Cookies are one of the methods available for adding persistent state to web sites. Over the years their capabilities have grown and evolved, but left the platform with some problematic legacy issues. To address this, browsers (including Chrome, Firefox, and Edge) are changing their behavior to enforce more privacy-preserving defaults.
Each cookie is a
key=value pair along with a number of attributes that control when and where that cookie is used. You've probably already used these attributes to set things like expiration dates or indicating the cookie should only be sent over HTTPS. Servers set cookies by sending the aptly-named
Set-Cookie header in their response. For all the detail you can dive into RFC6265bis, but for now here's a quick refresher.
Say you have a blog where you want to display a "What's new" promo to your users. Users can dismiss the promo and then they won't see it again for a while. You can store that preference in a cookie, set it to expire in a month (2,600,000 seconds), and only send it over HTTPS. That header would look like this:
Set-Cookie: promo_shown=1; Max-Age=2600000; Secure
When your reader views a page that meets those requirements—they're on a secure connection and the cookie is less than a month old—their browser will send this header in its request:
document.cookie. Making an assignment to
→ document.cookie = "promo_shown=1; Max-Age=2600000; Secure"
← "promo_shown=1; Max-Age=2600000; Secure"
document.cookie will output all the cookies accessible in the current context, with each cookie separated by a semicolon:
← "promo_shown=1; color_theme=peachpuff; sidebar_loc=left"
If you try this on a selection of popular sites you will notice that most of them set significantly more than just three cookies. In most cases, those cookies are sent on every single request to that domain, which has a number of implications. Upload bandwidth is often more restricted than download for your users, so that overhead on all outbound requests is adding a delay on your time to first byte. Be conservative in the number and size of cookies you set. Make use of the
Max-Age attribute to help ensure that cookies don't hang around longer than needed.
What are first-party and third-party cookies? #
If you go back to that same selection of sites you were looking at before, you probably noticed that there were cookies present for a variety of domains, not just the one you were currently visiting. Cookies that match the domain of the current site, that is, what's displayed in the browser's address bar, are referred to as first-party cookies. Similarly, cookies from domains other than the current site are referred to as third-party cookies. This isn't an absolute label but is relative to the user's context; the same cookie can be either first-party or third-party depending on which site the user is on at the time.
Continuing the example from above, let's say one of your blog posts has a picture of a particularly amazing cat in it and it's hosted at
/blog/img/amazing-cat.png. Because it's such an amazing image, another person uses it directly on their site. If a visitor has been to your blog and has the
promo_shown cookie, then when they view
amazing-cat.png on the other person's site that cookie will be sent in that request for the image. This isn't particularly useful for anyone since
promo_shown isn't used for anything on this other person's site, it's just adding overhead to the request.
If that's an unintended effect, why would you want to do this? It's this mechanism that allows sites to maintain state when they are being used in a third-party context. For example, if you embed a YouTube video on your site then visitors will see a "Watch later" option in the player. If your visitor is already signed in to YouTube, that session is being made available in the embedded player by a third-party cookie—meaning that "Watch later" button will just save the video in one go rather than prompting them to sign in or having to navigate them away from your page and back over to YouTube.
One of the cultural properties of the web is that it's tended to be open by default. This is part of what has made it possible for so many people to create their own content and apps there. However, this has also brought a number of security and privacy concerns. Cross-site request forgery (CSRF) attacks rely on the fact that cookies are attached to any request to a given origin, no matter who initiates the request. For example, if you visit
evil.example then it can trigger requests to
your-blog.example, and your browser will happily attach the associated cookies. If your blog isn't careful with how it validates those requests then
evil.example could trigger actions like deleting posts or adding their own content.
Users are also becoming more aware of how cookies can be used to track their activity across multiple sites. However until now there hasn't been a way to explicitly state your intent with the cookie. Your
promo_shown cookie should only be sent in a first-party context, whereas a session cookie for a widget meant to be embedded on other sites is intentionally there for providing the signed-in state in a third-party context.
