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{{HTTP}}
 
An '''HTTP cookiescookie''' (also called '''web cookiescookie''', '''Internet cookiescookie''', '''browser cookiescookie''', or simply '''cookiescookie''') areis a small blocksblock of [[Datadata (computing)|data]] created by a [[web server]] while a [[Useruser (computing)|user]] is [[browsing]] a [[website]] and placed on the user's computer or other device by the user's [[web browser]]. Cookies are placed on the device used to access a website, and more than one cookie may be placed on a user's device during a session.
 
Cookies serve useful and sometimes essential functions on the [[World Wide Web|web]]. They enable web servers to store [[program state|stateful]] information (such as items added in the shopping cart in an [[Online shopping|online store]]) on the user's device or to track the user's browsing activity (including clicking particular buttons, [[access control|logging in]], or recording which [[Web browsing history|pages were visited in the past]]).<ref>{{Cite web|title=What are cookies? What are the differences between them (session vs. persistent)?|url=https://www.cisco.com/c/en/us/support/docs/security/web-security-appliance/117925-technote-csc-00.html|id=117925|date=2018-07-17|website=Cisco|language=en}}</ref> They can also be used to save information that the user previously entered into [[Form (HTML)|form field]]s, such as names, addresses, [[password]]s, and [[payment card number]]s for subsequent use.
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==Background==
[[File:Cookie stack.jpg|thumb|HTTP cookies share their name with [[cookie|a popular baked treat]].]]
 
===Origin of the name===
[[File:Cookie stack.jpg|thumb|HTTP cookies share their name with [[cookie|a popular baked treat]].]]
The term ''cookie'' was coined by web-browser programmer [[Lou Montulli]]. It was derived from the term ''[[magic cookie]]'', which is a packet of data a program receives and sends back unchanged, used by [[Unix]] programmers.<ref name="wdi6I">{{cite web|url=http://dominopower.com/article/where-cookie-comes-from/|title=Where cookie comes from :: DominoPower|website=dominopower.com|access-date=19 October 2017|archive-url=https://web.archive.org/web/20171019174750/http://dominopower.com/article/where-cookie-comes-from/|archive-date=19 October 2017|url-status=live}}</ref><ref name="xVtjM">{{cite web|editor-last1=Raymond|editor-first1=Eric |title=magic cookie|url=http://catb.org/jargon/html/M/magic-cookie.html|website=The Jargon File (version 4.4.7)|access-date=8 September 2017|archive-url=https://web.archive.org/web/20170906230653/http://www.catb.org/jargon/html/M/magic-cookie.html|archive-date=6 September 2017|url-status=live}}</ref>
 
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Together with John Giannandrea, Montulli wrote the initial Netscape cookie specification the same year. Version 0.9beta of [[Netscape Navigator|Mosaic Netscape]], released on October 13, 1994,<ref name="JgNeY">{{cite web |url=http://wp.netscape.com/newsref/pr/newsrelease1.html |title=Press Release: Netscape Communications Offers New Network Navigator Free On The Internet |access-date=2010-05-22 |archive-url = https://web.archive.org/web/20061207145832/http://wp.netscape.com/newsref/pr/newsrelease1.html |archive-date=2006-12-07}}</ref><ref name="8YpTv">{{cite web |url=https://groups.google.com/group/comp.infosystems.www.users/msg/9a210e5f72278328 |title=Usenet Post by Marc Andreessen: Here it is, world! |date=1994-10-13 |access-date=2010-05-22 |archive-url=https://web.archive.org/web/20110427123350/http://groups.google.com/group/comp.infosystems.www.users/msg/9a210e5f72278328 |archive-date=2011-04-27 |url-status=live}}</ref> supported cookies.<ref name="kristol" /> The first use of cookies (out of the labs) was checking whether visitors to the Netscape website had already visited the site. Montulli applied for a patent for the cookie technology in 1995, which was granted in 1998.<ref>{{Cite patent|country=US|number=5774670|pubdate=1998-06-30|title=Persistent client state in a hypertext transfer protocol based client-server system|assign1=[[Netscape Communications Corp.]]|inventor1-last=Montulli|inventor1-first=Lou}}</ref> Support for cookies was integrated with [[Internet Explorer]] in version 2, released in October 1995.<ref name="95BiI">{{cite news |first=Sandi |last=Hardmeier |url=https://www.microsoft.com/windows/IE/community/columns/historyofie.mspx |title=The history of Internet Explorer |publisher=Microsoft |date=2005-08-25 |access-date=2009-01-04 |archive-url=https://web.archive.org/web/20051001113951/http://www.microsoft.com/windows/IE/community/columns/historyofie.mspx |archive-date=2005-10-01 |url-status=live}}</ref>
 
