HTTP cookies are small bits of text that are included with each HTTP request made from a user’s web browser to web servers. HTTP is a stateless protocol which means each requests is treated as a unique command, and no state data is maintained. Cookies were designed to fill this gap, and allow servers to associate data with user’s as they traverse sites. Cookies are commonly used by sites to hold authentication tokens, tracking ids, or save user preferences.

Alike real life cookies, HTTP cookies come in many different flavors. The three main variations are:

Session cookies are temporary cookies that are kept to manage a user’s state as they browse a website. These are used to retain preferences, log actions, or global variables across multiple pages. They do not have a expiration date, but are deleted automatically when the user closes their browser.

Unlike session cookies, persistent cookies are not deleted when the browser closes. Instead these have an expiration date, and/or lifespan that dictates when browsers should consider them expired, and need to be deleted. Persistent cookies have no upper limit to their lifespan, but user’s can delete them at anytime. These are typically used to keep a user logged into their account.

Third-party cookies are cookies that have a different domain associated with them than the site they were set on. These typically come from externally hosted content such as advertisements. Advertisers like to use these to track users, and their browsing habits.

Cookies are represented as strings, and are composed of a name, value, and several attributes. Cookies must be kept small in size, and may not have a length greater than 4096 bytes. To calculate the length of a cookie it should be “measured by the sum of the length of the cookie’s name, value, and attributes” [1].

Cookies are constrained to such a small size by design as they are automatically sent with every HTTP request a browser makes. While 4096 bytes may seem small in size, you should remember that a single site may use multiple cookies (up to 50), and each extra byte increases the amount of time a round trip from the client to server takes.

At minimum, a server can set a cookie on the user with only a name, and value. For example a site may offer multiple color themes that the user can choose from, and to prevent the user from having to select the theme each page they visit, the site could store a theme cookie on the user via a Set-Cookie header on it’s HTTP response.

Set-Cookie: theme=dark

The above cookie has a name of ‘theme’, and a value of ‘dark’

It’s worth mentioning that when the domain / path attributes of a cookie are omitted, the browser will automatically apply the domain / path that the cookie was received from. Therefore if the above cookie was sent from youtube.com a web browser would interpret the header as:

Set-Cookie: theme=dark Domain=youtube.com; Path=/

Cookies can be set by both the web browser, and the web server. A web server can set a cookie in the user’s browser using the Set-Cookie header on HTTP responses. There is no set limit on how many Set-Cookie headers may be included per HTTP response. Web browsers on the other hand can set cookies via JavaScript, and can access any cookie that does not have an HttpOnly attribute.

There’s no direct command for a server to use to tell a web browser to delete a cookie. Instead the server must send a Set-Cookie header with the cookies name, and an expiration date that has already passed.

Attributes can be used to control specific behaviors of the cookie. Attribute headers are considered case insensitive. And are provided after the name, and content of the cookie. When a cookie has more than one attribute a ; must be used to delimit them.

The expiration date dictates how long the browser should consider the cookie valid before it is deleted. Setting this to an exorbitantly high value such as in the year 2525 will not guarantee the cookie will be kept for that long as user’s can purge cookies from their browser at any time.

When no expiration date is provided the cookie is treated as a session cookie and is deleted once the user closes the tab or window with the web page loaded.

Expiration dates should be in UTC, or GMT format

foo=bar Expires=Thu, 24 Jan 2019 00:00:00 UTC

The max-age attribute is the number of seconds a cookie has to live before it is considered expired. Cookies may have both a max-age, and expires attribute but the max-age attribute will take precedence.

foo=bar Max-Age=3600

The domain attribute indicates the site that owns the cookie. A site such as widgets.com would specify their domain with the attribute as: Domain=widgets.com; It should be noted that a server may only set a domain with a URL that indicates the origin. Referring back to widgets.com while a domain attribute of Domain=widgets.com; is perfectly acceptable a domain attribute of Domain=google.com would be rejected.

foo=bar Domain=google.com

The path attribute can be used to further limit the scope of a cookie. If a cookie was set with a path of Path=/store for widgets.com then the cookie would only be supplied by the user when they visited a matching URL such as widgets.com/store

foo=bar Domain=google.com; Path=/

The secure attribute restricts cookies to only be used when the user is navigating the site via HTTPS. A cookie may not be set with the secure attribute unless the server is already communicating with the user through HTTPS.

foo=bar Secure

Normally cookies can be accessed by browsers via the document.cookie property in JavaScript. However, when the HttpOnly attribute is set, the cookie cannot be accessed by any code on the web page, and document.cookie will not include it. This attribute can be set with or without the secure attribute as they are independent of each other.

foo=bar HttpOnly

[1] RFC6265 HTTP State Management Mechanism. A. Barth. April 2011. (Format: TXT=79724 bytes) (Obsoletes RFC2965) (Status: PROPOSED STANDARD) (DOI: 10.17487/RFC6265) https://tools.ietf.org/html/rfc6265

[2] Wikipedia contributors. (2019, January 8). HTTP cookie. In Wikipedia, The Free Encyclopedia. Retrieved 01:12, January 25, 2019, from https://en.wikipedia.org/w/index.php?title=HTTP_cookie&oldid=877337327

[Image] By Cel Lisboa on Unsplash https://unsplash.com/photos/YnrSLOAjOEA