Computer Virus is a program or programming code that replicates by being copied or initiating its copying to another program, computer boot sector or document. Viruses can be transmitted as attachments to an e-mail note or in a downloaded file, or be present on a diskette or CD. Most common types of viruses are listed below:
Types of Viruses
1. Resident Viruses: A resident virus is a computer virus which stays in the memory on a computer, activating whenever the operating system performs a specific function so that it can infect files on the computer. Examples include: Randex, CMJ, Meve, and MrKlunky.
2. Direct Action Viruses: The main purpose of this virus is to replicate and take action when it is executed. When a specific condition is met, the virus will go into action and infect files in the directory or folder that it is in and in directories that are specified in the AUTOEXEC.BAT file PATH. This batch file is always located in the root directory of the hard disk and carries out certain operations when the computer is booted. it usually works immediately to load itself into memory, infect other files, and then to unload itself
3. Overwrite Viruses: An overwrite virus is a type of computer virus that overwrites a file with its own code, helping spread the virus to other files and computers. The only way to clean a file infected by an overwrite virus is to delete the file completely, thus losing the original content. Examples of this virus include: Way, Trj.Reboot, and Trivial.88.D.
4. Boot Virus: A virus that infects the boot records on floppy diskettes and hard drives and is designed to self-replicate from one disk to another. The best way of avoiding boot viruses is to take special precautions when using usb stick or floppy disk. Examples of boot viruses include: Polyboot.B, and AntiEXE.
5. Macro Virus: A macro virus is a virus that is written in a macro language, a language built into a software application such as a word processor. Since some applications allow macro programs to be embedded in documents, so that the programs may be run automatically when the document is opened. Examples of macro viruses: Relax, Melissa.A, Bablas, and O97M/Y2K.
6. Directory Virus: Directory viruses change the paths that indicate the location of a file. By running a program with the extension ".EXE" or ".COM" which has been infected by a virus, you are unknowingly starting the virus program. Once infected, it becomes impossible to locate the original files.
7. Polymorphic Virus: A polymorphic virus is a computer virus which is capable of encrypting or encoding themselves in a different way when it replicates, making it more difficult to identify with ordinary antivirus software and also enables them to create a large number of copies of themselves. Examples include: Elkern, Marburg, Satan Bug, and Tuareg.
8. File Infectors: This type of virus infects programs or executable files with an ".EXE" or ".COM" extension. When one of these programs is run, directly or indirectly, the virus is activated. The majority of existing viruses belong to this category.
9. Companion Viruses: It is a specific type of virus where the infected code is stored not in the host program, but in a separate "companion" file. For example, the virus might rename the standard NOTEPAD.EXE file to NOTEPAD.EXD and create a new NOTEPAD.EXE containing the virus code. When the user subsequently runs the Notepad application, the virus will run first and then pass control to the original program, so the user doesnt see anything suspicious. It can be considered file infector viruses like resident or direct action types. Some examples include: Stator, Asimov.1539, and Terrax.1069.
10. Worms: A worm is a self-replicating computer program. It uses a network to send copies of itself to other computers on the network and it may do so without any user intervention. This is due to the poor security the computers infected have. Unlike a virus, it does not need to attach itself to an existing program. Worms almost always cause at least some harm to the network, if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer. Examples of worms include: PSWBugbear.B, Lovgate.F, Trile.C, Sobig.D, and Mapson.
11. Trojans or Trojan Horses: It is non-self-replicating malware that appears to perform a desirable function for the user but instead facilitates unauthorized access to the user's computer system.
12. Logic Bombs: A logic bomb is a piece of code intentionally inserted into a software system that will set off a malicious function when specified conditions are met. Its objective is to destroy data on the computer once certain conditions have been met. Logic bombs go undetected until launched, and the results can be destructive.
How to create a virus to know how it works?
To let you know how some viruses work, you can try a simple harmless virus below. This virus just displays the same message repeatedly without doing any harm to your computer. If the code in the following file is harmful like deleting files and you send it to someone in an email attachment, it will be a nightmare to the receiver who opens the attachment. The following are the steps for you to follow:
1. Open a notepad.
2. Add the following code to it.
@ECHO off
:Begin
msg * Hello
msg * Are you Ok?
msg * I am!
GOTO BEGIN
3. Save it as "hello.bat".
4. Double click "hello.bat" to run it.
You should notice that the last step for you is to click the program, a virus. That means that usually the virus needs to be executed. So you should be very careful when executing programs which you download from internet or in the email attachment. Please remember "No Click, No Infection!
