NationMaster - Encyclopedia: HTTP cookie

HTTP cookies, sometimes known as web cookies or just cookies, are parcels of text sent by a server to a web browser and then sent back unchanged by the browser each time it accesses that server. HTTP cookies are used for authenticating, tracking, and maintaining specific information about users, such as site preferences or the contents of their electronic shopping carts. The term "cookie" is derived from "magic cookie," a well-known concept in UNIX computing which inspired both the idea and the name of HTTP cookies. This article is about the food. ... Look up cookie in Wiktionary, the free dictionary. ... The inside/front of a Dell PowerEdge web server The term Web server can mean one of two things: A computer program that is responsible for accepting HTTP requests from clients, which are known as Web browsers, and serving them HTTP responses along with optional data contents, which usually are... WWWs historical logo designed by Robert Cailliau The World Wide Web (commonly shortened to the Web) is a system of interlinked, hypertext documents accessed via the Internet. ... An example of a Web browser (Mozilla Firefox) A web browser is a software application that enables a user to display and interact with text, images, videos, music and other information typically located on a Web page at a website on the World Wide Web or a local area network. ... Hypertext Transfer Protocol (HTTP) is a communications protocol used to transfer or convey information on intranets and the World Wide Web. ... For other uses of the terms authentication, authentic and authenticity, see authenticity. ... Shopping cart software is software used in e-commerce to simulate a shopping cart. ... In computer programming, a magic cookie or cookie is a token or short packet of data passed between communicating programs, where the data is typically not meaningful to the recipient program. ... Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIXÂ®, sometimes also written as or Â® with small caps) is a computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ...

Cookies have been of concern for Internet privacy, since they can be used for tracking browsing behavior. As a result, they have been subject to legislation in various countries such as the United States and in the European Union. Cookies have also been criticized because the identification of users they provide is not always accurate and because they could potentially be a target of network attackers. Some alternatives to cookies exist, but each has its own uses, advantages and drawbacks. Internet privacy consists of privacy over the media of the Internet: the ability to control what information one reveals about oneself over the Internet, and to control who can access that information. ...

Cookies are also subject to a number of misconceptions, mostly based on the erroneous notion that they are computer programs. In fact, cookies are simple pieces of data unable to perform any operation by themselves. In particular, they are neither spyware nor viruses, despite the detection of cookies from certain sites by many anti-spyware products. A computer program is a collection of instructions that describe a task, or set of tasks, to be carried out by a computer. ... A large number of toolbars, some added by spyware, overwhelm an Internet Explorer session. ... A computer virus is a computer program that can copy itself and infect a computer without permission or knowledge of the user. ...

Most modern browsers allow users to decide whether to accept cookies, but rejection makes some websites unusable. For example, shopping baskets implemented using cookies do not work if cookies are rejected. A website (alternatively, Web site or web site) is a collection of Web pages, images, videos or other digital assets that is hosted on one or several Web server(s), usually accessible via the Internet, cell phone or a LAN. A Web page is a document, typically written in HTML...

Purpose

HTTP cookies are used by Web servers to differentiate users and to maintain data related to the user during navigation, possibly across multiple visits. HTTP cookies were introduced to provide a way for realizing a "shopping cart" (or "shopping basket"),^[1]^[2] a virtual device into which the user can "place" items to purchase, so that users can navigate a site where items are shown, adding or removing items from the shopping basket at any time. Shopping cart software is software used in e-commerce to assist people making purchases online, analagous to the US English term shopping cart. In British English it is generally known as a shopping basket, almost exclusively shortened on websites to basket. The software allows online shopping customers to place items...

Allowing users to log in to a website is another use of cookies. Users typically log in by inserting their credentials into a login page; cookies allow the server to know that the user is already authenticated, and therefore is allowed to access services or perform operations that are restricted to logged-in users.

Many websites also use cookies for personalization based on users' preferences. Sites that require authentication often use this feature, although it is also present on sites not requiring authentication. Personalization includes presentation and functionality. For example, the Wikipedia Web site allows authenticated users to choose the webpage skin they like best; the Google search engine allows users (even non-registered ones) to decide how many search results per page they want to see. Personalization is tailoring specifically to one individual. ... Wikipedia (IPA: , or ( ) is a multilingual, web-based, free content encyclopedia project, operated by the Wikimedia Foundation, a non-profit organization. ... In computing, skins and themes are custom graphical appearances (GUIs) that can be applied to certain software and websites in order to suit the different tastes of different users. ... This article is about the corporation. ...

Cookies are also used to track users across a website. Third-party cookies and Web bugs, explained below, also allow for tracking across multiple sites. Tracking within a site is typically done with the aim of producing usage statistics, while tracking across sites is typically used by advertising companies to produce anonymous user profiles, which are then used to target advertising (deciding which advertising image to show) based on the user profile. A Web bug is an object that is embedded in a web page or e-mail and is usually invisible to the user but allows checking that a user has viewed the page or e-mail. ...

