(FORTUNE Magazine) – You, the guest in room 201, thanks for letting me hijack your computer to spew Viagra ads over the Net.

And Mr. Hotel Manager, thanks for not securing the wireless Net you recently installed. You've made it much easier for me to sit in the lobby and digitally browse the laptops of half-a-dozen guests.

Spurred by the demand of business travelers, hotels all over the world are adding wired and wireless broadband connections. But before you jack your laptop into a public high-speed link, consider this: By the time you check out of the hotel, someone else may have checked out your computer.

I recently got a glimpse into how appallingly simple it is to log onto the networks using hacker tools easily downloaded from the Net. It's not that hotels are the only ones with security issues, but since laptop-toting executives may be carrying highly sensitive company files, the locations are an attractive hunting ground for predators, the modern equivalent of a saber-toothed tiger waiting for hairy mammoths at the watering hole.

My tour guide for the night of hotel hanky-panky was Brett Molen, chief technology officer of STSN, a leading provider of network services to the hospitality industry. STSN, based in Salt Lake City, brags about its rigid security, and the company regularly scouts hotels to test for flaws, not only in its own installations but also in those of the competition. Molen agreed to allow me to tag along. Not surprisingly, his probes found weaknesses in some of the competition's networks, but none in STSN's.

(FYI: Molen did not probe any of the vulnerable guest or hotel computers he was able to show me. He just demonstrated how easily it could be done by using his typically configured Windows laptop and connecting it to the hotel networks as any business traveler would. Nor did I actually violate any guest computers when I recreated the attacks on my own a few days later.)

Sitting in the lobby of a Holiday Inn, I connected an IBM ThinkPad to the guest network. But instead of directing my computer outward to the web, I used a popular security tool called NMAP, or Network Map, to see what else was on the local hotel network (for more information on NMAP, see www.insecure.org). To grossly simplify, NMAP enabled my computer to roam the corridors, knocking on doors, trying to find room numbers that just might contain a hairy mammoth. The next step is to see which doors are unlocked. Doors on Net-connected computers are called ports, and each machine has some 65,000. Hackers use a tool called a port scanner to see which are ajar.

Here's where the guests become enablers of the hotel's problems. When it comes to their computers, business travelers often have an open-door policy. Many Windows-based laptops are sold with the vulnerable file-sharing option turned on by default. Even virtual private networks, which create secure, encrypted tunnels to a corporate network over the Internet, are vulnerable to hackers. A VPN encrypts data traffic, but underneath in the OS layer you still have traffic you can exploit.

So when you use public networks, remember what your mama taught you: Lock your doors, don't talk to strangers, and don't leave your wallet out in the open (but forget what she said about sharing).

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