TL;DR: Signs include a public USB charging port or cable causing an unexpected "Trust This Computer?" or data-access prompt on your phone, or unfamiliar apps/files appearing after using an unknown charging station. Always select "Charge Only" if your phone prompts you when plugging into an unfamiliar USB port, use your own charger plugged into a wall outlet instead of a public USB port when possible, or use a USB data-blocker adapter for public charging stations.
Signs include a public USB charging port or cable causing an unexpected "Trust This Computer?" or data-access prompt on your phone, or unfamiliar apps/files appearing after using an unknown charging station.
What Usually Causes This
"Juice jacking" happens when a public USB port or cable has been modified to transfer data as well as power, allowing a connected device to attempt to access your phone's data if you accept the connection prompt.
What to Do About It
Always select "Charge Only" if your phone prompts you when plugging into an unfamiliar USB port, use your own charger plugged into a wall outlet instead of a public USB port when possible, or use a USB data-blocker adapter for public charging stations.
If this looks similar to something else you've seen, it's worth reading How to Check What Apps Have Access to Your Contacts.
This pattern shows up elsewhere too — see How to Check What Apps Have Access to Your Location.
Frequently Asked Questions
What are the real warning signs?
Signs include a public USB charging port or cable causing an unexpected "Trust This Computer?" or data-access prompt on your phone, or unfamiliar apps/files appearing after using an unknown charging station.
What usually causes this?
"Juice jacking" happens when a public USB port or cable has been modified to transfer data as well as power, allowing a connected device to attempt to access your phone's data if you accept the connection prompt.
What should I do about it?
Always select "Charge Only" if your phone prompts you when plugging into an unfamiliar USB port, use your own charger plugged into a wall outlet instead of a public USB port when possible, or use a USB data-blocker adapter for public charging stations.
Should I be worried if I only see one sign?
One sign alone (like a slower device) is usually not cause for alarm — most single symptoms have mundane explanations. Multiple signs together, especially unexpected data usage or unfamiliar apps, are worth investigating.