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Malware (a portmanteau for malicious software) is any software intentionally designed to cause disruption to a computer, server, client, or computer network, leak private information, gain unauthorized access to information or systems, deprive users access to information or which unknowingly interferes with the user's computer security and privacy.^[1][2][3][4] By contrast, software that causes harm due to some deficiency is typically described as a software bug.^[5] Malware poses serious problems to individuals and businesses.^[6] According to Symantec’s 2018 Internet Security Threat Report (ISTR), malware variants number has got up to 669,947,865 in 2017, which is twice as many malware variants in 2016.^[7]

Many types of malware exist, including computer viruses, worms, Trojan horses, ransomware, spyware, adware, rogue software, wiper and scareware. The defense strategies against malware differs according to the type of malware but most can be thwarted by installing antivirus software, firewalls, applying regular patches to reduce zero-day attacks, securing networks from intrusion, having regular backups and isolating infected systems. Malware is now being designed to evade antivirus software detection algorithms.^[7]

The notion of a self-reproducing computer program can be traced back to initial theories about the operation of complex automata.^[8] John von Neumann showed that in theory a program could reproduce itself. This constituted a plausibility result in computability theory. Fred Cohen experimented with computer viruses and confirmed Neumann's postulate and investigated other properties of malware such as detectability and self-obfuscation using rudimentary encryption. His 1987 doctoral dissertation was on the subject of computer viruses.^[9] The combination of cryptographic technology as part of the payload of the virus, exploiting it for attack purposes was initialized and investigated from the mid 1990s, and includes initial ransomware and evasion ideas.^[10]

Before Internet access became widespread, viruses spread on personal computers by infecting executable programs or boot sectors of floppy disks. By inserting a copy of itself into the machine code instructions in these programs or boot sectors, a virus causes itself to be run whenever the program is run or the disk is booted. Early computer viruses were written for the Apple II and Macintosh, but they became more widespread with the dominance of the IBM PC and MS-DOS system. The first IBM PC virus in the "wild" was a boot sector virus dubbed (c)Brain,^[11] created in 1986 by the Farooq Alvi brothers in Pakistan.^[12] Malware distributors would trick the user into booting or running from an infected device or medium. For example, a virus could make an infected computer add autorunnable code to any USB stick plugged into it. Anyone who then attached the stick to another computer set to autorun from USB would in turn become infected, and also pass on the infection in the same way.^[13]

Older email software would automatically open HTML email containing potentially malicious JavaScript code. Users may also execute disguised malicious email attachments. The 2018 Data Breach Investigations Report by Verizon, cited by CSO Online, states that emails are the primary method of malware delivery, accounting for 92% of malware delivery around the world.^[14][15]

The first worms, network-borne infectious programs, originated not on personal computers, but on multitasking Unix systems. The first well-known worm was the Internet Worm of 1988, which infected SunOS and VAX BSD systems. Unlike a virus, this worm did not insert itself into other programs. Instead, it exploited security holes (vulnerabilities) in network server programs and started itself running as a separate process.^[16] This same behavior is used by today's worms as well.^[17][18]

With the rise of the Microsoft Windows platform in the 1990s, and the flexible macros of its applications, it became possible to write infectious code in the macro language of Microsoft Word and similar programs. These macro viruses infect documents and templates rather than applications (executables), but rely on the fact that macros in a Word document are a form of executable code.^[19]

Many early infectious programs, including the first Internet Worm, were written as experiments or pranks.^[20] Today, malware is used by both black hat hackers and governments to steal personal, financial, or business information.^[21][22] Today, any device that plugs into a USB port – even lights, fans, speakers, toys, or peripherals such as a digital microscope – can be used to spread malware. Devices can be infected during manufacturing or supply if quality control is inadequate.^[13]

Malware is sometimes used broadly against government or corporate websites to gather guarded information,^[23] or to disrupt their operation in general. However, malware can be used against individuals to gain information such as personal identification numbers or details, bank or credit card numbers, and passwords.

