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Available in 16 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-16" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">16 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Polimorfismo_(malware)" title="Polimorfismo (malware) – Spanish" lang="es" hreflang="es" data-title="Polimorfismo (malware)" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%DA%A9%D8%AF_%DA%86%D9%86%D8%AF%D8%B1%DB%8C%D8%AE%D8%AA" title="کد چندریخت – Persian" lang="fa" hreflang="fa" data-title="کد چندریخت" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Virus_polymorphe" title="Virus polymorphe – French" lang="fr" hreflang="fr" data-title="Virus polymorphe" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/%EB%8B%A4%ED%98%95%EC%84%B1_%EC%BD%94%EB%93%9C" title="다형성 코드 – Korean" lang="ko" hreflang="ko" data-title="다형성 코드" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/Virus_polimorfo" title="Virus polimorfo – Italian" lang="it" hreflang="it" data-title="Virus polimorfo" data-language-autonym="Italiano" data-language-local-name="Italian" class="interlanguage-link-target"><span>Italiano</span></a></li><li class="interlanguage-link interwiki-hu mw-list-item"><a href="https://hu.wikipedia.org/wiki/Polimorf_k%C3%B3d" title="Polimorf kód – Hungarian" lang="hu" hreflang="hu" data-title="Polimorf kód" data-language-autonym="Magyar" data-language-local-name="Hungarian" class="interlanguage-link-target"><span>Magyar</span></a></li><li class="interlanguage-link interwiki-ms mw-list-item"><a href="https://ms.wikipedia.org/wiki/Kod_polimorf" title="Kod polimorf – Malay" lang="ms" hreflang="ms" data-title="Kod polimorf" data-language-autonym="Bahasa Melayu" data-language-local-name="Malay" class="interlanguage-link-target"><span>Bahasa Melayu</span></a></li><li class="interlanguage-link interwiki-mn mw-list-item"><a href="https://mn.wikipedia.org/wiki/%D0%9F%D0%BE%D0%BB%D0%B8%D0%BC%D0%BE%D1%80%D1%84%D0%B8%D0%BA" title="Полиморфик – Mongolian" lang="mn" hreflang="mn" data-title="Полиморфик" data-language-autonym="Монгол" data-language-local-name="Mongolian" class="interlanguage-link-target"><span>Монгол</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/%E3%83%9D%E3%83%AA%E3%83%A2%E3%83%AB%E3%83%95%E3%82%A3%E3%83%83%E3%82%AF%E3%82%B3%E3%83%BC%E3%83%89" title="ポリモルフィックコード – Japanese" lang="ja" hreflang="ja" data-title="ポリモルフィックコード" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/Kod_polimorficzny" title="Kod polimorficzny – Polish" lang="pl" hreflang="pl" data-title="Kod polimorficzny" data-language-autonym="Polski" data-language-local-name="Polish" class="interlanguage-link-target"><span>Polski</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/%D0%9F%D0%BE%D0%BB%D0%B8%D0%BC%D0%BE%D1%80%D1%84%D0%B8%D0%B7%D0%BC_%D0%BA%D0%BE%D0%BC%D0%BF%D1%8C%D1%8E%D1%82%D0%B5%D1%80%D0%BD%D1%8B%D1%85_%D0%B2%D0%B8%D1%80%D1%83%D1%81%D0%BE%D0%B2" title="Полиморфизм компьютерных вирусов – Russian" lang="ru" hreflang="ru" data-title="Полиморфизм компьютерных вирусов" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-simple mw-list-item"><a href="https://simple.wikipedia.org/wiki/Polymorphic_code" title="Polymorphic code – Simple English" lang="en-simple" hreflang="en-simple" data-title="Polymorphic code" data-language-autonym="Simple English" data-language-local-name="Simple English" class="interlanguage-link-target"><span>Simple English</span></a></li><li class="interlanguage-link interwiki-sk mw-list-item"><a href="https://sk.wikipedia.org/wiki/Polymorfn%C3%BD_v%C3%ADrus" title="Polymorfný vírus – Slovak" lang="sk" hreflang="sk" data-title="Polymorfný vírus" data-language-autonym="Slovenčina" data-language-local-name="Slovak" class="interlanguage-link-target"><span>Slovenčina</span></a></li><li class="interlanguage-link interwiki-sv mw-list-item"><a href="https://sv.wikipedia.org/wiki/Polymorfisk_kod" title="Polymorfisk