The introduction of cookies was not widely known to the public at the time. In particular, cookies were accepted by default, and users were not notified of their presence.<ref>{{citationCite journal needed|datelast=OctoberMiyazaki 2022|reasonfirst=ThisAnthony isD. most|date=2008 likely|title=Online incorrect,Privacy asand atthe leastDisclosure Internetof ExplorerCookie hadUse: veryEffects prominenton cookieConsumer warningsTrust and requiredAnticipated explicitPatronage permission|url=http://journals.sagepub.com/doi/10.1509/jppm.27.1.19 to|journal=Journal acceptof anyPublic Policy & Marketing |language=en |volume=27 |issue=1 |pages=19–33 |doi=10.1509/jppm.27.1.19 |issn=0743-9156|url-access=subscription }}</ref> The public learned about cookies after the ''[[Financial Times]]'' published an article about them on February 12, 1996.<ref name="B3JMd">{{cite news|last=Jackson|first=T|title=This Bug in Your PC is a Smart Cookie|newspaper=Financial Times|date=1996-02-12}}</ref> In the same year, cookies received a lot of media attention, especially because of potential privacy implications. Cookies were discussed in two U.S. [[Federal Trade Commission]] hearings in 1996 and 1997.<ref name="UjTred" />
 
The development of the formal cookie specifications was already ongoing. In particular, the first discussions about a formal specification started in April 1995 on the www-talk [[electronic mailing list|mailing list]]. A special working group within the [[Internet Engineering Task Force]] (IETF) was formed. Two alternative proposals for introducing state in HTTP transactions had been proposed by [[Brian Behlendorf]] and David Kristol respectively. But the group, headed by Kristol himself and Lou Montulli, soon decided to use the Netscape specification as a starting point. In February 1996, the working group identified third-party cookies as a considerable privacy threat. The specification produced by the group was eventually published as RFC 2109 in February 1997. It specifies that third-party cookies were either not allowed at all, or at least not enabled by default.<ref name="RFC2109">{{Cite ietf|rfc=2109 |section=8.3 }}</ref> At this time, advertising companies were already using third-party cookies. The recommendation about third-party cookies of RFC 2109 was not followed by Netscape and Internet Explorer. RFC 2109 was superseded by RFC 2965 in October 2000.
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A ''persistent cookie'' expires at a specific date or after a specific length of time. For the persistent cookie's lifespan set by its creator, its information will be transmitted to the server every time the user visits the website that it belongs to, or every time the user views a resource belonging to that website from another website (such as an advertisement).
 
For this reason, persistent cookies are sometimes referred to as ''tracking cookies''<ref>{{citationCite journal |last1=Bujlow |first1=Tomasz |last2=Carela-Espanol |first2=Valentin |last3=Lee |first3=Beom-Ryeol |last4=Barlet-Ros |first4=Pere needed|date=October2017 |title=A Survey on Web Tracking: Mechanisms, Implications, and Defenses |journal=Proceedings of the IEEE |volume=105 |issue=8 |pages=1476–1510 |doi=10.1109/JPROC.2016.2637878 |issn=0018-9219|hdl=2117/108437 |hdl-access=free }}</ref><ref>{{Citation |last1=Rasaii |first1=Ali |title=Exploring the Cookieverse: A Multi-Perspective Analysis of Web Cookies |date=2023 |work=Passive and Active Measurement |volume=13882 |pages=623–651 |editor-last=Brunstrom |editor-first=Anna |url=https://link.springer.com/10.1007/978-3-031-28486-1_26 |access-date=2024-08-24 |place=Cham |publisher=Springer Nature Switzerland |language=en |doi=10.1007/978-3-031-28486-1_26 |isbn=978-3-031-28485-4 |last2=Singh |first2=Shivani |last3=Gosain |first3=Devashish |last4=Gasser |first4=Oliver |editor2-last=Flores |editor2-first=Marcel |editor3-last=Fiore |editor3-first=Marco|url-access=subscription 2022}}</ref> because they can be used by advertisers to record information about a user's web browsing habits over an extended period of time. Persistent cookies are also used for reasons such as keeping users logged into their accounts on websites, to avoid re-entering login credentials at every visit. {{Crossreference|selfref=no|(See {{section link||Uses}}, below.)}}
 
