SameSite prevents the browser from sending this cookie along with cross-site requests. The main goal is mitigate the risk of cross-origin information leakage. It also provides some protection against cross-site request forgery attacks. Possible values for the flag are
strict value will prevent the cookie from being sent by the browser to the target site in all cross-site browsing context, even when following a regular link. For example, for a GitHub-like website this would mean that if a logged-in user follows a link to a private GitHub project posted on a corporate discussion forum or email, GitHub will not receive the session cookie and the user will not be able to access the project.
A bank website however most likely doesn't want to allow any transactional pages to be linked from external sites so the
strict flag would be most appropriate here.
lax value provides a reasonable balance between security and usability for websites that want to maintain user's logged-in session after the user arrives from an external link. In the above GitHub scenario, the session cookie would be allowed when following a regular link from an external website while blocking it in CSRF-prone request methods (e.g.
As of November 2017 the SameSite attribute is implemented in Chrome, Firefox, and Opera. Since version 12.1 Safari also supports this. Windows 7 with IE 11 lacks support as of December 2018, see caniuse.com below.