Malware

{{short description|Malicious software}} {{Use dmy dates|date=August 2017}} {{Computer hacking}} '''Malware''' (a portmanteau of '''''malicious software''''') is any software intentionally designed to cause disruption or destruction to a computer, server, client, or computer network, leak private information, gain unauthorized access to information or systems, deprive access to information, or interfere with the user's computer security and privacy without their knowledge.<ref>{{cite web |title=An Undirected Attack Against Critical Infrastructure |url=https://ics-cert.us-cert.gov/sites/default/files/recommended_practices/CaseStudy-002.pdf |url-status=live |archive-url=https://web.archive.org/web/20161224070034/https://ics-cert.us-cert.gov/sites/default/files/recommended_practices/CaseStudy-002.pdf |archive-date=24 December 2016 |access-date=28 September 2014 |publisher=United States Computer Emergency Readiness Team(Us-cert.gov)}}</ref><ref name="Cani 2014">{{Cite book |last1=Cani |first1=Andrea |title=Proceedings of the 29th Annual ACM Symposium on Applied Computing |last2=Gaudesi |first2=Marco |last3=Sanchez |first3=Ernesto |last4=Squillero |first4=Giovanni |last5=Tonda |first5=Alberto |date=2014-03-24 |publisher=Association for Computing Machinery |isbn=978-1-4503-2469-4 |series=SAC '14 |location=New York, NY, USA |pages=157–160 |chapter=Towards automated malware creation |doi=10.1145/2554850.2555157 |chapter-url=https://doi.org/10.1145/2554850.2555157 |s2cid=14324560}}</ref><ref>{{Cite journal |last=Brewer |first=Ross |date=2016-09-01 |title=Ransomware attacks: detection, prevention and cure |url=https://www.sciencedirect.com/science/article/pii/S1353485816300861 |url-status=live |journal=Network Security |language=en |volume=2016 |issue=9 |pages=5–9 |doi=10.1016/S1353-4858(16)30086-1 |issn=1353-4858 |url-access=subscription |archive-url=https://web.archive.org/web/20190410072104/https://www.sciencedirect.com/science/article/pii/S1353485816300861 |archive-date=10 April 2019 |access-date=2 December 2021}}</ref><ref>{{Cite journal |last1=Zhong |first1=Fangtian |last2=Chen |first2=Zekai |last3=Xu |first3=Minghui |last4=Zhang |first4=Guoming |last5=Yu |first5=Dongxiao |last6=Cheng |first6=Xiuzhen |author6-link=Xiuzhen Cheng |date=2022 |title=Malware-on-the-Brain: Illuminating Malware Byte Codes with Images for Malware Classification |journal=IEEE Transactions on Computers |volume=72 |issue=2 |pages=438–451 |arxiv=2108.04314 |doi=10.1109/TC.2022.3160357 |issn=0018-9340 |s2cid=236965755}}</ref> Researchers tend to classify malware into one or more sub-types (i.e. computer viruses, worms, Trojan horses, logic bombs, ransomware, spyware, adware, rogue software, wipers and keyloggers).<ref>https://nvlpubs.nist.gov/nistpubs/specialpublications/nist.sp.800-83r1.pdf Protection from malware strategies</ref>{{cn|date=March 2026}}

Malware poses serious threats to individuals and businesses on the Internet.<ref name="Kim 2018">{{Cite journal |last1=Kim |first1=Jin-Young |last2=Bu |first2=Seok-Jun |last3=Cho |first3=Sung-Bae |date=2018-09-01 |title=Zero-day malware detection using transferred generative adversarial networks based on deep autoencoders |url=https://www.sciencedirect.com/science/article/pii/S0020025518303475 |url-status=live |journal=Information Sciences |language=en |volume=460–461 |pages=83–102 |doi=10.1016/j.ins.2018.04.092 |issn=0020-0255 |s2cid=51882216 |url-access=subscription |archive-url=https://web.archive.org/web/20200623174252/https://www.sciencedirect.com/science/article/pii/S0020025518303475 |archive-date=23 June 2020 |access-date=2 December 2021}}</ref><ref>{{Cite journal |last1=Razak |first1=Mohd Faizal Ab |last2=Anuar |first2=Nor Badrul |last3=Salleh |first3=Rosli |last4=Firdaus |first4=Ahmad |date=2016-11-01 |title=The rise of "malware": Bibliometric analysis of malware study |url=https://linkinghub.elsevier.com/retrieve/pii/S1084804516301904 |url-status=live |journal=Journal of Network and Computer Applications |language=en |volume=75 |pages=58–76 |doi=10.1016/j.jnca.2016.08.022 |url-access=subscription |archive-url=https://web.archive.org/web/20220626040626/https://linkinghub.elsevier.com/retrieve/pii/S1084804516301904 |archive-date=26 June 2022 |access-date=30 April 2022}}</ref> According to Symantec's 2018 Internet Security Threat Report (ISTR), the number of malware variants increased to 669,947,865 in 2017, which is twice as many malware variants as in 2016.<ref name="Xiao 2020">{{Cite journal |last1=Xiao |first1=Fei |last2=Sun |first2=Yi |last3=Du |first3=Donggao |last4=Li |first4=Xuelei |last5=Luo |first5=Min |date=2020-03-21 |title=A Novel Malware Classification Method Based on Crucial Behavior |journal=Mathematical Problems in Engineering |volume=2020 |pages=1–12 |doi=10.1155/2020/6804290 |issn=1024-123X |doi-access=free}}</ref> Cybercrime, which includes malware attacks as well as other crimes committed by computer, was predicted to cost the global economy US$6 trillion in 2021, and is increasing at a rate of 15% per year.<ref name="Morgan">{{cite web |last=Morgan |first=Steve |date=13 November 2020 |title=Cybercrime To Cost The World $10.5 Trillion Annually By 2025 |url=https://cybersecurityventures.com/hackerpocalypse-cybercrime-report-2016/ |url-status=live |archive-url=https://web.archive.org/web/20220305072352/https://cybersecurityventures.com/hackerpocalypse-cybercrime-report-2016/ |archive-date=5 March 2022 |accessdate=5 March 2022 |work=Cybercrime magazine website |publisher=Cybersecurity ventures}}</ref> Since 2021, malware has been designed to target computer systems that run critical infrastructure such as the electricity distribution network.<ref>{{Cite journal |last1=Eder-Neuhauser |first1=Peter |last2=Zseby |first2=Tanja |last3=Fabini |first3=Joachim |date=2019-06-01 |title=Malware propagation in smart grid networks: metrics, simulation and comparison of three malware types |journal=Journal of Computer Virology and Hacking Techniques |language=en |volume=15 |issue=2 |pages=109–125 |doi=10.1007/s11416-018-0325-y |issn=2263-8733 |s2cid=255164530 |doi-access=free}}</ref>

The defense strategies against malware differ according to its type, but most can be prevented by installing antivirus software or firewalls, applying regular patches, securing networks, creating backups, and isolating infected systems. Malware can be designed to evade antivirus software detection algorithms.<ref name="Xiao 2020" />

==History== {{main|History of computer viruses}} {{For timeline|Timeline of computer viruses and worms}}

The notion of a self-reproducing computer program can be traced back to initial theories about the operation of complex automata.<ref>John von Neumann, "Theory of Self-Reproducing Automata", Part 1: Transcripts of lectures given at the University of Illinois, December 1949, Editor: A. W. Burks, University of Illinois, USA, 1966.</ref> 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.<ref>Fred Cohen, "Computer Viruses", PhD Thesis, University of Southern California, ASP Press, 1988.</ref> 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.<ref>{{cite book |last1=Young |first1=Adam |url=https://archive.org/details/maliciouscryptog00youn_705 |title=Malicious cryptography - exposing cryptovirology |last2=Yung |first2=Moti |date=2004 |publisher=Wiley |isbn=978-0-7645-4975-5 |pages=[https://archive.org/details/maliciouscryptog00youn_705/page/n27 1]–392 |url-access=limited}}</ref>

