All it takes is a few minutes on laptop.
"You could steal someone's identity. I believe I've already done that at this point," said Raj.
He is a digital security analyst, who now works for a company called Private WIFI. He's showed us how to cyber "sniff."
"This person is on Samsung's website," Raj said.
Armed with just a laptop, Raj set up shop near 3 public Wi-Fi hot spots: Lincoln Center, a major hotel and popular log-in spot, Starbucks.
"I'm invisible. No one can see that I'm there," he explained.
But if you're on unsecure Wi-Fi, he can see you and target your lap top, tablet and even your smart phone.
We won't disclose how it's done, but hackers use software and other tools to get between you and a router to see everything you're doing online.
In one case, he sniffed out someone on Kickstarter.com. If a hacker's watching, kiss your credit card info goodbye. Raj says a hacker needs only minutes to steal your information.
He showed us how it takes just minutes to translate computer gibberish to a person's g-mail account. For a hacker, that's the holy grail of personal information.
"I can see those bills. I can see where you live. How much you pay. How much you owe," he said.
Ensure you surf securely by subscribing to a Virtual Private Network service or VPN.
"A personal VPN will encrypt everything that goes from your device, whether it's a laptop or a phone or a tablet, and the WiFi network, making sure that you cannot be hacked," Raj said.
VPN's are pay services. His service costs about 10 bucks a month.
When you're on public WiFi:
1) Avoid doing important activities like banking or shopping over public networks.
2) Log out of sites and close browser windows.
3) If you use Wi-Fi at home, you should make sure you use a password to secure your router.