Types of Viruses

• Difficulty Level : Hard
• Last Updated : 03 Aug, 2021

A virus is a fragment of code embedded in a legitimate program. Viruses are self-replicating and are designed to infect other programs. They can wreak havoc in a system by modifying or destroying files causing system crashes and program malfunctions. On reaching the target machine a virus dropper(usually a trojan horse) inserts the virus into the system. 
For more details, refer to this. 

Various types of viruses: 

• File Virus: 
  This type of virus infects the system by appending itself to the end of a file. It changes the start of a program so that the control jumps to its code. After the execution of its code, the control returns back to the main program. Its execution is not even noticed. It is also called a Parasitic virus because it leaves no file intact but also leaves the host functional.
   
• Boot sector Virus: 
  It infects the boot sector of the system, executing every time system is booted and before the operating system is loaded. It infects other bootable media like floppy disks. These are also known as memory viruses as they do not infect the file systems. 
   

•  Macro Virus: 
  Unlike most viruses which are written in a low-level language(like C or assembly language), these are written in a      high-level language like Visual Basic. These viruses are triggered when a program capable of executing a macro is run. For example, the macro viruses can be contained in spreadsheet files.
   
• Source code Virus: 
  It looks for source code and modifies it to include virus and to help spread it.
   
• Polymorphic Virus: 
  A virus signature is a pattern that can identify a virus(a series of bytes that make up virus code). So in order to avoid detection by antivirus a polymorphic virus changes each time it is installed. The functionality of the virus remains the same but its signature is changed.
   
• Encrypted Virus: 
  In order to avoid detection by antivirus, this type of virus exists in encrypted form. It carries a decryption algorithm along with it. So the virus first decrypts and then executes.
   
• Stealth Virus: 
  It is a very tricky virus as it changes the code that can be used to detect it. Hence, the detection of viruses becomes very difficult. For example, it can change the read system call such that whenever the user asks to read a code modified by a virus, the original form of code is shown rather than infected code.
   
• Tunneling Virus: 
  This virus attempts to bypass detection by antivirus scanner by installing itself in the interrupt handler chain. Interception programs, which remain in the background of an operating system and catch viruses, become disabled during the course of a tunneling virus. Similar viruses install themselves in device drivers.
   
• Multipartite Virus: 
  This type of virus is able to infect multiple parts of a system including the boot sector, memory, and files. This makes it difficult to detect and contain.
   
• Armored Virus: 
  An armored virus is coded to make it difficult for antivirus to unravel and understand. It uses a variety of techniques to do so like fooling antivirus to believe that it lies somewhere else than its real location or using compression to complicate its code.
   
• Browser Hijacker: 
  As the name suggests this virus is coded to target the user’s browser and can alter the browser settings. It is also called the browser redirect virus because it redirects your browser to other malicious sites that can harm your computer system.
• Resident Virus:

Resident viruses installation store for your RAM and meddle together along with your device operations. They’re so sneaky that they could even connect themselves for your anti-virus software program files.