SameSite Cookies

SameSite Cookies

October 1, 2020
Category Defense

SameSite cookies are one of the most impactful modern security mechanisms for fixing security issues that involve cross-site requests. This mechanism allows applications to force browsers to only include cookies in requests that are issued same-site 1. This type of cookie has three modes: None, Lax, and Strict.

The following SameSite cookie modes are available:

  • None – Disables all protections and restores the old behavior of cookies. This mode is not recommended.

    important

    The None attribute must be accompanied by the Secure flag 1.

    1. SameSite cookies explained, link ↩︎

  • Strict – Causes the browser to not include cookies in any cross-site requests. This means <script src="example.com/resource">, <img src="example.com/resource">, fetch(), and XHR will all make requests without the SameSite Strict cookies attached. Even if the user clicks on a link to example.com/resource, their cookies are not included.

  • Lax – The only difference between Lax and Strict is that Lax mode allows cookies to be added to requests triggered by cross-site top-level navigations. This makes Lax cookies much easier to deploy since they won’t break incoming links to your application. Unfortunately, an attacker can trigger a top-level navigation via window.open that allows the attacker to maintain a reference to the window object.

Considerations #

Strict cookies provide the strongest security guarantees, but it can be very difficult to deploy Strict same-site cookies in an existing application.

SameSite cookies are neither bulletproof 2 nor can they fix everything. To complement this defense strategy against XS-Leaks, applications should consider implementing other, additional protections. For example, COOP can prevent an attacker from controlling pages using a window reference after the first navigation even if SameSite cookies in Lax mode are used.

important

Some browers may not use the default of Lax, So explicitly set the SameSite attrbute to ensure its enforced. By default, cookies in Chrome without SameSite attribute will default to Lax mode. However, there is an exception for that behavior for cookies set less than 2 minutes ago that are sent via POST requests. 1

  1. Cookies default to SameSite=Lax, link ↩︎

Deployment #

Anyone interested in deploying this mechanism in web applications should take a careful look at this web.dev article.

References #

  1. SameSite cookies explained, link ↩︎

  2. Bypass SameSite Cookies Default to Lax and get CSRF, link ↩︎