Computer Virus - Online Article


This report briefly introduces computer viruses and how they affect network security. I have introduced today's virus situation. Many people are afraid of viruses, mostly because they do not know much about them. This report will guide you in the event of a virus infection.

Computer viruses and network security is important. There are things that are not public information. Therefore it is good to be aware of possible network security problems.


The person might have a computer virus infection when the computer starts acting differently. For instance getting slow or when they turn the computer on, it says that all the data is erased or when they start writing a document, it looks different, some chapters might be missing or something else abnormal has happened.

The next thing usually the person whose computer might be infected with virus, panics. The person might think that all the work that has been done is missing. That could be true, but in most cases viruses have not done any harm jet, but when one start doing something and are not sure what you do, that might be harmful. When some people try to get rid of viruses they delete files or they might even format the whole hard disk like my cousin did. That is not the best way to act when the person think that he has a virus infection.

What people do when they get sick? They go to see a doctor if they do not know what is wrong with them. It is the same way with viruses, if the person does not know what to do they call someone who knows more about viruses and they get professional help.

If the person read email at their PC or if they use diskettes to transfer files between the computer at work and the computer at home, or if they just transfer files between the two computers they have a good possibility to get a virus. They might get viruses also when they download files from any internet site. There was a time when people were able to be sure that some sites we secure, that those secure sites did not have any virus problems, but nowadays the people can not be sure of anything. There have been viruses even in Microsoft's download sites.

In this report I am going to introduce different malware types and how they spread out and how to deal with them. Most common viruses nowadays are macro viruses and I am going to spend a little more time with them. I am going to give an example of Trojan horses stealing passwords.

General information about computer viruses

Different malware types

Malware is a general name for all programs that are harmful; viruses, Trojan, worms and all other similar programs


A computer virus is a program, a block of executable code, which attaches itself to, overwrite or otherwise replace another program in order to reproduce itself without knowledge of a PC user. There are a couple of different types of computer viruses: boot sector viruses, parasitic viruses, multi-partite viruses, companion viruses, link viruses and macro viruses. These classifications take into account the different ways in which the virus can infect different parts of a system. The manner in which each of these types operates has one thing in common: any virus has to be executed in order to operate. Most viruses are pretty harmless. The user might not even notice the virus for years. Sometimes viruses might cause random damage to data files and over a long period they might destroy files and disks. Even benign viruses cause damage by occupying disk space and main memory, by using up CPU processing time. There is also the time and expense wasted in detecting and removing viruses.


A Trojan horse is a program that does something else that the user thought it would do. It is mostly done to someone on purpose. The Trojan Horses are usually masked so that they look interesting, for example a saxophone.wav file that interests a person collecting sound samples of instruments. A Trojan horse differs from a destructive virus in that it doesn't reproduce. There has been a password Trojan out in AOL land (the American on Line). Password30 and Pasword50 which some people thought were wav. Files, but they were disguised and people did not know that they had the Trojan in their systems until they tried to change their passwords. According to an administrator of AOL, the Trojan steals passwords and sends an E-mail to the hacker's fake name and then the hacker has your account in his hands.


A worm is a program which spreads usually over network connections. Unlike a virus which attaches itself to a host program, worms always need a host program to spread. In practice, worms are not normally associated with one person computer systems. They are mostly found in multi-user systems such as UNIX environments. A classic example of a worm is Robert Morrisis Internet-worm 1988.

Macro virus

Macro viruses spread from applications which use macros. The macro viruses which are receiving attention currently are specific to Word 6, WordBasic and Excel. However, many applications, not all of them Windows applications, have potentially damaging and infective macro capabilities too.

A CAP macro virus, now widespread, infects macros attached to Word 6.0 for Windows, Word 6.0.1 for Macintosh, Word 6.0 for Windows NT, and Word for Windows 95 documents.

What makes such a virus possible is that the macros are created by Word BASIC and even allows DOS commands to be run. Word BASIC in a program language which links features used in Word to macros.