Realization

Technically, cookies are arbitrary pieces of data chosen by the Web server and sent to the browser. The browser returns them unchanged to the server, introducing a state (memory of previous events) into otherwise stateless HTTP transactions. Without cookies, each retrieval of a Web page or component of a Web page is an isolated event, mostly unrelated to all other views of the pages of the same site. By returning a cookie to a web server, the browser provides the server a means of connecting the current page view with prior page views. Other than being set by a web server, cookies can also be set by a script in a language such as JavaScript, if supported and enabled by the Web browser. Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... The inside/front of a Dell PowerEdge web server The term Web server can mean one of two things: A computer program that is responsible for accepting HTTP requests from clients, which are known as Web browsers, and serving them HTTP responses along with optional data contents, which usually are... In information processing, a state is the complete set of properties (for example, its energy level, etc. ... A screenshot of a web page. ... Scripting programming languages (commonly called scripting languages or script languages) are computer programming languages designed for scripting the operation of a computer. ... JavaScript is a scripting language most often used for client-side web development. ...

Cookie specifications^[3]^[4] suggest that browsers should support a minimal number of cookies or amount of memory for storing them. In particular, an internet browser is expected to be able to store at least 300 cookies of 4 kilobytes each, and at least 20 cookies per server or domain. It has been suggested that this article or section be merged into Domain Name System. ...

In practice cookies must be smaller than 4k. MSIE imposes a 4k total for all cookies stored in a given domain. is the 226th day of the year (227th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 226th day of the year (227th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ...

The cookie setter can specify a deletion date, in which case the cookie will be removed on that date. If the cookie setter does not specify a date, the cookie is removed once the user quits his or her browser. As a result, specifying a date is a way for making a cookie survive across sessions. For this reason, cookies with an expiration date are called persistent. As an example application, a shopping site can use persistent cookies to store the items users have placed in their basket. This way, if users quit their browser without making a purchase and return later, they still find the same items in the basket so they do not have to look for these items again. If these cookies were not given an expiration date, they would expire when the browser is closed, and the information about the basket content would be lost.

Misconceptions

Since their introduction on the Internet, misconceptions about cookies have circulated on the Internet and in the media.^[5]^[6] In 1998, CIAC, a computer incident response team of the United States Department of Energy, found the security vulnerability "essentially nonexistent" and explained that "information about where you come from and what web pages you visit already exists in a web server's log files".^[7] In 2005, Jupiter Research published the results of a survey,^[8] according to which a consistent percentage of respondents believed some of the following false claims: This article is about the Computer Incident services provided by the DOE. For the Connecticut Interscholastic Athletic Conference, see Connecticut Association of Schools. ... The United States Department of Energy (DOE) is a Cabinet-level department of the United States government responsible for energy policy and nuclear safety. ... JupiterMedia, Inc. ...

Cookies are in fact only data, not program code: they cannot erase or read information from the user's computer.^[9] However, cookies allow for detecting the Web pages viewed by a user on a given site or set of sites. This information can be collected in a profile of the user. Such profiles are often anonymous, that is, they do not contain personal information of the user (name, address, etc.) More precisely, they cannot contain personal information unless the user has made it available to some sites. Even if anonymous, these profiles have been the subject of some privacy concerns. A computer worm is a self-replicating computer program. ... A computer virus is a computer program that can copy itself and infect a computer without permission or knowledge of the user. ... A large number of toolbars, some added by spyware, overwhelm an Internet Explorer session. ... Dozens of pop-up ads cover a desktop. ... This article is about electronic spam. ... Advert redirects here. ...

According to the same survey, a large percentage of Internet users do not know how to delete cookies.

Browser settings

Most modern browsers support cookies. However, a user can usually also choose whether cookies should be used or not. The following are common options:^[10]

The browser may include the possibility of better specifying which cookies have to be accepted or not. In particular, the user can typically choose one or more of the following options: reject cookies from specific domains; disallow third-party cookies (see below); accept cookies as non-persistent (expiring when the browser is closed); and allow a server to set cookies for a different domain. Additionally, browsers may also allow users to view and delete individual cookies. Look up whitelist in Wiktionary, the free dictionary. ... A blacklist is a list or register of people who, for one reason or another, are being denied a particular privilege, service, or mobility. ... The Platform for Privacy Preferences Project, or P3P, is a protocol designed to give users more control of their personal information when browsing Internet Websites. ... Image File history File links Firefox_Cookie_Manager. ... Image File history File links Firefox_Cookie_Manager. ... Firefox may refer to: Firefox (novel), written by Craig Thomas, published in 1978 Firefox (film), the 1982 movie starring Clint Eastwood, based on the novel Firefox (arcade game), the laserdisc arcade game based on the movie Mozilla Firefox, a web browser The Red Fox or the Red Panda, based on...

Most browsers supporting JavaScript allow the user to see the cookies that are active with respect to a given page by typing javascript:alert("Cookies: "+document.cookie) in the browser URL field. Some browsers incorporate a cookie manager for the user to see and selectively delete the cookies currently stored in the browser. â€œURLâ€ redirects here. ...

The P3P specification includes the possibility for a server to state a privacy policy, which specifies which kind of information it collects and for which purpose. These policies include (but are not limited to) the use of information gathered using cookies. According to the P3P specification, a browser can accept or reject cookies by comparing the privacy policy with the stored user preferences or ask the user, presenting them the privacy policy as declared by the server. The Platform for Privacy Preferences Project, or P3P, is a protocol designed to give users more control of their personal information when browsing Internet Websites. ...

Privacy and third-party cookies

Cookies have some important implications on the privacy and anonymity of Web users. While cookies are only sent to the server setting them or one 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. Privacy is the ability of an individual or group to control the flow of information about themselves and thereby reveal themselves selectively. ... Anonymous redirects here. ... It has been suggested that this article or section be merged into Domain Name System. ...

