Are free phone chargers safe?

The FBI has issued a warning of the dangers of using free phone charging stations in place like airports, shopping centers and hotels. Free public USB ports can be hijacked and introduce malware or monitoring software on your devices. Instead, the FBI suggests bringing your own cord and USB charger plugged in to an electrical outlet. We also recommend carrying your own portable charger while traveling, just in case there isn't an electric outlet when you need one.