Classification based on the techniques used to infect boot sectors or files

Computer viruses can also be classified based on the techniques they use to infect boot sectors or files. They are of the following types: macro viruses, polymorphic viruses, stealth viruses, parasitic viruses and script viruses.

1. Macro Viruses
Most application programs let you create macros, small programs that automate tasks. These macros are stored as part of the application’s documents and are transferred along with them. Macro is used in word processing and spreadsheet applications. The data files created with these applications contain macro procedures and various other embedded objects. Macro viruses infect data files either by modifying macro procedure or by attaching macro procedures with these data files. They spread rapidly as data files are used and transferred frequently than executable files. The examples of macro viruses are W97M.Melissa, WM.NiceDay and W97M.Groov.

2. Polymorphic Viruses
Polymorphic code is a code that mutates while keeping the original algorithm intact. This technique is sometimes used by computer viruses to hide their presence. A virus that can change its appearance with every infection is called polymorphic virus. Polymorphic viruses are also known as encrypted viruses because they use the encryption technique to hide from antivirus software. These viruses encrypt their main code and use random sets of commands to decrypt the code. These viruses are difficult to detect. Antivirus software must perform algorithm scanning to detect these viruses. The examples of polymorphic viruses are Involuntary, Stimulate, Cascade, Phoenix, Evil and Proud.

3. Stealth Viruses
A stealth virus infects files and boot records. It is difficult to detect because it attempts to hide from the operating system and antivirus software on the computer. It hides itself from antivirus software by either masking the size of the file that it hides in or temporarily removing itself from the infected file and placing a copy of itself in another location on the drive, replacing the infected file with an uninfected one that it has stored on the hard drive. It also encrypts the virus code using various encryption techniques. The examples of stealth viruses are Frodo, Joshi and Whale.

4. Parasitic Viruses
Parasitic viruses attach themselves to programs or executables. When the programs are launched, the parasitic virus gets activated. The operating system assigns the same rights to the parasitic virus as the program to which it is attached because the operating system considers the virus as a part of the program. The rights assigned to a parasitic virus by the operating system enables the virus to replicate itself, damage data and applications on the computer. This virus, once activated slows down the processing of the computer and deletes any program that the user starts. An example of a parasitic virus is Jerusalem.

5. Script Viruses

Script viruses infect programs written in scripting languages, such as Visual Basic Script and JavaScript. These languages are hosted under Windows Scripting Host (WHS), an active scripting component that is available on the Windows 98 and Windows 2000 operating systems. Script viruses spread by attaching themselves to data files. Script viruses can spread though e-mail, floppy disks and CDs. Script viruses can also infect computer while downloading programs from the Internet. Script viruses can propogate through Internet Relay Chat (IRC). Script viruses can infect a computer through various scripting environments, such as Windows Help, Windows installation files, and Windows registry files. Script viruses is easy to write code for script viruses because the code for this viruses can be written using a high-level language.