Here's some things you can do to increase the level of security for your websites / forums:
1. Always upgrade to the latest stable version.
2. Do not install any unofficial hacks or plugins as they are not written or reviewed by our developers.
3. Password protect your Administrator and Moderator Control Panels directories as well as the install and includes directories using .htaccess/.htpassword Comprehensive guide to .htaccess- password protection
4. Make sure the tools.php (vB3) file is NOWHERE on your website.
5. Remove the ImpEx files if you had used this import system.
6. If you have phpMyAdmin make sure it's password protected.
7. If you suspect a hacking attempt, ask your host to change the login password for your web account.
8. Make sure all the Admin and Mod passwords are secure. Change them if you have any doubts. And use hard to guess passwords.
9. NEVER allow HTML in posts, PMs or in sigs.
10. Make absolutely sure there are no viruses, trojans or keylogger spyware on your PC. Any of these could steal your password and other personal info.
11. Do NOT upload the directory called do_not_upload
12. Use a different password for each forum you sign up with. Use a different password for your forum as you do for the .htaccess directory password.
13. Update the config.php file and set yourself as undeletable user so they can't touch your admin account.
14. Do Not Upload config.php.new when upgrading your forums.