Computer Viruses


Computer Viruses Essay, Research Paper

Computer Viruses

A virus is a program that copies itself without the knowledge of the computer

user. Typically, a virus spreads from one computer to another by adding itself

to an existing piece of executable code so that it is executed when its host

code is run. If a virus if found, you shouldn’t panic or be in a hurry, and

you should work systematically. Don’t rush!

A Viruse may be classified by it’s method of concealment (hiding). Some are

called stealth viruses because of the way that they hide themselves, and some

polymorphic because of the way they change themselves to avoid scanners from

detecting them.

The most common classification relates to the sort of executable code which the

virus attaches itself to. These are:

? Partition Viruses ? Boot Viruses ? File Viruses ? Overwriting Viruses

As well as replicating, a virus may carry a Damage routine.

There is also a set of programs that are related to viruses by virtue of their

intentions, appearances, or users likely reactions. For example:

? Droppers ? Failed viruses ? Packagers ? Trojans ? Jokes ? Test files


Damage is defined as something that you would prefer not to have happened. It is

measured by the amount of time it takes to reverse the damage.

Trivial damage happens when all you have to do is get rid of the virus. There

may be some audio or visual effect; often there is no effect at all.

Minor damage occurs when you have to replace some or all of your executable

files from clean backups, or by re-installing. Remember to run FindVirus again


Moderate damage is done when a virus trashes the hard disk, scrambles the FAT,

or low-level formats the drive. This is recoverable from your last backup. If

you take backups every day you lose, on average, half a day’s work.

Major damage is done by a virus that gradually corrupts data files, so that you

are unaware of what is happening. When you discover the problem, these corrupted

files are also backed up, and you might have to restore a very old backup to get

valid data.

Severe damage is done by a virus that gradually corrupts data files, but you

cannot see the corruption (there is no simple way of knowing whether the data is

good or bad). And, of course, your backups have the same problem.

Unlimited damage is done by a virus that gives a third party access to your

network, by stealing the supervisor password. The damage is then done by the

third party, who has control of the network.


Stealth Viruses

If a stealth virus is in memory, any program attempting to read the file (or

sector) containing the virus is fooled into believing that the virus is not

there, as it is hiding. The virus in memory filters out its own bytes, and only

shows the original bytes to the program.

There are three ways to deal with this:

1. Cold Boot from a clean DOS floppy, and make sure that nothing on the hard

disk is executed. Run any anti-virus software from floppy disk. Unfortunately,

although this method is foolproof, relatively few people are willing to do it.

2. Search for known viruses in memory. All the virus scanners do this when the

programs are run.

3. Use advanced programming techniques to probe the confusion that the virus

causes. A process known as the “Anti-Stealth Methodology” in some scanners can

be used for this.

Polymorphic Viruses

A polymorphic virus is one that is encrypted, and the decryptor/loader for the

rest of the virus is very variable. With a polymorphic virus, two instances of

the virus have no sequence of bytes in common. This makes it more difficult for

scanners to detect them.

Many scanners use the “Fuzzy Logic” technique and a “Generic Decryption Engine”

to detect these viruses.

The Partition and Partition Viruses

The partition sector is the first sector on a hard disk. It contains information

about the disk such as the number of sectors in each partition, where the DOS

partition starts, plus a small program. The partition sector is also called the

“Master Boot Record” (MBR).

When a PC starts up, it reads the partition sector and executes the code it

finds there. Viruses that use the partition sector modify this code.

Since the partition sector is not part of the normal data storage part of a disk,

utilities such as DEBUG will not allow access to it. However, it is possible to

use Inspect Disk to examine the partition sector. A floppy disk does not have a

partition sector.

How to Remove a Partition Sector (MBR) Virus

1. Cold Boot from a clean DOS diskette.

2. Run the DOS scanner.

3. Select the drive to clean and “Repair” it.

4. Follow the instructions.

The Boot Sector and Boot Sector Viruses

The boot sector is the first sector on a floppy disk. On a hard disk it is the

first sector of a partition. It contains information about the disk or partition,

such as the number of sectors, plus a small program.