===Secure cookie===
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==={{anchor|SameSite cookie}}Same-site cookie===
In 2016 [[Google Chrome]] version 51 introduced<ref name="T8177">{{Cite web|url=https://www.chromestatus.com/feature/4672634709082112|title='SameSite' cookie attribute, Chrome Platform tatus|website=Chromestatus.com|access-date=2016-04-23|archive-url=https://web.archive.org/web/20160509064447/https://www.chromestatus.com/feature/4672634709082112|archive-date=2016-05-09|url-status=live}}</ref> a new kind of cookie with attribute <code>SameSite</code> with possible values of <code>Strict</code>, <code>Lax</code> or <code>None</code>.<ref name="oCqyo">{{Cite journal|url=https://tools.ietf.org/html/draft-ietf-httpbis-cookie-same-site-00|title=Same-Site Cookies draft-ietf-httpbis-cookie-same-site-00|last1=Goodwin|last2=West|first1=M.|newspaper=Ietf Datatracker|date=20 June 2016|access-date=2016-07-28|archive-url=https://web.archive.org/web/20160816182604/https://tools.ietf.org/html/draft-ietf-httpbis-cookie-same-site-00|archive-date=2016-08-16|url-status=live}}</ref> With attribute <code>SameSite=Strict</code>, the browsers would only send cookies to a target ___domain that is the same as the origin ___domain. This would effectively mitigate [[cross-site request forgery]] (CSRF) attacks.<ref name="wi75q">{{Cite web|title=Using the Same-Site Cookie Attribute to Prevent CSRF Attacks|url=https://www.netsparker.com/blog/web-security/same-site-cookie-attribute-prevent-cross-site-request-forgery/|access-date=2021-04-05|website=www.netsparker.com|date=23 August 2016 |language=en}}</ref> With <code>SameSite=Lax</code>, browsers would send cookies with requests to a target ___domain even it is different from the origin ___domain, but only for ''safe'' requests such as GET (POST is unsafe) and not third-party cookies (inside iframe). Attribute <code>SameSite=None</code> would allow third-party (cross-site) cookies, however, most browsers require [[#Secure cookie|secure attribute]] on SameSite=None cookies.<ref name="vQf6D">{{Cite web|title=Require "Secure" for "SameSite=None". by miketaylr · Pull Request #1323 · httpwg/http-extensions|url=https://github.com/httpwg/http-extensions/pull/1323|access-date=2021-04-05|website=GitHub|language=en}}</ref>
 
The Same-site cookie is incorporated into a new RFC draft for "Cookies: HTTP State Management Mechanism"<ref>{{Cite report |url=https://datatracker.ietf.org/doc/draft-ietf-httpbis-rfc6265bis/07/ |title=Cookies: HTTP State Management Mechanism |lastlast1=West |firstfirst1=Mike |last2=Wilander |first2=John |date=2020-12-07 |publisher=Internet Engineering Task Force |issue=draft-ietf-httpbis-rfc6265bis-07}}</ref> to update RFC 6265 (if approved).
 
Chrome, Firefox, and Edge started to support Same-site cookies.<ref name="sJzIz">{{Cite web|url=https://www.lambdatest.com/SameSite-cookie-attribute|title = Browser Compatibility Testing of 'SameSite' cookie attribute}}</ref> The key of rollout is the treatment of existing cookies without the SameSite attribute defined, Chrome has been treating those existing cookies as if SameSite=None, this would let all website/applications run as before. Google intended to change that default to <code>SameSite=Lax</code> in Chrome 80 planned to be released in February 2020,<ref name="QPJhf">{{Cite web|title=SameSite Cookie Changes in February 2020: What You Need to Know|url=https://blog.chromium.org/2020/02/samesite-cookie-changes-in-february.html|access-date=2021-04-05|website=Chromium Blog|language=en}}</ref> but due to potential for breakage of those applications/websites that rely on third-party/cross-site cookies and [[COVID-19]] circumstances, Google postponed this change to Chrome 84.<ref name="Ne4hV">{{Cite webnews|title=Temporarily rolling back SameSite Cookie Changes|url=https://blog.chromium.org/2020/04/temporarily-rolling-back-samesite.html|access-date=2021-04-05|website=Chromium Blog|language=en}}</ref><ref>{{Cite web |last=Schuh |first=Justin |date=2020-05-28 |title=Resuming SameSite Cookie Changes in July |url=https://blog.chromium.org/2020/05/resuming-samesite-cookie-changes-in-july.html |access-date=2024-02-18 |website=Chromium Blog |language=en}}</ref>
 
===Supercookie===
A ''supercookie'' is a cookie with an origin of a [[top-level ___domain]] (such as <code>.com</code>) or a public suffix (such as <code>.co.uk</code>). Ordinary cookies, by contrast, have an origin of a specific ___domain name, such as <code>example.com</code>.
 
Supercookies can be a potential security concern and are therefore often blocked by web browsers. If unblocked by the browser, an attacker in control of a malicious website could set a supercookie and potentially disrupt or impersonate legitimate user requests to another website that shares the same top-level ___domain or public suffix as the malicious website. For example, a supercookie with an origin of <code>.com</code>, could maliciously affect a request made to <code>example.com</code>, even if the cookie did not originate from <code>example.com</code>. This can be used to fake logins or change user information.
 
