also edits the begining of the program. When the user runs the infected program it will jump to the end of the program where the virus is located, perform the functions of the virus, then return and continue to run the host program. To the user, the program is functioning normally. This method of infection causes infected programs to increase in size.
• Some appending viruses are unable to tell whether or not they have already infected a program and will continue to infect the program hundreds of times, causing it to grow considerably in size.
• DISK INFECTORS - Other viruses will infect the boot record or partition table. This is an executable area of the disk that is automatically run every time you boot up from the disk. This means that as soon as the computer boots up, the virus is in memory.
• TSR - A virus may or may not become resident in memory. If it does go TSR, then its chances of infecting files are greatly increased. Otherwise it can only do its stuff when an infected program is run. If the virus is in memory it can infect files any time it chooses. Partition table and boot sector infecting viruses are always TSRs.
• STEALTH - Some TSR viruses use a sophisticated technique called Stealth cloaking. What this means is the virus will fool the system so that everything appears to be normal. When a user does a directory listing the virus will intercept the disk read, and alter the data so that the file sizes appear to be unchanged, when in actuality they have increased in size.
• Boot sector infectors may use stealth so that when the user attempts to view the boot record, instead of showing the actual boot record, a copy of the old boot record is returned instead. Because of stealth techniques it may be impossible to detect a virus once it has become resident in memory. The only sure way to check for a stealth virus is to boot from a clean, write-protected floppy, then scan the hard drive. It is a good idea to prepare such a floppy disk ahead of time, and adding anti-virus software such as Scan and F-Prot.
• ACTIVATION CRITERIA AND EFFECT - The other area that gives a virus its personality is the activation criteria, or what makes it go off. Some activate by the date, others activate when a certain program is run, and other will activate when they can't find any more files that haven't been infected yet.
• When a virus activates it will take a certain action. I will refer to this as the activation effect. The efffect may be as simple and harmless as displaying a message or as malicious as trashing the victim's hard drive. Obviously, you want to find the virus BEFORE it activates

Q: What are the ways that I can catch a virus?

Just as with the AIDS virus, there is alot of bullshit concerning the conditions under which a virus may infect your system. A virus can only be caught by executing a program that has been infected with a virus or by ATTEMPTING to boot up from an infected disk. You cannot get a virus by merely LOOKING at an infected program or disk. A virus can infect just about any executable file EXE COM OVL SYS DRV BIN and the partition table and master boot record of floppies and hard disks.

Notice that above I said "attempting" to boot up from an infected disk. Even if you attempt to boot up from A: and it tells you, "Non-System disk" and then you boot from C: instead, the virus can still be active if A: was infected. This is very important. It doesn't have to be a succesful boot for the virus to get into memory. The first thing it will probably do is infect C: drive. Then if you put a new disk in A:, that will in turn be infected. That is why it is important to keep a clean, write-protected floppy.

So, to sum it up:

• You can catch a virus by executing an infected program, wether you realize the program was run or not. This includes overlay files, system drivers, EXE and COM files, etc.
• You can catch a virus by ATTEMPTING to boot from an infected floppy disk or hard disk, without regard as to whether that attempt was succesful.
• A cold boot will remove a virus from memory, a warm boot won't necessarily do it. So press the button on your computer instead of using CTRL-ALT-DEL.
• You CAN'T get a virus just from looking at an infected disk or file.
• You CAN'T get a virus from a data file, unless it is actually an executable and some other program renames it.

So in order to keep yourself in the clear, always check any new program for viruses before running it, and never leave a disk in the floppy drive when you boot up˛

Q: What are the signs that a virus is present?

There are several things that may indicate the presence of a virus on your system.

