At most businesses that offer wireless access, connecting is as simple as setting up an account and choosing how you'll be billed (either on a pay-as-you-go basis or at a flat rate for unlimited access). Then you sign in with the user name and password that you set up in advance.

But a group of clever independent security analysts has created a program that makes it easy for a hacker to slurp up your log-in information before you've even quaffed the foam on your cappuccino. The miscreant can then use the information to obtain free wireless Internet access and make you foot the bill.

The new tool, called Airsnarf, broadcasts a powerful signal that disconnects any nearby hot spot users from the Internet. Then it broadcasts a sign-in page that looks like the log-in site of the legitimate Wi-Fi provider. When users, figuring they were knocked off the Internet momentarily, log in again, their user name and password go to the hackers, not the ISP.

The Airsnarf program could be running on the laptop--or even the PDA--of the person sitting next to you. With the right antennas, crackers intent on stealing passwords wouldn't even need to get out of their cars. All they'd have to do is park in front of the cafe, sit for a while running Airsnarf, and then move on.

The program was never intended to be used as a tool for theft, according to its creators, members of a loosely affiliated group of computer security experts who call themselves the Shmoo Group. "Airsnarf was developed and released to demonstrate an inherent vulnerability of public 802.11b hot spots," the group writes on its Web site.

Spokespeople for two of the largest wireless access providers, T-Mobile (the provider for Starbucks) and Wayport (which serves many airports across the U.S.), say they don't know of any subscribers whose log-in information was stolen this way and don't anticipate the problem being widespread enough to warrant major changes to the way they run their services. Both say that if you notice odd usage patterns in your account, you should report them to your provider's customer service department, which will issue credits for stolen service. But can a hacker use your log-in information to get at more sensitive personal data? Both companies say no. Though you may use the same user name and password to connect to the Internet and to manage your account online, the ISPs report that credit card numbers and other sensitive data are hidden from view when you log in to your account information.

If you use a wireless hot spot, the best defense against this kind of service theft is to change your password regularly--at least once a month. And keep close tabs on your monthly bill, even if you're on a flat-rate plan; you may not be losing money, but you still shouldn't let crime pay for data thieves, who might be using your wireless account for other nefarious purposes.

Andrew Brandt is senior associate editor for PC World. E-mail him at privacywatch@pcworld.com. Click here to read more Privacy Watch columns.