Securing your Config.php File 
The config.php has several settings that can help protect your vBulletin installation. These settings are listed below. Since these are security features, less is more. Give out as few permissions as possible, otherwise your forum will not be as secure. Ultimately you should only include yourself as the owner in any of the settings. It is recommended to leave the "Users with Query Running Permissions" setting empty until you need to actually run a query. At that time, change the setting and reupload your config.php. The change will take effect immediately. When you are done running your queries, change the setting back to the default. With this one setting and your password, an attacker has complete control of your database.
// ****** USERS WITH ADMIN LOG VIEWING PERMISSIONS ******
// The users specified here will be allowed to view the admin log in the control panel.
// Users must be specified by *ID number* here. To obtain a user's ID number,
// view their profile via the control panel. If this is a new installation, leave
// the first user created will have a user ID of 1. Seperate each userid with a comma.
$config['SpecialUsers']['canviewadminlog'] = '1';
// ****** USERS WITH ADMIN LOG PRUNING PERMISSIONS ******
// The users specified here will be allowed to remove ("prune") entries from the admin
// log. See the above entry for more information on the format.
$config['SpecialUsers']['canpruneadminlog'] = '1';
// ****** USERS WITH QUERY RUNNING PERMISSIONS ******
// The users specified here will be allowed to run queries from the control panel.
// See the above entries for more information on the format.
// Please note that the ability to run queries is quite powerful. You may wish
// to remove all user IDs from this list for security reasons.
$config['SpecialUsers']['canrunqueries'] = '';
// ****** UNDELETABLE / UNALTERABLE USERS ******
// The users specified here will not be deletable or alterable from the control panel by any users.
// To specify more than one user, separate userids with commas.
$config['SpecialUsers']['undeletableusers'] = '';
The section of code to look for is:
// ****** UNDELETABLE / UNALTERABLE USERS ******
// The users specified here will not be deletable or alterable from the control panel by any users.
// To specify more than one user, separate userids with commas.
$config['SpecialUsers']['undeletableusers'] = '';
Securing your config.php file
Making sure no one edits this file after you upload it to the server is a large priority. If an attacker can change the contents of this file they can easily take control of your community. The first thing you want to do is restrict access to this file via file permissions. Make sure no one can access this except you. Use the techniques described under Restricting Access to secure this file.

One thing you might consider doing is denying access via a Web Browser at all times. This file only needs to be read internally via PHP and should not be accessed with a Web Browser. On most installations, this would never occur. However should your version of PHP stop working for some reason, then the file can be served as plain text and any prying eyes can see it. You can counter this on the webserver level with tools like .htaccess and NTFS Permissions.

Here is an example .htaccess file that would prevent access to the config.php. You would place this file within your /includes directory.
<Files config.php>
order deny,allow
deny from all
</Files>
For details on securing this file in IIS on a Windows Server please see:
http://www.microsoft.com/windows2000/en/server/iis/htm/core/iidfpsc.htm
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