Since the rise of widespread broadband Internet access, malicious software has more frequently been designed for profit. Since 2003, the majority of widespread viruses and worms have been designed to take control of users' computers for illicit purposes.^[24] Infected "zombie computers" can be used to send email spam, to host contraband data such as child pornography,^[25] or to engage in distributed denial-of-service attacks as a form of extortion.^[26]

Programs designed to monitor users' web browsing, display unsolicited advertisements, or redirect affiliate marketing revenues are called spyware. Spyware programs do not spread like viruses; instead they are generally installed by exploiting security holes. They can also be hidden and packaged together with unrelated user-installed software.^[27] The Sony BMG rootkit was intended to prevent illicit copying; but also reported on users' listening habits, and unintentionally created extra security vulnerabilities.^[28]

Ransomware prevents a user from accessing their files until a ransom is paid. There are two variations of ransomware, being crypto ransomware and locker ransomware.^[29] Locker ransomware just locks down a computer system without encrypting its contents, whereas crypto ransomware locks down a system and encrypts its contents. For example, programs such as CryptoLocker encrypt files securely, and only decrypt them on payment of a substantial sum of money.^[30]

Some malware is used to generate money by click fraud, making it appear that the computer user has clicked an advertising link on a site, generating a payment from the advertiser. It was estimated in 2012 that about 60 to 70% of all active malware used some kind of click fraud, and 22% of all ad-clicks were fraudulent.^[31]

In addition to criminal money-making, malware can be used for sabotage, often for political motives. Stuxnet, for example, was designed to disrupt very specific industrial equipment. There have been politically motivated attacks which spread over and shut down large computer networks, including massive deletion of files and corruption of master boot records, described as "computer killing." Such attacks were made on Sony Pictures Entertainment (25 November 2014, using malware known as Shamoon or W32.Disttrack) and Saudi Aramco (August 2012).^[32][33]

These categories are not mutually exclusive, so malware may use multiple techniques.^[34]

A Trojan horse is a harmful program that misrepresents itself to masquerade as a regular, benign program or utility in order to persuade a victim to install it. A Trojan horse usually carries a hidden destructive function that is activated when the application is started. The term is derived from the Ancient Greek story of the Trojan horse used to invade the city of Troy by stealth.^{[35][36][37][38][39]}

Trojan horses are generally spread by some form of social engineering, for example, where a user is duped into executing an email attachment disguised to be unsuspicious, (e.g., a routine form to be filled in), or by drive-by download. Although their payload can be anything, many modern forms act as a backdoor, contacting a controller (phoning home) which can then have unauthorized access to the affected computer, potentially installing additional software such as a keylogger to steal confidential information, cryptomining software or adware to generate revenue to the operator of the trojan.^[40] While Trojan horses and backdoors are not easily detectable by themselves, computers may appear to run slower, emit more heat or fan noise due to heavy processor or network usage, as may occur when cryptomining software is installed. Cryptominers may limit resource usage and/or only run during idle times in an attempt to evade detection.

Unlike computer viruses and worms, Trojan horses generally do not attempt to inject themselves into other files or otherwise propagate themselves.^[41]

In spring 2017 Mac users were hit by the new version of Proton Remote Access Trojan (RAT)^[42] trained to extract password data from various sources, such as browser auto-fill data, the Mac-OS keychain, and password vaults.^[43]

Once malicious software is installed on a system, it is essential that it stays concealed, to avoid detection. Software packages known as rootkits allow this concealment, by modifying the host's operating system so that the malware is hidden from the user. Rootkits can prevent a harmful process from being visible in the system's list of processes, or keep its files from being read.^[44]

Some types of harmful software contain routines to evade identification and/or removal attempts, not merely to hide themselves. An early example of this behavior is recorded in the Jargon File tale of a pair of programs infesting a Xerox CP-V time sharing system:

Each ghost-job would detect the fact that the other had been killed, and would start a new copy of the recently stopped program within a few milliseconds. The only way to kill both ghosts was to kill them simultaneously (very difficult) or to deliberately crash the system.^[45]

A backdoor is a method of bypassing normal authentication procedures, usually over a connection to a network such as the Internet. Once a system has been compromised, one or more backdoors may be installed in order to allow access in the future,^[46] invisibly to the user.

