Computer Viruses: Some Basic Information

What Are They?

A computer virus is a program designed and written to affect adversely your computer by altering the way it works. They hide in your files and spread systematically from one file to another. When the infected programs are then executed (started up) the embedded virus is executed as well.

A virus may do nothing other than display messages on your screen or play tricks with your desktop display. Other viruses can be more damaging. For example, some viruses make many copies of themselves, consuming system resources and slowing or halting tasks. These are often referred to as worms.

Viruses can hide in the macros of such files as Microsoft Word documents or Excel spreadsheets. When these are opened they cause the virus to start its work. The virus then replicates by infecting document templates, so causing all the new files that are created with the program to have the virus too. This type is the most common being found today.

Viruses may do irreversible damage, by deleting files, or by destroying the information that tells the computer where to find things on your hard disk, or even stopping the computer from working altogether.

Once a virus is in a computer's memory it can infect any vulnerable file, including those stored elsewhere on a network.

How are They Spread?

A computer virus cannot spread from computer to computer without assistance. They spread when a computer accesses an infected file (i.e. runs a program or opens a document). Sources of infection are:

  • Files on floppy disks;
  • Files as e-mail attachments (this is now the most common source);
  • Files transferred over a network;
  • Infected floppy disks.

In the first 3 virus-affected files are copied on to a new computer and accessed. The last is slightly different as the virus can only become active if the computer is switched on with the infected floppy disk left in the drive.

An Example

The ZippedFiles (or ExploreZip) worm was first reported in early June 1999.

If you send an e-mail to a person whose machine is infected by the worm it tries to infect yours by sending you an e-mail that looks like this:

From: [user of the infected machine]
Subject: RE: [subject of the original message]
To: [your e-mail address]

Hi [your name]!

I received your email and I shall send you a reply ASAP.
Till then, take a look at the attached zipped docs.
Sincerely
[user of the infected machine]

Attachment: zipped_files.exe

The attachment zipped_files.exe is the worm. If you access the attachment it infects your machine. Once this happens it will make various types of file on your machine unusable (e.g. Word documents), will try to spread to all machines on your network, and will try to spread to others by sending out e-mails with the attachment.

What Can I Do About Them?

  1. Use anti-virus software
    You should have anti-virus software running on your computer. Names of popular anti-virus programs with contact details can be found at the end of this factsheet. It is extremely important that you update your anti-virus software on a regular basis. Anti-virus software companies have updates to their programs within hours of the discovery of a new virus. Some have a live update feature, which will grab the latest virus information from the company's Web site and automatically update their program on your computer.

    Anti-virus software can:
    • Check your computer's memory for infection when it is switched on.
    • Check files for infection as you try to access or copy them.
    • Check for infection on hard and floppy disks.


Operation of the anti-virus software can be set up to be automatic.

  1. Be vigilant!
    • Opening e-mail attachments
      Always be very careful about accessing e-mail attachments. You should save them to disk (don't choose the open option) and then scan them with the anti-virus software installed on your computer. Also look out for unexpected e-mails like the one for the ZippedFiles worm
    • Copying files onto your machine
      Do not access files copied from floppy disks, other machines on your network, or from the Internet, without scanning them for virus infection first. Also do not leave floppy disks in the drive when you switch off the machine.
  2. Back up your data
    As a precaution you should back up your data files regularly. Remember the back up may contain infection.

Virus Hoaxes

In addition to the viruses themselves there is also a problem with virus hoaxes. These are sent as e-mails. This is a typical example:

If you receive an e-mail titled "PLEASE HELP POOR DOG. Win A Holiday" DO NOT open it. It will erase everything on your hard drive. Forward this letter out to as many people as you can. This is a new, very malicious virus and not many people know about it. This information was announced yesterday morning from Microsoft, please share it with everyone who might access the Internet

Other recent hoaxes have warned about viruses with names like, Good Times, Join the Crew and It takes guts to say 'Jesus'.

The characteristic elements of a hoax e-mail such as the one above are:

  • The hoax suggests you avoid e-mail with a particular title and suggests you do not open it.
  • The hoax asks you to pass on the hoax e-mail, usually to everyone in your address book.
  • The hoax mentions a well-known company in order to gain some credibility.

When you see such language in an e-mail, it is probably an hoax. If you are unsure check the e-mail against one of the Web sites dealing with viruses and virus hoaxes. Do not forward it!

The important points to remember about these hoaxes are:

  • The text of an e-mail in itself cannot contain a virus, only an e-mail attachment.
  • Forwarding the hoax e-mail is the aim of the hoaxer - it generates a great many new hoax e-mails flooding on to the Internet.

Web Sites

Anti-virus software:

There are a number of anti-virus packages manufacturers. Here are links to the Web sites of some of the best known:

F-Secure (http://www.f-secure.com)
Dr Soloman (Dr Soloman's Anti-Virus) (http://www.drsolomon.com)
McAfee (McAfee VirusScan) (http://www.mcafee.com)
Symantec (Norton AntiVirus) (http://www.symantec.com)

General and hoax information:

Most anti-virus package manufacturers have the latest information about viruses and virus hoaxes on their Web sites. An example is the F-secure site:

http://www.f-secure.com/v-descs/
(for virus information)

and

http://www.f-secure.com/virus-info/hoax/
(for hoaxes)

Current Threats

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Last modified: 20-Feb-2010 21:56:32