kod – Swedish" lang="sv" hreflang="sv" data-title="Polymorfisk kod" data-language-autonym="Svenska" data-language-local-name="Swedish" class="interlanguage-link-target"><span>Svenska</span></a></li><li class="interlanguage-link interwiki-uk mw-list-item"><a href="https://uk.wikipedia.org/wiki/%D0%9F%D0%BE%D0%BB%D1%96%D0%BC%D0%BE%D1%80%D1%84%D0%BD%D0%B8%D0%B9_%D0%BA%D0%BE%D0%B4" title="Поліморфний код – Ukrainian" lang="uk" hreflang="uk" data-title="Поліморфний код" data-language-autonym="Українська" data-language-local-name="Ukrainian" class="interlanguage-link-target"><span>Українська</span></a></li><li class="interlanguage-link interwiki-vi mw-list-item"><a href="https://vi.wikipedia.org/wiki/Virus_%C4%91a_h%C3%ACnh" title="Virus đa hình – Vietnamese" lang="vi" hreflang="vi" data-title="Virus đa hình" data-language-autonym="Tiếng Việt" data-language-local-name="Vietnamese" class="interlanguage-link-target"><span>Tiếng Việt</span></a></li> </ul> <div class="after-portlet after-portlet-lang"><span class="wb-langlinks-edit wb-langlinks-link"><a href="https://www.wikidata.org/wiki/Special:EntityPage/Q950981#sitelinks-wikipedia" title="Edit interlanguage links" class="wbc-editpage">Edit links</a></span></div> </div> </div> </div> </header> <div class="vector-page-toolbar"> <div class="vector-page-toolbar-container"> <div id="left-navigation"> <nav aria-label="Namespaces"> <div id="p-associated-pages" class="vector-menu vector-menu-tabs mw-portlet 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<div class="vector-body-before-content"> <div class="mw-indicators"> </div> <div id="siteSub" class="noprint">From Wikipedia, the free encyclopedia</div> </div> <div id="contentSub"><div id="mw-content-subtitle"></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Self-modifying program code designed to defeat anti-virus programs or reverse engineering</div> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">Not to be confused with <a href="/wiki/Polymorphism_(computer_science)" title="Polymorphism (computer science)">Polymorphism (computer science)</a>.</div> <style data-mw-deduplicate="TemplateStyles:r1251242444">.mw-parser-output .ambox{border:1px solid #a2a9b1;border-left:10px solid #36c;background-color:#fbfbfb;box-sizing:border-box}.mw-parser-output .ambox+link+.ambox,.mw-parser-output .ambox+link+style+.ambox,.mw-parser-output .ambox+link+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+style+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+link+.ambox{margin-top:-1px}html body.mediawiki .mw-parser-output .ambox.mbox-small-left{margin:4px 1em 4px 0;overflow:hidden;width:238px;border-collapse:collapse;font-size:88%;line-height:1.25em}.mw-parser-output .ambox-speedy{border-left:10px solid #b32424;background-color:#fee7e6}.mw-parser-output .ambox-delete{border-left:10px solid #b32424}.mw-parser-output .ambox-content{border-left:10px solid #f28500}.mw-parser-output .ambox-style{border-left:10px solid #fc3}.mw-parser-output .ambox-move{border-left:10px solid #9932cc}.mw-parser-output .ambox-protection{border-left:10px solid #a2a9b1}.mw-parser-output .ambox .mbox-text{border:none;padding:0.25em 0.5em;width:100%}.mw-parser-output .ambox .mbox-image{border:none;padding:2px 0 2px 0.5em;text-align:center}.mw-parser-output .ambox .mbox-imageright{border:none;padding:2px 0.5em 2px 0;text-align:center}.mw-parser-output .ambox .mbox-empty-cell{border:none;padding:0;width:1px}.mw-parser-output .ambox .mbox-image-div{width:52px}@media(min-width:720px){.mw-parser-output .ambox{margin:0 10%}}@media print{body.ns-0 .mw-parser-output .ambox{display:none!important}}</style><table class="box-More_citations_needed plainlinks metadata ambox ambox-content ambox-Refimprove" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><a href="/wiki/File:Question_book-new.svg" class="mw-file-description"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/50px-Question_book-new.svg.png" decoding="async" width="50" height="39" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/75px-Question_book-new.