Antivirus software

thumb|300x300px|[[ClamTk, an open-source antivirus based on the ClamAV antivirus engine, was originally developed by Tomasz Kojm in 2001.]]

Antivirus software (abbreviated to AV software), also known as anti-malware, is software intended to prevent, detect, and remove malware.

Antivirus software was originally developed to detect and remove computer viruses, hence the name. However, with the proliferation of other malware, antivirus software started to protect against other computer threats. Some products also include protection from malicious URLs, spam, and phishing.

History

1971–1980: Pre-antivirus

The first known computer virus appeared in 1971 and was dubbed the "Creeper virus". This computer virus infected Digital Equipment Corporation's (DEC) PDP-10 mainframe computers running the TENEX operating system.

The Creeper virus was eventually deleted by a program created by Ray Tomlinson and known as "The Reaper". Some people consider "The Reaper" the first antivirus software ever written – it may be the case, but the Reaper was actually a virus itself specifically designed to remove the Creeper virus.

The Creeper virus was followed by several other viruses. The first known that appeared "in the wild" was "Elk Cloner", in 1981, which infected Apple II computers.

In 1983, the term "computer virus" was coined by Fred Cohen in one of the first ever published academic papers on computer viruses. Cohen used the term "computer virus" to describe programs that: "affect other computer programs by modifying them in such a way as to include a (possibly evolved) copy of itself." (note that a more recent definition of computer virus has been given by the Hungarian security researcher Péter Szőr: "a code that recursively replicates a possibly evolved copy of itself").

The first IBM PC compatible "in the wild" computer virus, and one of the first real widespread infections, was "Brain" in 1986. From then, the number of viruses has grown exponentially. Most of the computer viruses written in the early and mid-1980s were limited to self-reproduction and had no specific damage routine built into the code. That changed when more and more programmers became acquainted with computer virus programming and created viruses that manipulated or even destroyed data on infected computers.

Before internet connectivity was widespread, computer viruses were typically spread by infected floppy disks. Antivirus software came into use, but was updated relatively infrequently. During this time, virus checkers essentially had to check executable files and the boot sectors of floppy disks and hard disks. However, as internet usage became common, viruses began to spread online.

1980–1990: Early days

There are competing claims for the innovator of the first antivirus product. Possibly, the first publicly documented removal of an "in the wild" computer virus (the "Vienna virus") was performed by Bernd Fix in 1987.

In 1985, in United Kingdom, Jan Hruska and Peter Lammer founded the security firm Sophos and began producing their first antivirus and encryption products. In the same period, in Hungary, VirusBuster was founded (and subsequently incorporated by Sophos).

In 1987, Andreas Lüning and Kai Figge, who founded G Data Software in 1985, released their first antivirus product for the Atari ST platform. In 1987, the Ultimate Virus Killer (UVK) was also released. This was the de facto industry standard virus killer for the Atari ST and Atari Falcon, the last version of which (version 9.0) was released in April 2004. In 1987, in the United States, John McAfee founded the McAfee company and, at the end of that year, he released the first version of VirusScan. Also in 1987 (in Czechoslovakia), Peter Paško, Rudolf Hrubý, and Miroslav Trnka created the first version of NOD antivirus.

In 1987, Fred Cohen wrote that there is no algorithm that can perfectly detect all possible computer viruses.

Finally, at the end of 1987, the first two heuristic antivirus utilities were released: Flushot Plus by Ross Greenberg and Anti4us by Erwin Lanting. In his O'Reilly book, Malicious Mobile Code: Virus Protection for Windows, Roger Grimes described Flushot Plus as "the first holistic program to fight malicious mobile code (MMC)."

However, the kind of heuristic used by early AV engines was totally different from those used today. The first product with a heuristic engine resembling modern ones was F-PROT in 1991. Early heuristic engines were based on dividing the binary into different sections: data section, code section (in a legitimate binary, it usually starts always from the same location). Indeed, the initial viruses re-organized the layout of the sections, or overrode the initial portion of a section in order to jump to the very end of the file where malicious code was located—only going back to resume execution of the original code. This was a very specific pattern, not used at the time by any legitimate software, which represented an elegant heuristic to catch suspicious code. Other kinds of more advanced heuristics were later added, such as suspicious section names, incorrect header size, regular expressions, and partial pattern in-memory matching.