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 Mac, but they became more widespread with the dominance of the IBM PC and MS-DOS. The first IBM PC virus in the wild was a boot sector virus dubbed (c)Brain, created in 1986 by the Farooq Alvi brothers in Pakistan.<ref name="Avoine">{{cite book |last=Avoine |first=Gildas |title=Computer system security: basic concepts and solved exercises |author2=Pascal Junod |author3=Philippe Oechslin |publisher=EFPL Press |year=2007 |isbn=978-1-4200-4620-5 |pages=20 |quote=The first PC virus is credited to two brothers, Basit Farooq Alvi and Amjad Farooq Alvi, from Pakistan}}</ref> 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.<ref name="dodusb">{{cite web |title=USB devices spreading viruses |url=http://www.cnet.com/uk/news/usb-devices-spreading-viruses/ |url-status=dead |archive-url=https://web.archive.org/web/20150924055323/http://www.cnet.com/uk/news/usb-devices-spreading-viruses/ |archive-date=24 September 2015 |access-date=18 February 2015 |work=CNET |publisher=CBS Interactive}}</ref>

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 96% of malware delivery around the world.<ref>{{cite report |url=https://enterprise.verizon.com/resources/reports/DBIR_2018_Report.pdf |title=2018 Data Breach Investigations Report |date=2018 |publisher=Verizon |edition=11th |page=18 |access-date=26 September 2022 |archive-url=https://web.archive.org/web/20211016232230/https://enterprise.verizon.com/resources/reports/DBIR_2018_Report.pdf |archive-date=16 October 2021 |url-status=dead}}</ref><ref name="Fruhlinger 2018">{{cite web |last=Fruhlinger |first=Josh |date=October 10, 2018 |title=Top cybersecurity facts, figures and statistics for 2018 |url=https://www.csoonline.com/article/3153707/top-cybersecurity-facts-figures-and-statistics.html |url-status=live |archive-url=https://web.archive.org/web/20190602063948/https://www.csoonline.com/article/3153707/top-cybersecurity-facts-figures-and-statistics.html |archive-date=2 June 2019 |access-date=January 20, 2020 |website=CSO Online}}</ref>

The first worms, network-borne infectious programs, originated not on personal computers, but on multitasking Unix systems. The first well-known worm was the Morris 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.<ref>{{cite web |author=William A Hendric |date=4 September 2014 |title=Computer Virus history |url=https://antivirus.comodo.com/blog/computer-safety/short-history-computer-viruses/ |url-status=live |archive-url=https://web.archive.org/web/20180510050522/https://antivirus.comodo.com/blog/computer-safety/short-history-computer-viruses/ |archive-date=10 May 2018 |access-date=29 March 2015 |work=The Register}}</ref> This same behavior is used by today's worms as well.<ref>{{Cite news |date=2018-05-02 |title=Cryptomining Worm MassMiner Exploits Multiple Vulnerabilities - Security Boulevard |url=https://securityboulevard.com/2018/05/cryptomining-worm-massminer-exploits-multiple-vulnerabilities/ |url-status=live |archive-url=https://web.archive.org/web/20180509221215/https://securityboulevard.com/2018/05/cryptomining-worm-massminer-exploits-multiple-vulnerabilities/ |archive-date=9 May 2018 |access-date=2018-05-09 |work=Security Boulevard |language=en-US}}</ref>

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.<ref>{{Cite web |title=Beware of Word Document Viruses |url=https://us.norton.com/yoursecurityresource/detail.jsp?aid=feature_word |url-status=live |archive-url=https://web.archive.org/web/20170926001833/https://us.norton.com/yoursecurityresource/detail.jsp?aid=feature_word |archive-date=26 September 2017 |access-date=2017-09-25 |website=us.norton.com}}</ref>

Many early infectious programs, including the Morris Worm, the first internet worm, were written as experiments or pranks.<ref>{{Cite book |last=Tipton |first=Harold F. |url=https://books.google.com/books?id=oR_UHxm7QBwC&q=history+of+malware+pranks&pg=PA982 |title=Information Security Management Handbook |date=2002-12-26 |publisher=CRC Press |isbn=978-1-4200-7241-9 |language=en |access-date=16 November 2020 |archive-url=https://web.archive.org/web/20230227060934/https://books.google.com/books?id=oR_UHxm7QBwC&q=history+of+malware+pranks&pg=PA982 |archive-date=27 February 2023 |url-status=live}}</ref> Today, malware is used by both black hat hackers and governments to steal personal, financial, or business information.<ref>{{cite web |title=Malware |url=http://www.consumer.ftc.gov/articles/0011-malware |url-status=live |archive-url=https://web.archive.org/web/20170320191553/https://www.consumer.ftc.gov/articles/0011-malware |archive-date=20 March 2017 |access-date=27 March 2014 |publisher=FEDERAL TRADE COMMISSION- CONSUMER INFORMATION}}</ref><ref>{{cite magazine |last=Hernandez |first=Pedro |title=Microsoft Vows to Combat Government Cyber-Spying |url=http://www.eweek.com/security/microsoft-vows-to-combat-government-cyber-spying.html |url-status=dead |archive-url=https://archive.today/20140123094411/http://www.eweek.com/security/microsoft-vows-to-combat-government-cyber-spying.html |archive-date=23 January 2014 |access-date=15 December 2013 |magazine=eWeek}}</ref> 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.<ref name="dodusb" />

==Purposes== 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.<ref>{{cite web |date=March 2007 |title=Malware Revolution: A Change in Target |url=https://technet.microsoft.com/en-us/library/cc512596.aspx |url-status=live |archive-url=https://web.archive.org/web/20081016211216/https://technet.microsoft.com/en-us/library/cc512596.aspx |archive-date=16 October 2008 |access-date=26 August 2017}}</ref> Infected "zombie computers" can be used to send email spam, to host contraband data such as child pornography,<ref>{{cite web |date=November 2009 |title=Child Porn: Malware's Ultimate Evil |url=http://www.itworld.com/security/84077/child-porn-malwares-ultimate-evil |url-status=dead |archive-url=https://web.archive.org/web/20131022155627/http://www.itworld.com/security/84077/child-porn-malwares-ultimate-evil |archive-date=22 October 2013 |access-date=22 November 2010}}</ref> or to engage in distributed denial-of-service attacks as a form of extortion.<ref>{{cite web |title=Zombie PCs: Silent, Growing Threat |url=http://www.pcworld.com/article/id,116841-page,1/article.html |url-status=dead |archive-url=https://web.archive.org/web/20080727001520/http://www.pcworld.com/article/id,116841-page,1/article.html |archive-date=27 July 2008 |website=PC World}}</ref> Malware is used broadly against government or corporate websites to gather sensitive information,<ref>{{cite web |last=Kovacs |first=Eduard |date=27 February 2013 |title=MiniDuke Malware Used Against European Government Organizations |url=http://news.softpedia.com/news/MiniDuke-Malware-Used-Against-European-Government-Organizations-333006.shtml |url-status=live |archive-url=https://web.archive.org/web/20161011191453/http://news.softpedia.com/news/MiniDuke-Malware-Used-Against-European-Government-Organizations-333006.shtml |archive-date=11 October 2016 |access-date=27 February 2013 |publisher=Softpedia}}</ref> or to disrupt their operation in general. Further, malware can be used against individuals to gain information such as personal identification numbers or details, bank or credit card numbers, and passwords.<ref name="theregister/2022-raccoon">{{cite news |last1=Claburn |first1=Thomas |date=October 26, 2022 |title=Ukrainian indicted by US govt on cybercrime charges |url=https://www.theregister.com/2022/10/26/feds_indict_ukrainian_raccoon/ |url-status=live |archive-url=https://web.archive.org/web/20221026235426/https://www.theregister.com/2022/10/26/feds_indict_ukrainian_raccoon/ |archive-date=26 October 2022 |access-date=27 October 2022 |work=theregister.com |language=en |quote=Those deploying Raccoon used phishing messages and other tricks to get the malware onto potentially millions of victims' computers worldwide. Once installed, the code provided access to login credentials and other data stored on the compromised system.}}</ref><ref name="raccoon.ic3.gov">{{cite web |title=Raccoon Infostealer Disclosure |url=https://raccoon.ic3.gov/ |url-status=dead |archive-url=https://web.archive.org/web/20230227063646/https://raccoon.ic3.gov/home |archive-date=27 February 2023 |access-date=27 October 2022 |website=raccoon.ic3.gov}}</ref>

