The SameSite is a cookie attribute that tells browser if the cookie value should be send to the server or not when the request is made from a different domain of the website.
For example, when you dont want to sent the cookie from a different url.
mycookie=value; expires=Wen, 1 Jan 2019 12:00:00 UTC; path=/; SameSite=Strict
mycookie=value; expires=Wen, 1 Jan 2019 12:00:00 UTC; path=/; SameSite=Lax
The default scenario is to sent it always.
The great benefit of this feature is the extra security it provides from CSRF attacks.
Like the following example: You have an endpoint that updates a user:
The new data should come from a POST method, so if you open the endpoint fromm a link, the update will not happen because the request method in this case is GET. But someone could build this simple html file:
<form id="f" action="http://mysite.com/user/update/2" method="post"> <input type="hidden" name="name" value="New Username"> <input type="hidden" name="email" value="email@example.com"> <input type="hidden" name="password" value="new-password"> </form> <script> f.submit() </script>
This index.html can be placed in any website (like clickhereandwhathappens.com), and the link is send it to the website administrator with an email. When the administrator opens this html in the browser, the form will be submited to mysite.com and the user will be updated. This will happen because the sessionId cookie of the user will be sent to the server and the application understants that this request was made from the administrator.
SameSite the sessionId cookie will not be sent from a different domain from mysite.com
This feature is already supported from all major browsers https://caniuse.com/#feat=same-site-cookie-attribute
So this is an extra layer of security for your website and very easy to implement.
To create a strict cookie with php:
header("Set-cookie: mycookie=value; path=/; HttpOnly; SameSite=Lax");
From php 7.3 you can use setcookie function with the options (8th) parameter:
setcookie('mycookie', 'value', time()+86400, '/', null, null, true, ['samesite'=>'Strict']);
To set the expiration date with header() you must print date with the expected format
$expire = date('D, d M Y H:i:s', time() + (86400 * 30)); // one month from now header("Set-cookie: mycookie=value; epires=$expire; path=/; HttpOnly; SameSite=Lax");