Advertising companies use third-party cookies to track a user across multiple sites. In particular, an advertising company can track a user across all pages where it has placed advertising images or web bugs. Knowledge of the pages visited by a user allows the advertisement company to target advertisement to the user's presumed preferences. Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... A Web bug is an object that is embedded in a web page or e-mail and is usually invisible to the user but allows checking that a user has viewed the page or e-mail. ...

The possibility of building a profile of users has been considered by some a potential privacy threat, even when the tracking is done on a single domain but especially when tracking is done across multiple domains using third-party cookies. For this reason, some countries have legislation about cookies.

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 to see if they then visited sites about drug making and drug use. In 2002, privacy activist Daniel Brandt found that the CIA had been leaving persistent cookies on computers for ten years. When notified it was violating policy, CIA stated that these cookies were not intentionally set and stopped setting them.^[11] On December 25, 2005, Brandt discovered that the National Security Agency had been leaving two persistent cookies on visitors' computers due to a software upgrade. After being informed, the National Security Agency immediately disabled the cookies.^[12] The White House Office of National Drug Control Policy (ONDCP), a component of the Executive Office of the President of the United States, was established in 1988 by the Anti-Drug Abuse Act. ... CIA redirects here. ... is the 359th day of the year (360th in leap years) in the Gregorian calendar. ... Year 2005 (MMV) was a common year starting on Saturday (link displays full calendar) of the Gregorian calendar. ... â€œNSAâ€ redirects here. ...

The 2002 European Union telecommunication privacy Directive contains rules about the use of cookies. In particular, Article 5, Paragraph 3 of this directive mandates that storing data (like cookies) in a user's computer can only be done if: 1) the user is provided information about how this data is used; and 2) the user is given the possibility of denying this storing operation. However, this article also states that storing data that is necessary for technical reasons is exempted from this rule. This directive was expected to have been applied since October 2003, but a December 2004 report says (page 38) that this provision was not applied in practice, and that some member countries (Slovakia, Latvia, Greece, Belgium, and Luxembourg) did not even implement the provision in national law. The same report suggests a thorough analysis of the situation in the Member States.

Microsoft Internet Explorer versions 6 and 7 support P3P which allows the web browser to determine whether to allow 3rd party cookies to be stored.

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 at odds with the Representational State Transfer (REST) software architectural style^{[citation needed]}. Rest may refer to: rest (fitness), a period of relative inactivity to allow recovery and growth. ...

Inaccurate identification

If more than one browser is used on a computer, each 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 a computer and browser, if they do not use different user accounts. Note: to create a user account for Wikipedia, go to the login page. ...

Cookie Hijacking

During normal operation, cookies are sent back and forth between a server (or a group of servers in the same domain) and the computer of the browsing user. Since cookies may contain sensitive information (user name, a token used for authentication, etc.), their values should not be accessible to other computers. Cookie theft is the act of intercepting cookies by an unauthorized party. Image File history File links Cookie-sniffing. ... Image File history File links Cookie-sniffing. ... For other uses of the terms authentication, authentic and authenticity, see authenticity. ...

Cookies can be stolen via packet sniffing in an attack called session hijacking. Traffic on a network can be intercepted and read by computers on the network other than its sender and its receiver, (particularly on unencrypted public Wi-Fi networks.) This traffic includes cookies sent on ordinary unencrypted http sessions. Where network traffic is not encrypted, malicious users can therefore read the communications of other users on the network, including their cookies, using programs called packet sniffers. Packet sniffers (also known as Network Analyzers or Ethernet Sniffers) are usually software programs (but sometimes implemented in hardware) which can intercept and log traffic passing over a network or part of a network. ... The term Session Hijacking refers to the exploitation of a valid computer session - sometimes also called a session key - to gain unauthorised access to information or services in a computer system. ... In data communications, cleartext is the form of a message or data which is transferred or stored without cryptographic protection. ... Official Wi-Fi logo Wi-Fi (pronounced wye-fye, IPA: ) is a wireless technology brand owned by the Wi-Fi Alliance intended to improve the interoperability of wireless local area network products based on the IEEE 802. ... HTTP (for HyperText Transfer Protocol) is the primary method used to convey information on the World Wide Web. ... A packet sniffer (also known as a network analyzer or protocol analyzer or, for particular types of networks, an Ethernet sniffer or wireless sniffer) is computer software or computer hardware that can intercept and log traffic passing over a digital network or part of a network. ...

This issue can be overcome 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 secure flag while setting a cookie; the browser will then send it only over a secure channel, such as an SSL connection.^[13] Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide secure communications on the Internet for such things as web browsing, e-mail, Internet faxing, instant messaging and other data transfers. ... https is a URI scheme used to indicate a secure HTTP connection. ... Secure Sockets Layer (SSL) and Transport Layer Security (TLS), its successor, are cryptographic protocols which provide secure communications on the Internet. ...

However a large number of websites, although using secure https communication for user authentication (i.e. the login page), subsequently send session cookies and other data over ordinary unencrypted http connections for performance reasons. Hackers can therefore easily intercept the cookies of other users and impersonate them on the relevant websites.^[14] https is a URI scheme used to indicate a secure HTTP connection. ... For other uses of the terms authentication, authentic and authenticity, see authenticity. ... In computer security, logging (or signing) in and out is the process by which individual access to a computer system is controlled by identification of the user in order to obtain credentials to permit access. ... In data communications, cleartext is the form of a message or data which is in a form that is immediately comprehensible to a human being without additional processing. ... HTTP (for HyperText Transfer Protocol) is the primary method used to convey information on the World Wide Web. ...