Beyond its use in criminal enterprises, malware has also been deployed as a tool for sabotage, often driven by political objectives. A notable example is Stuxnet, which was engineered to interfere with specific industrial control systems.<ref>{{Cite news |last=Zetter |first=Kim |title=How Digital Detectives Deciphered Stuxnet, the Most Menacing Malware in History |url=https://www.wired.com/2011/07/how-digital-detectives-deciphered-stuxnet/ |access-date=2026-02-24 |work=Wired |language=en-US |issn=1059-1028}}</ref>

In other cases, politically motivated malware attacks have targeted entire networks, causing widespread disruption. These incidents have included the mass deletion of files and damage to master boot records—actions sometimes described as "computer killing." High-profile examples include the attack on Sony Pictures Entertainment in November 2014, which involved malware known as Shamoon (also referred to as W32.Disttrack), and a similarstrike against Saudi Aramco in August 2012.<ref>{{cite web |title=Shamoon is latest malware to target energy sector |url=http://www.computerweekly.com/news/2240161674/Shamoon-is-latest-malware-to-target-energy-sector |url-status=dead |archive-url=https://web.archive.org/web/20150923222648/http://www.computerweekly.com/news/2240161674/Shamoon-is-latest-malware-to-target-energy-sector |archive-date=23 September 2015 |access-date=18 February 2015}}</ref><ref>{{cite web |title=Computer-killing malware used in Sony attack a wake-up call |url=http://www.computerweekly.com/news/2240235919/Computer-killing-malware-used-in-Sony-attack-a-wake-up-call-to-business?asrc=EM_MDN_37122786 |url-status=dead |archive-url=https://web.archive.org/web/20171207140213/http://www.computerweekly.com/news/2240235919/Computer-killing-malware-used-in-Sony-attack-a-wake-up-call-to-business?asrc=EM_MDN_37122786 |archive-date=7 December 2017 |access-date=18 February 2015}}</ref>

In 2024, a botnet owner was arrested for engaging in a pay-per-install operation for financial gain.<ref>{{cite web |date=29 May 2024 |title=Office of Public Affairs | 911 S5 Botnet Dismantled and Its Administrator Arrested in Coordinated International Operation | United States Department of Justice |url=https://www.justice.gov/archives/opa/pr/911-s5-botnet-dismantled-and-its-administrator-arrested-coordinated-international-operation}}</ref>

==Types== Malware can be classified in numerous ways, and certain malicious programs may fall into two or more categories simultaneously.{{cn|date=March 2026}} Broadly, software can be categorised into three types:<ref name="Molina-Coronado 2023">{{Cite journal |last1=Molina-Coronado |first1=Borja |last2=Mori |first2=Usue |last3=Mendiburu |first3=Alexander |last4=Miguel-Alonso |first4=Jose |date=2023-01-01 |title=Towards a fair comparison and realistic evaluation framework of android malware detectors based on static analysis and machine learning |url=https://www.sciencedirect.com/science/article/pii/S0167404822003881 |url-status=live |journal=Computers & Security |language=en |volume=124 |arxiv=2205.12569 |doi=10.1016/j.cose.2022.102996 |issn=0167-4048 |s2cid=252734950 |archive-url=https://web.archive.org/web/20230110063747/https://www.sciencedirect.com/science/article/pii/S0167404822003881 |archive-date=10 January 2023 |access-date=10 January 2023 |article-number=102996}}</ref> (i) goodware; (ii) grayware and (iii) malware. {|class="wikitable" |+Classification of potentially malicious software<br/>Data sourced from: Molina-Coronado et al. (2023)<ref name="Molina-Coronado 2023" /> !Type !Characteristics !Examples |- |Goodware |Obtained from trustworthy source |{{Plainlist|* Google Play apps * Buggy software}} |- |Grayware |Insufficient consensus or metrics |{{Plainlist|* Potentially unwanted programs * Spyware * Adware}} |- |Malware |Broad consensus among antivirus software that program is malicious or obtained from flagged sources. |{{Plainlist|* Viruses * Worms * Root kits * Backdoors * Ransomware * Trojan horses}} |}

===Malware===

====Virus==== {{Main|Computer virus}} thumb|Output of the MS-DOS "Kuku" virus A computer virus is software usually hidden within another seemingly harmless 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).<ref>{{cite web |title=What are viruses, worms, and Trojan horses? |url=https://kb.iu.edu/d/aehm |url-status=live |archive-url=https://web.archive.org/web/20160904162213/https://kb.iu.edu/d/aehm |archive-date=4 September 2016 |access-date=23 February 2015 |website=Indiana University |publisher=The Trustees of Indiana University}}</ref> They have been likened to biological viruses.<ref name="Cani 2014" /> 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.<ref name="Szor2005">{{cite book |author=Peter Szor |url=https://books.google.com/books?id=XE-ddYF6uhYC&pg=PT204 |title=The Art of Computer Virus Research and Defense |date=3 February 2005 |publisher=Pearson Education |isbn=978-0-672-33390-3 |page=204}}</ref> 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.

====Worm==== [[File:Blaster hex dump.png|thumb|Hex dump of the Blaster worm, showing a message left for Microsoft co-founder Bill Gates by the worm's programmer]] A worm is a stand-alone malware software that {{em|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.<ref>{{cite encyclopedia |title=computer virus – Encyclopædia Britannica |encyclopedia=Britannica.com |url=https://www.britannica.com/EBchecked/topic/130688/computer-virus |access-date=28 April 2013 |url-status=live |archive-url=https://web.archive.org/web/20130513221333/http://www.britannica.com/EBchecked/topic/130688/computer-virus |archive-date=13 May 2013}}</ref>

====Rootkits==== {{Main|Rootkit}}

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.<ref>{{cite web |last=McDowell |first=Mindi |title=Understanding Hidden Threats: Rootkits and Botnets |url=http://www.us-cert.gov/ncas/tips/ST06-001 |url-status=dead |archive-url=https://web.archive.org/web/20170329025139/https://www.us-cert.gov/ncas/tips/ST06-001 |archive-date=29 March 2017 |access-date=6 February 2013 |publisher=US-CERT}}</ref>

Some types of harmful software contain routines to evade identification 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:

{{blockquote|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.<ref>{{cite web |title=The Meaning of 'Hack' |url=http://catb.org/jargon/html/meaning-of-hack.html |url-status=live |archive-url=https://web.archive.org/web/20161013133924/http://www.catb.org/jargon/html/meaning-of-hack.html |archive-date=13 October 2016 |access-date=15 April 2010 |publisher=Catb.org}}</ref>}}

====Backdoors==== {{Main|Backdoor (computing)}}

A backdoor is a broad term for a computer program that allows an attacker persistent unauthorised remote access to a victim's machine often without their knowledge.<ref name="Gill 2022">{{Citation |last=Gill |first=Harjeevan |title=Malware: Types, Analysis and Classifications |date=2022-06-21 |url=https://engrxiv.org/preprint/view/2423 |access-date=2024-06-22 |language=en |doi=10.31224/2423 |doi-access=free}}</ref> The attacker typically uses another attack (such as a trojan, worm or virus) to bypass authentication mechanisms usually over an unsecured network such as the Internet to install the backdoor application. A backdoor can also be a side effect of a software bug in legitimate software that is exploited by an attacker to gain access to a victim's computer or network.

