An extra protection can be added to drupal update function by restricting which machines can access it.

Add this to the .htaccess

<FilesMatch "update.php">
  Order deny,allow
  Deny from all
  Allow from 192.168.3.1
  Allow from 192.168.3.*
  Allow from .host.tld
  Allow from hostname.host.tld
</FilesMatch>