First published: 23 Jun 2020
Last updated: 10 Nov 2023

Content written for

Individuals & families
Small & medium business
Large organisations & infrastructure

Malware is the term used to refer to any type of code or program that is used for a malicious purpose.

Cybercriminals use malware for many different reasons. Common types of malware are used for:

  • stealing your information and account details
  • encrypting your data for ransom
  • installing other software without your knowledge.

A malware attack can have serious and ongoing impacts. Malware can also act as an entry point for cybercriminals, opening the door to further malicious activity.

Malware is distributed in several ways:

  • by spam email or messages (either as a link or an attachment)
  • by malicious websites that attempt to install malware when you visit
  • by exploiting weaknesses in software on your devices
  • by posing as a trusted application that you download and install yourself.

Some malware may even pretend to be antivirus or security products.

Potential warning signs of malware

Malware can behave in many different ways. There are a number of signs you may notice that could be due to a malware infection.

  • You notice unusual account activity, for example, logins from an unusual location or at an unusual time, or your passwords have been changed and you are unable to access your accounts.
  • Your device consistently slows down, overheats, battery drains fast or runs its cooling fan faster than usual (these are signs that your processor is running at capacity).
  • Unexpected files and programs on your device. You may notice new programs, toolbars and icons have been installed.
  • Unable to access files, or ransom demands for release of your files.
  • You consistently see error messages that you never used to see.
  • Your web browser automatically takes you to a web page you did not intend to open.
  • Suspicious pop-up ads about updating or downloading a program.
  • Someone knows something that they could only have found out if they had access to your device.

Learn more about malware

There are many different types of malware but most are used to either steal your information, your computer’s resources or your money. This table lists some of the most common types of malware affecting people and businesses in the wild today.


What it does


A type of malware that encrypts all your personal files and prevent access to them, unless a ransom is paid to restore access to the files.

For guidance on preventing ransomware, read our Ransomware Advice.


A way of harvesting personal information, where a cybercriminal puts malicious code on your device that redirects you to a fake website.

Trojans and backdoors

Traditionally, trojans are programs that appear to serve a useful purpose but do something malicious when run. Trojans may steal information, download additional malicious files or even provide a ‘backdoor’ into your device – allowing a cybercriminal to do almost anything they like.


Records which keys you press and sends that information to a cybercriminal. This could include passwords and credit card details.

Viruses and Worms

Viruses infect files by inserting themselves into the file’s code and then running whenever the file is opened. Worms are standalone malicious programs that spread themselves from computer to computer. Similar to Trojans, viruses and worms can have many different payloads – for example, they can steal your information, download and install other malicious files, delete your files or even send spam.

Web Shell Malware

Malicious scripts that allow cyber criminals to compromise web servers and use it as a permanent backdoor to launch additional attacks.

ASD and NSA have jointly produced a Cybersecurity Information Sheet: Detect and Prevent Web Shell Malware (PDF)


Adware is a type of malware that gathers information to show you targeted advertising. In most cases, it is not dangerous but occasionally it can interfere with your system. For instance, it could open a webpage in your browser, which contains another type of malware.

More information

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