Cookies are not software. They cannot be programmed, cannot carry viruses, and cannot install malware on the host computer.[2] However, they can be used by spyware to track user's browsing activities – a major privacy concern that prompted European and US law makers to take action.[3][4] Cookies can also be stolen by hackers to gain access to a victim's web account.[5]
History
The term "cookie" was derived from "magic cookie", which is the packet of data a program receives and sends again unchanged. Magic cookies were already used in computing when computer programmer Lou Montulli had the idea of using them in Web communications in June 1994.[6] 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.[7][8]
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,[9][10] supported cookies. The first use of cookies (out of the labs) was checking whether visitors to the Netscape website had already visited the site. Montulli applied for a patent for the cookie technology in 1995, and US 5774670 was granted in 1998. Support for cookies was integrated in Internet Explorer in version 2, released in October 1995.[11]
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. 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 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.
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 state in 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. In February 1996, the working group identified third-party cookies as a considerable privacy threat. The specification produced by the group was eventually published as RFC 2109 in February 1997. It specifies that third-party cookies were either not allowed at all, or at least not enabled by default.
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.
A definitive specification for cookies as used in the real world was published as RFC 6265 in April 2011.
Session cookie
A session cookie[12] only lasts for the duration of users using the website. A web browser normally deletes session cookies when it quits. A session cookie is created when no Expires directive is provided when the cookie is created.
Persistent cookie
A persistent cookie[12] will outlast user sessions. If a persistent cookie has its Max-Age set to 1 year, then, within the year, the initial value set in that cookie would be sent back to the server every time the user visited the server. This could be used to record a vital piece of information such as how the user initially came to this website. For this reason, persistent cookies are also called tracking cookies or in-memory cookies.
Secure cookie
A secure cookie is only used when a browser is visiting a server via HTTPS, ensuring that the cookie is always encrypted when transmitting from client to server. This makes the cookie less likely to be exposed to cookie theft via eavesdropping.
HttpOnly cookie
The HttpOnly cookie is supported by most modern browsers.[13][14] On a supported browser, an HttpOnly session cookie will be used only when transmitting HTTP (or HTTPS) requests, thus restricting access from other, non-HTTP APIs (such as JavaScript). This restriction mitigates but does not eliminate the threat of session cookie theft via Cross-site scripting.[15] This feature applies only to session-management cookies, and not other browser cookies.
Third-party cookie
First-party cookies are cookies set with the same domain (or its subdomain) in your browser's address bar. Third-party cookies are cookies being set with different domains than the one shown on the address bar (i.e. the web pages on that domain may feature content from a third-party domain - e.g. an advertisement run by www.advexample.com showing advert banners).
For example: Suppose a user visits www.example1.com, which sets a cookie with the domain ad.foxytracking.com. When the user later visits www.example2.com, another cookie is set with the domain ad.foxytracking.com. Eventually, both of these cookies will be sent to the advertiser when loading their ads or visiting their website. The advertiser can then use these cookies to build up a browsing history of the user across all the websites this advertiser has footprints on.
Supercookie
A "supercookie" is a cookie with a public suffix domain, like .com, .co.uk or k12.ca.us.[16]
Most browsers, by default, allow first-party cookies—a cookie with domain to be the same or sub-domain of the requesting host. For example, a user visiting www.example.com can have a cookie set with domain www.example.com or .example.com, but not .com.[17] A supercookie with domain .com would be blocked by browsers; otherwise, a malicious website, like attacker.com, could set a supercookie with domain .com and potentially disrupt or impersonate legitimate user requests to example.com. The Public Suffix List is a cross-vendor initiative to provide an accurate list of domain name suffixes changing.[18] Older versions of browsers may not have the most up-to-date list, and will therefore be vulnerable to certain supercookies.
The term "supercookies" is erroneously used in the media for tracking technologies that do not rely on HTTP cookies. Two such "supercookie" mechanisms were found on Microsoft websites: cookie syncing that respawned MUID cookies, and ETag cookies.[19] Due to media attention, Microsoft later disabled this code:
In response to recent attention on "supercookies" in the media, we wanted to share more detail on the immediate action we took to address this issue, as well as affirm our commitment to the privacy of our customers. According to researchers, including Jonathan Mayer at Stanford University, "supercookies" are capable of re-creating users' cookies or other identifiers after people deleted regular cookies. Mr. Mayer identified Microsoft as one among others that had this code, and when he brought his findings to our attention we promptly investigated. We determined that the cookie behavior he observed was occurring under certain circumstances as a result of older code that was used only on our own sites, and was already scheduled to be discontinued. We accelerated this process and quickly disabled this code. At no time did this functionality cause Microsoft cookie identifiers or data associated with those identifiers to be shared outside of Microsoft.
—Mike Hintze[20]
Zombie cookie
Main article: Zombie cookie
A zombie cookie is any cookie that is automatically recreated after a user has deleted it. This is accomplished by a script storing the content of the cookie in some other locations, such as the local storage available to Flash content, HTML5 storages and other client side mechanisms, and then recreating the cookie from backup stores when the cookie's absence is detected.
Implementation
A possible interaction between a Web browser and a server holding a Web page, in which the server sends a cookie to the browser and the browser sends it back when requesting another page.
