I'm interested what about that you think is vulnerable.
The input is sanitised, so you cannot insert actual code and invalid actions are ignored. There is CSRF protection built in to Roundcube but if you want you can enable secure URLs for additional security see,
https://github.com/roundcube/roundcubemail/blob/master/config/defaults.inc.php#L627I suppose because the value is put into rcmai.env.action there could be a way to get roundcube actions executed from inside the application - like a button was clicked or something else done in Roundcube which used the value from rcmail.env.action but there is no way to specify the particular parameters for that action and so It would fail.