In 1988, the growth of antivirus companies continued. In Germany, Tjark Auerbach founded Avira (H+BEDV at the time) and released the first version of AntiVir (named "Luke Filewalker" at the time). In Spain, Carlos Jiménez released the first version of his antivirus (he founded Anyware Seguridad Informática, now part of McAfee, one year later). In Bulgaria, Vesselin Bontchev released his first freeware antivirus program (he later joined FRISK Software). Also Frans Veldman released the first version of ThunderByte Antivirus, also known as TBAV (he sold his company to Norman Safeground in 1998). In Czechoslovakia, Pavel Baudiš and Eduard Kučera founded Avast Software (at the time ALWIL Software) and released their first version of avast! antivirus. In June 1988, in South Korea, Ahn Cheol-Soo released its first antivirus software, called V1 (he founded AhnLab later in 1995). Finally, in autumn 1988, in the United Kingdom, Alan Solomon founded S&S International and created his Dr. Solomon's Anti-Virus Toolkit (although he launched it commercially only in 1991 – in 1998 Solomon's company was acquired by McAfee, then known as Network Associates Inc.).

Also in 1988, a mailing list named VIRUS-L was started on the BITNET/EARN network where new viruses and the possibilities of detecting and eliminating viruses were discussed. Some members of this mailing list were: Alan Solomon, Eugene Kaspersky (Kaspersky Lab), Friðrik Skúlason (FRISK Software), John McAfee (McAfee), Luis Corrons (Panda Security), Mikko Hyppönen (F-Secure), Péter Szőr, Tjark Auerbach (Avira) and Vesselin Bontchev (FRISK Software). SAM 2.0, released March 1990, incorporated technology allowing users to easily update SAM to intercept and eliminate new viruses, including many that didn't exist at the time of the program's release.

1990–2000: Emergence of the antivirus industry

In 1990, in Spain, Mikel Urizarbarrena founded Panda Security (Panda Software at the time). In Hungary, the security researcher Péter Szőr released the first version of Pasteur antivirus.

In 1990, the Computer Antivirus Research Organization (CARO) was founded. In 1991, CARO released the "Virus Naming Scheme", originally written by Friðrik Skúlason and Vesselin Bontchev. Although this naming scheme is now outdated, it remains the only existing standard that most computer security companies and researchers ever attempted to adopt. CARO members includes: Alan Solomon, Costin Raiu, Dmitry Gryaznov, Eugene Kaspersky, Friðrik Skúlason, Igor Muttik, Mikko Hyppönen, Morton Swimmer, Nick FitzGerald, Padgett Peterson, Peter Ferrie, Righard Zwienenberg and Vesselin Bontchev.

In 1991, in the United States, Symantec released the first version of Norton AntiVirus. In the same year, in the Czech Republic, Jan Gritzbach and Tomáš Hofer founded AVG Technologies (Grisoft at the time) while, in Italy, Gianfranco Tonello founded TG Soft, although both companies released the first version of their anti-virus products (Anti-Virus Guard (AVG) and VirIT eXplorer) only in 1992-1993. On the other hand, in Finland, F-Secure (founded in 1988 by Petri Allas and Risto Siilasmaa – with the name of Data Fellows) released the first version of their antivirus product. F-Secure claims to be the first antivirus firm to establish a presence on the World Wide Web.

In 1991, the European Institute for Computer Antivirus Research (EICAR) was founded to further antivirus research and improve development of antivirus software.

In 1992, in Russia, Igor Danilov released the first version of SpiderWeb, which later became Dr.Web.

In 1994, AV-TEST reported that there were 28,613 unique malware samples (based on MD5) in their database.

Over time other companies were founded. In 1996, in Romania, Bitdefender was founded and released the first version of Anti-Virus eXpert (AVX). In 1997, in Russia, Eugene Kaspersky and Natalya Kaspersky co-founded security firm Kaspersky Lab.

In 1996, there was also the first "in the wild" Linux virus, known as "Staog".

In 1999, AV-TEST reported that there were 98,428 unique malware samples (based on MD5) in their database.

In 2001, Tomasz Kojm released the first version of ClamAV, the first ever open source antivirus engine to be commercialised. In 2007, ClamAV was bought by Sourcefire, which in turn was acquired by Cisco Systems in 2013.

In 2002, in United Kingdom, Morten Lund and Theis Søndergaard co-founded the antivirus firm BullGuard.

In 2005, AV-TEST reported that there were 333,425 unique malware samples (based on MD5) in their database.

Over the years it has become necessary for antivirus software to use several different strategies (e.g. specific email and network protection or low level modules) and detection algorithms, as well as to check an increasing variety of files, rather than just executables, for several reasons:

• Powerful macros used in word processor applications, such as Microsoft Word, presented a risk. Virus writers could use the macros to write viruses embedded within documents. This meant that computers could now also be at risk from infection by opening documents with hidden attached macros.
• The possibility of embedding executable objects inside otherwise non-executable file formats can make opening those files a risk.
• Later email programs, in particular Microsoft's Outlook Express and Outlook, were vulnerable to viruses embedded in the email body itself. A user's computer could be infected by just opening or previewing a message.

