SameSiteCookies

SameSiteCookies refers to the SameSite cookie attribute, a mechanism used to control whether cookies are sent with cross-site requests. It is primarily designed to reduce the risk of cross-site request forgery and to give developers control over cookie scope in web applications.

The attribute offers three values. SameSite=Strict restricts cookies to be sent only in first-party contexts, meaning they are not sent when requests originate from a different site. SameSite=Lax allows cookies to be sent on top-level navigations to the site but blocks most cross-site subrequests, such as images or iframes loaded from another site. SameSite=None permits cookies to be sent in all contexts, including cross-site requests; however, cookies using None must be Secure, i.e., transmitted over HTTPS.

Behavior for cookies without a SameSite attribute varies by browser and version. Historically, cookies without the attribute were sent with all requests to the origin. In modern browsers, a default of SameSite=Lax is commonly applied when the attribute is omitted, but developers should not rely on this and should explicitly set SameSite to ensure predictable behavior across clients.

Implications for web applications are significant. Strict can break legitimate cross-origin flows such as OAuth, third-party widgets, or content loaded from a different domain. Lax often provides a middle ground that preserves some cross-site navigations while reducing CSRF risk. None is required for cross-site usage by trusted integrations but demands Secure, otherwise the cookie may be blocked by browsers.

Implementation occurs at cookie creation, for example in the Set-Cookie header, by including SameSite=Strict, SameSite=Lax, or SameSite=None, along with Secure if None is used. All major modern browsers support SameSite, though exact behavior may vary by version, so explicit configuration and testing are advised.