What is malware?

Malware, or malicious software, is any program or file that is harmful to a computer user. Types of malware can include computer viruses, worms, Trojan horses and spyware. These malicious programs can perform a variety of functions such as stealing, encrypting or deleting sensitive data, altering or hijacking core computing functions and monitoring users' computer activity.

What does malware do?

Malware can infect networks and devices and is designed to harm those devices, networks and/or their users in some way. Depending on the type of malware, this harm can take many forms and may present itself differently to the user. In some cases, the effect malware has is relatively mild and benign, and in others, it can be disastrous. No matter the method, all types of malware are designed to exploit devices at the expense of the user and to the benefit of the hacker -- the person who has designed and/or deployed the malware.

How do malware infections happen?

Malware authors use a variety of physical and virtual means to spread malware that infects devices and networks. For example, malicious programs can be delivered to a system with a USB drive or can spread over the internet through drive-by downloads, which automatically download malicious programs to systems without the user's approval or knowledge. Phishing attacks are another common type of malware delivery where emails disguised as legitimate messages contain malicious links or attachments that can deliver the malware executable file to unsuspecting users. Sophisticated malware attacks often feature the use of a command-and-control server that enables threat actors to communicate with the infected systems, exfiltrate sensitive data and even remotely control the compromised device or server.

Emerging strains of malware include new evasion and obfuscation techniques that are designed to not only fool users but security administrators and antimalware products as well. Some of these evasion techniques rely on simple tactics, such as using web proxies to hide malicious traffic or source IP addresses. More sophisticated threats include polymorphic malware that can repeatedly change its underlying code to avoid detection from signature-based detection tools; anti-sandbox techniques that enable the malware to detect when it is being analyzed and to delay execution until after it leaves the sandbox; and fileless malware that resides only in the system's RAM to avoid being discovered.

Common types of malware

Types of malware
A diagram of the various types of malware.

Different types of malware contain unique traits and characteristics. Types of malware include:

  • A virus is the most common type of malware that can execute itself and spread by infecting other programs or files.
  • A worm can self-replicate without a host program and typically spreads without any human interaction or directives from the malware authors.
  • A Trojan horse is designed to appear as a legitimate software program to gain access to a system. Once activated following installation, Trojans can execute their malicious functions.
  • Spyware is made to collect information and data on the device and user, as well as observe the user's activity without their knowledge.
  • Ransomware is designed to infect a user's system and encrypt its data. Cybercriminals then demand a ransom payment from the victim in exchange for decrypting the system's data.
  • A rootkit is created to obtain administrator-level access to the victim's system. Once installed, the program gives threat actors root or privileged access to the system.
  • A backdoor virus or remote access Trojan (RAT) secretly creates a backdoor into an infected computer system that enables threat actors to remotely access it without alerting the user or the system's security programs.
  • Adware is used to track a user's browser and download history with the intent to display pop-up or banner advertisements that lure the user into making a purchase. For example, an advertiser might use cookies to track the webpages a user visits to better target advertising.
  • Keyloggers, also called system monitors, are used to track nearly everything a user does on their computer. This includes emails, opened webpages, programs and keystrokes.

How to detect malware

A user may be able to detect malware if they observe unusual activity such as a sudden loss of disc space, unusually slow speeds, repeated crashes or freezes, or an increase in unwanted internet activity and pop-up advertisements. Antivirus software may also be installed on the device to detect and remove malware. These tools can provide real-time protection or detect and remove malware by executing routine system scans.

Windows Defender, for example, is Microsoft anti-malware software included in the Windows 10 operating system (OS) under the Windows Defender Security Center. Windows Defender protects against threats such as spyware, adware and viruses. Users can set automatic "Quick" and "Full" scans, as well as set low, medium, high and severe priority alerts.

Malware response plan
The steps involved in an organization's malware response plan.

How to remove malware

As mentioned, many security software products are designed to both detect and prevent malware, as well as remove it from infected systems.