A virus, named "Concept," has no destructive payload; it merely spreads, after a document containing the virus is opened. Concept copies itself to other documents when they are saved, without affecting the contents of documents. Since then, however, other macro viruses have been discovered, and some of them contain destructive routines. Microsoft suggests opening files without macros to prevent macro viruses from spreading, unless the user can verify that the macros contained in the document will not cause damage. This does NOT work for all macro viruses.

Why are macro viruses so successful? Today people share so much data, email documents and use the Internet to get programs and documents. Macros are also very easy to write. The problem is also that Word for Windows corrupts macros inadvertently creating new macro viruses.

Corruption's also creates "remnant" macros which are not infectious, but look like viruses and cause false alarms. Known macro virus can get together and create wholly new viruses.

Virus sources

Viruses don not just appear, there is always somebody that has made it and they have own reason to so. Viruses are written everywhere in the world. Now when the information flow in the net and Internet grows, it does not matter where the virus is made.

Most of the writers are young men. There are also few university students, professors, computer store managers, writers and even a doctor has written a virus. One thing is common to these writers, all of them are men, and women do not waste their time writing viruses. Women are either smarter or they are just so good that never get caught.

Why do people write and spread viruses?

It is difficult to know why people write them. Everyone has their own reasons. Some general reasons are to experiment how to write viruses or to test their programming talent. Some people just like to see how the virus spreads and gets famous around the World. The following is a list from news group postings alt.comp.virus and tries to explain why people write and spread viruses.

  • They don't understand or prefer not to think about the consequences for other people
  • They simply don't care.
  • They don't consider it to be their problem if someone else is inconvenienced
  • They draw a false distinction between creating/publishing viruses and distributing them.
  • They consider it to be the responsibility of someone else to protect systems from their creations.
  • They get a buzz, acknowledged or otherwise, from vandalism.
  • They consider they're fighting authority.
  • They like 'matching wits' with anti virus vendors
  • It's a way of getting attention, getting recognition from their peers and their names (or at least that of their virus) in the papers and the Wild List.
  • They're keeping the antivirus vendors in a job.

How viruses act

Viruses main mission is to spread out and then get active. Some viruses just spread out and never activate. Viruses when they spread out, they make copies of self and spreading is harmful.

How viruses spread out

Viruses mission is to hop from program to other and this should happen as quickly as possible. Usually viruses join to the host program in some way. They even write over part of the host program.

A computer is infected with a boot sector virus if it is booted from an infected floppy disk. Boot sector infections cannot normally spread across a network. These viruses spread normally via floppy disks which may come from virtually any source:

Unsolicited demonstration disks.

  • Brand-new software.
  • Disks used on your PC by salesmen or engineers.
  • Repaired hardware.

A file virus infects other files, when the program to which it is attached is run, and so a file virus can spread across a network and often very quickly. They may be spread from the same sources as boot sector viruses, but also from sources such as Internet FTP sites and newsgroups. Trojan horses spread just like file viruses.

A multipartite virus infects boot sectors and files. Often, an infected file is used to infect the boot sector: thus, this is one case where a boot sector infection could spread across a network.

How viruses activate

We are always afraid that viruses do something harmful to files when they get active, but not all the viruses activate. Some viruses just spread out, but when viruses activate they do very different things. Might play a part of melody or play music in the background, show a picture or animated picture, show text, format hard disk or do changes to files.

As an example, in one unnamed company: over a long period of time, the files in a server were corrupted just a bit. So backup copies were taken from the corrupted files. And after they noticed that something was wrong, it was too late to get back the data from the backups. That kind of event is the worst that can happen for the uses.

There is also talk that viruses have done something to hardware like hard disk or monitor. Viruses can not do any harm to hardware but they can do harm to programs and for example to BIOS so that computer does not start after that. Usually you can start the computer from a boot diskette if the computer does not start otherwise.

Viruses in different platforms

PC viruses

Viruses are mostly written for PC-computers and DOS environment. Even though viruses are made for DOS environment, they are working also in Windows, Windows95, Windows NT and OS/2 operating systems. Some viruses like boot sector viruses, do not care what about operating systems.