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.<ref>{{cite news |last=Staff |first=SPIEGEL |date=2013-12-29 |title=Inside TAO: Documents Reveal Top NSA Hacking Unit |url=http://www.spiegel.de/international/world/the-nsa-uses-powerful-toolbox-in-effort-to-spy-on-global-networks-a-940969-3.html |url-status=live |archive-url=https://web.archive.org/web/20170420112316/http://www.spiegel.de/international/world/the-nsa-uses-powerful-toolbox-in-effort-to-spy-on-global-networks-a-940969-3.html |archive-date=20 April 2017 |access-date=23 January 2014 |newspaper=Spiegel Online |publisher=SPIEGEL}}</ref> Backdoors may be installed by Trojan horses, worms, implants, or other methods.<ref>{{cite web |last=Edwards |first=John |title=Top Zombie, Trojan Horse and Bot Threats |url=http://www.itsecurity.com/features/top-zombie-trojan-bots-092507 |url-status=dead |archive-url=https://web.archive.org/web/20170209142725/http://www.itsecurity.com/features/top-zombie-trojan-bots-092507/ |archive-date=9 February 2017 |access-date=25 September 2007 |publisher=IT Security}}</ref><ref>{{cite news |last=Appelbaum |first=Jacob |date=2013-12-29 |title=Shopping for Spy Gear:Catalog Advertises NSA Toolbox |url=http://www.spiegel.de/international/world/catalog-reveals-nsa-has-back-doors-for-numerous-devices-a-940994.html |url-status=live |archive-url=https://web.archive.org/web/20170420112319/http://www.spiegel.de/international/world/catalog-reveals-nsa-has-back-doors-for-numerous-devices-a-940994.html |archive-date=20 April 2017 |access-date=29 December 2013 |newspaper=Spiegel Online |publisher=SPIEGEL}}</ref>

====Trojan horse==== {{Main|Trojan Horse (Computing)}} A Trojan horse 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.<ref>{{Cite conference |last=Landwehr |first=C. E |author2=A. R Bull |author3=J. P McDermott |author4=W. S Choi |year=1993 |title=A taxonomy of computer program security flaws, with examples |url=https://apps.dtic.mil/sti/pdfs/ADA465587.pdf |publisher=DTIC Document |archive-url=https://web.archive.org/web/20130408133230/http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA465587 |archive-date=8 April 2013 |access-date=5 April 2012 |url-status=live}}</ref><ref>{{Cite web |title=Trojan Horse: [coined By MIT-hacker-turned-NSA-spook Dan Edwards] N. |url=http://www.anvari.org/fortune/Miscellaneous_Collections/291162_trojan-horse-coined-by-mit-hacker-turned-nsa-spook-dan-edwards-n.html |url-status=live |archive-url=https://web.archive.org/web/20170705103553/http://www.anvari.org/fortune/Miscellaneous_Collections/291162_trojan-horse-coined-by-mit-hacker-turned-nsa-spook-dan-edwards-n.html |archive-date=5 July 2017 |access-date=5 April 2012}}</ref>

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.<ref>{{cite web |title=What is the difference between viruses, worms, and Trojan horses? |url=http://www.symantec.com/business/support/index?page=content&id=TECH98539 |url-status=dead |archive-url=https://web.archive.org/web/20150213213523/http://www.symantec.com/business/support/index?page=content&id=TECH98539 |archive-date=13 February 2015 |access-date=10 January 2009 |publisher=Symantec Corporation}}</ref> 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 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.<ref>{{Cite web |date=9 October 1995 |title=VIRUS-L/comp.virus Frequently Asked Questions (FAQ) v2.00 (Question B3: What is a Trojan Horse?) |url=http://www.faqs.org/faqs/computer-virus/faq/ |url-status=live |archive-url=https://web.archive.org/web/20150924041119/http://www.faqs.org/faqs/computer-virus/faq/ |archive-date=24 September 2015 |access-date=13 September 2012}}</ref> Modern Trojans are often disguised within legitimate-looking applications, making them particularly effective at bypassing basic user awareness and simple antivirus measures.

In spring 2017, Mac users were hit by the new version of Proton Remote Access Trojan (RAT)<ref>{{cite web |date=14 March 2017 |title=Proton Mac Trojan Has Apple Code Signing Signatures Sold to Customers for $50k |url=http://appleinsider.com/articles/17/03/14/proton-mac-trojan-has-apple-code-signing-signatures-sold-to-customers-for-50k |url-status=live |archive-url=https://web.archive.org/web/20171019163245/http://appleinsider.com/articles/17/03/14/proton-mac-trojan-has-apple-code-signing-signatures-sold-to-customers-for-50k |archive-date=19 October 2017 |access-date=19 October 2017 |publisher=AppleInsider}}</ref> trained to extract password data from various sources, such as browser auto-fill data, the Mac-OS keychain, and password vaults.<ref>{{cite web |date=24 August 2017 |title=Non-Windows Malware |url=https://betanews.com/2017/08/24/non-windows-malware |url-status=live |archive-url=https://web.archive.org/web/20171020033721/https://betanews.com/2017/08/24/non-windows-malware/ |archive-date=20 October 2017 |access-date=19 October 2017 |publisher=Betanews}}</ref>

====Droppers==== {{Main|Dropper (malware)}}

Droppers are a sub-type of Trojans that solely aim to deliver malware upon the system that they infect with the desire to subvert detection through stealth and a light payload.<ref>{{cite web |date=2020-01-30 |title=Trojan Dropper |url=https://www.malwarebytes.com/blog/threats/trojan-dropper |url-status=live |archive-url=https://web.archive.org/web/20221031235424/https://www.malwarebytes.com/blog/threats/trojan-dropper |archive-date=31 October 2022 |access-date=31 October 2022 |publisher=MalwareBytes}}</ref> It is important not to confuse a dropper with a loader or stager. A loader or stager will merely load an extension of the malware (for example a collection of malicious functions through reflective dynamic link library injection) into memory. The purpose is to keep the initial stage light and undetectable. A dropper merely downloads further malware to the system.

====Ransomware==== {{Main|Ransomware}} 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.<ref>{{Cite journal |last1=Richardson |first1=Ronny |last2=North |first2=Max |date=2017-01-01 |title=Ransomware: Evolution, Mitigation and Prevention |url=https://digitalcommons.kennesaw.edu/facpubs/4276 |url-status=live |journal=International Management Review |volume=13 |issue=1 |pages=10–21 |archive-url=https://web.archive.org/web/20221005110429/https://digitalcommons.kennesaw.edu/facpubs/4276/ |archive-date=5 October 2022 |access-date=23 November 2019}}</ref> 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.<ref>{{cite news |last=Fruhlinger |first=Josh |date=2017-08-01 |title=The 5 biggest ransomware attacks of the last 5 years |url=https://www.csoonline.com/article/3212260/ransomware/the-5-biggest-ransomware-attacks-of-the-last-5-years.html |url-status=dead |archive-url=https://web.archive.org/web/20180324041022/https://www.csoonline.com/article/3212260/ransomware/the-5-biggest-ransomware-attacks-of-the-last-5-years.html |archive-date=24 March 2018 |access-date=2018-03-23 |publisher=CSO}}</ref>

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.<ref>{{cite web |title=Rise of Android Ransomware, research |url=https://www.welivesecurity.com/wp-content/uploads/2016/02/Rise_of_Android_Ransomware.pdf |url-status=live |archive-url=https://web.archive.org/web/20171019221531/https://www.welivesecurity.com/wp-content/uploads/2016/02/Rise_of_Android_Ransomware.pdf |archive-date=19 October 2017 |access-date=19 October 2017 |publisher=ESET}}</ref> Jisut and SLocker impact Android devices more than other lock-screens, with Jisut making up nearly 60 percent of all Android ransomware detections.<ref>{{cite web |title=State of Malware, research |url=https://www.malwarebytes.com/pdf/white-papers/stateofmalware.pdf |url-status=dead |archive-url=https://web.archive.org/web/20170521075657/https://www.malwarebytes.com/pdf/white-papers/stateofmalware.pdf |archive-date=21 May 2017 |access-date=19 October 2017 |publisher=Malwarebytes}}</ref>

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 ad 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.<ref name="w174">{{cite journal |last1=O'Kane |first1=Philip |last2=Sezer |first2=Sakir |last3=Carlin |first3=Domhnall |date=2018 |title=Evolution of ransomware |journal=IET Networks |volume=7 |issue=5 |pages=321–327 |doi=10.1049/iet-net.2017.0207 |issn=2047-4954}}</ref>

According to Microsoft's Digital Crimes Unit in May 2025, Lumma Stealer ("Lumma"), which steals passwords, credit cards, bank accounts, and cryptocurrency wallets, is the favored info-stealing malware used by hundreds of cyber threat actors and enables criminals to empty bank accounts, hold schools for ransom, and disrupt critical services.<ref>{{cite news |last=Masada |first=Steven |date=21 May 2025 |title=Disrupting Lumma Stealer: Microsoft leads global action against favored cybercrime tool |url=https://blogs.microsoft.com/on-the-issues/2025/05/21/microsoft-leads-global-action-against-favored-cybercrime-tool/ |archive-url=https://web.archive.org/web/20250523194945/https://blogs.microsoft.com/on-the-issues/2025/05/21/microsoft-leads-global-action-against-favored-cybercrime-tool/ |archive-date=23 May 2025 |access-date=23 May 2025 |work=Microsoft}}</ref>

