In my React application with a backend of Wordpress, how can I specify the SameSite attribute to determine whether a cookie should be sent in a cross-site request, as currently there is an error?

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To set the SameSite attribute for cookies in a React application with a backend of WordPress, you need to modify the server configuration to add the attribute to the cookie header. WordPress, by default, sets the SameSite attribute to Lax, but if you need to set it to strict, you can modify the code.

Add the following code to the WordPress theme's functions.php file:

add_filter('wp_headers', 'set_cookie_samesite');
function set_cookie_samesite($headers) {
    if ( isset($headers['Set-Cookie']) ) {
        $headers['Set-Cookie'] = str_replace('secure;', 'secure; SameSite=None; Secure;', $headers['Set-Cookie']);
    }
    return $headers;
}

This will modify the WordPress Set-Cookie header to include the SameSite=None; Secure; attributes, which will allow the cookie to be sent in a cross-site request.

Note: Make sure your back-end is configured to use a secure connection (HTTPS) as the Secure option is employed.