Macintosh viruses

Macintosh viruses are not as a big problem as PC viruses are. There are not so many viruses in Macintosh operating system. Macintosh viruses have been found mostly from schools.

How many Mac viruses there are? I found out that there are about 2-300 Mac-specific viruses. There are virtually no macro viruses which have a Mac-specific payload, but all macro viruses can infect on Macs and other platforms which runs Word 6.x of better.

Other platforms

Viruses can be found from in almost any kind of computer, such as HP calculators used by students like HP 48-calculators and old computers like Commodore 64 and UNIX computers too.

In general, there are virtually no non-experimental UNIX viruses. There have been a few Worm incidents, most notably the Morris Worm, The Internet Worm, of 1988.

There are products which scan some UNIX systems for PC viruses. Any machine used as a file server (Novell, UNIX etc.) can be scanned for PC viruses by a DOS scanner if it can be mounted as a logical drive on a PC running appropriate network client software such as PC-NFS.

Intel-based PCs running UNIX e.g. Linux, etc. can also be infected by a DOS boot-sector virus if booted from an infected disk. The same goes for other PC-hosted operating systems such as NetWare.

While viruses are not a major risk on UNIX platforms, integrity checkers and audit packages are frequently used by system administrators to detect file changes made by other kinds of attack.

How to deal with viruses

What are the signs of viruses

Almost anything odd a computer may do, can blamed on a computer "virus," especially if no other explanation can readily be found. Many operating systems and programs also do strange things; therefore there is no reason to immediately blame a virus.  In most cases, when an anti-virus program is then run, no virus can be found.

A computer virus can cause unusual screen displays, or messages - but most don't do that.  A virus may slow the operation of the computer - but many times that doesn't happen.   Even longer disk activity, or strange hardware behavior can be caused by legitimate software, harmless "prank" programs, or by hardware faults.  A virus may cause a drive to be accessed unexpectedly and the drive light to go on but legitimate programs can do that also.

One usually reliable indicator of a virus infection is a change in the length of executable (*.com/*.exe) files, a change in their content, or a change in their file date/time in the Directory listing.  But some viruses don't infect files, and some of those which do can avoid showing changes they've made to files, especially if they're active in RAM.

Another common indication of a virus infection is a change to the reassignment of system resources.  Unaccounted use of memory or a reduction in the amount normally shown for the system may be significant.

In short, observing "something funny" and blaming it on a computer virus is less productive than scanning regularly  for potential viruses, and not scanning, because "everything is running OK" is equally inadvisable.

What to do when you find viruses?

First thing what you should do when you find virus is count to ten and stay cool. You should keep notes on what you do and write down what your virus programs and you computer tells you. If you are not sure what to do, you should call the administrator for future action. In some cases it is not good to start you computer from hard disk, because the virus may active and then do some harm.

Second, make sure that you should get sure that it is virus and what virus it is. It is important to know what kind of virus we are dealing with. Companies that make anti-virus programs knows what different viruses does and you can ether call them and ask about that viruses or you can go to their web pages and read about the virus you have.

When you start you computer you should do it from a clean (non-infected) floppy diskette and after that run the virus program. The boot diskette should be write protected so that virus can not infect the boot diskette too.

It is good to take a backup of the file that was infected. Virus program could do some damage to the file and that is why it is good to have a backup.

It is good to let you administrator to know about the virus, so viruses would not spread around so much. In TKK PC classes are protected by anti-virus program and that virus program reports to a person, responsible for virus protection.

How to protect from viruses

Best way to protect yourself is to prepare your computer against viruses in advance. One way to protect you computer is to use updated anti-virus program. When you get an email attachment, you should first check the attachment by checking the file with a anti-virus program.

As an example in one unnamed Finnish company all information was mailed in email attachments. There was this one Word document that was mailed to everybody. That email attachment was infected by a macro virus. Everyone got the infected attachment and those who opened that attachment by Word got that CAP-macro virus. After all there were a few thousand infections. It took lots of time and money to clear that virus.

One can protect the computer against boot sector viruses by setting the BIOS to start from a hard disk rather than from a floppy disk.