The idea has often been suggested that computer manufacturers preinstall backdoors on their systems to provide technical support for customers, but this has never been reliably verified. It was reported in 2014 that US government agencies had been diverting computers purchased by those considered "targets" to secret workshops where software or hardware permitting remote access by the agency was installed, considered to be among the most productive operations to obtain access to networks around the world.^[47] Backdoors may be installed by Trojan horses, worms, implants, or other methods.^[48][49]

The best-known types of malware, viruses and worms, are known for the manner in which they spread, rather than any specific types of behavior and have been likened to biological viruses.^[3]

A worm is a stand-alone malware software that actively transmits itself over a network to infect other computers and can copy itself without infecting files. These definitions lead to the observation that a virus requires the user to run an infected software or operating system for the virus to spread, whereas a worm spreads itself.^[50]

A computer virus is software usually hidden within another seemingly innocuous program that can produce copies of itself and insert them into other programs or files, and that usually performs a harmful action (such as destroying data).^[51] An example of this is a portable execution infection, a technique, usually used to spread malware, that inserts extra data or executable code into PE files.^[52] A computer virus is software that embeds itself in some other executable software (including the operating system itself) on the target system without the user's knowledge and consent and when it is run, the virus is spread to other executable files.

Lock-screens, or screen lockers is a type of “cyber police” ransomware that blocks screens on Windows or Android devices with a false accusation in harvesting illegal content, trying to scare the victims into paying up a fee.^[53] Jisut and SLocker impact Android devices more than other lock-screens, with Jisut making up nearly 60 percent of all Android ransomware detections.^[54]

Encryption-based ransomware, like the name suggests, is a type of ransomware that encrypts all files on an infected machine. These types of malware then display a pop-up informing the user that their files have been encrypted and that they must pay (usually in Bitcoin) to recover them. Some examples of encryption-based ransomware are CryptoLocker and WannaCry.^[55]

Grayware (sometimes spelled as greyware) is a term, coming into use around 2004, that applies to any unwanted application or file that can worsen the performance of computers and may cause security risks but which is not typically considered malware.^[56][57] Greyware are applications that behave in an annoying or undesirable manner, and yet are less serious or troublesome than malware. Grayware encompasses spyware, adware, fraudulent dialers, joke programs, remote access tools and other unwanted programs that may harm the performance of computers or cause inconvenience. For example, at one point, Sony BMG compact discs silently installed a rootkit on purchasers' computers with the intention of preventing illicit copying.^[28]

Potentially unwanted programs (PUPs) or potentially unwanted applications (PUAs) are applications that would be considered unwanted despite being downloaded often by the user, possibly after failing to read a download agreement.^[58] PUPs include spyware, adware, and fraudulent dialers. Many security products classify unauthorised key generators as grayware, although they frequently carry true malware in addition to their ostensible purpose. Malwarebytes lists several criteria for classifying a program as a PUP.^[59] Some types of adware (using stolen certificates) turn off anti-malware and virus protection; technical remedies are available.^[60]

Since the beginning of 2015, a sizable portion of malware has been utilizing a combination of many techniques designed to avoid detection and analysis.^[61] From the more common, to the least common:

1. evasion of analysis and detection by fingerprinting the environment when executed.^[62]
2. confusing automated tools' detection methods. This allows malware to avoid detection by technologies such as signature-based antivirus software by changing the server used by the malware.^[63]
3. timing-based evasion. This is when malware runs at certain times or following certain actions taken by the user, so it executes during certain vulnerable periods, such as during the boot process, while remaining dormant the rest of the time.
4. obfuscating internal data so that automated tools do not detect the malware.^[64]

