SSI, Directory Traversal.
This one is tricky, so make sure to read the clue carefully. We know that the password file is stored in /var/www/hackthissite.org/html/missions/basic/9/, and since we do not have an input box to run commands, perhaps there is another way to display the files in this particular directory.
Do not post spoilers!
"The quieter you become, the more you are able to hear..."
"Drink all the booze, hack all the things."