It can even happen if you've never signed on to that network before.
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"There could be any number of compromises that could occur," said Shawn Kanady of Trustwave, an information security company.
It can happen while you're on a banking app, looking at sensitive information.
Technology experts from Trustwave said you could be on an unsecure, open Wi-Fi network even if you've never signed on to it or granted it permission.
"A bad guy could be on that same network that's sniffing traffic, and since it is open and unsecure, he may be able to get at usernames, passwords, things like that," said Kanady.
Or the open network's router could have been hacked.
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"You are sharing that network with a lot of other devices and you don't know what's going on with their devices," Kanady said.
It can make you susceptible to a "man in the middle" attack where you may accidentally join a hacker's fake network with a similar name.
Kanady said there is a solution to this security risk.
"I would turn off my Wi-Fi, I would disable it completely when I'm out and about," he said.
Don't want to take such extreme measures? You can turn off "Ask to join" on iPhones, Kanady said.
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On other phones, turn off anything that says, "Use open Wi-Fi automatically."
You can also open individual Wi-Fi networks you don't want to use and "forget" the network.
"When you're going to join an open unsecure network, treat it like a pool of sharks with a sign that says 'Enter at your own risk,'" said Kanady.
Technology experts are not saying to never go on free, open Wi-Fi. The warning is that you should be aware of when you are signed on, and be careful about the type of websites and work you are doing on your phone while you're connected to free Wi-Fi.