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/100px-Question_book-new.svg.png 2x" data-file-width="512" data-file-height="399" /></a></span></div></td><td class="mbox-text"><div class="mbox-text-span">This article <b>needs additional citations for <a href="/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability">verification</a></b>.<span class="hide-when-compact"> Please help <a href="/wiki/Special:EditPage/Polymorphic_code" title="Special:EditPage/Polymorphic code">improve this article</a> by <a href="/wiki/Help:Referencing_for_beginners" title="Help:Referencing for beginners">adding citations to reliable sources</a>. Unsourced material may be challenged and removed.<br /><small><span class="plainlinks"><i>Find sources:</i> <a rel="nofollow" class="external text" href="https://www.google.com/search?as_eq=wikipedia&q=%22Polymorphic+code%22">"Polymorphic code"</a> – <a rel="nofollow" class="external text" href="https://www.google.com/search?tbm=nws&q=%22Polymorphic+code%22+-wikipedia&tbs=ar:1">news</a> <b>·</b> <a rel="nofollow" class="external text" href="https://www.google.com/search?&q=%22Polymorphic+code%22&tbs=bkt:s&tbm=bks">newspapers</a> <b>·</b> <a rel="nofollow" class="external text" href="https://www.google.com/search?tbs=bks:1&q=%22Polymorphic+code%22+-wikipedia">books</a> <b>·</b> <a rel="nofollow" class="external text" href="https://scholar.google.com/scholar?q=%22Polymorphic+code%22">scholar</a> <b>·</b> <a rel="nofollow" class="external text" href="https://www.jstor.org/action/doBasicSearch?Query=%22Polymorphic+code%22&acc=on&wc=on">JSTOR</a></span></small></span> <span class="date-container"><i>(<span class="date">November 2010</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table> <p>In computing, <b>polymorphic code</b> is code that uses a <a href="/wiki/Polymorphic_engine" title="Polymorphic engine">polymorphic engine</a> to mutate while keeping the original <a href="/wiki/Algorithm" title="Algorithm">algorithm</a> intact - that is, the <i>code</i> changes itself every time it runs, but the <i>function</i> of the code (its <a href="/wiki/Semantics" title="Semantics">semantics</a>) stays the same. For example, the simple math expressions 3+1 and 6-2 both achieve the same result, yet run with different <a href="/wiki/Machine_code" title="Machine code">machine code</a> in a <a href="/wiki/Central_processing_unit" title="Central processing unit">CPU</a>. This technique is sometimes used by <a href="/wiki/Computer_virus" title="Computer virus">computer viruses</a>, <a href="/wiki/Shellcode" title="Shellcode">shellcodes</a> and <a href="/wiki/Computer_worm" title="Computer worm">computer worms</a> to hide their presence.<sup id="cite_ref-rugha_1-0" class="reference"><a href="#cite_note-rugha-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p><p><a href="/wiki/Encryption" title="Encryption">Encryption</a> is the most common method to hide code. With encryption, the main body of the code (also called its <a href="/wiki/Payload_(computing)" title="Payload (computing)">payload</a>) is encrypted and will appear meaningless. For the code to function as before, a decryption function is added to the code. When the code is <i>executed</i>, this function reads the payload and decrypts it before executing it in turn. </p><p>Encryption alone is not polymorphism. To gain polymorphic behavior, the encryptor/decryptor pair is mutated with each copy of the code. This allows different versions of some code which all function the same.