Did you see small 30 bytes only index.php files in the folders of WordPress installation? If you don’t know for what reason those files included into WordPress package please read this post “Silence is Golden”.
This plugin can scan your WordPress blog installation subdirectories for the presence of such dummy index.php files and create it if index.php file doesn’t exist in the directory. As the second line of defence against directory listing plugin can add special “-Indexes” option into Apache Web Server
.htaccessfile placed at the WordPress root directory.