====Click fraud==== 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.<ref>{{cite web |title=Another way Microsoft is disrupting the malware ecosystem |url=http://blogs.technet.com/b/mmpc/archive/2012/11/29/another-way-microsoft-is-disrupting-the-malware-ecosystem.aspx |url-status=dead |archive-url=https://web.archive.org/web/20150920143940/http://blogs.technet.com/b/mmpc/archive/2012/11/29/another-way-microsoft-is-disrupting-the-malware-ecosystem.aspx |archive-date=20 September 2015 |access-date=18 February 2015}}</ref>

===Grayware=== {{See also|Privacy-invasive software|Potentially unwanted program}}

Grayware is any unwanted application or file that can worsen the performance of computers and may cause security risks but which there is insufficient consensus or data to classify them as malware.<ref name="Molina-Coronado 2023" /> Types of grayware typically include spyware, adware, fraudulent dialers, joke programs ("jokeware") and remote access tools.<ref name="Gill 2022" /> For example, at one point, Sony BMG compact discs silently installed a rootkit on purchasers' computers with the intention of preventing illicit copying.<ref name="russinovich">{{cite web |last=Russinovich |first=Mark |date=31 October 2005 |title=Sony, Rootkits and Digital Rights Management Gone Too Far |url=http://blogs.technet.com/markrussinovich/archive/2005/10/31/sony-rootkits-and-digital-rights-management-gone-too-far.aspx |url-status=dead |archive-url=https://web.archive.org/web/20120602231838/http://blogs.technet.com/b/markrussinovich/archive/2005/10/31/sony-rootkits-and-digital-rights-management-gone-too-far.aspx |archive-date=2 June 2012 |access-date=29 July 2009 |work=Mark's Blog |publisher=Microsoft MSDN}}</ref>

====Potentially unwanted program==== Potentially unwanted programs (PUPs) are applications that would be considered unwanted despite often being intentionally downloaded by the user.<ref>{{cite web |date=2009-12-15 |title=Rating the best anti-malware solutions |url=https://arstechnica.com/security/2009/12/av-comparatives-picks-eight-antipua-winners/ |url-status=live |archive-url=https://web.archive.org/web/20140202092753/http://arstechnica.com/security/2009/12/av-comparatives-picks-eight-antipua-winners/ |archive-date=2 February 2014 |access-date=28 January 2014 |publisher=Arstechnica}}</ref> PUPs include spyware, adware, and fraudulent dialers.

Many security products classify unauthorised key generators as PUPs, although they frequently carry true malware in addition to their ostensible purpose.<ref name="Kammerstetter 2012">{{Cite book |last1=Kammerstetter |first1=Markus |title=Proceedings of the 2012 ACM conference on Computer and communications security |last2=Platzer |first2=Christian |last3=Wondracek |first3=Gilbert |date=2012-10-16 |publisher=Association for Computing Machinery |isbn=978-1-4503-1651-4 |series=CCS '12 |location=New York, NY, USA |pages=809–820 |chapter=Vanity, cracks and malware |doi=10.1145/2382196.2382282 |chapter-url=https://doi.org/10.1145/2382196.2382282 |s2cid=3423843}}</ref> In fact, Kammerstetter et al. (2012)<ref name="Kammerstetter 2012" /> estimated that as much as 55% of key generators could contain malware and that about 36% malicious key generators were not detected by antivirus software.

====Adware==== Some types of adware turn off anti-malware and virus protection; technical remedies are available.<ref name="Casey">{{cite web |last1=Casey |first1=Henry T. |date=25 November 2015 |title=Latest adware disables antivirus software |url=https://www.yahoo.com/tech/s/latest-adware-disables-antivirus-software-152920421.html |url-status=live |archive-url=https://web.archive.org/web/20151127070904/https://www.yahoo.com/tech/s/latest-adware-disables-antivirus-software-152920421.html |archive-date=27 November 2015 |access-date=25 November 2015 |work=Tom's Guide |publisher=Yahoo.com}}</ref>

====Spyware==== 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.<ref>{{cite web |title=Peer To Peer Information |url=http://oit.ncsu.edu/resnet/p2p |url-status=dead |archive-url=https://web.archive.org/web/20150702143115/http://oit.ncsu.edu/resnet/p2p |archive-date=2 July 2015 |access-date=25 March 2011 |publisher=NORTH CAROLINA STATE UNIVERSITY}}</ref> The Sony BMG rootkit was intended to prevent illicit copying; but also reported on users' listening habits, and unintentionally created extra security vulnerabilities.<ref name="russinovich" />

==Detection== Antivirus software typically uses two techniques to detect malware: (i) static analysis and (ii) dynamic/heuristic analysis.<ref name="Si̇ngh 2018">{{Cite journal |last1=Si̇ngh |first1=Jagsir |last2=Si̇ngh |first2=Jaswinder |date=2018-09-01 |title=Challenge of Malware Analysis: Malware obfuscation Techniques |url=https://dergipark.org.tr/en/pub/ijiss/issue/67171/1048753 |url-status=live |journal=International Journal of Information Security Science |language=en |volume=7 |issue=3 |pages=100–110 |archive-url=https://web.archive.org/web/20230110063749/https://dergipark.org.tr/en/pub/ijiss/issue/67171/1048753 |archive-date=10 January 2023 |access-date=10 January 2023}}</ref> Static analysis involves studying the software code of a potentially malicious program and producing a signature of that program. This information is then used to compare scanned files by an antivirus program. Because this approach is not useful for malware that has not yet been studied, antivirus software can use dynamic analysis to monitor how the program runs on a computer and block it if it performs unexpected activity.

The aim of any malware is to conceal itself from detection by users or antivirus software.{{cn|date=March 2026}} Detecting potential malware is difficult for two reasons. The first is that it is difficult to determine if software is malicious.<ref name="Molina-Coronado 2023" /> The second is that malware uses technical measures to make it more difficult to detect it.<ref name="Si̇ngh 2018" /> An estimated 33% of malware is not detected by antivirus software.<ref name="Kammerstetter 2012" />

