templates/proxy.php.erb
<?php
$allowed_urls = [
<% @css_sheets.select { |x| !x['local'] }.each do |sheet| -%>
"<%= sheet[ 'filename' ] %>",
<% end -%>
];
if( isset( $_GET[ 'url' ] ) && is_string( $_GET[ 'url' ] ) && in_array( $_GET[ 'url' ], $allowed_urls ) )
readFile( $_GET[ 'url' ] );