security for drupal multisites in ISPConfig 3 hosting environment

for the drupal multisites with webmail interface for each of the sites (e.g. http://www.site-1.com/webmail, http://www.site-2.com/webmail) installed under the same server, the ISPConfig 3 hosting server, what should be considered in terms of the security issues of the multisites(shared drupal codebase), database(mysql), email(roundcube), webserver(apache), OS(Ubuntu) etc?

your knowledge, information and insight on the above are much appreciated!