A different way to steal cookies is cross-site scripting and making the browser itself send cookies to servers that should not receive them. Modern browsers allow execution of pieces of code retrieved from the server. If cookies are accessible during execution, their value may be communicated in some form to servers that should not access them. Encrypting cookies before sending them on the network does not help against this attack.^[15] Image File history File links Cookie-theft. ... Image File history File links Cookie-theft. ... Cross-site scripting (XSS) is a type of computer security vulnerability typically found in web applications which allow code injection by malicious web users into the web pages viewed by other users. ...

This type of cross-site scripting is typically exploited by attackers on sites that allow users to post HTML content. By embedding a suitable piece of code in an HTML post, an attacker may receive cookies of other users. Knowledge of these cookies can then be exploited by connecting to the same site using the stolen cookies, thus being recognised as the user whose cookies have been stolen. HTML, an initialism of Hypertext Markup Language, is the predominant markup language for web pages. ...

A way for preventing such attacks is by the HttpOnly flag;^[16] this is a Microsoft option that makes a cookie inaccessible to client side script. However, website designers should consider designing their websites so that they are immune to cross-site scripting.^[17]

Image File history File links Cookie-poison. ... Image File history File links Cookie-poison. ...

Cookie poisoning

While cookies are supposed to be stored and sent back to the server unchanged, an attacker may modify the value of cookies before sending them back to the server. If, for example, a cookie contains the total value a user has to pay for the items in their shopping basket, changing this value exposes the server to the risk of making the attacker pay less than the supposed price. The process of tampering with the value of cookies is called cookie poisoning, and is sometimes used after cookie theft to make an attack persistent.

Most websites, however, only store a session identifier — a randomly generated unique number used to identify the user's session — in the cookie itself, while all the other information is stored on the server. In this case, the problem of cookie poisoning is largely eliminated. Image File history File links Cookie-cooking. ... Image File history File links Cookie-cooking. ...

Cross-site cooking

Each site is supposed to have its own cookies, so a site like evil.net should not be able to alter or set cookies for another site, like good.net. Cross-site cooking vulnerabilities in web browsers allow malicious sites to break this rule. This is similar to cookie poisoning, but the attacker exploits non-malicious users with vulnerable browsers, instead of attacking the actual site directly. The goal of such attacks may be to perform session fixation. In cross-site cooking, the attacker exploits a browser bug to send an invalid cookie to a server. ... Session fixation attacks attempt to exploit the vulnerability of a system which allows one person to fixate (set) another persons session identifier (SID). ...

Users are advised to use the more recent versions of web browsers in which such issue is mitigated.

Inconsistent state on client and server

The use of cookies may generate an inconsistency between the state of the client and the state as stored in the cookie. If the user acquires a cookie and then clicks the "Back" button of the browser, the state on the browser is generally not the same as before that acquisition. As an example, if the shopping cart of an online shop is realized using cookies, the content of the cart may not change when the user goes back in the browser's history: if the user presses a button to add an item in the shopping cart and then clicks on the "Back" button, the item remains in the shopping cart. This might not be the intention of the user, who possibly wanted to undo the addition of the item. This can lead to unreliability, confusion and bugs. The website designers should therefore be aware of this issue and implement measures to handle such situation as fits.

Cookie Expiration

Persistent cookies have been criticized by privacy experts for not being set to expire soon enough, and thereby allowing some websites to track users and build up a profile of them over time.^[18] This aspect of cookies also compounds the issue of session hijacking, because a stolen persistent cookie can potentially be used to impersonate a user for a considerable period of time. The term Session Hijacking refers to the exploitation of a valid computer session - sometimes also called a session key - to gain unauthorised access to information or services in a computer system. ...

Alternatives to cookies

Some of the operations that can be realised using cookies can also be realised using other mechanisms. However, these alternatives to cookies have their own drawbacks, which make cookies usually preferred to them in practice. Most of the following alternatives allow for user tracking, even if not as reliably as cookies. As a result, privacy is an issue even if cookies are rejected by the browser or not set by the server.

IP address

An unreliable technique for tracking users is based on storing the IP addresses of the computers requesting the pages. This technique has been available since the introduction of the World Wide Web, as downloading pages requires the server holding them to know the IP address of the computer running the browser or the proxy, if any is used. This information is available for the server to be stored regardless of whether cookies are used or not. An IP address (Internet Protocol address) is a unique address that certain electronic devices use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)â€”in simpler terms, a computer address. ... In computer networks, a proxy server is a server (a computer system or an application program) which services the requests of its clients by forwarding requests to other servers. ...

However, these addresses are typically less reliable in identifying a user than cookies because computers and proxies may be shared by several users, and the same computer may be assigned different Internet addresses in different work sessions (this is often the case for dial-up connections). The reliability of this technique can be improved by using another feature of the HTTP protocol: when a browser requests a page because the user has followed a link, the request that is sent to the server contains the URL of the page where the link is located. If the server stores these URLs, the path of page viewed by the user can be tracked more precisely. However, these traces are less reliable than the ones provided by cookies, as several users may access the same page from the same computer, NAT router, or proxy and then follow two different links. Moreover, this technique only allows tracking and cannot replace cookies in their other uses. In telecommunication, the term dial-up has the following meanings: Dial-up access, typically to the Internet A service feature in which a user initiates service on a previously arranged trunk or transfers, without human intervention, from an active trunk to a standby trunk. ... In computer networking, Network Address Translation (NAT, also known as Network Masquerading, Native Address Translation or IP Masquerading) is a technique of transceiving network traffic through a router that involves re-writing the source and/or destination IP addresses and usually also the TCP/UDP port numbers of IP packets...

