Malware—a blanket term for viruses, worms, trojans, and other harmful computer programs—has been with us since the early days of computing. But malware is constantly evolving and hackers use it to wreak destruction and gain access to sensitive information; fighting malware takes up much of the day-to-day work of infosec professionals.

Malware definition

Malware is short for malicious software and, as Microsoft puts it, “is a catch-all term to refer to any software designed to cause damage to a single computer, server, or computer network.” In other words, software is identified as malware based on its intended use, rather than a particular technique or technology used to build it.

This means that the question of, say, what the difference is between malware and a virus misses the point a bit: a virus is a type of malware, so all viruses are malware (but not every piece of malware is a virus).

Types of malware

There are a number of different ways of categorizing malware; the first is by how the malicious software spreads. You’ve probably heard the words virus, trojan, and worm used interchangeably, but as Symantec explains, they describe three subtly different ways malware can infect target computers:

  • A worm is a standalone piece of malicious software that reproduces itself and spreads from computer to computer.
  • A virus is a piece of computer code that inserts itself within the code of another standalone program, then forces that program to take malicious action and spread itself.
  • A trojan is a program that cannot reproduce itself but masquerades as something the user wants and tricks them into activating it so it can do its damage and spread.

Malware can also be installed on a computer “manually” by the attackers themselves, either by gaining physical access to the computer or using privilege escalation to gain remote administrator access. 

Another way to categorize malware is by what it does once it has successfully infected its victim’s computers. There are a wide range of potential attack techniques used by malware:

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