During the Great Depression, hobos would mark a nearby fence post or wall with a symbol if they found a safe barn to sleep in, so the next guy could take advantage of the discovery. In a modern twist, some traveling Wi-Fi users mark curbs or walls in locations where they find a freely usable wireless network.

The trend is called war chalking. (It derives its name from the now-ancient practice of war dialing, in which hackers would set their modems to dial every telephone number in town, in search of other modem-connected computers.) Nowadays, some ingenious mobile hackers do a war drive through cities, mapping any wireless network they find as they cruise past at 35 miles per hour. Some return to war-chalk the locations with the best networks.

At last summer's DefCon hacker conference in Las Vegas, I rode along with a team in the first-ever war-driving contest. More than 80 contestants raced through town for 2 hours, waving antennas out the windows of their cars. The team I observed found more than 200 wireless access points that had either no security or limited security within a 9-mile radius from our starting point just off the Strip. And the winning team found more than 500 vulnerable access points in the same amount of time.

War driving is so easy, and it is becoming such a popular pastime among hackers, that any individuals running a wireless network--from administrators in large offices to families--should expect strangers to try to hop onto their network. The team I rode with found banks, law offices, and other businesses running wireless networks that were completely unprotected.

The privacy and security risks of having strangers on your network are not to be taken lightly. The war-driving contestants did not trespass on the networks that they found, but people with malicious intent can (and sometimes do) break into networks and steal valuable data simply because the wireless network has not been properly protected.

Vendors ship wireless products with their built-in security turned off by default. Even though this security, called Wired Equivalent Privacy (WEP), isn't bulletproof, if you run a wireless network, you should enable it (see Internet Tips for a full discussion of safeguards you can take). At the very least, enabling WEP keeps out casual hackers.

If you see war-chalk markings outside your home or business (to see what they look like, go to Warchalking), take it as a wake-up call to secure your network immediately. I learned from the war-driving contest that many users never change their wireless network's default settings, let alone secure them. Don't wait until your data has been stolen to protect your network.

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