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
User preferences, themes, and other settings
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 (
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.
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.
Set-Cookie HTTP response header sends cookies from the server to the user agent. A simple cookie is set like this:
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
GET /sample_page.html HTTP/2.0 Host: www.example.org Cookie: yummy_cookie=choco; tasty_cookie=strawberry
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
Expiresattribute, or after a period of time specified by the
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.
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
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 man-in-the-middle attackers 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
HttpOnly attribute isn't set) can read and modify the information.
A cookie with 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
Path attributes define the scope of a cookie: what URLs the cookies should be sent to.
Domain attribute specifies which hosts can receive a cookie. If the server does not specify a
Domain, the browser defaults the domain 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
Path attribute indicates a URL path that must exist in the requested URL in order to send the
%x2F ("/") character is considered a directory separator, and subdirectories match as well.
For example, if you set
Path=/docs, these request paths match:
But these request paths don't:
SameSite attribute lets servers specify whether/when cookies are sent with cross-site requests (where Site is defined by the registrable domain and the scheme: http or https).
This provides some protection against cross-site request forgery attacks (CSRF).
It takes three possible values:
Strict, the browser only sends the cookie with requests from the cookie's origin site.
Lax is similar, except the browser also sends the cookie when the user navigates to the cookie's origin site (even if the user is coming from a different 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).
SameSite attribute is set, the cookie is treated as
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=Laxis the new default if
SameSiteisn't specified. Previously, cookies were sent for all requests by default.
- Cookies with
SameSite=Nonemust now also specify the
Secureattribute (they require a secure context).
- Cookies from the same domain are no longer considered to be from the same site if sent using a different scheme (
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:
If a cookie name has this prefix, it's accepted in a
Set-Cookieheader only if it's also marked with the
Secureattribute, was sent from a secure origin, does not include a
Domainattribute, and has the
Pathattribute set to
/. This way, these cookies can be seen as "domain-locked".
If a cookie name has this prefix, it's accepted in a
Set-Cookieheader only if it's marked with the
Secureattribute and was sent from a secure origin. This is weaker than the
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
For more information about cookie prefixes and the current state of browser support, see the Prefixes section of the Set-Cookie reference article.
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"
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
- Cookies that are used for sensitive information (such as indicating authentication) should have a short lifetime, with the
SameSiteattribute set to
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.
A cookie is associated with a particular domain and scheme (such as
https), and may also be associated with subdomains if the
Domain attribute is set.
If the cookie domain and scheme match the current page, the cookie is considered to be from the same site as the page, and is referred to as a first-party cookie.
If the domain and scheme are different, the cookie is not considered to be from the same site, and is referred to as 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.
There are some useful features available for developers who wish to respect user privacy, and minimize third-party tracking:
- Servers can (and should) set the cookie SameSite attribute to specify whether or not third-party cookies may be sent.
- Cookies Having Independent Partitioned State (CHIPS) enables developers to opt-in their cookies to partitioned storage, with a separate cookie jar per top-level site. This enables valid non-tracking uses of third-party cookies to continue working in browsers that do not allow cookies to be used for third-party tracking.
- 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:
- Allowing users to opt out of receiving some or all cookies.
- Allowing users to use the bulk of your service without receiving cookies.
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.