Using HTTP cookies

An HTTP cookie (web cookie, browser cookie) is a small piece of data that a server sends to a user's web browser. The browser may store the cookie and send it back to the same server with later requests. Typically, an HTTP cookie is used to tell if two requests come from the same browser—keeping a user logged in, for example. It remembers stateful information for the stateless HTTP protocol.

Cookies are mainly used for three purposes:

Session management

Logins, shopping carts, game scores, or anything else the server should remember

Personalization

User preferences, themes, and other settings

Tracking

Recording and analyzing user behavior

Cookies were once used for general client-side storage. While this made sense when they were the only way to store data on the client, modern storage APIs are now recommended. Cookies are sent with every request, so they can worsen performance (especially for mobile data connections). Modern APIs for client storage are the Web Storage API (localStorage and sessionStorage) and IndexedDB.

Note: To see stored cookies (and other storage that a web page can use), you can enable the Storage Inspector in Developer Tools and select Cookies from the storage tree.

Creating cookies

After receiving an HTTP request, a server can send one or more Set-Cookie headers with the response. The browser usually stores the cookie and sends it with requests made to the same server inside a Cookie HTTP header. You can specify an expiration date or time period after which the cookie shouldn't be sent. You can also set additional restrictions to a specific domain and path to limit where the cookie is sent. For details about the header attributes mentioned below, refer to the Set-Cookie reference article.

The Set-Cookie HTTP response header sends cookies from the server to the user agent. A simple cookie is set like this:

Set-Cookie: <cookie-name>=<cookie-value>

This instructs the server sending headers to tell the client to store a pair of cookies:

HTTP/2.0 200 OK
Content-Type: text/html
Set-Cookie: yummy_cookie=choco
Set-Cookie: tasty_cookie=strawberry

[page content]

Then, with every subsequent request to the server, the browser sends all previously stored cookies back to the server using the Cookie header.

GET /sample_page.html HTTP/2.0
Host: www.example.org
Cookie: yummy_cookie=choco; tasty_cookie=strawberry

Note: Here's how to use the Set-Cookie header in various server-side applications:

The lifetime of a cookie can be defined in two ways:

  • Session cookies are deleted when the current session ends. The browser defines when the "current session" ends, and some browsers use session restoring when restarting. This can cause session cookies to last indefinitely.
  • Permanent cookies are deleted at a date specified by the Expires attribute, or after a period of time specified by the Max-Age attribute.

For example:

Set-Cookie: id=a3fWa; Expires=Thu, 31 Oct 2021 07:28:00 GMT;

Note: When you set an Expires date and time, they're relative to the client the cookie is being set on, not the server.

If your site authenticates users, it should regenerate and resend session cookies, even ones that already exist, whenever a user authenticates. This approach helps prevent session fixation attacks, where a third party can reuse a user's session.

Restrict access to cookies

You can ensure that cookies are sent securely and aren't accessed by unintended parties or scripts in one of two ways: with the Secure attribute and the HttpOnly attribute.

A cookie with the Secure attribute is only sent to the server with an encrypted request over the HTTPS protocol. It's never sent with unsecured HTTP (except on localhost), which means attackers man-in-the-middle can't access it easily. Insecure sites (with http: in the URL) can't set cookies with the Secure attribute. However, don't assume that Secure prevents all access to sensitive information in cookies. For example, someone with access to the client's hard disk (or JavaScript if the HttpOnly attribute isn't set) can read and modify the information.

A cookie with the HttpOnly attribute is inaccessible to the JavaScript Document.cookie API; it's only sent to the server. For example, cookies that persist in server-side sessions don't need to be available to JavaScript and should have the HttpOnly attribute. This precaution helps mitigate cross-site scripting (XSS) attacks.

Here's an example:

Set-Cookie: id=a3fWa; Expires=Thu, 21 Oct 2021 07:28:00 GMT; Secure; HttpOnly

Define where cookies are sent

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

Domain attribute

The Domain attribute specifies which hosts can receive a cookie. If unspecified, the attribute defaults to the same host that set the cookie, excluding subdomains. If Domain is specified, then subdomains are always included. Therefore, specifying Domain is less restrictive than omitting it. However, it can be helpful when subdomains need to share information about a user.

For example, if you set Domain=mozilla.org, cookies are available on subdomains like developer.mozilla.org.

Path attribute

The Path attribute 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 match as well.

For example, if you set Path=/docs, these request paths match:

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

But these request paths don't:

  • /
  • /docsets
  • /fr/docs

SameSite attribute

The SameSite attribute lets servers specify whether/when cookies are sent with cross-site requests (where Site is defined by the registrable domain). This provides some protection against cross-site request forgery attacks (CSRF). It takes three possible values: Strict, Lax, and None.

With Strict, the cookie is only sent to the site where it originated. Lax is similar, except that cookies are sent when the user navigates to the cookie's origin site. For example, by following a link from an external site. None specifies that cookies are sent on both originating and cross-site requests, but only in secure contexts (i.e., if SameSite=None then the Secure attribute must also be set). If no SameSite attribute is set, the cookie is treated as Lax.

