One of the reasons people seem so fond of network firewalls and other network-based security measures is that they assume they can allow host security to lapse if they secure the perimeter of their network. They reason that it is far easier to keep a single firewall and a half-dozen public (external) machines secure than many thousand. In this they are correct, but also gravely mistaken.
Most network security is really host security. If host administrators assume that the firewall will protect their hosts from access violations, they are in for a rude awakening. Hosts should protect themselves from network intruders as much as they would from local intruders, if not more. The network can only help host security, not replace it. Securing the perimeter of the network but ignoring internal security measures has been likened to having a piece of candy with a hard crunchy exterior and a soft, gooey interior. Once the exterior shell is breached, the insides just ooze out. Consider what happens if an intruder gets through your firewall; he of she can easily move from machine to machine causing untold damage, loss of sensitive information, and disruption. If your hosts were providing additional security measures of their own, the damage might be confined to a smaller set of hosts, rather than extending throughout your network.
A firewall also won't help you guard against threats from your own users. Studies show that 90 percent of most corporate security budgets are spent to protect against outside threats, but 90 percent of the damage is done by employees and others inside the company. While you can take action against internal users after the fact, the damage may already be done. Furthermore, internal users often cause problems through ignorance rather than malice. Good host security helps protect against mistakes as well as intentional attacks.
Finally, consider what happens when a user installs an unauthorized modem on a workstation so he or she can get around the firewall and reach the workstation from home. Security holes like this often go undiscovered until an intruder finds and exploits this trivial way to bypass your firewall.
SO the answer to the question, "Is host security still necessary when I have a firewall?" is an unqualified "Yes!" There are a few simple ideas you can keep in mind.
- Keep up with any security patches from your vendor.
- Disable any services that are not necessary.
- Configure your machines to permit access to themselves only from IP addresses that are properly registered with the DNS.
- Practice good password security.
- Audit your systems regularly.