I have to make sure, that the Wordpress cookie set after successful log in, "wordpress_logged_in_[salt-sth]" is set to be available for secure connections only.
It looks like that it´s a filter used in pluggable.php, line 653
$secure_logged_in_cookie = apply_filters('secure_logged_in_cookie', false, $user_id, $secure);
How can I set it to secure, WITHOUT modifiyng the wordpress core files? If I modify the core (set to true) - it´s not possible to logout anymore...