A virus is a software program which is used by an attacker to destroy hardware and software. Computer system and pc viruses can be broadly classified as: executable program virus, overwriting virus, parasitic virus, cavity virus, macro virus, source code virus, companion virus, memory resident virus, boot sector virus, and device driver virus:
1) Executable program virus: An executable program virus is the one which affects the executable programs in a computer by various methods. This type of virus keeps multiplying whenever an executable program is run. It can be further classified as:
a) Overwriting virus: An overwriting virus will overwrite an executable program with its own code. Thus, it will keep spreading in the computer by overwriting executable programs.
b) Parasitic virus: A parasitic virus will attach itself anywhere to an executable program. The original program will run as normal and the user won’t be able to detect it.
c) Cavity virus: A cavity virus finds an empty region in the executable program and resides in there. It remains hidden in the cavity of the executable program.
2) Macro virus: A micro virus uses word processor program or a spreadsheet program to run. It is usually transmitted as an attachment in an email. When the user opens the word or the excel file having the macro virus, his computer is infected. A macro virus can delete other word processing files and can infect them. Macro virus can be avoided by scanning the attachment each time you download a file from your email.
3) Source code virus: A source code virus is the one which is called from the source code of a C program or any other program. The destination of this virus could be a local or a remote computer. Whenever the program runs the virus is also executed.
4) Companion virus: A companion virus is the one which stays in the computer but doesn’t affects it at the moment it is inserted in the compute. It will be present in the system and will be there unless a user tries to access it. For example, an attacker can create a companion virus which will look like a genuine program. It will be a .exe file which will have the icons of a regular program used by the owner of the computer except for the target file of this icon will be changed by the attacker. Now, the user will click on the icon presuming that it is a regular program which he’s using from a long time. However, unknowingly he has just activated the companion virus.
5) Memory resident virus: A memory resident virus is the one which stays hidden in the RAM of the computer. It runs in kernel mode for a system call and remain undetected to the regular user of the computer.
6) Boot sector virus: A boot sector virus is the one which overwrites the boot sector. I remains hidden and allocates memory as if it was a bad sector. When the computer is booted it creates a copy of itself and moves to the RAM. Then, it picks a spot within the interrupt vectors and stays there. At a later time, it acquires the system call trap vector.
7) Device driver virus: Device driver virus copies itself as a device driver and loads at boot time. Then, it acquires system call trap vector.
You can keep your computer safe from the above given viruses by following these steps.