An increasingly common technique (2015) is adware that uses stolen certificates to disable anti-malware and virus protection; technical remedies are available to deal with the adware.^[60]

Nowadays, one of the most sophisticated and stealthy ways of evasion is to use information hiding techniques, namely stegomalware. A survey on stegomalware was published by Cabaj et al. in 2018.^[65]

Another type of evasion technique is Fileless malware or Advanced Volatile Threats (AVTs). Fileless malware does not require a file to operate. It runs within memory and utilizes existing system tools to carry out malicious acts. Because there are no files on the system, there are no executable files for antivirus and forensic tools to analyze, making such malware nearly impossible to detect. The only way to detect fileless malware is to catch it operating in real time. Recently these types of attacks have become more frequent with a 432% increase in 2017 and makeup 35% of the attacks in 2018. Such attacks are not easy to perform but are becoming more prevalent with the help of exploit-kits. ^[66][67]

A vulnerability is a weakness, flaw or software bug in an application, a complete computer, an operating system, or a computer network that is exploited by malware to bypass defences or gain privileges it requires to run. For example, TestDisk 6.4 or ealier contained a vulnerability that allowed attackers to inject code into Windows.^[68] Malware can exploit security defects (security bugs or vulnerabilities) in the operating system, applications (such as browsers, e.g. older versions of Microsoft Internet Explorer supported by Windows XP^[69]), or in vulnerable versions of browser plugins such as Adobe Flash Player, Adobe Acrobat or Reader, or Java SE.^[70][71] For example, a common method is exploitation of a buffer overrun vulnerability, where software designed to store data in a specified region of memory does not prevent more data than the buffer can accommodate being supplied. Malware may provide data that overflows the buffer, with malicious executable code or data after the end; when this payload is accessed it does what the attacker, not the legitimate software, determines.

Malware can exploit recently discovered vulnerabilities before developers have had time to release a suitable patch.^[6] Even when new patches addressing the vulnerability have been released, they may not necessarily be installed immediately, allowing malware to take advantage of systems lacking patches. Sometimes even applying patches or installing new versions does not automatically uninstall the old versions. Security advisories from plug-in providers announce security-related updates.^[72] Common vulnerabilities are assigned CVE IDs and listed in the US National Vulnerability Database. Secunia PSI^[73] is an example of software, free for personal use, that will check a PC for vulnerable out-of-date software, and attempt to update it. Other approaches involve using firewalls and intrusion prevention systems to monitor unusual traffic patterns on the local computer network.^[74]

Users and programs can be assigned more privileges than they require, and malware can take advantage of this. For example, of 940 Android apps sampled, one third of them asked for more privileges than they required.^[75] Apps targeting the Android platform can be a major source of malware infection but one solution is to use third party software to detect apps that have been assigned excessive privileges.^[76]

Some systems allow all users to modify their internal structures, and such users today would be considered over-privileged users. This was the standard operating procedure for early microcomputer and home computer systems, where there was no distinction between an administrator or root, and a regular user of the system. In some systems, non-administrator users are over-privileged by design, in the sense that they are allowed to modify internal structures of the system. In some environments, users are over-privileged because they have been inappropriately granted administrator or equivalent status.^[77] This can be because users tend to demand more privileges than they need, so often end up being assigned unnecessary privileges.^[78]

Some systems allow code executed by a user to access all rights of that user, which is known as over-privileged code. This was also standard operating procedure for early microcomputer and home computer systems. Malware, running as over-privileged code, can use this privilege to subvert the system. Almost all currently popular operating systems, and also many scripting applications allow code too many privileges, usually in the sense that when a user executes code, the system allows that code all rights of that user.

A crendential attack occurs when a user account with administrative privileges is cracked and that account is used to provide malware with appropriate privileges.^[79] Typically, the attack succeeds because the weakest form of account security is used, which is typically a short password that can be cracked using a dictionary or brute force attack. Using strong passwords and enabling two-factor authentication can reduce this risk. With the latter enabled, even if an attacker can crack the password, they cannot use the account without also having the token possessed by the legitimate user of that account.