<sup id="cite_ref-wongstamp_2-0" class="reference"><a href="#cite_note-wongstamp-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Malicious_code">Malicious code</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Polymorphic_code&action=edit&section=1" title="Edit section: Malicious code"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Most <a href="/wiki/Anti-virus_software" class="mw-redirect" title="Anti-virus software">anti-virus software</a> and <a href="/wiki/Intrusion_detection_system" title="Intrusion detection system">intrusion detection systems</a> (IDS) attempt to locate malicious code by searching through computer files and data packets sent over a <a href="/wiki/Computer_network" title="Computer network">computer network</a>. If the security software finds patterns that correspond to known computer viruses or worms, it takes appropriate steps to neutralize the threat. Polymorphic algorithms make it difficult for such software to recognize the offending code because it constantly mutates. </p><p>Malicious <a href="/wiki/Programmer" title="Programmer">programmers</a> have sought to protect their encrypted code from this virus-scanning strategy by rewriting the unencrypted decryption engine (and the resulting encrypted payload) each time the virus or worm is propagated. Anti-virus software uses sophisticated pattern analysis to find underlying patterns within the different mutations of the decryption engine, in hopes of reliably detecting such <a href="/wiki/Malware" title="Malware">malware</a>. </p><p>Emulation may be used to defeat polymorphic obfuscation by letting the malware demangle itself in a virtual environment before utilizing other methods, such as traditional signature scanning. Such a virtual environment is sometimes called a <a href="/wiki/Sandbox_(computer_security)" title="Sandbox (computer security)">sandbox</a>. Polymorphism does not protect the virus against such emulation if the decrypted payload remains the same regardless of variation in the decryption algorithm. <a href="/wiki/Metamorphic_code" title="Metamorphic code">Metamorphic code</a> techniques may be used to complicate detection further, as the virus may execute without ever having identifiable code blocks in memory that remains constant from infection to infection. </p><p>The first known polymorphic virus was written by Mark Washburn. The virus, called <a href="/wiki/1260_(computer_virus)" title="1260 (computer virus)">1260</a>, was written in 1990. A better-known polymorphic virus was created in 1992 by the hacker <a href="/wiki/Dark_Avenger" title="Dark Avenger">Dark Avenger</a> as a means of avoiding pattern recognition from antivirus software. A common and very virulent polymorphic virus is the file infecter <a href="/wiki/Virut" title="Virut">Virut</a>. </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Polymorphic_code&action=edit&section=2" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Metamorphic_code" title="Metamorphic code">Metamorphic code</a></li> <li><a href="/wiki/Self-modifying_code" title="Self-modifying code">Self-modifying code</a></li> <li><a href="/wiki/Alphanumeric_shellcode" class="mw-redirect" title="Alphanumeric shellcode">Alphanumeric shellcode</a></li> <li><a href="/wiki/Shellcode" title="Shellcode">Shellcode</a></li> <li><a href="/wiki/Obfuscated_code" class="mw-redirect" title="Obfuscated code">Obfuscated code</a></li> <li><a href="/wiki/Oligomorphic_code" title="Oligomorphic code">Oligomorphic code</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Polymorphic_code&action=edit&section=3" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-references-wrap"><ol class="references"> <li id="cite_note-rugha-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-rugha_1-0">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite id="CITEREFRaghunathan2007" class="citation thesis cs1">Raghunathan, Srinivasan (2007). <i>Protecting anti-virus software under viral attacks</i> (M.Sc.). Arizona State University. <a href="/wiki/CiteSeerX_(identifier)" class="mw-redirect" title="CiteSeerX (identifier)">CiteSeerX</a> <span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.93.796">10.1.1.93.796</a></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Adissertation&rft.title=Protecting+anti-virus+software+under+viral+attacks&rft.inst=Arizona+State+University&rft.date=2007&rft_id=https%3A%2F%2Fciteseerx.ist.psu.edu%2Fviewdoc%2Fsummary%3Fdoi%3D10.1.1.93.796%23id-name%3DCiteSeerX&rft.aulast=Raghunathan&rft.aufirst=Srinivasan&rfr_id=info%3Asid%2Fen.wikipedia.org%3APolymorphic+code" class="Z3988"></span></span> </li> <li id="cite_note-wongstamp-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-wongstamp_2-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFWongStamp2006" class="citation journal cs1">Wong, Wing; Stamp, M. (2006). "Hunting for Metamorphic Engines". <i>Journal in Computer Virology</i>. <b>2</b> (3): <span class="nowrap">211–</span>229. <a href="/wiki/CiteSeerX_(identifier)" class="mw-redirect" title="CiteSeerX (identifier)">CiteSeerX</a> <span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.108.3878">10.1.1.108.3878</a></span>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1007%2Fs11416-006-0028-7">10.1007/s11416-006-0028-7</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:8116065">8116065</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Journal+in+Computer+Virology&rft.atitle=Hunting+for+Metamorphic+Engines&rft.volume=2&rft.issue=3&rft.pages=%3Cspan+class%3D%22nowrap%22%3E211-%3C%2Fspan%3E229&rft.date=2006&rft_id=https%3A%2F%2Fciteseerx.ist.psu.edu%2Fviewdoc%2Fsummary%3Fdoi%3D10.1.1.108.3878%23id-name%3DCiteSeerX&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A8116065%23id-name%3DS2CID&rft_id=info%3Adoi%2F10.1007%2Fs11416-006-0028-7&rft.aulast=Wong&rft.aufirst=Wing&rft.au=Stamp%2C+M.&rfr_id=info%3Asid%2Fen.wikipedia.org%3APolymorphic+code" class="Z3988"></span></span> </li> </ol></div> <style data-mw-deduplicate="TemplateStyles:r1239549316">.mw-parser-output .refbegin{margin-bottom:0.5em}.mw-parser-output .refbegin-hanging-indents>ul{margin-left:0}.mw-parser-output .refbegin-hanging-indents>ul>li{margin-left:0;padding-left:3.2em;text-indent:-3.2em}.mw-parser-output .refbegin-hanging-indents ul,.mw-parser-output .refbegin-hanging-indents ul li{list-style:none}@media(max-width:720px){.mw-parser-output .refbegin-hanging-indents>ul>li{padding-left:1.6em;text-indent:-1.6em}}.mw-parser-output .refbegin-columns{margin-top:0.3em}.mw-parser-output .refbegin-columns ul{margin-top:0}.mw-parser-output .refbegin-columns li{page-break-inside:avoid;break-inside:avoid-column}@media screen{.mw-parser-output .refbegin{font-size:90%}}</style><div class="refbegin" style=""> <ul><li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFSpinellis2003" class="citation journal cs1">Spinellis, Diomidis (January 2003). <a rel="nofollow" class="external text" href="http://www.spinellis.gr/pubs/jrnl/2002-ieeetit-npvirus/html/npvirus.html">"Reliable identification of bounded-length viruses is NP-complete"</a>. <i>IEEE Transactions on Information Theory</i>. <b>49</b> (1): <span class="nowrap">280–</span>4. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1109%2FTIT.2002.806137">10.1109/TIT.2002.806137</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=IEEE+Transactions+on+Information+Theory&rft.atitle=Reliable+identification+of+bounded-length+viruses+is+NP-complete&rft.volume=49&rft.issue=1&rft.pages=%3Cspan+class%3D%22nowrap%22%3E280-%3C%2Fspan%3E4&rft.date=2003-01&rft_id=info%3Adoi%2F10.1109%2FTIT.2002.806137&rft.aulast=Spinellis&rft.aufirst=Diomidis&rft_id=http%3A%2F%2Fwww.spinellis.gr%2Fpubs%2Fjrnl%2F2002-ieeetit-npvirus%2Fhtml%2Fnpvirus.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3APolymorphic+code" class="Z3988"></span></li></ul> </div>    </div><noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?useformat=desktop&type=1x1&usesul3=0" alt="" width="1" height="1" style="border: none; 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