Symantec Europe
global sites
products and services
purchase
support
partners
security response
downloads
about symantec
search
feedback


© 1995-2008 Symantec Corporation.
All rights reserved.

Legal Notices
Privacy Policy


Home and Home Office

Viruses, Worms, and Trojan Horses: How do you get them?

Do you feel lost among virus, worm, and Trojan Horse alerts? These threats cannot be countered in the same way. What lies beneath these mysterious names? Details.


Carelessly opening file attachments because they come from a friend, thinking that one's antivirus software provides sufficient protection, and thinking that Internet downloads are harmless are all examples of how many users expose themselves to the risks involved in these exchanges. But users are not the sole culprits. With a dozen viruses being discovered each day along with the 50,000 that are already out there, it is difficult to stay on top of all the latest information and newest threats. Many users get lost among notices of new viruses, worms, and Trojan Horses. But without knowing the difference between these malicious programs, it is impossible to effectively protect your computer from them. Each type of threat demands a certain type of precaution, because the respective prevention and recovery methods are not the same.

Viruses: The Agent Smith's of the real world
Fans of the Matrix films can easily define what a virus is: a program that runs automatically and replicates itself by infecting other programs just as Agent Smith does in the film. The primary objective of a virus is to multiply and spread. Some viruses can be harmless, and stop at displaying messages declaring their presence. But others, such as the Vote-B virus that randomly reformats its victim's computers, can be truly destructive by damaging or deleting files.
Among the major types of viruses are boot viruses, which infect the computer's bootstrap sector. This sector is the part of the hard disk that starts up before the operating system boots. If a diskette is forgotten in the floppy drive, this sector can be infected upon startup. Although most viruses today are spread through email attachments, boot viruses are still active.
Program viruses attack computer software and replace part of the programming code. As a result, all documents that open with the infected application will also be infected. Among the most dangerous of this type of virus has been the Chernobyl virus that infects all of the victim's software, and then attacks the computer's BIOS.
Macro viruses directly target files themselves, the most frequently affected being Microsoft Office documents (such as Word and Excel). These viruses are based on the Visual Basic programming language, and are transmitted through email attachments (as in the case of the Melissa virus), or through file sharing by diskette. Certain worms (see below) are spread in the same way through infected files.
Logic bombs are viruses that are programmed to attack on a specified date. They are capable of launching a simultaneous attack on a large number of computers to saturate a network.
- How you can protect yourself
Do not open suspicious email attachments (ending in .exe, .vbs, etc.), check if the file has a hidden extension and always scan attachments with antivirus software before opening them.

Worms: Journey to the end of the network
Just like viruses, worms seek to propagate themselves and find new victims. But unlike the former, these malicious programs do not always need a vehicle (such as a program or a file) to spread, and can pass themselves from one machine to another through a local area network or through the Internet (as was the case with Code Red). Some worms can also be spread through email attachments (as in the case of Klez, Bugbear, and Nimda). These are called virus-worms.
- How you can protect yourself
Antivirus software (especially if it is not up-to-date) may not offer sufficient protection against viruses that spread via networks. It is advisable to use a firewall to monitor incoming and outgoing data.

Trojan Horses: Poison gifts

Trojan Horses take their name from the deceptive gift the Greeks gave to the citizens of the city of Troy a wooden horse containing soldiers hidden inside it. A Trojan Horse is a malicious code that is hidden inside a program that appears to be useful. Unlike viruses and worms, their mission is not to duplicate themselves. Instead, they are transmitted either through email sent by ill-intentioned users or through Internet downloads. They can be disguised as screensavers, games, or any other program that can be downloaded, but contain damaging code that allows hackers to take control of the victim's computer remotely.
- How you can protect yourself
Do not download programs from personal or unauthorized websites. In addition to antivirus software, install a firewall to prevent hackers from accessing your computer.