When the PC starts up, it attempts to read the boot sector of a disk in drive A:.

If this fails because there is no disk, it reads the boot sector of drive C:. A

boot sector virus replaces this sector with its own code and moves the original

elsewhere on the disk.

Even a non-bootable floppy disk has executable code in its boot sector. This

displays the “not bootable” message when the computer attempts to boot from the

disk. Therefore, a non-bootable floppy can still contain a virus and infect a PC

if it is inserted in drive A: when the PC starts up.

File Viruses

File viruses append or insert themselves into executable files, typically .COM

and .EXE programs.

A direct-action file virus infects another executable file on disk when its

‘host’ executable file is run.

An indirect-action (or TSR – Terminate and Stay Resident) file virus installs

itself into memory when its ‘host’ is executed, and infects other files when

they are subsequently accessed.

Overwriting Viruses

Overwriting viruses overwrite all or part of the original program. As a result,

the original program doesn’t run. Overwriting viruses are not, therefore, a real

problem – they are extremely obvious, and so cannot spread effectively.



Droppers are programs that have been written to perform some apparently useful

job but, while doing so, write a virus out to the disk. In some cases, all that

they do is install the virus (or viruses).

A typical example is a utility that formats a floppy disk, complete with Stoned

virus installed on the boot sector.

Failed Viruses

Sometimes a file is found that contains a ‘failed virus’. This is the result of

either a corrupted ‘real’ virus or simply a result of bad programming on the

part of an aspiring virus writer. The virus does not work – it hangs when run,

or fails to infect.

Many viruses have severe bugs that prevent their design goals – some will not

reproduce successfully or will fail to perform their intended final actions

(such as corrupting the hard disk). In general many virus authors are very poor



Packagers are programs that in some way wrap something around the original

program. This could be as an anti-virus precaution, or for file compression.

Packagers can mask the existence of a virus inside.

Trojans and Jokes

A Trojan is a program that deliberately does unpleasant things, as well as (or

instead of) its declared function. They are not capable of spreading themselves

and rely on users copying them.

A Joke is a harmless program that does amusing things, perhaps unexpectedly. We

include the detection of a few jokes in the Toolkit, where people have found

particular jokes that give concern or offence.

Test files

Test files are used to test and demonstrate anti-virus software, in the context

of viruses. They are not viruses – simply small files that are recognised by

the software and cause it to simulate what would happen if it had found a virus.

This allows users to see what happens when it is triggered, without needing a

live virus.


How to Remove a Boot Virus from a Hard Disk

1. Cold Boot from a clean DOS diskette.

2. Run the scanner.

3. Select the drive to clean and “Repair” it.

An alternative method is as follows:

1. Cold Boot from a clean DOS diskette.

2. Type:

SYS C: at the DOS prompt. (if C drive is infected)

The clean DOS diskette should be the same version of DOS that is on the hard


How to Remove a Boot Virus from a Floppy

1. Cold Boot from a clean DOS diskette.

2. Run the scanner.

3. Make sure to “Replace the Boot Sector” of the floppy drive.

If you find a new virus…

If you have some symptoms that you think are a virus, then:

1. Format a floppy disk in the infected computer.

2. Copy any infected files to that floppy.

3. Copy your FORMAT and CHKDSK programs too.

As you can see in this essay, viruses are very appalling, and since a virus

spreads from one computer to another, it gets worse! Just like a contagious

human virus which causes more harm, as more people are infected and more need to

be treated. This same concept applies to a computer virus infecting computers

continually. Also, in this essay, various techniques have been explained on how

to remove and deal with computer viruses, of different types, inflicting

different components in a computer. So, next time you have suspicions that your

computer has been damaged by a virus, read through this essay and exercise the

remedies indicated.

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