In my security presentations, another basic I talk about is defense-in-depth. The idea here is to produce multiple layers of protection against a particular attack. For instance, imagine malicious code against your home computer. This is a case where you likely already practice defense in depth, as this illustration shows.
Now if you're using a Mac or if you aren't using Vista or if you're using Firefox with NoScript there may be more or less or slightly different layers. But hopefully the picture conveys the idea. Some attacks will get stopped right away. But other attacks, might require multiple layers. For instance, a worm that tries to infect via RPC but can also be triggered by starting an executable would require all the layers, and in some cases even that may not be enough (for instance, if the user double clicks said executable before AV definitions become available and accepts the prompt by the Vista UAC to run as an administrator). Blaster and its variants come to mind right away, though they proceeded Vista. The idea is that the more different layers we have, the more ways an attack is going to have to work or the more security measures that attack is going to have to beat in order to be effective.
With respect to SQL Server we can put multiple layers in place. Here are some ideas:
Now these layers don't help us if a web server with access to the SQL Server is compromised, but hopefully defense in depth has been practiced there, as well. But what it does do is make it that much harder for a rogue system on the network to do any direct damage to the SQL Server. There is always the possibility of coming through shared components (in this case the domain controller is the weak link), but if the DC is up-to-date on patches and the IDS/IPS is up to snuff, it may be very difficult to exploit that system to get around to the SQL Server. That's why we should plan a strategy that involves defense in depth when considering the security architecture for a system. We want to make it as difficult as possible for an attack to succeed while still staying within the constraints of what business has provided for overall security of a system.
Another reason to consider defense in depth is when any one control isn't particularly strong or is flat-out missing. For instance, if a third party application has a hard-coded and weak password for a SQL Server login (I've seen it), then the valid credentials to logon just isn't there. After all, anyone who owns the application (or has supported the application) has that login/password combination. As a result, the IPSEC policy restricting what IPs can connect, along with the hardware firewall doing the same thing, and the NAC/NAP ensuring that a rogue system can't grab that IP may all be what we call compensating controls to counteract that login weakness.