The Federal Trade Commission recently sounded the alarm on smartphones interfacing with the onboard computers of rental cars, saying it could expose your personal information to future renters, employees or even hackers. There are other sources of danger for your data, including those charging stations at the airport and, of course, those "free" Internet hot spots in your hotel lobby.

Cars are essentially computers on wheels, says John Michelsen, chief product officer at Zimperium, an enterprise mobile security company. Consider the Ford F-150, whose onboard computer system has 150 million lines of code. That's more programming than a Boeing 787, which has 7 million lines of code says USA Today. You don't know what the computer will do with the data you shared once you return the car to the lot, and chances are, neither does the car rental company.

According to The Telegraph, it's also possible to hack into a smartphone that is charging via USB in a public place, such as an airport, cafe or on public transport. To avoid being a victim, only plug your phone into trusted computers when using a USB cable.

Maybe there's a broader lesson about the hidden dangers of everything the travel industry claims is free but is not. If you'd rather not hurt your brain thinking about that, you can take steps to avoid these hacks (see below), but perhaps the easiest way is to walk away. To paraphrase Robert Heinlein, there ain't no such thing as "free."