Tracking by IP address can be impossible with some systems that are used to retain Internet anonymity, such as Tor. With such systems, not only could one browser carry multiple addresses throughout a session, but multiple users could appear to be coming from the same IP address, thus making IP address use for tracking wholly unreliable. Anonymity is the state of not being identifiable within a set, called the anonymity set. ... Tor (The Onion Router) is a free software implementation of second-generation onion routing â€“ a system enabling its users to communicate anonymously on the Internet. ...

Some major ISPs, including AOL, route all web traffic through a small number of proxies which makes this scheme particularly unworkable.

URL (query string)

A more precise technique is based on embedding information into URLs. The query string part of the URL is the one that is typically used for this purpose, but other parts can be used as well. The PHP session mechanism uses this method if cookies are not enabled. In the World Wide Web, a query string is the part of a URL that contains data to be passed to CGI programs. ... â€œURLâ€ redirects here. ... For other uses, see PHP (disambiguation). ...

This method consists of the Web server appending query strings to the links of a Web page it holds when sending it to a browser. When the user follows a link, the browser returns the attached query string to the server.

Query strings used in this way and cookies are very similar, both being arbitrary pieces of information chosen by the server and sent back by the browser. 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 is sent to the server. 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. Wikipedia does not yet have an article with this exact name. ...

Moreover, even if the same user accesses the same page two times, there is no guarantee that the same query string is used in both views. For example, if the same user arrives to the same page but coming from a page internal to the site the first time and from an external search engine the second time, the relative query strings are typically different while the cookies would be the same. For more details, see query string. A search engine is an information retrieval system designed to help find information stored on a computer system. ... In the World Wide Web, a query string is the part of a URL that contains data to be passed to CGI programs. ...

Other drawbacks of query strings are related to security: storing data that identifies a session in a query string enables or simplifies session fixation attacks, referer logging attacks and other security exploits. Transferring session identifiers as HTTP cookies is more secure. Session fixation attacks attempt to exploit the vulnerability of a system which allows one person to fixate (set) another persons session identifier (SID). ... Referer is a common misspelling of the word referrer; so common, in fact, that it made it into the official specification of HTTP - the communication protocol of the world wide web. ... An exploit is a piece of software, a chunk of data, or sequence of commands that take advantage of a bug, glitch or vulnerability in order to cause unintended or unanticipated behavior to occur on computer software, hardware, or something electronic (usually computerized). ...

Hidden form fields

A form of session tracking, used by ASP.NET, 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. However, it presents two advantages from the point of view of the tracker: first, having the tracking information placed in the HTML source rather than the URL means that it is not noticed by the average user; second, the session information is not copied when the user copies the URL (to save the page on disk or send it via email, for example). A drawback of this technique is that session information is in the HTML code; therefore, each web page must be generated dynamically each time someone requests it, placing an additional workload on the web server. ASP.NET logo ASP.NET is a web application framework marketed by Microsoft that programmers can use to build dynamic web sites, web applications and XML web services. ... A webform on a web page allows a user to enter data that is, typically, sent to a server for processing and to mimic the usage of paper forms. ...

HTTP authentication

As for 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 credential for granting access to a Web page, the browser requests them to the user; once obtained, the browser stores and uses them also for accessing subsequent pages, without requiring the user to provide them again. From the point of view of the user, the effect is the same as if cookies were used: username and password are only requested once, and from that point on the user is given access to the site. In the basic access authentication protocol, a combination of username and password is sent to the server in every browser request. This means that someone listening in on this traffic can simply read this information and store for later use. This problem is overcome in the digest access authentication protocol, in which the username and password are encrypted using a random nonce created by the server. In the context of an HTTP transaction, the basic access authentication is a method designed to allow a web browser, or other client program, to provide credentials â€“ in the form of a user name and password â€“ when making a request. ... Digest access authentication is one of the agreed methods a web page can use to negotiate credentials with a web user (using the HTTP protocol). ... In security engineering, a nonce is a number used once. ...

Macromedia Flash Local Stored Objects

If a browser includes the Macromedia Flash Player plugin, its Local Shared Objects function can be used in a way very similar to cookies. Local Stored Objects may be an attractive choice to web developers because a majority of Windows users have Flash Player installed, the default size limit is 100 kB, and the security controls are distinct from the user controls for cookies, so Local Shared Objects may be enabled when cookies are not. // == Macromedia Flash == ==]] Using Macromedia Flash 8 (bundled in Studio 8) in Windows XP. Maintainer: Adobe Systems (formerly Macromedia) Latest release: 8 / September 30th, 2005 OS: Windows (no native Windows XP Professional x64 Edition support), Mac OS X, Linux (i386 only, via wine [1]) Use: Multimedia Content Creator License: Proprietary Website... For other uses, see Plug in. ... Local Shared Object (LSO) is a cookie-like data entity used by Adobe Flash Player. ... Windows redirects here. ...

