Daniel’s Internet Saftey Blog

There are many viruses.

These are called File infector viruses e.g. these infect program files, such as applications, games or utilities.

 Boot sector viruses e.g. Boot sector viruses attach themselves to this program Multi-partite or polypeptide viruses these infect both boot records and program files and are very difficult to repair

Macro viruses: these infect data files, such as Word documents or Excel spreadsheets, rather than programs. They are very common and can be difficult and expensive to repair

Trojans normally claim to do something useful but are actually malicious. One major difference between Trojans and true viruses is that they do not replicate. Trojans contain malicious code that can cause loss or theft of data. Trojans must be invited onto your computer, e.g.: by opening an email attachment or downloading and running a file. Trojan. Un.do is an example.

There are two types of Trojan:

• A program that pretends to do something useful but is actually a virus

• A real program that has been altered to contain a virus

Trojans differ from other types of virus in that rather than attaching themselves to existing programs they are complete programs in themselves.

Worms replicate themselves from computer to computer without infecting a host program, unlike viruses, which must be carried by a host. Although worms are often carried inside another file, usually a Word document or an Excel spreadsheet, they use the program in a different way from a virus.

A worm will usually release a document that already has the worm macro inside it. The entire document travels from system to system, so the entire document should be regarded as a worm. It was designed to allow spammers to send spam e-mail from infected computers

Like Trojans, worms are self contained programs that are designed to copy themselves from computer to computer. A common purpose of worms is to install a backdoor into a computer. These programs gain access to your computer.

Logic Bombs are designed to cause harm to your computer. Logic bombs, however, only trigger when specific conditions are met. For example a computer programmer may insert code into a payroll program that deletes files if her name is not found on the list. This means that files will be deleted if she is ever fired..

Other viruses, called time bombs, may only detonate on a specific date or time. An example of this is the Jerusalem virus.

On a Microsoft Windows computer a .SCR file is a screen saver. As screen savers are just computer programs they are a useful way of inserting a virus into your computer. Typically SCR viruses come as an attachment to an e-mail which shows a funny picture which you can click.

Once clicked the virus then installs itself onto your computer. As the SCR file pretends to be something else it is another example of a Trojan.

Perhaps the most famous example of a SCR virus is the SOBIG worm, which installed software to send spam.

Image viruses cannot cause any damage to a computer by themselves. Instead the code they contain exploits problems in other programs. An infected image, transferred to your computer on a web page, in a document or in a chat window could access your system and plant a virus.

Images are not usually seen as a threat, so these viruses are not stopped by normal anti-virus software

The viruses stay in a computer’s memory ready to attack program that is run on the computer. These are resident, or memory-resident, viruses.

The concept of a resident virus is simple. A computer can run several programs at the same time. This is called multi-tasking. At any moment there may be various programs such as a word processor, a web browser and a media player running simultaneously.

A resident virus is written to take advantage of multi-tasking. However, unlike other programs, however, resident viruses hide in a system’s Random Access Memory (RAM), waiting to be activated when something happens, such as when a user clicks a mouse to start a program. In this way they can easily infect other programs.

As soon as new viruses are created, anti-virus software is created to neutralise and remove them. Virus writers have adopted various strategies to fool anti-virus software. One of the most popular and successful ways of fooling anti-virus software is to create a stealth virus.

Stealth viruses contain code that inserts itself between the operating system and the anti-virus software. Whenever a check is made for a virus, the stealth virus tells the operating system that there is no problem. This means the virus is still active and may cause lots of various problems to your system.

A mail bomb is a form of denial of service attack. The idea is to flood someone’s system with more e-mail than it can cope with. There are two ways to make a mail bomb. The first is to send millions of e-mails to one address simultaneously and send small compressed files that, when decompressed, expand to extremely large files.

A backdoor is a way of accessing a computer without going through the normal access routines, such as entering a name and password. It can be installed by a virus.

This is used to conceal programs or files to help hackers avoid detection which may cause many problems to your identity.