Danger of Passive Monitoring

Earlier we mentioned the link between passive monitoring and active attacks. Everybody is subject to eavesdropping. People overhear conversations or peek over shoulders at laptops on planes. Business users should be sensitive to this issue and take steps with confidential information. However, there is usually a clear distinction between eavesdropping and interfering. You might not notice someone peeking at your laptop screen and you might not care if the information were not confidential. You would certainly notice if someone leaned across and typed on your keyboard.

The same is not true for an unsecured Wi-Fi LAN. Unless security is in place, you may not be able to detect when you are being attacked. The simplest form of attack is a connection to an unprotected access point. Access points send out wireless messages advertising themselves to anyone in range. If security is turned off, the enemy can request a connection just as an authorized user would. Magazine reports have written about driving around a city with a laptop and connecting directly into companies' unprotected wireless LANs (Hopper, 2002). This has become known as "war driving" and has been publicized by, among others, Pete Shipley in California. He created software, linked to a GPS receiver, that automatically logs the position of vulnerable networks (Poulsen, 2001). There is even a term "warchalking" for the practice of marking on the sidewalk where access to a Wi-Fi network has been detected.

The prospect of strangers connecting to your LAN is frightening, but surely the network operating system can protect you? Attackers can't log on to the server because it requires a username and password right? Well, yes and no. Attackers can't log on to the server; but, if they are clever, they don't need to. They can monitor the Wi-Fi LAN to find an existing user who has already logged on, and then they imitate that user's transmissions. If the attacker copies the user's network address information, the server may not realize that it is talking to an enemy and may allow all the same access rights as the valid user has.

These examples show just how vulnerable a Wi-Fi LAN can be unless security is put in place. Even if you don't care about eavesdropping and you use your home computer only for browsing the Web, don't assume that you are safe without activating security measures. If the enemy can see you, he can always touch you.