The most commonly employed anti-detection technique involves encrypting the malware payload in order to prevent antivirus software from recognizing the signature.<ref name="Molina-Coronado 2023" /> Tools such as crypters come with an encrypted blob of malicious code and a decryption stub. The stub decrypts the blob and loads it into memory. Because antivirus does not typically scan memory and only scans files on the drive, this allows the malware to evade detection. Advanced malware has the ability to transform itself into different variations, making it less likely to be detected due to the differences in its signatures. This is known as polymorphic malware. Other common techniques used to evade detection include, from common to uncommon:<ref name="Tripwire 2015" /> (1) evasion of analysis and detection by fingerprinting the environment when executed;<ref>{{cite conference |last1=Kirat |first1=Dhilung |last2=Vigna |first2=Giovanni |last3=Kruegel |first3=Christopher |date=2014 |title=Barecloud: bare-metal analysis-based evasive malware detection |url=https://dl.acm.org/citation.cfm?id=2671244 |publisher=ACM |pages=287–301 |isbn=978-1-931971-15-7 |url-access=subscription |archive-url=https://web.archive.org/web/20190801213541/https://dl.acm.org/citation.cfm?id=2671244 |archive-date=1 August 2019 |access-date=28 November 2018 |url-status=live}} <br /> Freely accessible at: {{cite web|title=Barecloud: bare-metal analysis-based evasive malware detection|url=https://seclab.cs.ucsb.edu/media/uploads/papers/kirat_barecloud_usenix_2014.pdf|access-date=28 November 2018|archive-date=4 March 2016|archive-url=https://web.archive.org/web/20160304013726/https://seclab.cs.ucsb.edu/media/uploads/papers/kirat_barecloud_usenix_2014.pdf|url-status=dead}}</ref> (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;<ref name="Tripwire 2015">[http://www.tripwire.com/state-of-security/security-data-protection/the-four-most-common-evasive-techniques-used-by-malware/ The Four Most Common Evasive Techniques Used by Malware] {{Webarchive|url=https://web.archive.org/web/20210529160838/https://www.tripwire.com/state-of-security/security-data-protection/the-four-most-common-evasive-techniques-used-by-malware/|date=29 May 2021}}. 27 April 2015.</ref> (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;<ref>{{cite conference |last1=Young |first1=Adam |last2=Yung |first2=Moti |date=1997 |title=Deniable Password Snatching: On the Possibility of Evasive Electronic Espionage |publisher=IEEE |pages=224–235 |isbn=0-8186-7828-3 |book-title=Symp. on Security and Privacy}}</ref> (v) information hiding techniques, namely stegomalware;<ref>{{cite journal |last1=Cabaj |first1=Krzysztof |last2=Caviglione |first2=Luca |last3=Mazurczyk |first3=Wojciech |last4=Wendzel |first4=Steffen |last5=Woodward |first5=Alan |last6=Zander |first6=Sebastian |date=May 2018 |title=The New Threats of Information Hiding: The Road Ahead |journal=IT Professional |volume=20 |issue=3 |pages=31–39 |arxiv=1801.00694 |bibcode=2018ITPro..20c..31C |doi=10.1109/MITP.2018.032501746 |s2cid=22328658}}</ref> and (5) fileless malware which runs within memory instead of using files and utilizes existing system tools to carry out malicious acts. The use of existing binaries to carry out malicious activities is a technique known as LotL, or Living off the Land.<ref>{{Cite journal |last1=Sudhakar |last2=Kumar |first2=Sushil |date=2020-01-14 |title=An emerging threat Fileless malware: a survey and research challenges |journal=Cybersecurity |volume=3 |issue=1 |doi=10.1186/s42400-019-0043-x |issn=2523-3246 |s2cid=257111442 |doi-access=free |article-number=1}}</ref> This reduces the amount of forensic artifacts available to analyze. 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.<ref>{{Cite journal |title=Penn State WebAccess Secure Login |url=https://webaccess.psu.edu/?cosign-scripts.libraries.psu.edu&https%3A%2F%2Fscripts.libraries.psu.edu%2Fscripts%2Fezproxyauth.php%3Furl=ezp.2aHR0cHM6Ly9kbC5hY20ub3JnL2RvaS8xMC4xMTQ1LzMzNjUwMDE- |url-status=dead |doi=10.1145/3365001 |s2cid=219884145 |archive-url=https://web.archive.org/web/20210308133613/https://webaccess.psu.edu/?cosign-scripts.libraries.psu.edu&https%3A%2F%2Fscripts.libraries.psu.edu%2Fscripts%2Fezproxyauth.php%3Furl=ezp.2aHR0cHM6Ly9kbC5hY20ub3JnL2RvaS8xMC4xMTQ1LzMzNjUwMDE- |archive-date=8 March 2021 |access-date=2020-02-29 |website=webaccess.psu.edu}}</ref><ref>{{cite arXiv | last1=Afianian | first1=Amir | last2=Niksefat | first2=Salman | last3=Sadeghiyan | first3=Babak | last4=Baptiste | first4=David | title=Malware Dynamic Analysis Evasion Techniques: A Survey | date=2018 | class=cs.CR | eprint=1811.01190 }}</ref>

==Risks==

===Vulnerable software=== 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 earlier contained a vulnerability that allowed attackers to inject code into Windows.<ref>{{cite book |last1=Nemeth |first1=Zoltan L. |title=2015 IEEE 13th International Symposium on Intelligent Systems and Informatics (SISY) |year=2015 |isbn=978-1-4673-9388-1 |pages=275–280 |chapter=Modern binary attacks and defences in the windows environment — Fighting against microsoft EMET in seven rounds |doi=10.1109/SISY.2015.7325394 |chapter-url=https://doi.org/10.1109/SISY.2015.7325394 |s2cid=18914754}}</ref> 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<ref>{{cite web |date=November 2012 |title=Global Web Browser... Security Trends |url=http://www.kaspersky.com/images/Kaspersky_Report_Browser_Usage_ENG_Final.pdf |url-status=dead |archive-url=https://web.archive.org/web/20130202153249/http://www.kaspersky.com/images/Kaspersky_Report_Browser_Usage_ENG_Final.pdf |archive-date=2 February 2013 |access-date=17 January 2013 |publisher=Kaspersky lab}}</ref>), or in vulnerable versions of browser plugins such as Adobe Flash Player, Adobe Acrobat or Reader, or Java SE.<ref>{{cite web |last=Rashid |first=Fahmida Y. |date=27 November 2012 |title=Updated Browsers Still Vulnerable to Attack if Plugins Are Outdated |url=http://securitywatch.pcmag.com/none/305385-updated-browsers-still-vulnerable-to-attack-if-plugins-are-outdated |url-status=dead |archive-url=https://web.archive.org/web/20160409063012/http://securitywatch.pcmag.com/none/305385-updated-browsers-still-vulnerable-to-attack-if-plugins-are-outdated |archive-date=9 April 2016 |access-date=17 January 2013 |publisher=pcmag.com}}</ref><ref>{{cite web |last=Danchev |first=Dancho |date=18 August 2011 |title=Kaspersky: 12 different vulnerabilities detected on every PC |url=http://www.zdnet.com/blog/security/kaspersky-12-different-vulnerabilities-detected-on-every-pc/9283 |url-status=dead |archive-url=https://web.archive.org/web/20140705182539/http://www.zdnet.com/blog/security/kaspersky-12-different-vulnerabilities-detected-on-every-pc/9283 |archive-date=5 July 2014 |access-date=17 January 2013 |publisher=pcmag.com}}</ref> 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 from 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.<ref name="Kim 2018" /> 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.

There are several ways the users can stay informed and protected from security vulnerabilities in software. Software providers often announce updates that address security issues.<ref>{{cite web |title=Adobe Security bulletins and advisories |url=https://www.adobe.com/support/security/ |url-status=live |archive-url=https://web.archive.org/web/20131115002036/http://www.adobe.com/support/security/ |archive-date=15 November 2013 |access-date=19 January 2013 |publisher=Adobe.com}}</ref> Common vulnerabilities are assigned unique identifiers (CVE IDs) and listed in public databases like the National Vulnerability Database. Tools like Secunia PSI,<ref>{{cite magazine |last=Rubenking |first=Neil J. |title=Secunia Personal Software Inspector 3.0 Review & Rating |url=https://www.pcmag.com/article2/0,2817,2406767,00.asp |url-status=live |archive-url=https://web.archive.org/web/20130116064450/http://www.pcmag.com/article2/0,2817,2406767,00.asp |archive-date=16 January 2013 |access-date=19 January 2013 |magazine=PCMag.com}}</ref> free for personal use, can scan a computer for outdated software with known vulnerabilities and attempt to update them. Firewalls and intrusion prevention systems can monitor the network traffic for suspicious activity that might indicate an attack.<ref>{{Cite book |last1=Morales |first1=Jose Andre |title=Security and Privacy in Communication Networks |last2=Al-Bataineh |first2=Areej |last3=Xu |first3=Shouhuai |last4=Sandhu |first4=Ravi |date=2010 |publisher=Springer |isbn=978-3-642-16161-2 |editor-last=Jajodia |editor-first=Sushil |series=Lecture Notes of the Institute for Computer Sciences, Social Informatics and Telecommunications Engineering |volume=50 |location=Berlin, Heidelberg |pages=20–34 |language=en |chapter=Analyzing and Exploiting Network Behaviors of Malware |doi=10.1007/978-3-642-16161-2_2 |access-date=2 December 2021 |editor2-last=Zhou |editor2-first=Jianying |chapter-url=https://link.springer.com/chapter/10.1007/978-3-642-16161-2_2 |archive-url=https://web.archive.org/web/20211202085918/https://link.springer.com/chapter/10.1007/978-3-642-16161-2_2 |archive-date=2 December 2021 |url-status=live}}</ref>

