It's seriously handy to charge your devices via any available USB port, but it's also a data and malware security risk, depending on configuration. It's feasible to hack USB connectivity to lessen the risks.
Photo by Paul
The issue relates to the fact that USB carries both data and power connections. You could hack to sever the data lines, but this removes a negotiation component that allows USB to deliver full power. Without that negotiation, you'll only ever get 0.5w out of a USB port, or in other words, very slow charging indeed.
The trick is to allow the low-level communication of USB to permit higher capacity charging, while blocking the communications that allow security risks to pass to your device. Reclaimer Labs runs through the basics of how this works (and, surprise, surprise, they've got a device to sell) if you're interested in sorting out secure USB charging for yourself.