The [[Public Suffix List]]<ref name="oN0iG">{{cite web|url=https://publicsuffix.org/learn/|title=Learn more about the Public Suffix List|website=Publicsuffix.org|access-date=28 July 2016|archive-url=https://web.archive.org/web/20160514000450/https://publicsuffix.org/learn/|archive-date=14 May 2016|url-status=live}}</ref> helps to mitigate the risk that supercookies pose. The Public Suffix List is a cross-vendor initiative that aims to provide an accurate and up-to-date list of ___domain name suffixes. Older versions of browsers may not have an up-to-date list, and will therefore be vulnerable to supercookies from certain domains.
 
====Other uses====
The term ''supercookie'' is sometimes used for tracking technologies that do not rely on HTTP cookies. Two such ''supercookie'' mechanisms were found on Microsoft websites in August 2011: [[cookie syncing]] that respawned MUID (machine unique identifier) cookies, and [[HTTP ETag|ETag]] cookies.<ref name="Ivyb7">{{cite web |last=Mayer |first=Jonathan |date=19 August 2011 |title=Tracking the Trackers: Microsoft Advertising |url=http://cyberlaw.stanford.edu/node/6715 |publisher=The Center for Internet and Society |access-date=28 September 2011 |archive-url=https://web.archive.org/web/20110926122412/http://cyberlaw.stanford.edu/node/6715 |archive-date=26 September 2011 |url-status=live}}</ref> Due to media attention, Microsoft later disabled this code.<ref name="8LKGy">{{cite web |last=Vijayan |first=Jaikumar |title=Microsoft disables 'supercookies' used on MSN.com visitors |website=Computerworld |date=Aug 19, 2011 |url=http://www.computerworld.com/article/2510494/data-privacy/microsoft-disables--supercookies--used-on-msn-com-visitors.html |access-date=23 November 2014 |archive-url=https://web.archive.org/web/20141127185609/http://www.computerworld.com/article/2510494/data-privacy/microsoft-disables--supercookies--used-on-msn-com-visitors.html |archive-date=27 November 2014 |url-status=dead }}</ref> In a 2021 blog post, Mozilla used the term ''supercookie'' to refer to [[#Browser cache|the use of browser cache]] as a means of tracking users across sites.<ref name="PyGKV">{{cite web |date=26 January 2021 |url=https://blog.mozilla.org/security/2021/01/26/supercookie-protections/ |title=Firefox 85 Cracks Down on Supercookies |first1=Steven|last1=Englehardt|first2=Arthur|last2=Edelstein |website=Mozilla Security Blog |url-status=live |archive-url= https://web.archive.org/web/20240225171134/https://blog.mozilla.org/security/2021/01/26/supercookie-protections/ |archive-date= Feb 25, 2024 }}</ref>
 
===Zombie cookie===
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==Structure==
A cookie consists of the following components:<ref name="Peng, Weihong 2000">{{cite journal|title=HTTP Cookies, A Promising Technology|journal=ProQuest|publisher=Online Information Review|last1=Peng|first1=Weihong|last2=Cisna|first2=Jennifer|year=2000|id={{ProQuest|194487945}}}}</ref><ref name="Stenberg, Daniel 2009">Jim Manico quoting Daniel Stenberg, [http://manicode.blogspot.it/2009/08/real-world-cookie-length-limits.html Real world cookie length limits] {{Webarchive|url=https://web.archive.org/web/20130702114435/http://manicode.blogspot.it/2009/08/real-world-cookie-length-limits.html |date=2013-07-02}}</ref><ref>{{Cite journal |last1=Lee |first1=Wei-Bin |last2=Chen |first2=Hsing-Bai |last3=Chang |first3=Shun-Shyan |last4=Chen |first4=Tzung-Her |date=2019-01-25 |title=Secure and efficient protection for HTTP cookies with self-verification |url=https://onlinelibrary.wiley.com/doi/10.1002/dac.3857 |journal=International Journal of Communication Systems |language=en |volume=32 |issue=2 |pages=e3857 |doi=10.1002/dac.3857|s2cid=59524143 |url-access=subscription }}</ref>
 
# Name
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==Implementation==
[[File:HTTP cookie exchange.svg|thumb|A possible interaction between a web browser and a web server holding a web page in which the server sends a cookie to the browser and the browser sends it back when requesting another page.]]
 
