mod_negotiation is an Apache module responsible for selecting the document that best matches the clients capabilities, from one of several available documents. If the client provides an invalid Accept header, the server will respond with a 406 Not Acceptable error containing a pseudo directory listing. This behaviour can help an attacker to learn more about his target, for example, generate a list of base names, generate a list of interesting extensions, look for backup files and so on.
Possible information disclosure: directory listing, filename bruteforcing, backup files.
Disable the MultiViews directive from Apache's configuration file and restart Apache.
You can disable MultiViews by creating a .htaccess file containing the following line:
mod_negotiation: directory listing, filename bruteforcing
Multiviews Apache, Accept Requests and free listing
Apache Module mod_negotiation