In 2005, F-Secure was the first security firm that developed an Anti-Rootkit technology, called BlackLight.

Because most users are usually connected to the Internet on a continual basis, Jon Oberheide first proposed a Cloud-based antivirus design in 2008.

In February 2008 McAfee Labs added the industry-first cloud-based anti-malware functionality to VirusScan under the name Artemis. It was tested by AV-Comparatives in February 2008 and officially unveiled in August 2008 in McAfee VirusScan.

Cloud AV created problems for comparative testing of security software – part of the AV definitions was out of testers control (on constantly updated AV company servers) thus making results non-repeatable. As a result, Anti-Malware Testing Standards Organisation (AMTSO) started working on method of testing cloud products which was adopted on May 7, 2009.

In 2011, AVG introduced a similar cloud service, called Protective Cloud Technology.

2014–present: Rise of next-gen, market consolidation

Following the 2013 release of the APT 1 report from Mandiant, the industry has seen a shift towards signature-less approaches to the problem capable of detecting and mitigating zero-day attacks. Numerous approaches to address these new forms of threats have appeared, including behavioral detection, artificial intelligence, machine learning, and cloud-based file detection. According to Gartner, it is expected the rise of new entrants, such Carbon Black, Cylance and Crowdstrike will force end point protection incumbents into a new phase of innovation and acquisition.

One method from Bromium involves micro-virtualization to protect desktops from malicious code execution initiated by the end user. Another approach from SentinelOne and Carbon Black focuses on behavioral detection by building a full context around every process execution path in real time, while Cylance leverages an artificial intelligence model based on machine learning.

Increasingly, these signature-less approaches have been defined by the media and analyst firms as "next-generation" antivirus and are seeing rapid market adoption as certified antivirus replacement technologies by firms such as Coalfire and DirectDefense. In response, traditional antivirus vendors such as Trend Micro, Symantec and Sophos have responded by incorporating "next-gen" offerings into their portfolios as analyst firms such as Forrester and Gartner have called traditional signature-based antivirus "ineffective" and "outdated".

As of Windows 8, Windows includes its own free antivirus protection under the Windows Defender brand. Despite bad detection scores in its early days, AV-Test now certifies Defender as one of its top products. While it isn't publicly known how the inclusion of antivirus software in Windows affected antivirus sales, Google search traffic for antivirus has declined significantly since 2010. In 2014, Intel bought McAfee.

Since 2016, there has been a notable amount of consolidation in the industry. Avast purchased AVG in 2016 for $1.3 billion. Avira was acquired by Norton owner Gen Digital (then NortonLifeLock) in 2020 for $360 million. In 2021, the Avira division of Gen Digital acquired BullGuard. The BullGuard brand was discontinued in 2022 and its customers were migrated to Norton. In 2022, Gen Digital acquired Avast, effectively consolidating four major antivirus brands under one owner.

In September 2024, following the US Commerce Department's ban on Kaspersky, Pango Group acquired its customers (about 1 million). The customers received continued services with no action required on their part. Then, in December 2024, Pango Group merged with Total Security, the provider of Total AV antivirus. The combined entity, now called Point Wild, has an enterprise value of $1.7 billion.

As of 2024, more than half of Americans use built-in antivirus protection for their devices like Microsoft Defender or XProtect from Apple. However, about 121 million adults still use third-party antivirus software. Half of these adults use paid products, and about 50% of third-party software users - the owners of personal computers and Windows operating systems. Antivirus programs on mobile devices are used by 17% of adults.

The 2025 antivirus market report confirms that most third-party antivirus users are on desktop devices, primarily aged between 35 and 45. In contrast, younger users (18–25) tend to rely on ad blockers instead. In the U.S., on average, 75–85% of people use antivirus software or some other form of protection on at least one device. Antivirus software for computers and mobile devices is predominantly used by residents of large cities. Mobile device users more often rely on password managers rather than antivirus software for digital security. Moreover, the majority of password‑manager users live in medium‑sized and small towns.

Identification methods

In 1987, Frederick B. Cohen demonstrated that the algorithm which would be able to detect all possible viruses can't possibly exist (like the algorithm which determines whether or not the given program halts). the antivirus engine can determine if the program is malicious or not. If not, then, the program is executed in the real environment. Although this technique has shown to be quite effective, given its heaviness and slowness, it is rarely used in end-user antivirus solutions.

• Data mining techniques: one of the latest approaches applied in malware detection. Data mining and machine learning algorithms are used to try to classify the behaviour of a file (as either malicious or benign) given a series of file features, that are extracted from the file itself.

