From Online Manual
Keep your SMF version up-to-date and make sure you install security updates. This is done by patching your forum.
The best way to be informed about new SMF versions is to subscribe to the News and Updates board at the SMF Community Forum. The most important announcements also appear directly within the Administration Center, in the information panel "Live from Simple Machines".
Password Security for Administrators
Be sure to change your passwords often and do not use the same password for everything. You should have different passwords for each of the following:
The most important password is the database password. This is the same password you use with phpMyAdmin or any other database management control panel. This password is stored in the Settings.php file. Do not re-use this password for your FTP or your forum administrator account.
The best passwords:
- use letters, numbers, and special characters
- vary in upper and lower case
- do not use dictionary words or common names
- are not easy to guess
- are fairly long
- are something you can remember without needing to write it down
You can choose the minimum password strength for your users. The higher the level, the less likely that it will be brute forced.
Additional Security Tips for Your Forum
- To keep anyone from taking over your forum, make sure your passwords are difficult to guess. You can do this by making sure your passwords do not relate to you (for example, your spouse's name), by mixing capital and lower-case letters with numbers, and by using special characters where applicable.
- Frequently back up your database and FTP files and make sure you understand how to restore these backups.
- Delete temporary files such as install.php, converters and recovery tools.
- Frequently check your error log for any suspicious or unusual errors.
- Delete any login failures made by administrators from the error log.
Additional Security Tips to Protect Your Users
- Do not allow guests to upload files -- guests could upload things you do not want on your forum.
- Do not allow Flash files to be uploaded or displayed -- you do not want users to upload malware and persuade other users to download and open/execute it.
Backups with Tilde (~)
Whenever some file is modified (by a mod operation for example) SMF creates a backup of it with a tilde (~). For example, if some mod has modified index.template.php, SMF will create this file: index.template.php~. Notice that these files can be accessed through the browser, so it can be a possible vulnerability. To prevent that, you can either disable this option in the Administration Center or you can periodically delete them.