Cookies are arbitrary pieces of data, usually chosen and first sent by the web server, and stored on the client computer by the web browser. The browser then sends them back to the server with every request, introducing [[state (computer science)|states]] (memory of previous events) into otherwise stateless [[HTTP]] transactions. Without cookies, each retrieval of a [[web page]] or component of a web page would be an isolated event, largely unrelated to all other page views made by the user on the website. Although cookies are usually set by the web server, they can also be set by the client using a scripting language such as [[JavaScript]] (unless the cookie's <code>HttpOnly</code> flag is set, in which case the cookie cannot be modified by scripting languages).
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The <code>Secure</code> attribute is meant to keep cookie communication limited to encrypted transmission, directing browsers to use cookies only via [[HTTPS|secure/encrypted]] connections. However, if a web server sets a cookie with a secure attribute from a non-secure connection, the cookie can still be intercepted when it is sent to the user by [[man-in-the-middle attack]]s. Therefore, for maximum security, cookies with the Secure attribute should only be set over a secure connection.
 
The <code>HttpOnly</code> attribute directs browsers not to expose cookies through channels other than HTTP (and HTTPS) requests. This means that the cookie cannot be accessed via client-side scripting languages (notably [[JavaScript]]), and therefore cannot be stolen easily via [[cross-site scripting]] (a pervasive attack technique).<ref name="Symantec-2007-2nd-exec">{{cite report |title=Symantec Internet Security Threat Report: Trends for July–December 2007 (Executive Summary) |publisher=Symantec Corp. |volume=XIII |pages=1–3 |date=April 2008 |url=http://eval.symantec.com/mktginfo/enterprise/white_papers/b-whitepaper_exec_summary_internet_security_threat_report_xiii_04-2008.en-us.pdf |access-date=May 11, 2008 |archive-url=https://web.archive.org/web/20080625065121/http://eval.symantec.com/mktginfo/enterprise/white_papers/b-whitepaper_exec_summary_internet_security_threat_report_xiii_04-2008.en-us.pdf |archive-date=June 25, 2008 |url-status=livedead}}</ref>
 
==Browser settings==
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{{as of|2014}}, some websites were setting cookies readable for over 100 third-party domains.<ref name="BYMXD">{{cite web |url=http://webcookies.org/third-party-cookies/ |title=Third party domains |publisher=WebCookies.org |access-date=2014-12-07 |archive-url=https://web.archive.org/web/20141209234746/http://webcookies.org/third-party-cookies/ |archive-date=2014-12-09 |url-status=live}}</ref> On average, a single website was setting 10 cookies, with a maximum number of cookies (first- and third-party) reaching over 800.<ref name="cOnAw">{{cite web |url=http://webcookies.org/number-of-cookies/ |title=Number of cookies |publisher=WebCookies.org |access-date=2014-12-07 |archive-url=https://web.archive.org/web/20141209235956/http://webcookies.org/number-of-cookies/ |archive-date=2014-12-09 |url-status=live}}</ref>
 
The older standards for cookies, RFC 2109<ref name="RFC2109"/> and RFC 2965, recommend that browsers should protect user privacy and not allow sharing of cookies between servers by default. However, the newer standard, RFC 6265, explicitly allows user agents to implement whichever third-party cookie policy they wish. Most modern web browsers contain [[privacy settings]] that can [[ad blocker|block]] third-party cookies. Since 2020, [[Apple Safari]],<ref name="zw6bb">{{Cite web|last=Statt|first=Nick|date=2020-03-24|title=Apple updates Safari's anti-tracking tech with full third-party cookie blocking|url=https://www.theverge.com/2020/3/24/21192830/apple-safari-intelligent-tracking-privacy-full-third-party-cookie-blocking|access-date=2020-07-24|website=The Verge|language=en}}</ref> [[Firefox]],<ref name="GSofz">{{Cite web|date=2019-06-04|title=Firefox starts blocking third-party cookies by default|url=https://venturebeat.com/2019/06/04/firefox-enhanced-tracking-protection-blocks-third-party-cookies-by-default/|access-date=2020-07-24|website=VentureBeat|language=en-US}}</ref> and [[Brave (web browser)|Brave]]<ref name="sUPt1">{{Cite web|last=Brave|date=2020-02-06|title=OK Google, don't delay real browser privacy until 2022|url=https://brave.com/ok-google/|access-date=2020-07-24|website=Brave Browser|language=en-US}}</ref> block all third-party cookies by default. Safari allows embedded sites to use Storage Access API to request permission to set first-party cookies. In May 2020, [[Google Chrome]] 83 introduced new features to block third-party cookies by default in its Incognito mode for private browsing, making blocking optional during normal browsing. The same update also added an option to block first-party cookies.<ref name="xiHRq">{{cite web |last1=Protalinski |first1=Emil |title=Chrome 83 arrives with redesigned security settings, third-party cookies blocked in Incognito |url=https://venturebeat.com/2020/05/19/google-chrome-83/ |website=VentureBeat |access-date=25 June 2020 |date=19 May 2020}}</ref> As ofIn April 2024, Chrome postponed third-party cookie blocking by default to 2025.<ref>{{Cite web |last=Amadeo |first=Ron |date=2024-04-24 |title=Google can’tcan't quit third-party cookies—delays shut down for a third time |url=https://arstechnica.com/gadgets/2024/04/google-delays-third-party-cookie-death-again-now-scheduled-for-2025/ |access-date=2024-04-25 |website=Ars Technica |language=en-us}}</ref> In July 2024, Google announced plan to avoid blocking third-party cookies by default and instead prompt users to allow third-party cookies.<ref>{{Cite web |last=Lawler |first=Richard |date=2024-07-22 |title=Google's plan to turn off third-party cookies in Chrome is dying |url=https://www.theverge.com/2024/7/22/24203893/google-cookie-tracking-prompt-ad-targeting-privacy-sandbox |access-date=2024-07-29 |website=The Verge |language=en}}</ref>
 