Malwarebytes is an example of an antimalware tool that handles both detection and removal of malware. It can remove malware from Windows, macOS, Android and iOS platforms. Malwarebytes can scan a user's registry files, running programs, hard drives and individual files. If detected, malware can then be quarantined and deleted. However, unlike some other tools, users cannot set automatic scanning schedules.

How to prevent malware infections

There are several ways users can prevent malware. In the case of protecting a personal computer, users can install antimalware software. Beyond that, users can prevent malware by practicing safe behavior on their computer or other personal devices. This includes not opening attachments from strange email addresses that may contain malware disguised as a legitimate attachment -- such emails may even claim to be from legitimate companies but have unofficial email domains. Users should also update their antimalware software regularly, as hackers are always adapting and developing new techniques to breach security software. Security software vendors respond by releasing updates that patch those vulnerabilities. If a user neglects to update their software, they may miss out on a patch that leaves them vulnerable to a preventable exploit.

In enterprise settings, networks are larger than home networks, and there is more at stake financially. There are proactive steps companies should take to enforce malware protection. Outward-facing precautions include:

  • Implementing dual approval for business-to-business (B2B) transactions.
  • Implementing second-channel verification for business-to-consumer (B2C) transactions.

Business facing, internal precautions include:

  • Implementing offline malware and threat detection to catch malicious software before it spreads.
  • Implementing allowlist security policies whenever possible.
  • Implementing robust web browser-level security.

Does malware affect Mac devices?

Malware can affect Mac devices as well as Windows devices. Windows devices are considered by some to be a larger target for malware than Macs, in part because applications for Apple devices can only be downloaded through the heavily vetted App Store. For this reason, jailbroken Apple devices are more vulnerable to malware and other cyberattacks than normal Macs.

The company Malwarebytes reported in 2020 that for the first time ever, malware on Macs is outpacing malware on PCs. This is in part due to the popularity of Apple devices, drawing more attention from hackers.

Does malware affect mobile devices?

Malware can also be found on mobile phones and can provide access to a device's components such as the camera, microphone, GPS or accelerometer. Malware can be contracted on a mobile device if a user downloads an unofficial application or clicks on a malicious link from an email or text message. A mobile device can also be infected through a Bluetooth or Wi-Fi connection.

Malware is found much more commonly on devices that run the Android OS compared to iOS devices. Malware on Android devices is usually downloaded through applications. Signs that an Android device is infected with malware include unusual increases in data usage, a quickly dissipating battery charge or calls, texts and emails being sent to the device contacts without the user's initial knowledge. Similarly, if a user receives a message from a recognized contact that seems suspicious, it may be from a type of mobile malware that spreads between devices.

Apple iOS devices are rarely infected with malware because Apple carefully vets the applications sold in the App Store. However, it is still possible for an iOS device to be infected with malicious code by opening an unknown link found in an email or text message. iOS devices will become more vulnerable if jailbroken.

History of malware

The term malware was first used by computer scientist and security researcher Yisrael Radai in 1990. However, malware existed long before this. One of the first known examples of malware was the Creeper virus in 1971, which was created as an experiment by BBN Technologies engineer Robert Thomas. Creeper was designed to infect mainframes on ARPANET. While the program did not alter functions -- or steal or delete data -- it moved from one mainframe to another without permission while displaying a teletype message that read, "I'm the creeper: Catch me if you can." Creeper was later altered by computer scientist Ray Tomlinson, who added the ability to self-replicate to the virus and created the first known computer worm.

The concept of malware took root in the technology industry, and examples of viruses and worms began to appear on Apple and IBM personal computers in the early 1980s before becoming popularized following the introduction of the World Wide Web and the commercial internet in the 1990s. Since then, malware, and the security strategies to prevent it, have only grown more complex.

Similar programs

There are other types of programs that share common traits with malware but are distinctly different, such as a PUP, or potentially unwanted program. These are typically applications that trick users into installing them on their system (such as browser toolbars) but do not execute any malicious functions once they have been installed. However, there are cases where a PUP may contain spyware-like functionality or other hidden malicious features, in which case the PUP would be classified as malware.

This was last updated in November 2020

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