===Excessive privileges=== 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.<ref>{{Cite book |last1=Felt |first1=Adrienne Porter |author1-link=Adrienne Porter Felt |title=Proceedings of the 18th ACM conference on Computer and communications security |last2=Chin |first2=Erika |last3=Hanna |first3=Steve |last4=Song |first4=Dawn |last5=Wagner |first5=David |date=2011-10-17 |publisher=Association for Computing Machinery |isbn=978-1-4503-0948-6 |series=CCS '11 |location=New York, NY, USA |pages=627–638 |chapter=Android permissions demystified |doi=10.1145/2046707.2046779 |chapter-url=https://doi.org/10.1145/2046707.2046779 |s2cid=895039}}</ref> 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.<ref>{{Cite book |last1=Wu |first1=Sha |title=ICC 2019 - 2019 IEEE International Conference on Communications (ICC) |last2=Liu |first2=Jiajia |date=May 2019 |isbn=978-1-5386-8088-9 |pages=1–6 |chapter=Overprivileged Permission Detection for Android Applications |doi=10.1109/ICC.2019.8761572 |s2cid=198168673}}</ref>

Some systems allow all users to make changes to the core components or settings of the system, which is considered over-privileged access today. 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.<ref>{{Cite web |title=Malware, viruses, worms, Trojan horses and spyware |url=https://list.ercacinnican.tk/ |url-status=dead |archive-url=https://web.archive.org/web/20210205072901/https://list.ercacinnican.tk/ |archive-date=5 February 2021 |access-date=2020-11-14 |website=list.ercacinnican.tk}}</ref> This can be because users tend to demand more privileges than they need, so often end up being assigned unnecessary privileges.<ref>{{Citation |last1=Mutch |first1=John |last2=Anderson |first2=Brian |title=The Hard and Soft Cost of Apathy |date=2011 |work=Preventing Good People from doing Bad Things: Implementing Least Privilege |pages=163–175 |editor-last=Mutch |editor-first=John |editor2-last=Anderson |editor2-first=Brian |url=https://doi.org/10.1007/978-1-4302-3922-2_10 |access-date=2021-12-02 |archive-url=https://web.archive.org/web/20230227061951/https://link.springer.com/chapter/10.1007/978-1-4302-3922-2_10 |archive-date=27 February 2023 |url-status=live |place=Berkeley, CA |publisher=Apress |language=en |doi=10.1007/978-1-4302-3922-2_10 |isbn=978-1-4302-3922-2 |url-access=subscription}}</ref>

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.{{Citation needed|date=July 2024|reason=This last sweeping statement needs a citation.}}

===Weak passwords=== A credential attack occurs when a user account with administrative privileges is cracked and that account is used to provide malware with appropriate privileges.<ref>{{Cite book |last1=Singh |first1=Vaishali |title=Rising Threats in Expert Applications and Solutions |last2=Pandey |first2=S. K. |date=2021 |publisher=Springer |isbn=978-981-15-6014-9 |editor-last=Rathore |editor-first=Vijay Singh |series=Advances in Intelligent Systems and Computing |volume=1187 |location=Singapore |pages=339–350 |language=en |chapter=Revisiting Cloud Security Attacks: Credential Attack |doi=10.1007/978-981-15-6014-9_39 |access-date=2 December 2021 |editor2-last=Dey |editor2-first=Nilanjan |editor3-last=Piuri |editor3-first=Vincenzo |editor4-last=Babo |editor4-first=Rosalina |editor5-last=Polkowski |editor5-first=Zdzislaw |editor6-last=Tavares |editor6-first=João Manuel R. S. |chapter-url=https://link.springer.com/chapter/10.1007/978-981-15-6014-9_39 |archive-url=https://web.archive.org/web/20220304031316/https://link.springer.com/chapter/10.1007/978-981-15-6014-9_39 |archive-date=4 March 2022 |url-status=live |s2cid=224940546}}</ref> 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.

===Use of the same operating system=== 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:<ref name="UKan">"LNCS 3786 – Key Factors Influencing Worm Infection", U. Kanlayasiri, 2006, web (PDF): [https://doi.org/10.1007%2F11604938_5 SL40-PDF] {{Webarchive|url=https://web.archive.org/web/20230227061952/https://link.springer.com/chapter/10.1007/11604938_5|date=27 February 2023}}.</ref> 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.<ref>{{Cite web |last=Cohen |first=Jason |date=2020-08-28 |title=Windows Computers Account for 83% of All Malware Attacks in Q1 2020 |url=https://au.pcmag.com/encryption/68294/windows-computers-account-for-83-of-all-malware-attacks-in-q1-2020 |url-status=live |archive-url=https://web.archive.org/web/20211202085917/https://au.pcmag.com/encryption/68294/windows-computers-account-for-83-of-all-malware-attacks-in-q1-2020 |archive-date=2 December 2021 |access-date=2021-12-02 |website=PCMag Australia |language=en-au}}</ref> This risk is mitigated by segmenting the networks into different subnetworks and setting up firewalls to block traffic between them.<ref>{{Cite book |last1=Wagner |first1=Neal |title=2016 IEEE Symposium Series on Computational Intelligence (SSCI) |last2=Şahin |first2=Cem Ş. |last3=Winterrose |first3=Michael |last4=Riordan |first4=James |last5=Pena |first5=Jaime |last6=Hanson |first6=Diana |last7=Streilein |first7=William W. |date=December 2016 |isbn=978-1-5090-4240-1 |pages=1–10 |chapter=Towards automated cyber decision support: A case study on network segmentation for security |doi=10.1109/SSCI.2016.7849908 |s2cid=9065830}}</ref><ref>{{Cite book |last1=Hemberg |first1=Erik |title=Proceedings of the Genetic and Evolutionary Computation Conference Companion |last2=Zipkin |first2=Joseph R. |last3=Skowyra |first3=Richard W. |last4=Wagner |first4=Neal |last5=O'Reilly |first5=Una-May |date=2018-07-06 |publisher=Association for Computing Machinery |isbn=978-1-4503-5764-7 |series=GECCO '18 |location=New York, NY, USA |pages=1648–1655 |chapter=Adversarial co-evolution of attack and defense in a segmented computer network environment |doi=10.1145/3205651.3208287 |chapter-url=https://doi.org/10.1145/3205651.3208287 |s2cid=51603533}}</ref>

==Mitigation==

===Antivirus / Anti-malware software=== 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) provide real-time protection. The Windows Malicious Software Removal Tool removes malicious software from the system.<ref>{{cite web |title=Malicious Software Removal Tool |url=http://www.microsoft.com/security/pc-security/malware-removal.aspx |url-status=dead |archive-url=https://web.archive.org/web/20120621103611/http://www.microsoft.com/security/pc-security/malware-removal.aspx |archive-date=21 June 2012 |access-date=21 June 2012 |publisher=Microsoft}}</ref> Additionally, several capable antivirus software programs are available for free download from the Internet (usually restricted to non-commercial use).<ref name="PCmag">{{cite web |last=Rubenking |first=Neil J. |date=22 January 2025 |title=The Best Free Antivirus Software for 2025 |url=https://www.pcmag.com/picks/the-best-free-antivirus-protection |url-status=live |archive-url=https://web.archive.org/web/20250212195340/https://www.pcmag.com/picks/the-best-free-antivirus-protection |archive-date=12 February 2025 |access-date=18 February 2025}}</ref> Tests found some free programs to be competitive with commercial ones.<ref name="PCmag" /><ref>{{cite news |title=Free antivirus profiles in 2025 |url=https://www.antivirusgratis.org |url-status=live |archive-url=https://web.archive.org/web/20250117052335/https://www.antivirusgratis.org/ |archive-date=17 January 2025 |access-date=18 February 2025 |website=antivirusgratis.org |language=es}}</ref><ref>{{cite web |title=Quickly identify malware running on your PC |url=https://www.techadvisor.co.uk/download/security/crowdinspect-1500-3329721 |url-status=dead |archive-url=https://web.archive.org/web/20180902220617/https://www.techadvisor.co.uk/download/security/crowdinspect-1500-3329721/ |archive-date=2 September 2018 |access-date=2 September 2018 |website=techadvisor.co.uk}}</ref>

