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).
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 categorise 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:
Spyware is defined by Webroot Cybersecurity as “malware used for the purpose of secretly gathering data on an unsuspecting user.” In essence, it spies on your behavior as you use your computer, and on the data you send and receive, usually with the purpose of sending that information to a third party. A keylogger is a specific kind of spyware that records all the keystrokes a user makes—great for stealing passwords.
A rootkit is, as described by TechTarget, “a program or, more often, a collection of software tools that gives a threat actor remote access to and control over a computer or other system.” It gets its name because it’s a kit of tools that (generally illicitly) gain root access (administrator-level control, in Unix terms) over the target system, and use that power to hide their presence.
Adware is malware that forces your browser to redirect to web advertisements, which often themselves seek to download further, even more malicious software. As The New York Times notes, adware often piggybacks onto tempting “free” programs like games or browser extensions.
Ransomware is a flavor of malware that encrypts your hard drive’s files and demands a payment, usually in Bitcoin, in exchange for the decryption key. Several high-profile malware outbreaks of the last few years, such as Petya, are ransomware. Without the decryption key, it’s mathematically impossible for victims to regain access to their files. So-called scareware is a sort of shadow version of ransomware; it claims to have taken control of your computer and demands a ransom, but actually is just using tricks like browser redirect loops to make it seem as if it’s done more damage than it really has, and unlike ransomware can be relatively easily disabled.
Any specific piece of malware has both a means of infection and a behavioural category. So, for instance, WannaCry is a ransomware worm. And a particular piece of malware might have different forms with different attack vectors: for instance, the Emotet banking malware has been spotted in the wild as both a trojan and a worm.
A look at the Centre for Internet Security’s top 10 malware offenders for June of 2018 gives you a good sense of the types of malware out there. By far the most common infection vector is via spam email, which tricks users into activating the malware, Trojan-style. WannaCry and Emotet are the most prevalent malware on the list, but many others, including NanoCore and Gh0st, are what’s called Remote Access Trojans or RATs—essentially, rootkits that propagate like Trojans. Cryptocurrency malware like CoinMiner rounds out the list.