When no default document is found on a web server, depending on the server configuration, users may be able to list the files in a given directory. For Apache servers, this may be prevented on a per-directory basis by adding add an IndexIgnore directive to an .htaccess file.
The syntax is:
IndexIgnore file [file] ...
IndexIgnore * will prevent listing of all files, or alternatively, individual files may be specified.
Full details may be found in the Apache HTTP Server documentation.