Реферат: What is computer virus

1. What iscomputer virus?

A virus is a piece of softwaredesigned and written to adversely affect your computer by altering the way itworks without your knowledge or permission. In more technical terms, a virus isa segment of program code that implants itself to one of your executable filesand spreads systematically from one file to another. Computer viruses do notspontaneously generate: They must be written and have a specific purpose.Usually avirus hastwodistinct functions:

·<span Times New Roman""> Spreadsitself from one file to another without your input or knowledge.

Technically,this is known as self-replication and propagation.

·<span Times New Roman"">Implements the symptom or damageplanned by the perpetrator. This could include erasing a disk, corrupting yourprograms or just creating havoc on your computer. Technically, this is known asthe virus payload, which can be benign or malignant at the whim of the viruscreator.

A benign virus is one that isdesigned to do no real damage to your computer. For example, a virus thatconceals itself until some predetermined date or time and then does nothingmore than display some sort of message is considered benign.

A malignant virus is one thatattempts to inflict malicious damage to your computer, although the damage maynot be intentional. There are a significant number of viruses that cause damagedue to poor programming and outright bugs in the viral code. A malicious virusmight alter one or more of your programs so that it does not work, as itshould. The infected program might terminate abnormally, write incorrectinformation into your documents. Or, the virus might alter the directoryinformation on one of your system area. This might prevent the partition frommounting, or you might not be able to launch one or more programs, or programsmight not be able to locate the documents you want to open.

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

What Viruses Do

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

What Viruses Don't Do

Computer viruses don't infect fileson write-protected disks and don't infect documents, except in the case of Wordmacro viruses, which infect only documents and templates written in Word 6.0 orhigher. They don't infect compressed files either. However, applications withina compressed file could have been infected before they were compressed. Virusesalso don't infect computer hardware, such as monitors or computer chips; theyonly infect software.

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

Finally, viruses don't necessarily let you knowthat they are there — even after they do something destructive. <span Times New Roman",«serif»;mso-fareast-font-family:«Times New Roman»;mso-ansi-language: EN-US;mso-fareast-language:RU;mso-bidi-language:AR-SA">[1]

2. Types of Computer Viruses Nowadaysnumber of viruses is about 55000. It increases constantly. New unknown types ofviruses appear. To classify them becomes more and more difficult. In commonthey can be divided by three basic signs: a place of situating, used operationsystem and work algorithms. For example according these three classificationsvirus <st1:City w:st=«on»><st1:place w:st=«on»>Chernobyl</st1:place></st1:City>   can be classified as file infector andresident Windows virus. Further it will be explained what it means.2.1 A place of existence2.1.1File Infectors

These are viruses that attachthemselves to (or replace) .COM and .EXE files, although in some cases they caninfect files with extensions .SYS, .DRV, .BIN, .OVL and .OVY. With this type ofvirus, uninfected programs usually become infected when they are executed withthe 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 thefiles in the directory is run from (a direct infector).

There are three groups of fileinfectors.

Viruses of the first group are calledoverwriting viruses because they overwrite their code into infected fileerasing contents. But these viruses are primitive and they can be found veryquickly.

Othergroupis calledparasitic or cavity viruses. Infected file is capable of work fully or partlybut 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.

Thirdgroupis called companionviruses. They don’t change files. They make double of infected file so wheninfected 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 doublefile 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 andthen a virus starts an EXE. file.

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

It is not only one way to make doublefiles. For example there is subgroup of companion viruses called path-companionviruses. They use special feature of DOS — PATH: hierarchical record of filelocation. Virus copies itself in file with the same name but situated one levelhigher. In this case DOS will find file with virus.   <span Times New Roman",«serif»;mso-fareast-font-family:«Times New Roman»;mso-ansi-language: EN-US;mso-fareast-language:RU;mso-bidi-language:AR-SA">[2]

 2.1.2Boot viruses Boot Sector Infectors

Every logical drive, both hard diskand floppy, contains a boot sector. This is true even of disks that are notbootable. This boot sector contains specific information relating to the formattingof the disk, the data stored there and also contains a small program called theboot program (which loads the DOS system files). The boot program displays thefamiliar «Non-system Disk or Disk Error» message if the DOS systemfiles 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 andrebooting the machine. When the boot sector program is read and executed, the virusgoes into memory and infects your hard drive. Remember, because every disk hasa boot sector, it is possible (and common) to infect a machine from a datadisk. NOTE: Both floppy diskettes and hard drives contain boot sectors.

