In Part 1, we learned what a firewall is, and what it can do. In today's conclusion, we'll take a look at what a firewall can't protect against, including their use against viruses.
What can't a firewall protect against?
Firewalls can't protect against attacks that don't go through the firewall. Many corporations that connect to the Internet are very concerned about proprietary data leaking out of the company through that route. Unfortunately for those concerned, a magnetic tape can just as effectively be used to export data. Many organizations that are terrified (at a management level) of Internet connections have no coherent policy about how dial-in access via modems should be protected. It's silly to build a 6-foot thick steel door when you live in a wooden house, but there are a lot of organizations out there buying expensive firewalls and neglecting the numerous other back-doors into their network. For a firewall to work, it must be a part of a consistent overall organizational security architecture. Firewall policies must be realistic and reflect the level of security in the entire network. For example, a site with top secret or classified data doesn't need a firewall at all: they shouldn't be hooking up to the Internet in the first place, or the systems with the really secret data should be isolated from the rest of the corporate network.
Another thing a firewall can't really protect you against is traitors or idiots inside your network. While an industrial spy might export information through your firewall, he's just as likely to export it through a telephone, FAX machine, or floppy disk. Floppy disks are a far more likely means for information to leak from your organization than a firewall! Firewalls also cannot protect you against stupidity. Users who reveal sensitive information over the telephone are good targets for social engineering; an attacker may be able to break into your network by completely bypassing your firewall, if he can find a "helpful" employee inside who can be fooled into giving access to a modem pool. Before deciding this isn't a problem in your organization, ask yourself how much trouble a contractor has getting logged into the network or how much difficulty a user who forgot his password has getting it reset. If the people on the help desk believe that every call is internal, you have a problem.
Lastly, firewalls can't protect against tunneling over most application protocols to trojaned or poorly written clients. There are no magic bullets and a firewall is not an excuse to not implement software controls on internal networks or ignore host security on servers. Tunneling "bad" things over HTTP, SMTP, and other protocols is quite simple and trivially demonstrated. Security isn't "fire and forget."
What about viruses?
Firewalls can't protect very well against things like viruses. There are too many ways of encoding binary files for transfer over networks, and too many different architectures and viruses to try to search for them all. In other words, a firewall cannot replace security-consciousness on the part of your users. In general, a firewall cannot protect against a data-driven attack - attacks in which something is mailed or copied to an internal host where it is then executed. This form of attack has occurred in the past against various versions of sendmail, ghostscript, and scripting mail user agents like OutLook.
Organizations that are deeply concerned about viruses should implement organization-wide virus control measures. Rather than trying to screen viruses out at the firewall, make sure that every vulnerable desktop has virus scanning software that is run when the machine is rebooted. Blanketing your network with virus scanning software will protect against viruses that come in via floppy disks, modems, and Internet. Trying to block viruses at the firewall will only protect against viruses from the Internet--and the vast majority of viruses are caught via floppy disks. Do not think that because "everyone" is using that mailer or because the vendor is a gargantuan multinational company, you're safe.
Nevertheless, an increasing number of firewall vendors are offering "virus detecting" firewalls. They're probably only useful for naive users exchanging Windows-on-Intel executable programs and malicious-macro-capable application documents. There are many firewall-based approaches for dealing with problems like the "ILOVEYOU" worm and related attacks, but these are really oversimplified approaches that try to limit the damage of something that is so stupid it never should have occurred in the first place. Do not count on any protection from attackers with this feature.
A strong firewall is never a substitute for sensible software that recognizes the nature of what it's handling--untrusted data from an unauthenticated party--and behaves appropriately. Do not think that because "everyone" is using that mailer or because the vendor is a gargantuan multinational company, you're safe. In fact, it isn't true that "everyone" is using any mailer, and companies that specialize in turning technology invented elsewhere into something that's "easy to use" without any expertise are more likely to produce software that can be fooled.
Hopefully you now have a basic understanding of the principles that make network firewalls work. These tools are a great means of mitigating unwanted intruders and their malicious attacks, and should be used on every computer connected to the Internet. Stay Safe!