Explicitly state cookie usage with the
SameSite attribute #
The introduction of the
SameSite attribute (defined in RFC6265bis) allows you to declare if your cookie should be restricted to a first-party or same-site context. It's helpful to understand exactly what 'site' means here. The site is the combination of the domain suffix and the part of the domain just before it. For example, the
www.web.dev domain is part of the
The public suffix list defines this, so it's not just top-level domains like
.com but also includes services like
github.io. That enables
my-project.github.io to count as separate sites.
SameSite attribute on a cookie provides three different ways to control this behaviour. You can choose to not specify the attribute, or you can use
Lax to limit the cookie to same-site requests.
If you set
Strict, your cookie will only be sent in a first-party context. In user terms, the cookie will only be sent if the site for the cookie matches the site currently shown in the browser's URL bar. So, if the
promo_shown cookie is set as follows:
Set-Cookie: promo_shown=1; SameSite=Strict
When the user is on your site, then the cookie will be sent with the request as expected. However when following a link into your site, say from another site or via an email from a friend, on that initial request the cookie will not be sent. This is good when you have cookies relating to functionality that will always be behind an initial navigation, such as changing a password or making a purchase, but is too restrictive for
promo_shown. If your reader follows the link into the site, they want the cookie sent so their preference can be applied.
SameSite=Lax comes in by allowing the cookie to be sent with these top-level navigations. Let's revisit the cat article example from above where another site is referencing your content. They make use of your photo of the cat directly and provide a link through to your original article.
<p>Look at this amazing cat!</p>
<img src="https://blog.example/blog/img/amazing-cat.png" />
<p>Read the <a href="https://blog.example/blog/cat.html">article</a>.</p>
And the cookie has been set as so:
Set-Cookie: promo_shown=1; SameSite=Lax
When the reader is on the other person's blog the cookie will not be sent when the browser requests
amazing-cat.png. However when the reader follows the link through to
cat.html on your blog, that request will include the cookie. This makes
Lax a good choice for cookies affecting the display of the site with
Strict being useful for cookies related to actions your user is taking.
Finally there is the option of not specifying the value which has previously been the way of implicitly stating that you want the cookie to be sent in all contexts. In the latest draft of RFC6265bis this is being made explicit by introducing a new value of
SameSite=None. This means you can use
None to clearly communicate that you intentionally want the cookie sent in a third-party context.
Changes to the default behavior without SameSite #
SameSite attribute is widely supported, it has unfortunately not been widely adopted by developers. The open default of sending cookies everywhere means all use cases work but leaves the user vulnerable to CSRF and unintentional information leakage. To encourage developers to state their intent and provide users with a safer experience, the IETF proposal, Incrementally Better Cookies lays out two key changes:
- Cookies without a
SameSiteattribute will be treated as
- Cookies with
SameSite=Nonemust also specify
Secure, meaning they require a secure context.
Chrome implements this default behavior as of version 84. Firefox has them available to test as of Firefox 69 and will make them default behaviors in the future. To test these behaviors in Firefox, open
about:config and set
network.cookie.sameSite.laxByDefault. Edge also plans to change its default behaviors.
SameSite=Lax by default #
While this is intended to apply a more secure default, you should ideally set an explicit
SameSite attribute rather than relying on the browser to apply that for you. This makes your intent for the cookie explicit and improves the chances of a consistent experience across browsers.
SameSite=None must be secure #
You can test this behavior as of Chrome 76 by enabling
about://flags/#cookies-without-same-site-must-be-secure and from Firefox 69 in
about:config by setting
You will want to apply this when setting new cookies and actively refresh existing cookies even if they are not approaching their expiry date.
Both of these changes are backwards-compatible with browsers that have correctly implemented the previous version of the
SameSite attribute, or just do not support it at all. By applying these changes to your cookies, you are making their intended use explicit rather than relying on the default behavior of the browser. Likewise, any clients that do not recognize
SameSite=None as of yet should ignore it and carry on as if the attribute was not set.
SameSite cookie recipes #
For further detail on exactly how to update your cookies to successfully handle these changes to
SameSite=None and the difference in browser behavior, head to the follow up article, SameSite cookie recipes.
Kind thanks for contributions and feedback from Lily Chen, Malte Ubl, Mike West, Rob Dodson, Tom Steiner, and Vivek Sekhar