Master Boot Record Infectors

The first physical sector of everyhard disk (Side Ш, Track Ш, Sector 1) contains the disk's Master BootRecord and Partition Table. The Master Boot Record has a small program withinit called the Master Boot Program, which looks up the values in the partitiontable for the starting location of the bootable partition, and then tells thesystem to go there and execute any code it finds. Assuming your disk is set upproperly, what it finds in that location (Side 1, Track Ш, Sector 1) is a valid boot sector. On floppydisks, these same viruses infect the boot sectors. You get a Master Boot Recordvirus in exactly the same manner you get a boot sector virus — by leaving aninfected diskette in a drive and rebooting the machine. When the boot sectorprogram is read and executed, the virus goes into memory and infects the MBR ofyour hard drive. Again, because every disk has a boot sector, it is possible(and common) to infect a machine from a data disk. <span Times New Roman",«serif»;mso-fareast-font-family:«Times New Roman»;mso-ansi-language: EN-US;mso-fareast-language:RU;mso-bidi-language:AR-SA">[3]

2.1.3 Multi-partiteViruses

Multi-partite viruses are acombination of the viruses listed above. They will infect both files and MBRsor both files and boot sectors. These types of viruses are currently rare, butthe number of cases is growing steadily.

2.1.4Macro Viruses

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

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

Because most application macro languages supportpassing 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 macrolanguage itself<span Times New Roman",«serif»;mso-fareast-font-family:«Times New Roman»;mso-ansi-language: EN-US;mso-fareast-language:RU;mso-bidi-language:AR-SA">[4]




2.2Used operation system.

Any computer or net virus can infectfiles of one or more operation systems: DOS, Windows, OS/2, Linux, MacOS andothers. It is a base of this way of classification. For example virus BOZAworking with Windows only is classified as Windows virus, virus BLISS – as Linuxvirus. 

2.3Work algorithms.

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

Firstly viruses can be divided onresident and nonresident.

Resident virus having come inoperation memory of computer doesn’t infect memory. They are capable of copyingwhen they are started only. We can call any macro virus resident. They presentin memory during application infected by them works.

Second   viruses are visible and invisible. To beinvisible means that users and antivirus programs can’t notice changes ofinfected file done by virus. Invisible virus catches all requires of operationsystem 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 invisiblefile infectors was FRODO and boot infector – BRAIN.

Almost any virus uses methods ofself-coding or polymorphism to escape antivirus programs. It means that theycan change itself. Changing itself helps virus to be able work.<span Times New Roman",«serif»;mso-fareast-font-family:«Times New Roman»;mso-ansi-language: EN-US;mso-fareast-language:RU;mso-bidi-language:AR-SA">[5]




In conclusion I would like to say few words aboutfuture of this classification. Nowadays computer technologies and all softwaredevelop very quickly. It helps new types of computer viruses to appear. Virusesare becoming more and more dangerous and ‘cleverer’. It means that viruses canbe found more and more hard. But I think that this classification can be saveda long time thank for principles of work of computer. It means that thisclassification will be changed when computers work by principles that differfrom principles of von Neiman. So this classification can be change by addingnew subtypes of basic types if virus makers have created something new. 
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      The paper: Types of computer viruses

       Presented by Nefyodov Yuri

Scientific advisor: Sodboyeva L.D.





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




Этот доклад посвящён классификации компьютерныхвирусов. В начале рассказывается, что такое компьютерный вирус, что вирусымогут делать и что не могут. Далее здесь описаны три основных способаклассификации: по среде обитания, используемой операционной системе и алгоритму работы. В заключении говорится обудущем классификации.


     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.



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

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Журнал «Наука ижизнь» №7 2000 год

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Наука и жизнь №7 2000 год стр. 100


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Могилёв, Пак, Хеннер  Информатика 2000 изд. «Академия»

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Наука и жизнь №7 2000 годстр. 101-102
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