 Rowan Merewood [GitHub](https://github.com/rowan-m) [LinkedIn](https://www.linkedin.com/in/rowanmerewood) [Mastodon](https://mastodon.social/@rowan_m) [Bluesky](https://bsky.app/profile/rowan.fyi) [Homepage](https://rowan.fyi)
> [!WARNING]
> **Warning:** Access to third-party cookies may be impacted by [user settings](https://developers.google.com/privacy-sandbox/cookies/basics/cookie-blocking#user_settings), [browser restrictions](https://developers.google.com/privacy-sandbox/cookies/basics/cookie-blocking#browser_restrictions) or [Enterprise policy](https://developers.google.com/privacy-sandbox/cookies/basics/cookie-blocking#enterprise_policy). If you've set `SameSite=None` on your cookies in the past you **will need** to take additional action. [Learn about third-party cookie restrictions](https://developers.google.com/privacy-sandbox/3pcd).
> [!NOTE]
> **Note:** This page is part of a series on the `SameSite` cookie attribute changes that includes the following:
>
> - [Understanding cookies](https://web.dev/articles/understanding-cookies.md.txt)
> - [SameSite cookies recipes](https://web.dev/articles/samesite-cookie-recipes.md.txt)
> - [Schemeful Same-Site](https://web.dev/articles/schemeful-samesite.md.txt)
Browser Support
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Every cookie contains a key-value pair along with a number of attributes that
control when and where that cookie is used.
The introduction of the `SameSite` attribute (defined in
[RFC6265bis](https://tools.ietf.org/html/draft-ietf-httpbis-cookie-same-site-00))
lets you declare whether your cookie is 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 `web.dev` site.
Key term: If the user is on `www.web.dev` and requests an image from
`static.web.dev`, that's a *same-site* request.
The [public suffix list](https://publicsuffix.org/) defines what pages count as
being on the same site. It doesn't just depend on top-level domains like `.com`,
but can also include services like `github.io`. This enables
`your-project.github.io` and `my-project.github.io` to count as separate sites.
Key term: If the user is on `your-project.github.io` and requests an image from
`my-project.github.io` that's a *cross-site* request.
If you're not sure how cookies work, take a look at [What are cookies?](https://goo.gle/cookie-basics)
## Use the `SameSite` attribute to declare cookie usage
The `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
`Strict` or `Lax` to limit the cookie to same-site requests.
If you set `SameSite` to `Strict`, your cookie can only be sent in a
first-party context; that is, if the site for the cookie matches the site shown
in the browser's address bar. So, if the `promo_shown` cookie is set as follows:
```text
Set-Cookie: promo_shown=1; SameSite=Strict
```
When the user is on your site, the cookie is sent with the request as expected.
However, if the user follows a link into your site from another one, the cookie
isn't sent on that initial request.
This is good for cookies relating to features that are always behind an initial
navigation, such as changing a password or making a purchase, but it's too
restrictive for a cookie like `promo_shown`. If your reader follows the link
into the site, they want the cookie sent so their preference can be applied.
`SameSite=Lax` allows the browser to send the cookie with these top-level
navigations. For example, if another site references your site's content, in
this case by using your cat photo and providing a link to your article as
follows:
```html
Look at this amazing cat!
Read the article.
```
With a cookie set to `Lax` as follows:
```text
Set-Cookie: promo_shown=1; SameSite=Lax
```
When the browser requests `amazing-cat.png` for the other person's blog, your
site doesn't send the cookie. However, when the reader follows the
link to `cat.html` on your site, that request does include the cookie.
We recommend using `SameSite` in this way, setting cookies that affect website
display to `Lax`, and cookies related to user actions to `Strict`.
> [!CAUTION]
> **Caution:** Neither `Strict` nor `Lax` is a complete solution for your site's security. Cookies are sent as part of the user's request and you should sanitize and validate them in the same way as any other user input. Never use a cookie to store data you consider a server-side secret.
You can also set `SameSite` to `None` to indicate that you want the cookie to be
sent in all contexts. If you provide a service that other sites consume such as
widgets, embedded content, affiliate programs, advertising, or sign-in across
multiple sites, use `None` to ensure your intent is clear.
 Explicitly mark the context of a cookie as `None`, `Lax`, or `Strict`.
## Changes to the default behavior without SameSite
Browser Support
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The `SameSite` attribute is widely supported, but it hasn't been widely adopted.
In the past, setting cookies without `SameSite` defaulted to sending them in
all contexts, which leaves users 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](https://tools.ietf.org/html/draft-west-cookie-incrementalism-00)
lays out two key changes:
- Cookies without a `SameSite` attribute are treated as `SameSite=Lax`.
- Cookies with `SameSite=None` must also specify `Secure`, meaning they require a secure context.
Both of these changes are backwards-compatible with browsers that have correctly
implemented the previous version of the `SameSite` attribute, as well as
browsers that don't support earlier `SameSite` versions. They're intended to
reduce developers' reliance on browsers' default behavior by making cookie
behavior and intended use explicit. Any clients that don't recognize
`SameSite=None` should ignore it.
### `SameSite=Lax` by default
If you send a cookie without specifying its `SameSite` attribute, the browser
treats that cookie as if it were set to `SameSite=Lax`. We still recommend
explicitly setting `SameSite=Lax` to make your user experience more consistent
across browsers.
> [!CAUTION]
> **Caution:** Chrome's default behavior is slightly more permissive than an explicit `SameSite=Lax`, because it lets sites send some cookies on top-level POST requests. For details, see [the blink-dev announcement](https://groups.google.com/a/chromium.org/d/msg/blink-dev/AknSSyQTGYs/YKBxPCScCwAJ). This is intended as a temporary mitigation. You'll still need to update your cross-site cookies to `SameSite=None; Secure` as described in the next section.
### `SameSite=None` must be secure
When you create cross-site cookies using `SameSite=None`, you must also set them
to `Secure` for the browser to accept them:
```text
Set-Cookie: widget_session=abc123; SameSite=None; 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
by setting `network.cookie.sameSite.noneRequiresSecure` in
[`about:config`](http://kb.mozillazine.org/About:config).
We also recommend updating existing cookies to `Secure` as soon as possible.
If you rely on services that provide third-party content on your site, make sure
your service provider updates their cookies, and update any snippets or
dependencies on your site to make sure it uses the new behavior.
> [!WARNING]
> **Warning:** Some earlier versions of browsers, including Chrome, Safari, and UC browser, are incompatible with the new `None` attribute, and might ignore or restrict the cookie. This behavior is fixed in current versions, but we recommend checking your traffic to determine what proportion of your users are affected. You can see the [list of known incompatible clients](https://www.chromium.org/updates/same-site/incompatible-clients) on the Chromium site.
## `SameSite` cookie recipes
For further details on updating your cookies to successfully handle these
changes to `SameSite=None` and the differences in browser behavior, see the
follow up-article, [SameSite cookie recipes](https://web.dev/articles/samesite-cookie-recipes.md.txt).
*Kind thanks for contributions and feedback from Lily Chen, Malte Ubl, Mike
West, Rob Dodson, Tom Steiner, and Vivek Sekhar.*