Signature-based detection

Traditional antivirus software relies heavily upon signatures to identify malware.

Substantially, when a malware sample arrives in the hands of an antivirus firm, it is analysed by malware researchers or by dynamic analysis systems. Then, once it is determined to be a malware, a proper signature of the file is extracted and added to the signatures database of the antivirus software.

Although the signature-based approach can effectively contain malware outbreaks, malware authors have tried to stay a step ahead of such software by writing "oligomorphic", "polymorphic" and, more recently, "metamorphic" viruses, which encrypt parts of themselves or otherwise modify themselves as a method of disguise, so as to not match virus signatures in the dictionary.

Heuristics

Many viruses start as a single infection and through either mutation or refinements by other attackers, can grow into dozens of slightly different strains, called variants. Generic detection refers to the detection and removal of multiple threats using a single virus definition.

For example, the Vundo trojan has several family members, depending on the antivirus vendor's classification. Symantec classifies members of the Vundo family into two distinct categories, Trojan.Vundo and Trojan.Vundo.B.

While it may be advantageous to identify a specific virus, it can be quicker to detect a virus family through a generic signature or through an inexact match to an existing signature. Virus researchers find common areas that all viruses in a family share uniquely and can thus create a single generic signature. These signatures often contain non-contiguous code, using wildcard characters where differences lie. These wildcards allow the scanner to detect viruses even if they are padded with extra, meaningless code. A detection that uses this method is said to be "heuristic detection".

Rootkit detection

Anti-virus software can attempt to scan for rootkits. A rootkit is a type of malware designed to gain administrative-level control over a computer system without being detected. Rootkits can change how the operating system functions and in some cases can tamper with the anti-virus program and render it ineffective. Rootkits are also difficult to remove, in some cases requiring a complete re-installation of the operating system.

Real-time protection

Real-time protection, on-access scanning, background guard, resident shield, autoprotect, and other synonyms refer to the automatic protection provided by most antivirus, anti-spyware, and other anti-malware programs. This monitors computer systems for suspicious activity such as computer viruses, spyware, adware, and other malicious objects. Real-time protection detects threats in opened files and scans apps in real-time as they are installed on the device. When inserting a CD, opening an email, or browsing the web, or when a file already on the computer is opened or executed.

Machine learning detection

Machine learning has emerged as a core detection method in modern antivirus software, using algorithms trained on large datasets to classify software as malicious or benign. ML-based approaches are diverse, but detectors typically extract features from files, such as API call sequences, byte n-grams, opcode distributions, behavioral characteristics, or even raw bytes, and train classifiers to identify malware based on learned patterns from this data.

ML-based detection can be highly effective, but still faces significant challenges. Concept drift occurs as malware continuously evolves, causing trained models to degrade in accuracy over time without regular retraining on fresh samples. Research has demonstrated that even simple obfuscation techniques can create adversarial variants that bypass ML-based detectors while preserving malicious functionality. Additionally, the highly imbalanced nature of real-world data, where benign files vastly outnumber malicious ones, makes acquiring training data difficult and requires careful tuning to avoid unacceptable false positive rates.

Issues of concern

Unexpected renewal costs

Some commercial antivirus software end-user license agreements include a clause that the subscription will be automatically renewed, and the purchaser's credit card automatically billed, at the renewal time without explicit approval. For example, McAfee requires users to unsubscribe at least 60 days before the expiration of the present subscription, while Bitdefender sends notifications to unsubscribe 30 days before the renewal. Norton AntiVirus also renews subscriptions automatically by default.

Rogue security applications

Some apparent antivirus programs are actually malware masquerading as legitimate software, such as WinFixer, MS Antivirus, and Mac Defender.

Problems caused by false positives

A "false positive" or "false alarm" is when antivirus software identifies a non-malicious file as malware. When this happens, it can cause serious problems. For example, if an antivirus program is configured to immediately delete or quarantine infected files, as is common on Microsoft Windows antivirus applications, a false positive in an essential file can render the Windows operating system or some applications unusable. Recovering from such damage to critical software infrastructure incurs technical support costs and businesses can be forced to close whilst remedial action is undertaken.

Examples of serious false-positives:

• May 2007: a faulty virus signature issued by Symantec mistakenly removed essential operating system files, leaving thousands of PCs unable to boot.
• May 2007: the executable file required by Pegasus Mail on Windows was falsely detected by Norton AntiVirus as being a Trojan and it was automatically removed, preventing Pegasus Mail from running. Norton AntiVirus had falsely identified three releases of Pegasus Mail as malware, and would delete the Pegasus Mail installer file when that happened. In response to this Pegasus Mail stated:

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