If you are optimizing a Magento vhost, you probably disabled the .htaccess necessities as they slow down by a nice amount.
Doing so however, lead to the following error:
Your web server is configured incorrectly. As a result, configuration files with sensitive information are accessible from the outside. Please contact your hosting provider.
That means that the files local.xml and config.xml are accessible from the user’s browser, to fix that you need to set the correct permissions for those 2 files, that is 500.