Write protection is a good way to prohibit against viruses. Write protection works well in floppy disks, Windows NT and UNIX, but not that well in Windows and Windows95.

Different anti-virus programs

There are three different kind of anti-viral packages: activity monitors, authentication or change-detection software, and scanners. Each type has its own strengths and weaknesses. Commercial anti-viral programs have a combination of the above mentioned functions.

There are over ten good anti-viral programs. Most knows programs are Data Fellows F-Prot, EliaShim ViruSafe, ESaSS ThunderBYTE, IBM AntiVirus, McAfee Scan, Microsoft Anti-Virus, Symantec Norton AntiVirus and S&S Dr Solomon's AVTK.

On a day-to-day basis, the average corporation should be very interested in the scan time; these impact strongly the users, who should be scanning hard drives and disks on a daily basis. If a product takes too long to carry out these basic tasks, users will be unwilling to wait, and will stop using it. This is clearly undesirable - the perfect anti-virus product would be one which takes no time to run and finds all viruses.

How computer viruses have spread out around the world. Computer viruses are a problem all over the world. The following picture tells us how many times people have accessed Data Fellows, a company that makes anti-virus program F-Prot, more than 1,672,846 per month. It means that people are interesting in virus information. One reason is that people have to deal with viruses. Viruses in not only a problem in Finland and USA, it is a problem around the world. Today's most common virus is the macro virus. Cap virus is one of the macro viruses. Last month there were 3100 Cap macro virus accesses during the last 30 days in Data Fellows. Next common virus was join the Crew with 1171 accesses and third common was Pen pal Greetings with 895 accesses.

Computer viruses and network security

Computer viruses are one network security problem. A few people when asked if computer viruses can cause network security problems answered as follows.

Dave Kenney answered from National Computer Security Assoc: "There is one macro virus for MSWord that is received as an attachment to MS Mail messages.  If a user has Word open, and double clicks to see the contents of the attachment, MS Word and the open document are infected. Then the document is mailed to three other users listed in the original user's address book."

"The only information that is leaked is the thing you should be worried about, your password!  The Trojan sends an E-mail to the hacker's fake name and then he has your account at his hands," wrote CJ from American Online.

"Rarely, a Word macro virus may accidentally pick up some user information and carry it along; we know of one case where a macro virus "snatched" an innocent user macro that contained a password, and spread it far outside the company where that happened. In the future, however, it is entirely possible that more network-aware viruses will cause significant network security problems," wrote David Chess from IBM.

Marko Helenius wrote from Virus Research Unit, that there has been some cases when hackers have used trojan horses to gain information. There is one example in one finnish corporation where some money were transferred illegally a year ago. There has been a Trojan in the University of Tampere too where the Trojan pretend to be a host transfer program. The Trojan saved users login name and password to hard disk.


  • There are lots of viruses in the world and new viruses are coming up every day. There are new anti-virus programs and techniques developed too. It is good to be aware of viruses and other malware and it is cheaper to protect you environment from them rather then being sorry.
  • There might be a virus in your computer if it starts acting differently. There is no reason to panic if the computer virus is found.
  • It is good to be a little suspicious of malware when you surf in the Internet and download files. Some files that look interesting might hide a malware.
  • A computer virus is a program that reproduces itself and its mission is to spread out. Most viruses are harmless and some viruses might cause random damage to data files.
  • A Trojan horse is not a virus because it doesn't reproduce. The Trojan horses are usually masked so that they look interesting. There are Trojan horses that steal passwords and formats hard disks.
  • Marco viruses spread from applications which use macros. Macro viruses spreads fast because people share so much data, email documents and use the Internet to get documents. Macros are also very easy to write.
  • Some people want to experiment how to write viruses and test their programming talent. At the same time they do not understand about the consequences for other people or they simply do not care.
  • Viruses mission is to hop from program to other and this can happen via floppy disks, Internet FTP sites, newsgroups and via email attachments. Viruses are mostly written for PC-computers and DOS environments.
  • Viruses are not any more something that just programmers and computer specialist have to deal with. Today everyday users have to deal with viruses.

About the Author:

No further information.


No comment yet. Be the first to post a comment.