SameSite cookie recipes

Update your site’s cookies to prepare for the upcoming changes to the SameSite attribute’s behavior.

Oct 30, 2019
• Updated Apr 9, 2020

Chrome,
Firefox,
Edge,
and others will be changing their default behavior in line with the IETF
proposal,
Incrementally Better Cookies
so that:

• Cookies without a SameSite attribute will be treated as SameSite=Lax,
  meaning the default behavior will be to restrict cookies to first party
  contexts only.
• Cookies for cross-site usage must specify SameSite=None; Secure to
  enable inclusion in third party context.

This feature was scheduled to be the default behavior from Chrome 80
onward but
has been temporarily rolled
back to
improve the stability of the platform. If you have not already done so, should
update the attributes for your third-party cookies so they will not be blocked
in the future.

Cross-browser support #

See the
Browser compatibility
section of MDN’s Set-Cookie
page.

Use cases for cross-site or third-party cookies #

There are a number of common use cases and patterns where cookies need to be
sent in a third-party context. If you provide or depend on one of these use
cases, ensure that either you or the provider are updating their cookies to
ensure the service continues to function correctly.

Content within an <iframe> #

Content from a different site displayed in an <iframe> is in a third-party
context. Standard use cases here are:

• Embedded content shared from other sites, such as videos, maps, code samples,
  and social posts.
• Widgets from external services such as payments, calendars, booking, and
  reservation functionality.
• Widgets such as social buttons or anti-fraud services that create less obvious
  <iframes>.

Cookies may be used here to, among other things, maintain session state, store
general preferences, enable statistics, or personalize content for users with
existing accounts.

Additionally, as the web is inherently composable, <iframes> are used to embed
content that is also viewed in a top-level or first-party context. Any cookies
used by that site will be considered as third-party cookies when the site is
displayed within the frame. If you’re creating sites that you intend to be
easily embedded by others while also relying on cookies to function, you will
also need to ensure those are marked for cross-site usage or that you can
gracefully fallback without them.

“Unsafe” requests across sites #

While “unsafe” may sound slightly concerning here, this refers to any request
that may be intended to change state. On the web that’s primarily POST requests.
Cookies marked as SameSite=Lax will be sent on safe top-level navigations,
e.g. clicking a link to go to a different site. However something like a
<form> submission via POST to a different site would not include cookies.

This pattern is used for sites that may redirect the user out to a remote
service to perform some operation before returning, for example redirecting to a
third-party identity provider. Before the user leaves the site, a cookie is set
containing a single use token with the expectation that this token can be
checked on the returning request to mitigate
Cross Site Request Forgery (CSRF)
attacks. If that returning request comes via POST then it will be necessary to
mark the cookies as SameSite=None; Secure.

Remote resources #

Any remote resource on a page may be relying on cookies to be sent with a
request, from <img> tags, <script> tags, and so on. Common use cases include
tracking pixels and personalizing content.

This also applies to requests initiated from your JavaScript by fetch or
XMLHttpRequest. If fetch() is called with the
credentials: 'include' option
this is a good indication that cookies may well be expected on those requests.
For XMLHttpRequest you should look for instances of the
withCredentials property
being set to true. This is a good indication that cookies may well be expected
on those requests. Those cookies will need to be appropriately marked to be
included in cross-site requests.

Content within a WebView #

A WebView in a native app is powered by a browser and you will need to test if
the same restrictions or issues apply. In Android, if the WebView is powered by
Chrome the new defaults will not immediately be applied with Chrome 80.
However the intent is to apply them in the future, so you should still test and
prepare for this. Additionally, Android allows native apps to set cookies
directly via the
CookieManager API.
As with cookies set via headers or JavaScript, consider including
SameSite=None; Secure if they are intended for cross-site use.

How to implement SameSite today #

For cookies where they are only needed in a first-party context you should
ideally mark them as SameSite=Lax or SameSite=Strict depending on your
needs. You can also choose to do nothing and just allow the browser to enforce
its default, but this comes with the risk of inconsistent behavior across
browsers and potential console warnings for each cookie.

Set-Cookie: first_party_var=value; SameSite=Lax

For cookies needed in a third-party context, you will need to ensure they are
marked as SameSite=None; Secure. Note that you need both attributes together.
If you just specify None without Secure the cookie will be rejected. There
are some mutually incompatible differences in browser implementations though, so
you may need to use some of the mitigating strategies described in
Handling incompatible clients below.

Set-Cookie: third_party_var=value; SameSite=None; Secure

Handling incompatible clients #

As these changes to include None and update default behavior are still
relatively new, there are inconsistencies amongst browsers as to how these
changes are handled. You can refer to the
updates page on chromium.org
for the issues currently known, however it’s not possible to say if this is
exhaustive. While this is not ideal, there are workarounds you can employ during
this transitionary phase. The general rule though is to treat incompatible
clients as the special case. Do not create an exception for browsers
implementing the newer rules.

The first option is to set both the new and old style cookies:

Set-cookie: 3pcookie=value; SameSite=None; Secure
Set-cookie: 3pcookie-legacy=value; Secure

Browsers implementing the newer behavior will set the cookie with the SameSite
value, while other browsers may ignore or incorrectly set it. However, those
same browsers will set the 3pcookie-legacy cookie. When processing included
cookies, the site should first check for the presence of the new style cookie
and if it’s not found, then fallback to the legacy cookie.

The example below shows how to do this in Node.js, making use of the
Express framework and its
cookie-parser middleware.

const express = require('express');
const cp = require('cookie-parser');
const app = express();
app.use(cp());
app.get('/set', (req, res) => {
res.cookie('3pcookie', 'value', { sameSite: 'none', secure: true });
res.cookie('3pcookie-legacy', 'value', { secure: true });
res.end();
});
app.get('/', (req, res) => {
let cookieVal = null;
if (req.cookies['3pcookie']) {
cookieVal = req.cookies['3pcookie'];
} else if (req.cookies['3pcookie-legacy']) {
cookieVal = req.cookies['3pcookie-legacy'];
}
res.end();
});
app.listen(process.env.PORT);

The downside is that this involves setting redundant cookies to cover all
browsers and requires making changes both at the point of setting and reading
the cookie. However, this approach should cover all browsers regardless of their
behavior and ensure third-party cookies continue to function as before.

Alternatively at the point of sending the Set-Cookie header, you can choose to
detect the client via the user agent string. Refer to the
list of incompatible clients
and then make use of an appropriate library for your platform, for example
ua-parser-js library on Node.js.
It’s advisable to find a library to handle user agent detection as you most
probably don’t want to write those regular expressions yourself.

The benefit of this approach is that it only requires making one change at the
point of setting the cookie. However, the necessary warning here is that user
agent sniffing is inherently fragile and may not catch all of the affected
users.

Support for SameSite=None in languages, libraries, and frameworks #

The majority of languages and libraries support the SameSite attribute for
cookies, however the addition of SameSite=None is still relatively new which
means that you may need to work around some of the standard behavior for now.
These are documented in the
SameSite examples repo on GitHub.

Getting help #

Cookies are all over the place and it’s rare for any site to have completely
audited where they’re set and used, especially once you throw cross-site use
cases in the mix. When you encounter an issue, it may well be the first time
anyone has encountered it – so don’t hesitate to reach out:

Cookie hero image by
Cayla1
on
Unsplash