1. Unexplained file growth in EXE and COM files may indicate an appending virus.
2. Programs that used to work now return with some type of error message and fail to work at all. This may indicate an overwriting virus. Some common messages are "Program to big to fit in memory" or "Unknown Command" and other similar messages. Thes should make you suspicious.
3. Unexplained directory changes. If you execute a program and then find that you are suddenly in a different directory, this may indicate that a virus has been hunting for files to infect.
4. A decrease in available system memory. You should know how much memory is usually free on your computer. If this number drops, it may indicate a TSR virus. This does not always work since some viruses do not protect the memory they use.
5. Unexplained ChkDsk errors. Stealth viruses will cause you to get a CHKDSK error because they are altering the info before it gets to CHKDSK. If you do a CHKDSK /F under this condition, it could CAUSE considerable damage to the directory structure when in actuality nothing was wrong in the first place.
6. Unexplained disk access. If the floppy or hard drive begin to light up all of a sudden for no reason, it could mean viral activity. It could also mean that you are running a disk cache with staged writes enabled.
7. An overall slowdown in system activity. Programs may take longer to execute than normal.

Q: How can I protect myself against viruses?

There is one fool-proof positive method. Never run any program that isn't already on your computer and never use anybody else's disks. Unfortunately, that is practical. So what is the next best thing?

• Backups - Make frequent backups of the files on your hard disk. Remember that at any given moment you may lose your entire hard drive and its contents. Do you have backups of all your important files? Things like Phone directories and passwords are especially hard to get back. So be prepared for the worst.
• Rescue Disk - Many programs such as TBAV and Norton Utilities will allow you to create a 'rescue disk', which is a floppy disk that can be booted from in an emergency. On this disk will be stored a copy of important system info that could be very hard, if not impossible to come up with manually. This includes a copy of the partition table, Master Boot Record (MBR), CMOS settings, and other important system info.

Also on this disk, you should store utilities that can be used to detect, clean, and remove viruses from your hard disk. This disk should be write-protected, and should be updated any time you make changes to your system.

• Knowledge - Keeping yourself well-informed about how viruses work, any new viruses, and that kind of info is very important. Most of the computer using public is entirely ignorant when it comes to viruses. By readin this article, you have already made a big step at reducing your odds of being hit by a virus.
• AV Software - There are plenty of good Anti-Virus programs available on the market. Most of the good ones are usually shareware or freeware. Some are commercial. Many of the commercial ones are lousy, too. Using some of the less effective virus software can provide a false sense of security.

Anti-Virus Software

What NOT to use

The following are products that I feel are not up to par as far as AV software goes. I would avoid using them if possible, opting for some of the products in the following list. However, if these programs are the only ones you can find, then they certainly are better than nothing at all.

• Norton Anti-Virus (NAV)
• Central Point Anti-Virus (CPAV)
• Dos v6.0 Anti-Virus

What TO use

These are some of the AV products that I DO recommend for you to use. The more Anti-Virus software, the better protected you are. Allow me to quickly explain what a Heuristic Scan is.

Normally, a virus scanner will look for a 'signature', a series of bytes that occur inside the virus that can be used to identify a specific virus. A huruistic scan takes a different approach. It evaluates the code and looks for virus-like programming techniques. This technique enables the scanner to find new or unknown viruses and variations but also tends to cause more false positives and takes longer. It is a very useful feature.

• VirusScan - by MacAfee, Also known as SCAN. This is the standard, and recognizes more than 1300 virus strains. This program is readily available and offers frequent updates. {Shareware}
• F-Prot Anti Virus- by Frisk Software, I highly recommend this program. It recognizes nearly as many viruses as SCAN and recognizes trojan horse programs, as well. It has both a menu driven and command line interface, huriustic scan, virus database, and detailed descriptions. {Free for personal use}
• Thunder Byte Anti Virus- This is a good package that does alot of interesting things. It will create a rescue disk, is highly configurable, does CRC test for changed files, and has an adjustable heruistic scan. It will also allow you to replace the bootstrap loader on your hard drive with a new one that will perform an automatic CRC check upon bootup. This will allow you to be instantly informed of any boot sector viruses. {ShareWare}
• Doctor Solomon's Anti Virus ToolKit - Although more expensive than the others, this program has some interesting utilities. It has "anti-stealth" technology, and an authorization TSR, and a Certify TSR, which only allows you to run programs that have been checked and had their CRC logged in. {Commercial}

Well, that's it for now. Now that we have covered the basics, that will allow us to get into the more fun stuff next time, like how to play with virues. See ya guys next time.