Homogeneity can be a vulnerability. For example, when all computers in a network run the same operating system, upon exploiting one, one worm can exploit them all:^[80] In particular, Microsoft Windows or Mac OS X have such a large share of the market that an exploited vulnerability concentrating on either operating system could subvert a large number of systems. It is estimated that approximately 83% of malware infections between January and March 2020 were spread via systems running Windows 10.^[81] This risk is mitigated by segmenting the networks into different subnetworks and setting up firewalls to block traffic between them.^[82][83]

Anti-malware (sometimes also called antivirus) programs block and remove some or all types of malware. For example, Microsoft Security Essentials (for Windows XP, Vista, and Windows 7) and Windows Defender (for Windows 8, 10 and 11) provides real-time protection. The Windows Malicious Software Removal Tool removes malicious software from the system.^[84] Additionally, several capable antivirus software programs are available for free download from the Internet (usually restricted to non-commercial use).^[85] Tests found some free programs to be competitive with commercial ones.^[85][86][87]

Typically, antivirus software can combat malware in the following ways:

1. Real-time protection: They can provide real time protection against the installation of malware software on a computer. This type of malware protection works the same way as that of antivirus protection in that the anti-malware software scans all incoming network data for malware and blocks any threats it comes across.
2. Removal: Anti-malware software programs can be used solely for detection and removal of malware software that has already been installed onto a computer. This type of anti-malware software scans the contents of the Windows registry, operating system files, and installed programs on a computer and will provide a list of any threats found, allowing the user to choose which files to delete or keep, or to compare this list to a list of known malware components, removing files that match.^[88]
3. Sandboxing: Provide sandboxing of apps considered dangerous (such as web browsers where most vulnerabilities are likely to be installed from).^[89]

A specific component of anti-malware software, commonly referred to as an on-access or real-time scanner, hooks deep into the operating system's core or kernel and functions in a manner similar to how certain malware itself would attempt to operate, though with the user's informed permission for protecting the system. Any time the operating system accesses a file, the on-access scanner checks if the file infected or not. Typically, when an infected file is found, execution is stopped and the file is quarantined to prevent further damage with the intention to prevent irreversible system damage. Most AVs allow users to override this behaviour. This can have a considerable performance impact on the operating system, though the degree of impact is dependent on how many pages it creates in virtual memory.^[90]

Because many malware components are installed as a result of browser exploits or user error, using security software (some of which are anti-malware, though many are not) to "sandbox" browsers (essentially isolate the browser from the computer and hence any malware induced change) can also be effective in helping to restrict any damage done.^[89]

Some viruses disable System Restore and other important Windows tools such as Task Manager and Command Prompt. Many such viruses can be removed by rebooting the computer into Windows safe mode and then using system tools or Microsoft Safety Scanner.^[91] The properiety Emsisoft Emergency Kit can be used to remove malware from an infected system.^[92]

As malware also harms the compromised websites (by breaking reputation, blacklisting in search engines, etc.), some websites offer vulnerability scanning.^[93]Such scans check the website, detect malware, may note outdated software, and may report known security issues.

As a last resort, computers can be protected from malware, and infected computers can be prevented from disseminating trusted information, by imposing an "air gap" (i.e. completely disconnecting them from all other networks). However, malware can still cross the air gap in some situations. Stuxnet is an example of malware that is introduced to the target environment via a USB drive.

AirHopper,^[94] BitWhisper,^[95] GSMem ^[96] and Fansmitter^[97] are four techniques introduced by researchers that can leak data from air-gapped computers using electromagnetic, thermal and acoustic emissions.

Look up malware in Wiktionary, the free dictionary.

Wikimedia Commons has media related to Malware.

• Malicious Software at Curlie
• Further Reading: Research Papers and Documents about Malware on IDMARCH (Int. Digital Media Archive)
• Advanced Malware Cleaning – a Microsoft video