(FORTUNE Magazine) – FORTUNE's Stephanie N. Mehta recently went on an undercover expedition around Manhattan, accompanied by a team from AirDefense, a small wireless security firm. Our snoops call it "war driving" -- testing how well big corporations are securing their wireless networks. To protect the careless, we've disguised our targets. But read on: One of these wireless laggards could be your firm.

OUR DETECTIVE'S KIT INCLUDES A SUPERANTENNA THAT LOOKS LIKE A SMALL PIZZA BOX AND A LAPTOP LOADED WITH OPEN-SOURCE SOFTWARE THAT SNIFFS OUT WIRELESS WEAK SPOTS.

OUR FIRST STOP: WALL STREET.

WE PARK OUR TOWN CAR IN FRONT OF A BUILDING THAT HOUSES TWO FINANCIAL INSTITUTIONS -- A COUPLE OF REALLY BIG OPERATORS. IT ISN'T LONG BEFORE OUR SNOOPING PAYS OFF. MEGA BANK HAS BARELY MASKED THE NICKNAMES OF ONE OF ITS WIRELESS NETWORKS. MIGHT AS WELL PUT OUT A WELCOME MAT FOR HACKERS.

AND LOOK! I'VE DETECTED A ROGUE ACCESS POINT. SOME WISE GUY PROBABLY BOUGHT ONE AT AN ELECTRONICS STORE AND INSTALLED IT HIMSELF. LITTLE DOES HE KNOW THAT DATA PASSING THROUGH THAT TINY ANTENNA AREN'T ENCRYPTED!

UH-OH ... WE'RE PICKING UP A LAPTOP THAT'S CONNECTED TO THE WIRED NETWORK AND SEEKING A WIRELESS CONNECTION TOO, AND IT'S SPEWING DATA FROM THE COMPANY'S COMPUTERS INTO THE AIR.

WE MOVE OUR STING OPERATION FARTHER UPTOWN, STOPPING IN FRONT OF A BUILDING THAT HOUSES A LOT OF TELECOM OPERATIONS. WE'RE NOT GETTING MUCH WIRELESS ACTIVITY HERE ... THE COMPANIES PROBABLY WISELY BAN WIRELESS NETWORKS. LET'S MOVE ON.

IN MIDTOWN, WE PULL UP IN FRONT OF ENTERTAINMENT INC. ... DANGER! A WIRELESS DEVICE IS TRYING TO SPREAD A VIRUS TO OTHER COMPUTERS ON THE COMPANY'S NETWORK!

FOR THE LOVE OF PRIVACY ... SOMEONE IS TRYING TO SEND AN E-MAIL WIRELESSLY, AND IT LOOKS LIKE A NEWS STORY OR PRESS RELEASE ...

LOOK OUT -- USER IDS AND PASSWORDS ARE FLYING EVERYWHERE. THAT MAKES ENTERTAINMENT INC. A CINCH TO HACK -- AND PUTS ITS MULTINATIONAL CONGLOMERATE PARENT AT RISK.

SO ARE COMPANIES ALWAYS SO CARELESS?

UNFORTUNATELY THIS IS PRETTY TYPICAL. AND IF ANY OF THE STUFF WE SAW TODAY ENDED UP IN THE WRONG HANDS, YOU WOULDN'T BE READING ABOUT IT IN THE FUNNY PAGES.