If you're reading this newsletter, you're connected in some way to the Internet—which means that you're vulnerable to various system attacks and compromises. Unless you work for the government, the military, or an incredibly security-conscious organization, you probably don't maintain an "air gap" (i.e., a total disconnection) between your computer and the outside world. If you're connected, you're vulnerable.

To limit your vulnerability, install and configure a firewall. If you have DSL or another high-speed Internet connection, your Internet router can act as a firewall. Routers typically support port blocking, which lets you limit access to specific ports. Nearly all routers available today support address-by-address blocking. However, if you maintain multiple internal machines, this solution can become cumbersome. Free firewall products include Zone Labs' ZoneAlarm, which is free for personal use, and Windows XP's Internet Connection Firewall (ICF), which Microsoft built into the OS.

Installing a firewall is an important first step toward securing your system, but you must take other steps as well. Watch your firewall logs to monitor malicious activity. Any traffic that you permit (e.g., through port 80, if you host a Web page) can contain malicious code. Several common vulnerabilities let remote intruders run code on a Microsoft IIS server and other Web servers. Locking down the services that you allow traffic to is equally important, because the ports you open to that traffic are visible to Internet port scanners—and as such, are targets.

Any software is a potential source of malicious code. For example, product downloads might include spyware, which is software that sends information back to the vendor. Other programs might use your resources and bandwidth—often without your knowledge. Still others contain back doors that grant remote access to your system with full administrator privileges.

Another way to limit your vulnerability is to use strong password protection. (I assume that you use a Windows client; security for Linux and other OSs are beyond the scope of this column.) Establish passwords that are easy to remember but hard for others to guess. Your passwords should include letters, numbers, and special symbols. Change your passwords regularly so that your system becomes a moving target. Changing passwords frequently limits the time intruders have to try to break a password and reduces the time during which intruders have access to your system if they do manage to crack a password.

Another defensive strategy is to physically protect your system. If intruders can gain physical access to a machine, they can launch a slew of additional attacks.

Do you have suggestions or comments about securing your system? Email me at gsmith@certtutor.net, and I might feature your comments in a future column!