First-party cookie recipes
The distinction between first-party and third-party context on the web isn't always obvious and the effect it has on different resources can vary. To address some of the challenges with how browsers treat first-party and third-party cookies, First-Party Sets proposes to allow related domain names owned and operated by the same entity to declare themselves as belonging to the same first party.
The good first-party cookie recipe
The following configuration is best practice, ensuring security and cross-browser compatibility for most first-party cookies. It will provide you with a safe foundation, which you can adjust to open up permissions only when necessary. This article also covers recipe variations for some specific use-cases.
The recipe
Set-Cookie:
__Host-cookie-name=cookie-value;
Secure;
Path=/;
HttpOnly;
Max-Age=7776000;
SameSite=Lax;
Details
__Host is an optional prefix that makes some attributes mandatory and forbids others:
Securemust be presentDomainmust be omittedPathmust be/
With __Host added, you can rely on the browser to check if attributes above are set in line with __Host rules and reject the cookie if not.
Secure protects cookies from being stolen on insecure networks because it only allows sending cookies over HTTPS connections. If you haven't fully migrated your site to HTTPS, make that a priority.
The Domain attribute specifies which hosts can receive a cookie. Omitting it restricts the cookie to the current document host, excluding subdomains: the cookie for example.com will be sent on every request to example.com but not on requests to images.example.com. If you have different apps running on different subdomains, this reduces the risk of one compromised domain allowing a door into the others.
Path indicates the path that must exist in the requested URL for the browser to send the Cookie header. Setting Path=/ means that the cookie is sent to all URL paths on that domain. The combination of no Domain and Path=/ makes the cookie bound to the origin as closely as possible, so it behaves similarly to other client-side storage such as LocalStorage—there's no confusion that example.com/a might receive different values to example.com/b.
The HttpOnly attribute adds some protection against malicious third-party scripts on your sites by restricting JavaScript access. It allows a cookie to be sent only in request headers and makes them unavailable to JavaScript via document.cookie.
Even with HttpOnly, you can still trigger requests from JavaScript, like fetch or XML HTTP requests—and if you've specified including credentials, cookies (including HTTP only cookies) will be sent on those requests—they're just not visible to those client-side scripts in any way. If any of those scripts on your site have been compromised or are malicious, their access to potentially sensitive cookie data is limited.
Max-Age limits the life of a cookie as browser sessions can last a pretty long time and you don't want stale cookies hanging around forever. It's good for short-term cookies, such as user sessions or even shorter ones such as tokens for form submission. Max-Age is defined in seconds and in the example above it's set to 7776000 seconds which is 90 days. This is a reasonable default, which you can change depending on your use case.
The maximum value of Max-Age must not be greater than 400 days (34560000 seconds). Another way to limit the life of a cookie is specifying the Expires attribute which sets an expiration date in the future, instead of an expiration duration. Note that the date and time set in the Expires attribute are relative to the client the cookie is being set on, not the server.
SameSite=Lax restricts the cookie to only be sent on same-site requests. That is, where the request matches the current browsing context–the top level site the user is currently visiting which is displayed in their location bar. SameSite=Lax is the default in modern browsers but it's good practice to specify it for compatibility across browsers which may have different defaults. By explicitly marking the cookie as same-site only, you are restricting it to your first-party contexts, and you should not have to make changes to that cookie when third-party cookies go away.
To learn more about different cookie attributes, check out Set-Cookie documentation on MDN.
First-party cookie recipe for sites with subdomains
If you have a site with subdomains and want to have one session across all of them, the Host prefix can be too restrictive. For example news.site could have subdomains for topics, such as finance.news.site and sport.news.site and you'd want one user session on all of them. In that case, use the __Secure prefix instead of __Host and specify Domain.
The recipe
Set-Cookie:
__Secure-cookie-name=cookie-value;
Secure;
Domain=news.site;
Path=/;
HttpOnly;
Max-Age=7776000;
SameSite=Lax;
Details
__Secure is an optional prefix which asserts fewer requirements than __Host: it only requires that the cookie be set with the Secure attribute.
The Secure attribute restricts the cookie to be sent only over the HTTPS protocol. The __Secure prefix tells the browser to check if the cookie is set with the Secure attribute and reject it if it's not. This ensures that an attacker can not overwrite secure cookies with another cookie that is missing the Secure attribute.\
Restricting first-party cookie access on requests initiated from third-party websites
While SameSite=Lax cookies are not sent on cross-site subrequests (for example, when loading embedded images or iframes on a third-party site), they are sent when a user is navigating to the origin site (for example, when following a link from a different site).
You can further restrict cookies access and disallow sending them along with requests initiated from third-party websites with SameSite=Strict. This is useful when you have cookies relating to functionality that will always be behind an initial navigation, such as changing a password or making a purchase.
The recipe
Set-Cookie:
__Host-cookie-name=cookie-value;
Secure;
Path=/;
HttpOnly;
Max-Age=7776000;
SameSite=Strict;
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