What is computer virus

1.
?

A virus is a piece of
software designed and written to adversely affect your computer by altering the
way it works without your knowledge or permission. In more technical terms, a
virus is a segment of program code that implants itself to one of your
executable files and spreads systematically from one file to another. Computer
viruses do not spontaneously generate: They must be written and have a specific
purpose. Usually a virus
has two distinct functions:

· Spreads itself from one
file to another without your input or knowledge. Technically, this is known as self-replication and
propagation.

· Implements the symptom or
damage planned by the perpetrator. This could include erasing a disk,
corrupting your programs or just creating havoc on your computer. Technically,
this is known as the virus payload, which can be benign or malignant at the
whim of the virus creator.

A benign virus is one that
is designed to do no real damage to your computer. For example, a virus that
conceals itself until some predetermined date or time and then does nothing
more than display some sort of message is considered benign.

A malignant virus is one
that attempts to inflict malicious damage to your computer, although the damage
may not be intentional. There are a significant number of viruses that cause
damage due to poor programming and outright bugs in the viral code. A malicious
virus might alter one or more of your programs so that it does not work, as it
should. The infected program might terminate abnormally, write incorrect
information into your documents. Or, the virus might alter the directory
information on one of your system area. This might prevent the partition from
mounting, or you might not be able to launch one or more programs, or programs
might not be able to locate the documents you want to open.

Some of the viruses
identified are benign; however, a high percentage of them are very malignant.
Some of the more malignant viruses will erase your entire hard disk, or delete
files.

What Viruses
Do

Some viruses
are programmed specifically to damage the data on your computer by corrupting
programs, deleting files, or erasing your entire hard disk. Many of the
currently known Macintosh viruses are not designed to do any damage. However,
because of bugs (programming errors) within the virus, an infected system may
behave erratically.

What Viruses Don't Do

Computer
viruses don't infect files on write-protected disks and don't infect documents,
except in the case of Word macro viruses, which infect only documents and
templates written in Word 6.0 or higher. They don't infect compressed files
either. However, applications within a compressed file could have been infected
before they were compressed. Viruses also don't infect computer hardware, such
as monitors or computer chips; they only infect software.

In addition, Macintosh
viruses don't infect DOS-based computer software and vice versa. For example,
the infamous Michelangelo virus does not infect Macintosh applications. Again,
exceptions to this rule are the Word and Excel macro viruses, which infect
spreadsheets, documents and templates, which can be opened by either Windows or
Macintosh computers.

Finally, viruses don't
necessarily let you know that they are there - even after they do something
destructive. [1]

2.  Types of Computer
Viruses

Nowadays
number of viruses is about 55000. It increases constantly. New unknown types of
viruses appear. To classify them becomes more and more difficult. In common
they can be divided by three basic signs: a place of situating, used operation
system and work algorithms. For example according these three classifications
virus Chernobyl   can be classified as file infector and resident Windows virus.
Further it will be explained what it means.

2.1 A place of existence

2.1.1File Infectors

These are
viruses that attach themselves to (or replace) .COM and .EXE files, although in
some cases they can infect files with extensions .SYS, .DRV, .BIN, .OVL and
.OVY. With this type of virus, uninfected programs usually become infected when
they are executed with the virus in memory. In other cases they are infected
when they are opened (such as using the DOS DIR command) or the virus simply
infects all of the files in the directory is run from (a direct infector).

There are
three groups of file infectors.

Viruses of the
first group are called
overwriting viruses because they overwrite their code into infected file
erasing contents. But these viruses are primitive and they can be found very
quickly.

Other
group
is called parasitic or cavity viruses. Infected file is capable of work fully
or partly but contents of last one are changed. Viruses can copy itself into
begin, middle or end of a file. They record their code in data known not to be
used.

Third
group
is called companion viruses. They don’t change files. They make double of
infected file so when infected file is being started a double file becomes
managing, it means virus. For example companion viruses working with DOS use
that DOS firstly runs COM. file and after if this file is not found runs EXE.
file. Viruses make double file with a same name and with extension COM and
copies itself in this file. During start of infected file DOS runs a COM. file
with a virus firstly and then a virus starts an EXE. file.

Sometime
companion viruses rename file will be infected and record their code in a
double file with old name. For example the file XCOPY.EXE is renamed into
XCOPY.EXD and virus record itself in file XCOPY.EXE. When this file is started
computer runs a virus code firstly and after virus starts original XCOPY, saved
as XCOPY.EXD. Viruses like this were found not only in DOS. They were found in
Windows and OS/2.

It is not only
one way to make double files. For example there is subgroup of companion
viruses called path-companion viruses. They use special feature of DOS - PATH:
hierarchical record of file location. Virus copies itself in file with the same
name but situated one level higher. In this case DOS will find file with
virus.   [2]

2.1.2Boot viruses

Boot Sector Infectors

Every logical
drive, both hard disk and floppy, contains a boot sector. This is true even of
disks that are not bootable. This boot sector contains specific information
relating to the formatting of the disk, the data stored there and also contains
a small program called the boot program (which loads the DOS system files). The
boot program displays the familiar "Non-system Disk or Disk Error"
message if the DOS system files are not present. It is also the program that
gets infected by viruses. You get a boot sector virus by leaving an infected
diskette in a drive and rebooting the machine. When the boot sector program is
read and executed, the virus goes into memory and infects your hard drive.
Remember, because every disk has a boot sector, it is possible (and common) to
infect a machine from a data disk. NOTE: Both floppy diskettes and hard drives
contain boot sectors.

