By far the most common way sites get hacked and defaced is by way of the following two exploits: |
The exploitation of know holes in PHP and CGI software
Software is not perfect. Just about every know software application (Message Board, Guest book, CMS software, you name it) goes through several revisions over the course of it's life. During this time, bugs and security holes are very often discovered and rectified (patched) by the developers.
In a nutshell, this means if your customers install any PHP or CGI software on their sites, the have to regularly check the developers site for updates. If they do not, be 100% certain that sooner or later (usually sooner!) the site will be hacked and defaced.
For you as the Web Host, this is very bad news. Often customers have no idea how to fix the problem and it takes you hours of your time to fix. Even worse, the customer paid someone to originally create the site and now has no more budget to get it fixed so s/he cancels instead and you lose a customer.
Bottom line - regularly inform your customers of updates and make sure you emphasize the need for updating on a regular basis.
Brute force attacks on passwords
Please do not allow your customers/clients to use weak passwords . Again, as with software updates, your reminders to your customers must be relentless.
Passwords must contain at least Upper Case and lower case characters and as a digit and be longer than 8 long and not be any known word.
For example, even a password like 'rumpelstilskin' (which is 14 characters long) is absolutely useless against a brute force attack simply because it is a known word. It can be cracked in under 1 minute by most decent password crackers.
OK, so why is a password so important after all? Here's a simple example:
Joe is one of your hosting customers. His password one one of his email accounts is Joe123 - a particularly bad password. A spammer brute forces it and gains access to the SMTP server on Joes site and sends out 1,555,000 SPAM emails over the next 6 hours. Joe's site and the main shared IP of the server gets blacklisted by just about every ISP on the planet. We shut you down and you lose all your hosting customers and we lose you as a customer. There are no winners here. We all lose.