samesite

SameSite is a cookie attribute used by web browsers to control whether a cookie is sent with cross-site requests. It is set in the Set-Cookie header as SameSite=value and works with other cookie attributes such as Secure and HttpOnly. The goal is to reduce the chances that a cookie is sent in contexts initiated by other sites, thereby lowering the risk of certain cross-site attacks such as CSRF (cross-site request forgery).

The attribute supports three values: Strict, Lax, and None. Strict means the cookie is sent only in first‑party contexts; cross-site requests will not include the cookie. Lax allows the cookie to be sent for top‑level navigations initiated by a cross‑site URL, but not for most subresource requests like images or iframes. None allows the cookie to be sent in all contexts, including cross‑site requests, but it requires the cookie to be Secure (transmitted over HTTPS) in modern browsers.

Compatibility and defaults: If a cookie omits the SameSite attribute, many browsers treat it as SameSite=Lax by default, which constrains cross‑site usage unless explicitly overridden. Developers should explicitly set SameSite to the intended value. For session cookies that authenticate a user, Lax is a common safe default; Strict provides stronger protection but can disrupt certain navigation flows. None is used for cross‑origin use cases such as third‑party widgets, but it should be paired with additional CSRF protections.

History and impact: The SameSite attribute gained prominence in the 2010s as browsers standardized anti‑CSRF measures. Today, major browsers support SameSite, with evolving default handling for cookies that do not specify the attribute and for those marked None. Proper configuration helps balance security with the functional needs of web applications.