Master Boot Record
Infectors

The first
physical sector of every hard disk (Side Ш, Track Ш, Sector 1) contains the
disk's Master Boot Record and Partition Table. The Master Boot Record has a
small program within it called the Master Boot Program, which looks up the
values in the partition table for the starting location of the bootable
partition, and then tells the system to go there and execute any code it finds.
Assuming your disk is set up properly, what it finds in that location (Side 1,
Track Ш, Sector 1) is a valid
boot sector. On floppy disks, these same viruses infect the boot sectors. You
get a Master Boot Record virus in exactly the same manner you get a boot sector
virus -- by leaving an infected diskette in a drive and rebooting the machine.
When the boot sector program is read and executed, the virus goes into memory
and infects the MBR of your hard drive. Again, because every disk has a boot
sector, it is possible (and common) to infect a machine from a data disk. [3]

2.1.3 Multi-partite Viruses

Multi-partite
viruses are a combination of the viruses listed above. They will infect both
files and MBRs or both files and boot sectors. These types of viruses are
currently rare, but the number of cases is growing steadily.

2.1.4
Macro Viruses

 Until recently, the macro
languages included with most applications were not powerful or robust enough to
support writing an effective virus. However, many of the more advanced applications
that are being developed today include built-in programming capabilities that
rival some of the larger development packages. This has recently been demonstrated
by the various strains of Microsoft Word viruses, including the so-called Word
Concept and Word Nuclear viruses. These viruses transport themselves through
Microsoft Word documents. When opened in Word, they perform various actions,
including spreading themselves into the user's installation of Word, thus
preparing to infect all future documents on the system.

An additional concern is
that macro viruses can be cross-platform. The Word Concept virus has the claim
to fame of being the first prominent cross-platform virus, because it can infect
both Windows and Macintosh systems.

Because most application
macro languages support passing execution to an external shell, such as COMMAND.COM
or CMD.EXE, the power of the macro virus is not limited to the constraints
of the macro language itself[4].

2.2 Used
operation system.

Any computer
or net virus can infect files of one or more operation systems: DOS, Windows,
OS/2, Linux, MacOS and others. It is a base of this way of classification. For
example virus BOZA working with Windows only is classified as Windows virus,
virus BLISS – as Linux virus. 

2.3 Work
algorithms.

Viruses can be
differed by used algorithms making them danger and hard for catching.

Firstly
viruses can be divided on resident and nonresident.

Resident virus
having come in operation memory of computer doesn’t infect memory. They are
capable of copying when they are started only. We can call any macro virus
resident. They present in memory during application infected by them works.

Second  
viruses are visible and invisible. To be invisible means that users and
antivirus programs can’t notice changes of infected file done by virus.
Invisible virus catches all requires of operation system to read file and to
record in file and shows uninfected version of file. So we can see only ‘clear’
programs during virus works. One of first invisible file infectors was FRODO
and boot infector – BRAIN.

Almost any
virus uses methods of self-coding or polymorphism to escape antivirus programs.
It means that they can change itself. Changing itself helps virus to be able
work.[5]

3.
Conclusion   

In conclusion I would like to say few
words about future of this classification. Nowadays computer technologies and
all software develop very quickly. It helps new types of computer viruses to
appear. Viruses are becoming more and more dangerous and ‘cleverer’. It means
that viruses can be found more and more hard. But I think that this
classification can be saved a long time thank for principles of work of computer.
It means that this classification will be changed when computers work by
principles that differ from principles of von Neiman. So this classification
can be change by adding new subtypes of basic types if virus makers have
created something new. 

Buryat State University

      The paper: Types of computer
viruses

       Presented by
Nefyodov Yuri

Scientific
advisor: Sodboyeva L.D.

                                    Ulan-Ude 2003

       Abstract 

This paper is about the classification of
computer viruses. Firstly, the paper tells what a computer virus is, what
viruses can do and what they can’t do. Then there are basic ways of
classification: a place of situation, used operation system and work
algorithms. In conclusion it’s said about future of classification.

Аннотация

Этот доклад посвящён классификации компьютерных
вирусов. В начале рассказывается, что такое компьютерный вирус, что вирусы
могут делать и что не могут. Далее здесь описаны три основных способа
классификации: по среде обитания, используемой  операционной системе и
алгоритму работы. В заключении говорится о будущем классификации.

Plan

     1. What is a computer virus?

     2. Types of computer viruses.

   2.1 a place of existence

2.1.1 file infectors

2.1.2 boot viruses

2.1.3 multi-partite viruses

2.1.4 macro viruses

    2.2 used operation system

    2.3 work algorithms                              

3. Conclusion.

References:

1.          
Могилёв, Хеннер, Пак 
«Информатика» Издательство «Академия» 2000г

2.          
Журнал «Наука и жизнь» №7 2000 год

3.          
сайт WWW.SEMANTEC.RU