Client-side persistence

Some web browsers support a script-based persistence mechanism that allows the page to store information locally for later retrieval. Internet Explorer, for example, supports persisting information in the browser's history, in favorites, in an XML store, or directly within a Web page saved to disk.^[19]

If JavaScript is enabled, the window.name property of the object window can be used to persistently store data. This property remains unaltered across the loading and unloading of other web pages. However, this method introduces a security risk, as the user may decide to then go to a site different than the intended one, and that site would be able to see the data.^[20]

A different mechanism relies on browsers normally caching (holding in memory instead of reloading) JavaScript programs used in web pages. As an example, a page may contain a link such as <script type="text/javascript" src="example.js">. The first time this page is loaded, the program example.js is loaded as well. At this point, the program remains cached and is not reloaded the second time the page is visited. As a result, if this program contains a statement such as id=3243242, this identifier remains valid and can be exploited by other JavaScript code the next times the page is loaded, or another page linking the same program is loaded.^[21]

History

The term "HTTP cookie" derives from "magic cookie", a packet of data a program receives but only uses for sending it again, possibly to its origin, unchanged. Magic cookies were already used in computing when Lou Montulli had the idea of using them in Web communications in June 1994.^[22] At the time, he was an employee of Netscape Communications, which was developing an e-commerce application for a customer. Cookies provided a solution to the problem of reliably implementing a virtual shopping cart.^[1]^[2] In computer programming, a magic cookie or cookie is a token or short packet of data passed between communicating programs, where the data is typically not meaningful to the recipient program. ... Lou Montulli is a programmer who is well known for his work in producing web browsers. ... Netscape Communications Corporation was the publisher of the Netscape Navigator web browser as well as many other internet and intranet client and server software products. ... Electronic commerce, EC, e-commerce or ecommerce consists primarily of the distributing, buying, selling, marketing, and servicing of products or services over electronic systems such as the Internet and other computer networks. ... Shopping cart software is software used in e-commerce to assist people making purchases online, analagous to the US English term shopping cart. In British English it is generally known as a shopping basket, almost exclusively shortened on websites to basket. The software allows online shopping customers to place items...

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^[23]^[24], supported cookies. The first actual use of cookies (out of the labs) was made for checking whether visitors to the Netscape Web site had already visited the site. Montulli applied for a patent for the cookie technology in 1995; it was granted in 1998. Support for cookies was integrated in Internet Explorer in version 2, released in October 1995.^[25] is the 286th day of the year (287th in leap years) in the Gregorian calendar. ... Year 1994 (MCMXCIV) The year 1994 was designated as the International Year of the Family and the International Year of the Sport and the Olympic Ideal by the United Nations. ...

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 the presence of cookies. Some people were aware of the existence of cookies as early as the first quarter of 1995,^[26] but the general public learned about them after the Financial Times published an article about them on February 12, 1996. In the same year, cookies received 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. The Financial Times (FT) is a British international business newspaper. ... is the 43rd day of the year in the Gregorian calendar. ... Year 1996 (MCMXCVI) was a leap year starting on Monday (link will display full 1996 Gregorian calendar). ... For other uses, see United States (disambiguation) and US (disambiguation). ... | logo_caption = | seal = US-FederalTradeCommission-Seal. ...

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 mailing list. A special working group within the IETF was formed. Two alternative proposals for introducing a state in an HTTP transactions had been proposed by Brian Behlendorf and David Kristol, respectively, but the group, headed by Kristol himself, soon decided to use the Netscape specification as a starting point. On 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. The Internet Engineering Task Force (IETF) develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standard bodies; and dealing in particular with standards of the TCP/IP and Internet protocol suite. ...

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 followed by RFC 2965 in October 2000.

Implementation

Setting a cookie

Transfer of Web pages follows the HyperText Transfer Protocol (HTTP). Regardless of cookies, browsers request a page from web servers by sending them a short text called HTTP request. For example, to access the page http://www.w3.org/index.html, browsers connect to the server www.w3.org sending it a request that looks like the following one: Hypertext Transfer Protocol (HTTP) is a communications protocol used to transfer or convey information on intranets and the World Wide Web. ... Hypertext Transfer Protocol (HTTP) is a communications protocol used to transfer or convey information on intranets and the World Wide Web. ...

The server replies by sending the requested page preceded by a similar packet of text, called HTTP header. This packet may contain lines requesting the browser to store cookies: Hypertext Transfer Protocol (HTTP) is a communications protocol used to transfer or convey information on intranets and the World Wide Web. ...

The line Set-cookie is only sent if the server wishes the browser to store a cookie. Indeed, it is a request for the browser to store the string name=value and send it back in all future requests to the server. If the browser supports cookies and cookies are enabled, every subsequent page request to the same server contains the cookie. For example, the browser requests the page http://www.w3.org/spec.html by sending the server www.w3.org a request like the following:

This is a request for another page from the same server, and differs from the first one above because it contains the string that the server has previously sent to the browser. This way, the server knows that this request is related to the previous one. The server answers by sending the requested page, possibly adding other cookies as well.

The value of a cookie can be modified by the server by sending a new Set-Cookie: name=newvalue line in response of a page request. The browser then replaces the old value with the new one.

The term "cookie crumb" is sometimes used to refer to the name-value pair.^[27] This is not the same as breadcrumb web navigation, which is the technique of showing in each page the list of pages the user has previously visited; this technique may however be implemented using cookies. Breadcrumbs or breadcrumb trails is a navigation technique used in user interfaces. ...

The Set-Cookie line is typically not created by the HTTP server itself but by a CGI program. The HTTP server only sends the result of the program (a document preceded by the header containing the cookies) to the browser. The Common Gateway Interface (CGI) is a standard protocol for interfacing external application software with an information server, commonly a web server. ...