==Privacy==
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Website operators who do not disclose third-party cookie use to consumers run the risk of harming consumer trust if cookie use is discovered. Having clear disclosure (such as in a [[privacy policy]]) tends to eliminate any negative effects of such cookie discovery.<ref name="dCj3R">Miyazaki, Anthony D. (2008), "Online Privacy and the Disclosure of Cookie Use: Effects on Consumer Trust and Anticipated Patronage," Journal of Public Policy & Marketing, 23 (Spring), 19–33</ref>{{Failed verification|date=October 2022|reason=The article says the opposite: that apriori disclosure of cookies reduces trust, and that nobody reads privacy policies anyway.}}
 
The [[United States]] government has set strict rules on setting cookies in 2000 after it was disclosed that the White House [[Office of National Drug Control Policy|drug policy office]] used cookies to track computer users viewing its online anti-drug advertising. In 2002, privacy activist Daniel Brandt found that the [[Central Intelligence Agency|CIA]] had been leaving persistent cookies on computers that had visited its website. When notified it was violating policy, CIA stated that these cookies were not intentionally set and stopped setting them. On December 25, 2005, Brandt discovered that the [[National Security Agency]] (NSA) had been leaving two persistent cookies on visitors' computers due to a software upgrade. After being informed, the NSA immediately disabled the cookies.<ref name="BnfI7">{{cite news |url=https://www.nytimes.com/2005/12/29/national/29cookies.html |newspaper=New York Times |title=Spy Agency Removes Illegal Tracking Files |date=2005-12-29 |access-date=2017-02-19 |archive-url=https://web.archive.org/web/20111112115242/http://www.nytimes.com/2005/12/29/national/29cookies.html |archive-date=2011-11-12 |url-status=live}}</ref>
 
===EU cookie directive===
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In 2009, the law was amended by Directive 2009/136/EC, which included a change to Article 5, Paragraph 3. Instead of having an option for users to opt out of cookie storage, the revised Directive requires consent to be obtained for cookie storage.<ref name="ICO reference" /> The definition of consent is cross-referenced to the definition in European data protection law, firstly the Data Protection Directive 1995 and subsequently the [[General Data Protection Regulation]] (GDPR). As the definition of consent was strengthened in the text of the GDPR, this had the effect of increasing the quality of consent required by those storing and accessing information such as cookies on users devices. In a case decided under the Data Protection Directive however, the [[Court of Justice of the European Union]] later confirmed however that the previous law implied the same strong quality of consent as the current instrument.<ref name="eur-lex.europa.eu">{{Cite web|title=EUR-Lex - 62017CN0673 - EN - EUR-Lex|url=https://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX:62017CN0673|access-date=2021-06-06|website=eur-lex.europa.eu}}</ref> In addition to the requirement of consent which stems from storing or accessing information on a user's terminal device, the information in many cookies will be considered personal data under the GDPR alone, and will require a legal basis to process. This has been the case since the 1995 Data Protection Directive, which used an identical definition of personal data, although the GDPR in interpretative Recital 30 clarifies that cookie identifiers are included. While not all data processing under the GDPR requires consent, the characteristics of behavioural advertising mean that it is difficult or impossible to justify under any other ground.<ref name="Veale">{{Citation |last1=Veale|first1=Michael|last2=Zuiderveen Borgesius|first2=Frederik|date=2021-04-01|title=Adtech and Real-Time Bidding under European Data Protection Law|url=https://osf.io/wg8fq|doi=10.31235/osf.io/wg8fq|s2cid=243311598|doi-access=free|hdl=2066/253518|hdl-access=free}}</ref><ref>{{Cite journal|last=Zuiderveen Borgesius|first=Frederik J.|date=August 2015|title=Personal data processing for behavioural targeting: which legal basis?|journal=International Data Privacy Law|language=en|volume=5|issue=3|pages=163–176|doi=10.1093/idpl/ipv011|issn=2044-3994|doi-access=free}}</ref>
 