Typically, antivirus software can combat malware in the following ways: # '''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. # '''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.<ref>{{cite web |title=How Antivirus Software Works? |url=https://antivirus.comodo.com/how-antivirus-software-works.php |url-status=live |archive-url=https://web.archive.org/web/20170112193703/https://antivirus.comodo.com/how-antivirus-software-works.php |archive-date=12 January 2017 |access-date=16 October 2015}}</ref>{{Failed verification|date=July 2024|reason=These statements are not matching the info provided by the reference, although they might still be accurate.}} #'''Sandboxing:''' Sandboxing confines applications within a controlled environment, restricting their operations and isolating them from other applications on the host while limiting access to system resources.<ref name="Souppaya 2013">{{Cite report |url=https://csrc.nist.gov/pubs/sp/800/83/r1/final |title=Guide to Malware Incident Prevention and Handling for Desktops and Laptops |last1=Souppaya |first1=Murugiah |last2=Scarfone |first2=Karen |date=2013-07-22 |publisher=National Institute of Standards and Technology |issue=NIST Special Publication (SP) 800-83 Rev. 1 |language=en}}</ref> Browser sandboxing isolates web processes to prevent malware and exploits, enhancing security.<ref name="g370" />

====Real-time protection==== 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 is 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.<ref>{{Cite journal |last1=Al-Saleh |first1=Mohammed Ibrahim |last2=Espinoza |first2=Antonio M. |last3=Crandall |first3=Jedediah R. |date=2013 |title=Antivirus performance characterisation: system-wide view |journal=IET Information Security |language=en |volume=7 |issue=2 |pages=126–133 |doi=10.1049/iet-ifs.2012.0192 |issn=1751-8717 |doi-access=free}}</ref>

====Sandboxing==== Sandboxing is a security model that confines applications within a controlled environment, restricting their operations to authorized "safe" actions and isolating them from other applications on the host. It also limits access to system resources like memory and the file system to maintain isolation.<ref name="Souppaya 2013" />

Browser sandboxing is a security measure that isolates web browser processes and tabs from the operating system to prevent malicious code from exploiting vulnerabilities. It helps protect against malware, zero-day exploits, and unintentional data leaks by trapping potentially harmful code within the sandbox. It involves creating separate processes, limiting access to system resources, running web content in isolated processes, monitoring system calls, and memory constraints. Inter-process communication (IPC) is used for secure communication between processes. Escaping the sandbox involves targeting vulnerabilities in the sandbox mechanism or the operating system's sandboxing features.<ref name="g370">{{cite web |date=2024-02-19 |title=What is Browser Sandboxing? |url=https://www.geeksforgeeks.org/what-is-browser-sandboxing/ |url-status=live |archiveurl=https://web.archive.org/web/20240707050014/https://www.geeksforgeeks.org/what-is-browser-sandboxing/ |archivedate=2024-07-07 |access-date=2024-07-07 |website=GeeksforGeeks}}</ref><ref name="a944">{{cite web |date=2024-06-15 |title=What is browser sandboxing? How to escape the sandbox? |url=https://misile00.github.io/notes/Browser-Sandboxing |url-status=live |archiveurl=https://web.archive.org/web/20240424000722/https://misile00.github.io/notes/Browser-Sandboxing |archivedate=2024-04-24 |access-date=2024-07-07 |website=misile00's personal website}}</ref>

While sandboxing is not foolproof, it significantly reduces the attack surface of common threats. Keeping browsers and operating systems updated is crucial to mitigate vulnerabilities.<ref name="g370" /><ref name="a944" />

===Website security scans=== Website vulnerability scans check the website, detect malware, may note outdated software, and may report known security issues, in order to reduce the risk of the site being compromised.

===Network Segregation=== Structuring a network as a set of smaller networks, and limiting the flow of traffic between them to that known to be legitimate, can hinder the ability of infectious malware to replicate itself across the wider network. Software-defined networking provides techniques to implement such controls.

==="Air gap" isolation or "parallel network"=== As a last resort, computers can be protected from malware, and the risk of infected computers disseminating trusted information can be greatly reduced by imposing an "air gap" (i.e. completely disconnecting them from all other networks) and applying enhanced controls over the entry and exit of software and data from the outside world. However, malware can still cross the air gap in some situations, not least due to the need to introduce software into the air-gapped network and can damage the availability or integrity of assets thereon. Stuxnet is an example of malware that is introduced to the target environment via a USB drive, causing damage to processes supported on the environment without the need to exfiltrate data.

AirHopper,<ref name="z758">{{cite conference |last1=Guri |first1=Mordechai |last2=Kedma |first2=Gabi |last3=Kachlon |first3=Assaf |last4=Elovici |first4=Yuval |date=2014 |title=2014 9th International Conference on Malicious and Unwanted Software: The Americas (MALWARE) |publisher=IEEE |pages=58–67 |arxiv=1411.0237 |doi=10.1109/MALWARE.2014.6999418 |isbn=978-1-4799-7329-3 |chapter=AirHopper: Bridging the air-gap between isolated networks and mobile phones using radio frequencies}}</ref> BitWhisper,<ref name="s209">{{cite conference |last1=Guri |first1=Mordechai |last2=Monitz |first2=Matan |last3=Mirski |first3=Yisroel |last4=Elovici |first4=Yuval |date=2015 |title=2015 IEEE 28th Computer Security Foundations Symposium |publisher=IEEE |pages=276–289 |arxiv=1503.07919 |doi=10.1109/CSF.2015.26 |isbn=978-1-4673-7538-2 |chapter=BitWhisper: Covert Signaling Channel between Air-Gapped Computers Using Thermal Manipulations}}</ref> GSMem<ref>{{cite conference |last1=Guri |first1=Mordechai |last2=Kachlon |first2=Assaf |last3=Hasson |first3=Ofer |last4=Kedma |first4=Gabi |last5=Mirsky |first5=Yisroel |last6=Elovici |first6=Yuval |date=2015 |title=GSMem: Data Exfiltration from Air-Gapped Computers over GSM Frequencies |url=https://www.usenix.org/system/files/sec15-paper-guri-update_v2.pdf |publisher=USENIX Security Symposium |isbn=978-1-939133-11-3 |url-status=live |archiveurl=https://web.archive.org/web/20240301215837/https://www.usenix.org/system/files/sec15-paper-guri-update_v2.pdf |archivedate=2024-03-01}}</ref> and Fansmitter<ref>{{Cite arXiv |eprint=1606.05915 |class=cs.CR |first1=Michael |last1=Hanspach |first2=Michael |last2=Goetz |title=Fansmitter: Acoustic Data Exfiltration from (Speakerless) Air-Gapped Computers |last3=Daidakulov |first3=Andrey |last4=Elovici |first4=Yuval |year=2016}}</ref> are four techniques introduced by researchers that can leak data from air-gapped computers using electromagnetic, thermal and acoustic emissions.

==Research== Utilizing bibliometric analysis, the study of malware research trends from 2005 to 2015, considering criteria such as impact journals, highly cited articles, research areas, number of publications, keyword frequency, institutions, and authors, revealed an annual growth rate of 34.1%. North America led in research output, followed by Asia and Europe. China and India were identified as emerging contributors.<ref>{{Cite journal |last1=Razak |first1=Mohd Faizal Ab |last2=Anuar |first2=Nor Badrul |last3=Salleh |first3=Rosli |last4=Firdaus |first4=Ahmad |date=2016-11-01 |title=The rise of "malware": Bibliometric analysis of malware study |url=https://www.sciencedirect.com/science/article/pii/S1084804516301904 |journal=Journal of Network and Computer Applications |volume=75 |pages=58–76 |doi=10.1016/j.jnca.2016.08.022 |issn=1084-8045 |url-access=subscription}}</ref>

==References== {{reflist|30em}}

==External links== {{Wiktionary|malware}} {{commons category}} * [http://www.idmarch.org/document/Malware Further Reading: Research Papers and Documents about Malware on IDMARCH (Int. Digital Media Archive)] {{Webarchive|url=https://web.archive.org/web/20160922195926/http://www.idmarch.org/document/Malware |date=22 September 2016 }} * [https://technet.microsoft.com/en-us/sysinternals/Video/gg618529 Advanced Malware Cleaning] – a Microsoft video {{Malware}} {{Software distribution}} {{Information security}}

Category:Malware Category:Computer programming Category:Cybercrime Category:Security breaches