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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 and the contents of their Electronic Shopping Cart s. 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. 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 criticised because the identification of users they provide is not always accurate and because they could potentially be used for network attacks. Some alternatives to cookies exist, but each has its own drawbacks. Cookies are also subject to a number of misconceptions, mostly based on the erroneous notion that they are Computer Program s. 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. Most modern browsers allow users to decide whether to accept cookies, but rejection makes some Website s unusable. For example, shopping baskets implemented using cookies do not work if cookies are rejected. 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"),Jay Kesan and Rajiv Shah. Deconstructing Code . Chapter II.B (Netscape's cookies). Yale Journal of Law and Technology, 6, 277–389.David Kristol. HTTP Cookies: Standards, privacy, and politics. ACM Transactions on Internet Technology, 1(2), 151–198, 2001. 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. 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. Several 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. Cookies are also used to track users across a website. Third-party cookies and Web Bug s, 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. 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. Cookie specifications Persistent client state - HTTP cookies - Preliminary specification (Netscape)RFC 2109 and RFC 2965 - HTTP State Management Mechanism ( IETF ) 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 . Relevant count of maximum stored cookies per domain for the major browsers are:
In practice cookies must be smaller than 4k. MSIE imposes a 4k total for all cookies stored in a given domain. Cookie names are case insensitive according to section 3.1 of RFC 2965 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. 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 In 2005, 2005 according to which a consistent percentage of respondents believed some of the following false claims:
Cookies are in fact only data, not program code: they cannot erase or read information from the user's computer.Adam Penenberg. Cookie Monsters . Slate , November 7 2005 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. 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: The unofficial cookie faq # cookies are never accepted, # the browser asks the user whether to accept every individual cookie or allows them to generate a White List of acceptable sites, or # cookies are always accepted, or # cookies are accepted, except from sites specified by the user, generating a Black List for cookies. Cookie Manager, showing the details of various cookies by domain]] The browser may also 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. 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.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. 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''. 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 Bug s. Knowledge of the pages visited by a user allows the advertisement company to target advertisement to the user's presumed preferences. 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 2005 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. 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. 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 Account s. Cookie theft 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. Cookies theft is any process allowing an unauthorised party to receive a cookie. A first way cookies can be stolen is via Packet Sniffing . Traffic on a network can be read by computers on the network other than its sender and its receiver. This traffic includes cookies sent on ordinary HTTP sessions. Users on these computers can read the traffic on the network, including the cookies, using programs called Packet Sniffer s. This problem can be overcome by using the Https URI scheme, which invokes Transport Layer Security 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. A different way to steal cookies is Cross-site Scripting , which makes 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. "Can you show me what XSS cookie theft looks like?" (excerpt from the Cgisecurity Cross-Site Scripting FAQ ) This possibility 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. A way for preventing such attacks is by the HttpOnly flag; Microsoft. Mitigating Cross-site Scripting With HTTP-only Cookies this is a Microsoft option that makes a cookie inaccessible to client side script. 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. Cross-site cooking Each site is supposed to have its own cookies, so a site like 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. 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 Address es 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. 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 , or proxy and then follow two different links. Moreover, this technique only allows tracking and cannot replace cookies in their other uses. 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. 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. 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. 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 . 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. 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. HTTP authentication As for authentication, the HTTP protocol includes the created by the server. 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. 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. Introduction to Persistence , MSDN 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.WWW FAQs: How do I pass data between JavaScript pages? Thomas Boutell 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 |