9 Types of Computer Viruses
Everyone is afraid being the recipient of a computer virus, but not everyone minds studying them. If you’re a conscious type, you’ll ask yourself – are there other types of computer viruses? if yes, what are the different kinds of computer viruses? how are they created? what kind of damage they do and how they spread?
It’s worth to know the basic knowledge about security threats. It’s sometimes hard to know how a risk must be dealt with before you know its consequences. With a computer virus, however, the consequence is sometimes complete loss of your data or identity theft – so it’s best to learn sooner rather than later.
1. Boot Sector Virus
This type of virus affects the boot sector of a floppy or hard disk. This is a crucial part of a disk, 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.
The best way of avoiding boot viruses is to ensure that floppy disks are write-protected and never start your computer with an unknown floppy disk in the disk drive.
Examples of boot viruses include: Polyboot.B, AntiEXE
2. Direct Action Virus
This type of virus, unlike most, only comes into action when the file containing the virus is executed. The payload is delivered and then the virus essentially becomes dormant – it takes no other action unless an infected file is executed again.
Most viruses do not use the direct action method of reproduction simply because it is not prolific, but viruses of this type have done damage in the past. The Vienna virus, which briefly threatened computers in 1988, is one such example of a direct action virus.
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.
3. File Infector Virus
Perhaps the most common type of virus, the file infector takes root in a host file and then begins its operation when the file is executed. The virus may completely overwrite the file that it infects, or may only replace parts of the file, or may not replace anything but instead re-write the file so that the virus is executed rather than the program the user intended.
Although called a “file virus” the definition doesn’t apply to all viruses in all files generally – for example, the macro virus below is not referred to by the file virus. Instead, the definition is usually meant to refer only to viruses which use an executable file format, such as .exe, as their host.
This type of virus infects programs or executable files (files with an .EXE or .COM extension). When one of these programs is run, directly or indirectly, the virus is activated, producing the damaging effects it is programmed to carry out. The majority of existing viruses belong to this category, and can be classified depending on the actions that they carry out.
4. Macro Virus
A wide variety of programs, including productivity applications like Microsoft Excel, provide support for Macros – special actions programmed into the document using a specific macro programming language. Unfortunately, this makes it possible for a virus to be hidden inside a seemingly benign document.
Macro viruses very widely in terms of payload. The most well known macro virus is probably Melissa, a Word document supposedly containing the passwords to pornographic websites. The virus also exploited Word’s link to Microsoft Outlook in order to automatically email copies of itself.
Examples of macro viruses: Relax, Melissa.A, Bablas, O97M/Y2K.
5. Multipartite Virus
While some viruses are happy to spread via one method or deliver a single payload, Multipartite viruses want it all. A virus of this type may spread in multiple ways, and it may take different actions on an infected computer depending on variables, such as the operating system installed or the existence of certain files.
6. Browser Hijacker
This type of virus, which can spread itself in numerous ways including voluntary download, effectively hijacks certain browser functions, usually in the form of re-directing the user automatically to particular sites. It’s usually assumed that this tactic is designed to increase revenue from web advertisements.
There are a lot of such viruses, and they usually have “search” included somewhere in their description. CoolWebSearch may be the most well known example, but others are nearly as common.
7. Polymorphic Virus
Another jack-of-all-trades, the Polymorphic virus actually mutates over time or after every execution, changing the code used to deliver its payload. Alternatively, or in addition, a Polymorphic virus may guard itself with an encryption algorithm that automatically alters itself when certain conditions are met.
The goal of this trickery is evasion. Antivirus programs often find viruses by the specific code used. Obscuring or changing the code of a virus can help it avoid detection.
8. Resident Virus
This broad virus definition applies to any virus that inserts itself into a system’s memory. It then may take any number of actions and run independently of the file that was originally infected.
A resident virus can be compared to a direct payload virus, which does not insert itself into the system’s memory and therefore only takes action when an infected file is executed.
9. Web Scripting Virus
Many websites execute complex code in order to provide interesting content. Displaying online video in your browser, for example, requires the execution of a specific code language that provides both the video itself and the player interface.
Of course, this code can sometimes be exploited, making it possible for a virus to infect a computer or take actions on a computer through a website. Although malicious sites are sometimes created with purposely infected code, many such cases of virus exist because of code inserted into a site without the webmaster’sknowledge.
Conclusion
I have included every type of computer virus that I felt was relevant, but this isn’t a list of every type of computer virus known to man. Also, I did not include any worms or trojans, two common threats that are similar to viruses but technically not the same.
If there is a specific type of virus that you feel was missed and is important, let us know in the comments. I’ll be glad to add them up there!
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