Consent under the combination of the GDPR and e-Privacy Directive has to meet a number of conditions in relation to cookies.<ref name=":0">{{Cite book|last1=Nouwens|first1=Midas|last2=Liccardi|first2=Ilaria|last3=Veale|first3=Michael|last4=Karger|first4=David|last5=Kagal|first5=Lalana|title=Proceedings of the 2020 CHI Conference on Human Factors in Computing Systems |chapter=Dark Patterns after the GDPR: Scraping Consent Pop-ups and Demonstrating their Influence |date=2020-04-21|chapter-url=https://dl.acm.org/doi/10.1145/3313831.3376321|series=Chi '20|language=en|___location=Honolulu HI USA|publisher=ACM|pages=1–13|doi=10.1145/3313831.3376321|arxiv=2001.02479|isbn=978-1-4503-6708-0|hdl=1721.1/129999|s2cid=210064317|hdl-access=free}}</ref> It must be freely given and unambiguous: preticked boxes were banned under both the Data Protection Directive 1995<ref name="eur-lex.europa.eu"/> and the GDPR (Recital 32).<ref name=":1">{{Cite web|title=EUR-Lex - 32016R0679 - EN - EUR-Lex|url=https://eur-lex.europa.eu/eli/reg/2016/679/oj|access-date=2021-06-06|website=eur-lex.europa.eu|language=en}}</ref> The GDPR is specific that consent must be as 'easy to withdraw as to give',<ref name=":1" /> meaning that a reject-all button must be as easy to access in terms of clicks and visibility as an 'accept all' button.<ref name=":0" /> It must be specific and informed, meaning that consent relates to particular purposes for the use of this data, and all organisations seeking to use this consent must be specifically named.<ref name=":2">{{Cite book|last=Information Commissioner's Office|url=https://cy.ico.org.uk/media/about-the-ico/documents/2615156/adtech-real-time-bidding-report-201906-dl191220.pdf|title=Update Report into Adtech and Real Time Bidding|year=2019|archive-url=https://web.archive.org/web/20210513192000/https://cy.ico.org.uk/media/about-the-ico/documents/2615156/adtech-real-time-bidding-report-201906-dl191220.pdf|archive-date=2021-05-13|url-status=live}}</ref><ref>{{Cite web|url=https://www.legifrance.gouv.fr/jorf/id/JORFTEXT000038783337|access-date=2021-06-06|title=Délibération n° 2019-093 du 4 juillet 2019 portant adoption de lignes directrices relatives à l'application de l'article 82 de la loi du 6 janvier 1978 modifiée aux opérations de lecture ou écriture dans le terminal d'un utilisateur (notamment aux cookies et autres traceurs) (rectificatif)|website=www.legifrance.gouv.fr}}</ref> The [[Court of Justice of the European Union]] has also ruled that consent must be 'efficient and timely', meaning that it must be gained before cookies are laid and data processing begins instead of afterwards.<ref>{{Cite web|title=EUR-Lex - 62017CC0040 - EN - EUR-Lex|url=https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:62017CC0040|access-date=2021-06-06|website=eur-lex.europa.eu}}</ref>
 
The industry's response has been largely negative. Robert Bond of the law firm Speechly Bircham describes the effects as "far-reaching and incredibly onerous" for "all UK companies". Simon Davis of [[Privacy International]] argues that proper enforcement would "destroy the entire industry".<ref name="sKaxf">{{cite magazine|title=EU cookie law: stop whining and just get on with it|magazine=Wired UK|url=https://www.wired.co.uk/news/archive/2012-05/24/eu-cookie-law-moaning|access-date=31 October 2012|archive-url=https://web.archive.org/web/20121115110013/http://www.wired.co.uk/news/archive/2012-05/24/eu-cookie-law-moaning|archive-date=15 November 2012|url-status=live|date=2012-05-24}}</ref> However, scholars note that the onerous nature of cookie pop-ups stems from an attempt to continue to operate a business model through convoluted requests that may be incompatible with the GDPR.<ref name="Veale"/>
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A [[W3C]] specification called [[P3P]] was proposed for servers to communicate their privacy policy to browsers, allowing automatic, user-configurable handling. However, few websites implement the specification, and the W3C has discontinued work on the specification.<ref>{{Cite web|title=P3P: The Platform for Privacy Preferences|url=https://www.w3.org/P3P/Overview.html|access-date=2021-10-15|website=W3C }}</ref>
 
