Public Wi-Fi networks - like those found in coffee shops, malls and public libraries - are convenient, but can be a risky proposition if you connect your mobile device to them without indulging in a little healthy paranoia.
Essentially, their security is typically more lax than is desirable, and being in public locations they tend to attract those who look to snoop on others for fun or profit.
There are a few common sense (if perhaps not immediately obvious) things you can do to minimize your risk when connected to a public Wi-Fi.
First, avoid activities like online banking or ordering things from a website. Surfing around to catch the latest baseball scores may be fine, but keep the sensitive stuff for home.
Second, make sure the Wi-Fi you are connecting to is what you think it is. Find out for sure what the buildings Wi-Fi is called - it's trivial for a bad guy to set up a dummy Wi-Fi hotspot in or close to the area you are using. You would not want to connect to that fake network.
Lastly, turn OFF your Wi-Fi in public places, or at least disconnect from the public network after you are finished doing what it is you are doing. If you can't get out, others can't get in. There is usually a physical on/off switch, or a key combination to do this (on Windows, typically the "Fn" key plus some other key).
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