A computer presents very little risk if it's by itself. The problem arises when it's hooked up to a modem. A modem is a communications device that allows your computer to talk with another computer. Modem is short for modulator/demodulator. It is, basically, a telephone for your computer. It converts the computer's output to a format that can be sent over telephone lines.
If your computer has a modem connected to the Internet, it is like you are living in a high-crime neighborhood. You must take appropriate precautions. The modem connection can be a significant vulnerability. Any unauthorized modem is a serious security concern.
Hackers commonly use a tool known as a "war-dialer" to identify the modems at a target organization. A war-dialer is a computer program that automatically dials phone numbers within a specified range of numbers. Most organizations have a block of sequential phone numbers. If you have one number for the organization, it is usually correct to assume that most other numbers are within a limited range of numbers either higher or lower than that number.
In one test of corporate security, a computer dialed a block of 1,500 numbers in the space of 16 hours and identified 55 modems.1 As a countermeasure to war-dialers, many organizations have equipment that detects rapid sequential dialing and shuts it down. On the other hand, some war-dialers are designed to avoid this type of detection.
Modems have become standard features on many desktop computers. People use them not just to connect to the Internet, but also to connect to their office so they can work from home. Software developers create applications that give people easy access to their computers by telephone from anywhere in the world.
The problem is that a modem is a means of bypassing the "firewall" that protects your network from outside intruders. A hacker using a "war-dialer" to identify the modem telephone number and a password cracker to break one weak password can gain access to the system. Due to the nature of computer networking, once a hacker connects to that one computer, the hacker can often connect to just about any other computer in the network.2
It is possible to have a secure connection to the Internet, but it must meet certain requirements. The connection must be configured properly with the latest security equipment, and all employees who are authorized to access their office computers via the Internet from home or while traveling must use strong passwords. Too often, however, these conditions are not met.