Cookies are arbitrary pieces of data 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. 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[14][21][22] suggest that browsers should be able to save and send back a minimal number of cookies. In particular, a web browser is expected to be able to store at least 300 cookies of four kilobytes each, and at least 20 cookies per server or domain.
[edit] 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 usually short text called HTTP request. For example, to access the page http://www.example.org/index.html, browsers connect to the server www.example.org sending it a request that looks like the following one:
GET /index.html HTTP/1.1
Host: www.example.org
browser
-------→
server
The server replies by sending the requested page preceded by a similar packet of text, called 'HTTP response'. This packet may contain lines requesting the browser to store cookies:
HTTP/1.1 200 OK
Content-type: text/html
Set-Cookie: name=value
Set-Cookie: name2=value2; Expires=Wed, 09 Jun 2021 10:18:14 GMT
(content of page)
browser
←-------
server
The server sends lines of Set-Cookie only if the server wishes the browser to store cookies. Set-Cookie is a directive for the browser to store the cookie and send it back in future requests to the server (subject to expiration time or other cookie attributes), if the browser supports cookies and cookies are enabled. For example, the browser requests the page http://www.example.org/spec.html by sending the server www.example.org a request like the following:
GET /spec.html HTTP/1.1
Host: www.example.org
Cookie: name=value; name2=value2
Accept: */*
browser
-------→
server
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.[23] 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, however, may be implemented using cookies.
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.[24]
Cookie attributes
Besides the name-value pair, servers can also set these cookie attributes: a cookie domain, a path, expiration time or maximum age, secure flag and httponly flag. Browsers will not send cookie attributes back to the server. They will only send the cookie’s name-value pair. Cookie attributes are used by browsers to determine when to delete a cookie, block a cookie or whether to send a cookie (name-value pair) to the servers.
Domain and Path
The cookie domain and path define the scope of the cookie—they tell the browser that cookies should only be sent back to the server for the given domain and path. If not specified, they default to the domain and path of the object that was requested. An example of Set-Cookie directives from a website after a user logged in:
Set-Cookie: LSID=DQAAAK…Eaem_vYg; Domain=docs.foo.com; Path=/accounts; Expires=Wed, 13-Jan-2021 22:23:01 GMT; Secure; HttpOnly
Set-Cookie: HSID=AYQEVn….DKrdst; Domain=.foo.com; Path=/; Expires=Wed, 13-Jan-2021 22:23:01 GMT; HttpOnly
Set-Cookie: SSID=Ap4P….GTEq; Domain=.foo.com; Path=/; Expires=Wed, 13-Jan-2021 22:23:01 GMT; Secure; HttpOnly
......
The first cookie LSID has default domain docs.foo.com and Path /accounts, which tells the browser to use the cookie only when requesting pages contained in docs.foo.com/accounts. The other 2 cookies HSID and SSID would be sent back by the browser while requesting any subdomain in .foo.com on any path, for example www.foo.com/.
Cookies can be set on only top domain and its sub domains. Setting cookies on www.foo.com from www.bar.com will not work for security reasons.[25]
Expires and Max-Age
The Expires directive tells the browser when to delete the cookie. It is specified in the form of “Wdy, DD-Mon-YYYY HH:MM:SS GMT”, indicating the exact date/time this cookie will expire. As an alternative to setting cookie expiration as an absolute date/time, RFC 6265 allows the use of the Max-Age attribute to set the cookie’s expiration as an interval of seconds in the future, relative to the time the browser received the cookie. An example of Set-Cookie directives from a website after a user logged in:
Set-Cookie: lu=Rg3vHJZnehYLjVg7qi3bZjzg; Expires=Tue, 15-Jan-2013 21:47:38 GMT; Path=/; Domain=.foo.com; HttpOnly
Set-Cookie: made_write_conn=1295214458; Path=/; Domain=.foo.com
Set-Cookie: reg_fb_gate=deleted; Expires=Thu, 01-Jan-1970 00:00:01 GMT; Path=/; Domain=.foo.com; HttpOnly
......
The first cookie lu is set to expire sometime in 15-Jan-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 his/her 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 – note that cookie will only be deleted when the domain and path attributes in the Set-Cookie field match the values used when the cookie was created.
Secure and HttpOnly
The Secure and HttpOnly attributes do not have associated values. Rather, the presence of the attribute names indicates that the Secure and HttpOnly behaviors are specified.
The Secure attribute is meant to keep cookie communication limited to encrypted transmission, directing browsers to use cookies only via secure/encrypted connections. Naturally, web servers should set Secure cookies via secure/encrypted connections, lest the cookie information be transmitted in a way that allows eavesdropping when first sent to the web browser.
The HttpOnly attribute directs browsers to use cookies via the HTTP protocol only. An HttpOnly cookie is not accessible via non-HTTP methods, such as calls via JavaScript (e.g., referencing "document.cookie"), and therefore cannot be stolen easily via cross-site scripting (a pervasive attack technique[26]). As shown in previous examples, both Facebook and Google use the HttpOnly attribute extensively.
tags:Zombie cookie,super cookie,Persistent cookie,Secure cookie
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