HTTP cookie

An HTTP cookie is a small piece of data sent from a website and stored on the user's computer by the user's web browser while the user is browsing. Cookies were designed to be a reliable mechanism for websites to remember stateful information or to record the user's browsing activity. They can also be used to remember arbitrary pieces of information that the user previously entered into form fields such as names, addresses, passwords, and credit card numbers.

Other kinds of Cookies perform essential functions in the modern web. Perhaps most importantly, authentication cookies are the most common method used by web servers to know whether the user is logged in or not,and which account they are logged in with.

Origin of the name

The term "cookie" was coined by web browser programmer Lou Montulli. It was derived from the term "magic cookie", which is a packet of data a program receives and sends back unchanged, used by Unix programmers

History

Magic cookies were already used in computing when computer programmer Lou Montulli had the idea of using them in web communications in June 1994. At the time, he was an employee of Netscape Communications, which was developing an e-commerce application for MCI. Vint Cerf and John Klensin represented MCI in technical discussions with Netscape Communications. MCI did not want its servers to have to retain partial transaction states, which led them to ask Netscape to find a way to store that state in each user's computer instead.

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Together with John Giannandrea, Montulli wrote the initial Netscape cookie specification the same year. Version 0.9beta of Mosaic Netscape, released on October 13, 1994, supported cookies[citation needed]. 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

The introduction of cookies was not widely known to the public at the time. In particular, cookies were accepted by default, and users were not notified of their presence. The general public learned about cookies after the Financial Times published an article about them on February 12, 1996. In the same year, cookies received a lot of media attention, especially because of potential privacy implications. Cookies were discussed in two U.S. Federal Trade Commission hearings in 1996 and 1997.

Session cookie

A session cookie, also known as an in-memory cookie or transient cookie, exists only in temporary memory while the user navigates the website. Web browsers normally delete session cookies when the user closes the browser. Unlike other cookies, session cookies do not have an expiration date assigned to them, which is how the browser knows to treat them as session cookies.

Java code is not required to be complete or self-contained within a single scriptlet block. It can straddle markup content, provided that the page as a whole is syntactically correct. For example, any Java if/for/while blocks opened in one scriptlet must be correctly closed in a later scriptlet for the page to successfully compile.

Persistent cookie

Instead of expiring when the web browser is closed as session cookies do, a persistent cookie expires at a specific date or after a specific length of time. This means that, for the cookie's entire lifespan, its information will be transmitted to the server every time the user visits the website that it belongs to, or every time the user views a resource belonging to that website from another website.
These cookies are however reset if the expiration time is reached or the user manually deletes the cookie.

Secure cookie

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

HttpOnly cookie

An HttpOnly cookie cannot be accessed by client-side APIs, such as JavaScript. This restriction eliminates the threat of cookie theft via cross-site scripting (XSS). However, the cookie remains vulnerable to cross-site tracing (XST) and cross-site request forgery (XSRF) attacks. A cookie is given this characteristic by adding the HttpOnly flag to the cookie.

SameSite cookie

In 2016 Google Chrome version 51 introduced a new kind of cookie which can only be sent in requests originating from the same origin as the target domain. This restriction mitigates attacks such as cross-site request forgery (XSRF). A cookie is given this characteristic by setting the SameSite flag to Strict or Lax

Third-party cookie

Normally, a cookie's domain attribute will match the domain that is shown in the web browser's address bar. This is called a first-party cookie. A third-party cookie, however, belongs to a domain different from the one shown in the address bar. This sort of cookie typically appears when web pages feature content from external websites, such as banner advertisements. This opens up the potential for tracking the user's browsing history, and is often used by advertisers in an effort to serve relevant advertisements to each user.

Supercookie

A supercookie is a cookie with an origin of a top-level domain (such as .com) or a public suffix (such as .co.uk). Ordinary cookies, by contrast, have an origin of a specific domain name, such as example.com.
Supercookies can be a potential security concern and are therefore often blocked by web browsers. If unblocked by the browser, an attacker in control of a malicious website could set a supercookie and potentially disrupt or impersonate legitimate user requests to another website that shares the same top-level domain or public suffix as the malicious website. For example, a supercookie with an origin of .com, could maliciously affect a request made to example.com, even if the cookie did not originate from example.com. This can be used to fake logins or change user information.
The Public Suffix List helps to mitigate the risk that supercookies pose. The Public Suffix List is a cross-vendor initiative that aims to provide an accurate and up-to-date list of domain name suffixes. Older versions of browsers may not have an up-to-date list, and will therefore be vulnerable to supercookies from certain domains.

Other uses

The term "supercookie" is sometimes used for tracking technologies that do not rely on HTTP cookies. Two such "supercookie" mechanisms were found on Microsoft websites in August 2011: cookie syncing that respawned MUID (machine unique identifier) cookies, and ETag cookies. Due to media attention, Microsoft later disabled this code.

Structure

A cookie consists of the following components:

● Name
● Value
Zero or more attributes (name/value pairs). Attributes store information such as the cookie’s expiration, domain, and flags.

Implementation

Cookies are arbitrary pieces of data, usually chosen and first sent by the web server, and stored on the client computer by the web browser. The browser then sends them back to the server with every request, introducing states (memory of previous events) into otherwise stateless HTTP transactions. Without cookies, each retrieval of a web page or component of a web page would be an isolated event, largely unrelated to all other page views made by the user on the website. Although cookies are usually set by the web server, they can also be set by the client using a scripting language such as JavaScript (unless the cookie's HttpOnly flag is set, in which case the cookie cannot be modified by scripting languages).

The cookie specifications require that browsers meet the following requirements in order to support cookies:

● Can support cookies as large as 4,096 bytes in size.
● Can support at least 50 cookies per domain.
● Can support at least 3,000 cookies in total.

Setting a cookie

Cookies are set using the Set-Cookie HTTP header, sent in an HTTP response from the web server. This header instructs the web browser to store the cookie and send it back in future requests to the server (the browser will ignore this header if it does not support cookies or has disabled cookies).

Example, the browser sends its first request for the homepage of the www.example.org website:

GET /index.html HTTP/1.1
Host: www.example.org


The server responds with two Set-Cookie headers:

HTTP/1.0 200 OK
Content-type: text/html
Set-Cookie: theme=light
Set-Cookie: sessionToken=abc123; Expires=Wed, 09 Jun 2021 10:18:14 GMT

The server's HTTP response contains the contents of the website's homepage. But it also instructs the browser to set two cookies. The first, "theme", is considered to be a session cookie, since it does not have an Expires or Max-Age attribute. Session cookies are intended to be deleted by the browser when the browser closes. The second, "sessionToken" is considered to be a persistent cookie, since it contains an Expires attribute, which instructs the browser to delete the cookie at a specific date and time.

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