iThemes Security requires Apache or LiteSpeed and mod_rewrite or NGINX to work. While this plugin should work on all hosts with Apache or LiteSpeed and mod_rewrite or NGINX, it has been known to experience problems in shared hosting environments where it runs out of resources such as available CPU or RAM. For this reason, it is extremely important that you make a backup of your site before installing on an existing site. If you run out of resources during an operation such as renaming your database table, you may need your backup to be able to restore access to your site. Finally, please make sure you have adequate RAM if you plan to use the file change detector or make large backups.