There are many files that are placed in the Bugzilla directory area that should not be accessible from the web. Because of the way Bugzilla is currently layed out, the list of what should and should not be accessible is rather complicated. A quick way is to run testserver.pl to check if your web server serves Bugzilla files as expected. If not, you may want to follow the few steps below.
Bugzilla ships with the ability to create .htaccess files that enforce these rules. Instructions for enabling these directives in Apache can be found in Section 220.127.116.11
In the main Bugzilla directory, you should:
Block: *.pl, *localconfig*
But allow: duplicates.rdf
If you use a remote webdot server:
But allow *.dot only for the remote webdot server
Otherwise, if you use a local GraphViz:
But allow: *.png, *.gif, *.jpg, *.map
And if you don't use any dot:
Be sure to test that data that should not be accessed remotely is properly blocked. Of particular interest is the localconfig file which contains your database password. Also, be aware that many editors create temporary and backup files in the working directory and that those should also not be accessible. For more information, see bug 186383 or Bugtraq ID 6501. To test, simply run testserver.pl, as said above.
Be sure to check Section 2.2.4 for instructions specific to the web server you use.