Here's an example:

Set-Cookie: mykey=myvalue; SameSite=Strict

Note: The standard related to SameSite recently changed (MDN documents the new behavior above). See the cookies Browser compatibility table for information about how the attribute is handled in specific browser versions:

  • SameSite=Lax is the new default if SameSite isn't specified. Previously, cookies were sent for all requests by default.
  • Cookies with SameSite=None must now also specify the Secure attribute (they require a secure context).

Because of the design of the cookie mechanism, a server can't confirm that a cookie was set from a secure origin or even tell where a cookie was originally set.

A vulnerable application on a subdomain can set a cookie with the Domain attribute, which gives access to that cookie on all other subdomains. This mechanism can be abused in a session fixation attack. See session fixation for primary mitigation methods.

As a defense-in-depth measure, however, you can use cookie prefixes to assert specific facts about the cookie. Two prefixes are available:

__Host-

If a cookie name has this prefix, it's accepted in a Set-Cookie header only if it's also marked with the Secure attribute, was sent from a secure origin, does not include a Domain attribute, and has the Path attribute set to /. This way, these cookies can be seen as "domain-locked".

__Secure-

If a cookie name has this prefix, it's accepted in a Set-Cookie header only if it's marked with the Secure attribute and was sent from a secure origin. This is weaker than the __Host- prefix.

The browser will reject cookies with these prefixes that don't comply with their restrictions. Note that this ensures that subdomain-created cookies with prefixes are either confined to the subdomain or ignored completely. As the application server only checks for a specific cookie name when determining if the user is authenticated or a CSRF token is correct, this effectively acts as a defense measure against session fixation.

Note: On the application server, the web application must check for the full cookie name including the prefix. User agents do not strip the prefix from the cookie before sending it in a request's Cookie header.

For more information about cookie prefixes and the current state of browser support, see the Prefixes section of the Set-Cookie reference article.

JavaScript access using Document.cookie

You can create new cookies via JavaScript using the Document.cookie property. You can access existing cookies from JavaScript as well if the HttpOnly flag isn't set.

document.cookie = "yummy_cookie=choco";
document.cookie = "tasty_cookie=strawberry";
console.log(document.cookie);
// logs "yummy_cookie=choco; tasty_cookie=strawberry"

Cookies created via JavaScript can't include the HttpOnly flag.

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

Security

Note: When you store information in cookies, keep in mind that all cookie values are visible to, and can be changed by, the end user. Depending on the application, you may want to use an opaque identifier that the server looks up, or investigate alternative authentication/confidentiality mechanisms such as JSON Web Tokens.

Ways to mitigate attacks involving cookies:

  • Use the HttpOnly attribute to prevent access to cookie values via JavaScript.
  • Cookies that are used for sensitive information (such as indicating authentication) should have a short lifetime, with the SameSite attribute set to Strict or Lax. (See SameSite attribute, above.) In browsers that support SameSite, this ensures that the authentication cookie isn't sent with cross-site requests. This would make the request effectively unauthenticated to the application server.

Tracking and privacy

Third-party cookies

A cookie is associated with a domain. If this domain is the same as the domain of the page you're on, the cookie is called a first-party cookie. If the domain is different, it's a third-party cookie. While the server hosting a web page sets first-party cookies, the page may contain images or other components stored on servers in other domains (for example, ad banners) that may set third-party cookies. These are mainly used for advertising and tracking across the web. For example, the types of cookies used by Google.

A third-party server can create a profile of a user's browsing history and habits based on cookies sent to it by the same browser when accessing multiple sites. Firefox, by default, blocks third-party cookies that are known to contain trackers. Third-party cookies (or just tracking cookies) may also be blocked by other browser settings or extensions. Cookie blocking can cause some third-party components (such as social media widgets) not to function as intended.

Note: Servers can (and should) set the cookie SameSite attribute to specify whether or not cookies may be sent to third party sites.

Legislation or regulations that cover the use of cookies include:

  • The General Data Privacy Regulation (GDPR) in the European Union
  • The ePrivacy Directive in the EU
  • The California Consumer Privacy Act

These regulations have global reach. They apply to any site on the World Wide Web that users from these jurisdictions access (the EU and California, with the caveat that California's law applies only to entities with gross revenue over 25 million USD, among things).

These regulations include requirements such as:

  • Notifying users that your site uses cookies.
  • Allowing users to opt out of receiving some or all cookies.
  • Allowing users to use the bulk of your service without receiving cookies.

There may be other regulations that govern the use of cookies in your locality. The burden is on you to know and comply with these regulations. There are companies that offer "cookie banner" code that helps you comply with these regulations.

Other ways to store information in the browser

Another approach to storing data in the browser is the Web Storage API. The window.sessionStorage and window.localStorage properties correspond to session and permanent cookies in duration, but have larger storage limits than cookies, and are never sent to a server. More structured and larger amounts of data can be stored using the IndexedDB API, or a library built on it.

There are some techniques designed to recreate cookies after they're deleted. These are known as "zombie" cookies. These techniques violate the principles of user privacy and user control, may violate data privacy regulations, and could expose a website using them to legal liability.

See also