Viruses, Worms and Trojans

Viruses

Like biological viruses, computer viruses rely on a host. In this case, the host is another program. Programs are often downloaded, which have viruses attached and this is how the viruses spread. They will come into action when the offending program is run, some embed themselves in the computer memory, these are known as residential viruses. Others immediately look for new programs to infect and run simultaneously when infected programs are running (non-residential viruses). Once running, the virus will go to work doing the function it was programmed for. This could be damaging files and sending copies of itself out to other files and on to other computers. It may even be programmed to damage the computer operating system. Commonly, a virus can is designed to take action when a particular trigger occurs, for example, at a certain time or date. The virus will lie dormant until the trigger. One common way that viruses are spread is through email attachments. You should always be careful opening these, especially if they are .exe files.

Worms

Similar to viruses, worms replicate themselves and can spread through a computer and cause catastrophic results. They differ from viruses in that they do not require a host. They are stand-alone programs and can make their way onto your computer from emails, without the use of an attachment. Worms are particularly bad when they find their way onto a network. They will use up bandwidth as they send themselves between computers and the damage caused varies. Some are designed to delete files or send emails. Others are used to open access to the network via a 'backdoor'. The backdoor is then used by hackers for further disruption or spying.

Trojans

Trojans will often be disguised as something else, hence the name from the Greek story of the Trojan horse. They do not make copies of themselves like viruses but can do a great deal of damage. Trojans are a type of spyware, which may piggy-back on other programs. They can steal information, such as credit card numbers and passwords, or record activity. Usually, the disguised function of the program seems to the user to be useful but, really, the purpose is for a remote hacker to be able to access the system. Depending on the complexity of the Trojan, the hacker can control certain operations, such as downloading files, uploading files, installing programs, watching the user screen as they see it, logging keystrokes.