Cookies can also be set by JavaScript or similar scripts running within the browser. In JavaScript, the object document.cookie is used for this purpose. For example, the instruction document.cookie = "temperature=20" creates a cookie of name temperature and value 20.^[28]

Cookie attributes

Beside the name/value pair, a cookie may also contain an expiration date, a path, a domain name, and whether the cookie is intended only for encrypted connections. RFC 2965 also specifies that cookies must have a mandatory version number, but this is usually omitted. These pieces of data follow the name=newvalue pair and are separated by semicolons. For example, a cookie can be created by the server by sending a line Set-Cookie: name=newvalue; expires=date; path=/; domain=.example.org. A path is the general form of a file or directory name, giving a files name and its unique location in a file system. ... The term domain name has multiple related meanings: A name that identifies a computer or computers on the internet. ... Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide secure communications on the Internet for such things as web browsing, e-mail, Internet faxing, instant messaging and other data transfers. ...

The domain and path tell the browser that the cookie has to be sent back to the server when requesting URLs of a given domain and path. If not specified, they default to the domain and path of the object that was requested. As a result, the domain and path strings may tell the browser to send the cookie when it normally would not. For security reasons, the cookie is accepted only if the server is a member of the domain specified by the domain string. Image File history File links HTTP-Cookie-Google. ... Image File history File links HTTP-Cookie-Google. ... This article is about the corporation. ...

Cookies are actually identified by the triple name/domain/path, not only the name (the original Netscape specification considers only the pair name/path). In other words, same name but different domains or paths identify different cookies with possibly different values. As a result, cookie values are changed only if a new value is given for the same name, domain, and path.

The expiration date tells the browser when to delete the cookie. If no expiration date is provided, the cookie is deleted at the end of the user session, that is, when the user quits the browser. As a result, specifying an expiration date is a means for making cookies to survive across browser sessions. For this reason, cookies that have an expiration date are called persistent.

The expiration date is specified in the "Wdy, DD-Mon-YYYY HH:MM:SS GMT" format. As an example, the following is a cookie sent by a Web server (the value string has been changed): GMT redirects here. ...

Set-Cookie: RMID=732423sdfs73242; expires=Fri, 31-Dec-2010 23:59:59 GMT; path=/; domain=.example.net

The name of this particular cookie is RMID, while its value is the string 732423sdfs73242. The server can use an arbitrary string as the value of a cookie. The server may collapse the value of a number of variables in a single string, like for example a=12&b=abcd&c=32. The path and domain strings / and .example.net tell the browser to send the cookie when requesting an arbitrary page of the domain .example.net, with an arbitrary path.

Expiration

Cookies expire, and are therefore not sent by the browser to the server, under any of these conditions:

Authentication

Cookies can be used by a server to recognize previously-authenticated users and to personalize the web pages of a site depending on the preferences of a user. This can be done for example as follows: This page deals with authentication in computing. ...

This is the method commonly used by many sites that allow logging in, such as Yahoo!, Wikipedia, and Facebook. (See "Cookie Theft" and "Cookie Expiration" sections of this article for security concerns around this mechanism) Yahoo redirects here. ... Wikipedia (IPA: , or ( ) is a multilingual, web-based, free content encyclopedia project, operated by the Wikimedia Foundation, a non-profit organization. ... Facebook headquarters in Palo Alto, CA Facebook is a social networking website that allows people to communicate with their friends and exchange information. ...

Personalisation

Cookies can be used for allowing users to express preferences about a Web site. For example, the Google search engine allows the user to choose how many results are to be shown for every query, and this choice is maintained across sessions. This article is about the corporation. ...

If a user was authenticated using the technique above, when they request a page the server is also sent the cookie associated with the user. It can therefore adapt the requested page to the stored user preferences. When authentication is not used, the user preferences are stored in a cookie. Users select their preferences by entering them in a Web form and submitting it to the server. The server encodes them in a cookie and sends it back to the browser. This way, every time the user accesses a page, the server is also sent the cookie where the preferences are stored, and can personalise the page according to the user preferences.

For example, Google stores the user preferences in a cookie of name PREF. This cookie is created with default values when the user accesses the site for the first time. For example, the cookie value contains the string NR=10, that indicates a default preference of ten hits displayed in each page. If the user changes this number to 20 in the preference page, the server modifies the cookie with NR=20. Every time the user queries the search engine, the cookie is sent to the server along with the query. This way, the server knows how many hits should be shown in each page.

Tracking

Cookies can also be used for tracking the path of a user while visiting the web pages of a site. This can also be done in part by using the IP address of the computer requesting the page or the referer field of the HTTP header, but cookies allows for a greater precision. This can be done for example as follows: An IP address (Internet Protocol address) is a unique address that certain electronic devices use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)â€”in simpler terms, a computer address. ... Referer is a common misspelling of the word referrer; so common, in fact, that it made it into the official specification of HTTP - the communication protocol of the world wide web. ... Hypertext Transfer Protocol (HTTP) is a communications protocol used to transfer or convey information on intranets and the World Wide Web. ...

By looking at the log file, it is then possible to find out which pages, and in which sequence, the user has visited. For example, if the log contains some requests done using the cookie id=dfhsiw, these requests all come from the same user. The URL and time/date stored with the cookie allows finding out which pages the user has visited, and at which time.

Third-party cookies

Images or other objects contained in a Web page may reside in servers different from the one holding the page. In order to show such a page, the browser downloads all these objects, possibly receiving cookies. These cookies are called third-party cookies if the server sending them is located outside the domain of the Web page.