Third-party cookies can be blocked by most browsers to increase privacy and reduce tracking by advertising and tracking companies without negatively affecting the user's web experience on all sites. Some sites operate 'cookie walls', which make access to a site conditional on allowing cookies either technically in a browser, through pressing 'accept', or both.<ref>{{Cite journal|last1=Zuiderveen Borgesius|first1=F.J.|last2=Kruikemeier|first2=S.|last3=C Boerman|first3=S.|last4=Helberger|first4=N.|date=2017|title=Tracking Walls, Take-It-Or-Leave-It Choices, the GDPR, and the ePrivacy Regulation|url=http://edpl.lexxion.eu/article/EDPL/2017/3/9|journal=European Data Protection Law Review|volume=3|issue=3|pages=353–368|doi=10.21552/edpl/2017/3/9|hdl=11245.1/dfb59b54-0544-4c65-815a-640eae10668a|hdl-access=free}}</ref> In 2020, the [[European Data Protection Board]], composed of all EU data protection regulators, stated that cookie walls were illegal.<blockquote>In order for consent to be freely given, access to services and functionalities must not be made conditional on the consent of a user to the storing of information, or gaining of access to information already stored, in the terminal equipment of a user (so called cookie walls).<ref>{{Cite web|title=Guidelines 05/2020 on consent under Regulation 2016/679 {{!}} European Data Protection Board|url=https://edpb.europa.eu/our-work-tools/our-documents/guidelines/guidelines-052020-consent-under-regulation-2016679_en|access-date=2021-06-06|website=edpb.europa.eu}}</ref></blockquote>Many advertising operators have an opt-out option to behavioural advertising, with a generic cookie in the browser stopping behavioural advertising.<ref name="nyt-bits">{{cite web|title=A Loophole Big Enough for a Cookie to Fit Through|url=http://bits.blogs.nytimes.com/2010/09/17/a-loophole-big-enough-for-a-cookie-to-fit-through/|work=Bits|publisher=The New York Times|access-date=31 January 2013|archive-url=https://web.archive.org/web/20130126032302/http://bits.blogs.nytimes.com/2010/09/17/a-loophole-big-enough-for-a-cookie-to-fit-through/|archive-date=26 January 2013|url-status=live|date=2010-09-17}}</ref><ref name="howtoblock">{{cite news |first=Rob |last=Pegoraro |url=https://www.washingtonpost.com/wp-dyn/content/article/2005/07/16/AR2005071600111.html |title=How to Block Tracking Cookies |page=F07 |date=July 17, 2005 |newspaper=Washington Post |access-date=2009-01-04 |archive-url=https://web.archive.org/web/20110427074701/http://www.washingtonpost.com/wp-dyn/content/article/2005/07/16/AR2005071600111.html |archive-date=April 27, 2011 |url-status=live}}</ref> However, this is often ineffective against many forms of tracking, such as first-party tracking that is growing in popularity to avoid the impact of browsers blocking third party cookies.<ref>{{Cite webnews|last=FranciscoClaburn|first=Thomas Claburn in |___location=San Francisco|title=What's CNAME of your game? This DNS-based tracking defies your browser privacy defenses|url=https://www.theregister.com/2021/02/24/dns_cname_tracking/|access-date=2021-06-06|website=www.theregister.com|language=en}}</ref><ref>{{cite arXiv|last1=Dimova|first1=Yana|last2=Acar|first2=Gunes|last3=Olejnik|first3=Lukasz|last4=Joosen|first4=Wouter|last5=Van Goethem|first5=Tom|date=2021-03-05|title=The CNAME of the Game: Large-scale Analysis of DNS-based Tracking Evasion|class=cs.CR|eprint=2102.09301}}</ref> Furthermore, if such a setting is more difficult to place than the acceptance of tracking, it remains in breach of the conditions of the e-Privacy Directive.<ref name=":0"/>
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===Network eavesdropping===
[[File:Cookie-sniffing.svg|thumb|A cookie can be stolen by another computer that is allowed reading from the network.]]
 
Traffic on a network can be intercepted and read by computers on the network other than the sender and receiver (particularly over [[plaintext|unencrypted]] open [[Wi-Fi]]). This traffic includes cookies sent on ordinary unencrypted [[HTTP sessions]]. Where network traffic is not encrypted, attackers can therefore read the communications of other users on the network, including HTTP cookies as well as the entire contents of the conversations, for the purpose of a [[man-in-the-middle attack]].
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<!-- Dead note "rfc2": RFC 2109 and RFC 2965, HTTP State Management Mechanism ([[Internet Engineering Task Force|IETF]]) -->
 
===Sources===
* Anonymous, 2011. Cookiejacking Attack Steals Website Access Credentials. Informationweek - Online, pp.&nbsp;Informationweek - Online, May 26, 2011.
 
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[[Category:Hypertext Transfer Protocol headers|Cookie]]
[[Category:Internet privacy]]
[[Category:Wikipedia articles with ASCII art]]
[[Category:Hacking (computer security)]]
[[Category:Tracking]]