Computer Viruses

Definition

Computer computer virus is self-copying computer software / program containing code that copied itself and that can infect other programs by modifying them or their environment such that a call to an infected program implies a call to a possibly evolved copy of the computer virus.

Types of Computer viruses

There are Different Types of Computer viruses which can be divided in (origin, techniques, types of files they infect, where they hide, the kind of damage they cause, the type of operating system or platform they attack) etc. Most common types of computer viruses are as follows,

1- Boot Computer viruses

This type of computer virus affects the boot sector of a floppy or hard disk. This is a very sensitive part, in which information on the disk itself is stored together with a program that makes it possible to boot (start) the computer from the disk.
Examples of boot computer viruses include: Polyboot.B, AntiEXE.

2- FAT Computer viruses

The file allocation table or FAT is the part of a disk used to connect information and is a vital part of the normal functioning of the computer.
This type of computer virus attack can be especially dangerous, by preventing access to certain sections of the disk where important files are stored. Damage caused can result in information losses from individual files or even entire directories.

3- Trojans or Trojan Horses

Another unsavory breed of malicious code are Trojans or Trojan horses, which unlike computer viruses do not reproduce by infecting other files, nor do they self-replicate like worms

4- Resident Computer viruses

This type of computer virus is a permanent which dwells in the RAM memory. From there it can overcome and interrupt all of the operations executed by the system: corrupting files and programs that are opened, closed, copied, renamed etc.
Examples include: Randex, CMJ, Meve, and MrKlunky.

5- Macro Computer viruses

Macro computer viruses infect files that are created using certain applications or programs that contain macros. These mini-programs make it possible to automate series of operations so that they are performed as a single action, thereby saving the user from having to carry them out one by one.
Examples of macro computer viruses: Relax, Melissa.A, Bablas, O97M/Y2K.

6- Direct Action Computer viruses

The main purpose of this computer virus is to replicate and take action when it is executed. When a specific condition is met, the computer 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.

7- Overwrite Computer viruses

Computer virus of this kind is characterized by the fact that it deletes the information contained in the files that it infects, rendering them partially or totally useless once they have been infected.
The only way to clean a file infected by an overwrite computer virus is to delete the file completely, thus losing the original content.
Examples of this computer virus include: Way, Trj.Reboot, Trivial.88.D.

8- Worms

A worm is a program very similar to a computer virus; it has the ability to self-replicate, and can lead to negative effects on your system and most importantly they are detected and eliminated by Anti Computer viruses.
Examples of worms include: PSWBugbear.B, Lovgate.F, Trile.C, Sobig.D, Mapson.

9- Companion Computer viruses

Companion computer viruses can be considered file infector computer viruses like resident or direct action types. They are known as companion computer viruses because once they get into the system they “accompany” the other files that already exist. In other words, in order to carry out their infection routines, companion computer viruses can wait in memory until a program is run (resident computer viruses) or act immediately by making copies of themselves (direct action computer viruses).
Some examples include: Stator, Asimov.1539, and Terrax.1069

10- Directory Computer viruses

Directory computer viruses change the paths that indicate the location of a file. By executing a program (file with the extension .EXE or .COM) which has been infected by a computer virus, you are unknowingly running the computer virus program, while the original file and program have been previously moved by the computer virus.
Once infected it becomes impossible to locate the original files.

11- Logic Bombs

They are not considered computer viruses because they do not replicate. They are not even programs in their own right but rather camouflaged segments of other programs.
Their 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.

12- Polymorphic Computer viruses

Polymorphic computer viruses encrypt or encode themselves in a different way (using different algorithms and encryption keys) every time they infect a system.
This makes it impossible for anti-computer viruses to find them using string or signature searches (because they are different in each encryption) and also enables them to create a large number of copies of themselves.
Examples include: Elkern, Marburg, Satan Bug, and Tuareg.

13- File Infectors

This type of computer virus infects programs or executable files (files with an .EXE or .COM extension). When one of these programs is run, directly or indirectly, the computer virus is activated, producing the damaging effects it is programmed to carry out. The majority of existing computer viruses belong to this category, and can be classified depending on the actions that they carry out.

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