This condition is common with on-line advertisement. Indeed, web banners are typically stored in servers of the advertising company, which are not in the domain of the Web pages showing them. If third-party cookies are not rejected by the browser, an advertising company can track a user across the sites where it has placed a banner. In particular, whenever a user views a page containing a banner, the browser retrieves the banner from a server of the advertising company. If this server has previously set a cookie, the browser sends it back, allowing the advertising company to link this access with the previous one. By choosing a unique banner URL for every Web page where it is placed or by using the HTTP referer field, the advertising company can then find out which pages the user has viewed. The same technique can be used with web bugs. These, unlike the obvious banners, are images embedded in the Web page that are undetectable by the user (e.g. they are tiny and/or transparent) A web banner or banner ad is a form of advertising on the World Wide Web. ... Referer is a common misspelling of the word referrer; so common, in fact, that it made it into the official specification of HTTP - the communication protocol of the world wide web. ... A Web bug is an object that is embedded in a web page or e-mail and is usually invisible to the user but allows checking that a user has viewed the page or e-mail. ...

Third-party cookies are used to create an anonymous profile of the user. This allows the advertising company to select the banner to show to a user based on the user's profile. The advertising industry has denied any other use of these profiles.

Many modern browsers, such as Mozilla Firefox, Internet Explorer and Opera block third party cookies if requested by the user. Internet Explorer version 6 allows a mild form of blocking, called leashing. A leashed cookie is a third-party cookie that is sent by the browser only when accessing a third-party document via the same first-party. For example, if third.com sets a cookie when an image is requested, and this cookie is set for the first time when the user views a document from first.com, the same cookie is not sent if the user downloads a document that contains the same image but the document is on another site other.com, if the cookie is leashed. A leashed cookie is different from a blocked cookie in that it is sent, in this example, if the image is contained in another document from the same site first.com.^[29] Firefox redirects here. ... Windows Internet Explorer (formerly Microsoft Internet Explorer abbreviated MSIE), commonly abbreviated to IE, is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems starting in 1995. ... Opera is an Internet suite which handles common internet-related tasks, including visiting web sites, sending and receiving e-mail messages, managing contacts, and online chat. ...

Basket

Some online shopping sites allow a user, even when not logged in, to store a number of items in a "virtual basket". The user starts navigating the site with an empty basket, and can add items to the basket while visiting the site. The list of items the user has chosen can be stored using cookies. For example, the server sends an empty cookie to the browser when the user visits the first page; whenever the user adds an item to the basket, the server adds the name of the item to the cookie.

This is a very insecure mechanism, because a malicious user can alter the cookie; a much more secure mechanism is to generate a random cookie as under "tracking", and using that as a lookup key in a database stored on the server.

Cookie theft

The cookie specifications constrain cookies to be sent back only to the servers in the same domain as the server from which they originate. However, the value of cookies can be sent to other servers using means different from the Cookie header.

In particular, scripting languages such as JavaScript and JScript are usually allowed access to cookie values and have some means to send arbitrary values to arbitrary servers on the Internet. These facts are used in combination with sites allowing users to post HTML content that other users can see. JavaScript is a scripting language most often used for client-side web development. ... JScript is the Microsoft implementation of the ECMAScript scripting programming language specification. ...

As an example, an attacker running the domain example.com may post a comment containing the following link to a popular blog they do not otherwise control:

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 of the user that are active for the page. As a result, this list of cookies is sent to the example.com server, and the attacker is then able to collect the cookies of other users.

This type of attack is difficult to detect on the user side, since the script is coming from the same domain that has set the cookie, and the operation of sending the value appears to be authorised by this domain. It is usually considered the responsibility of the administrators running sites where users can post to disallow the posting of such malicious code.

Cookies are not visible to client-side programs such as JavaScript if they have been sent with the HttpOnly flag. From the point of view of the server, the only difference with respect of the normal case is that the set-cookie header line is added a new field containing the string `HttpOnly':

When the browser receives such a cookie, it is supposed to use it as usual in the following HTTP exchanges, but not to make it visible to client-side scripts.^{[citation needed]} The `HttpOnly` flag is not part of any standard, and is not implemented in all common browsers.

References

^ ^a ^b Jay Kesan and Rajiv Shah. Deconstructing Code. Chapter II.B (Netscape's cookies). Yale Journal of Law and Technology, 6, 277–389.
^ ^a ^b David Kristol. HTTP Cookies: Standards, privacy, and politics. ACM Transactions on Internet Technology, 1(2), 151–198, 2001. doi:10.1145/502152.502153
^ Persistent client state - HTTP cookies - Preliminary specification (Netscape)
^ RFC 2109 and RFC 2965 - HTTP State Management Mechanism (IETF)
^ Contrary to popular belief, cookies are good for you! (on the Internet)
^ Keith C. Ivey Untangling the Web Cookies: Just a Little Data Snack. 1998
^ I-034: Internet Cookies. CIAC, United States Department of Energy (ciac.org) (March 12, 1998, revised February 1, 2007). Retrieved on 2007-11-05.
^ Brian Quinton. Study: Users Don't Understand, Can’t Delete Cookies. Direct. May 18, 2005
^ Adam Penenberg. Cookie Monsters. Slate, November 7, 2005
^ The unofficial cookie faq
^ CBS News. CIA Caught Sneaking Cookies.

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