Is it safe to charge your phone at a public USB port?

A USB port carries data as well as power

The same connector that charges your phone is the one it uses to sync photos with a computer. That is the whole basis of what is called juice jacking: a charging port or a cable that has been tampered with so that, while the battery fills up, something on the other end tries to read data from the phone or install software on it.

INCIBE, Spain's national cybersecurity institute, describes it as an attack that takes advantage of public USB ports to get at the data on mobile devices, and it adds a detail that is easy to miss: cables left behind at charging stations can be the trap too. A wall socket on its own is a different thing. It only supplies power, and the plug and cable between it and your phone are the ones you brought.

What the agencies actually say

The FBI's Denver field office posted a warning telling people to avoid free charging stations in airports, hotels and shopping centres, because bad actors had found ways to use public USB ports to put malware and monitoring software on devices. Its advice was to carry your own charger and USB cord and use an electrical outlet. The FBI later told reporters it was a general advisory and was not prompted by a specific case.

The FCC's consumer guide says much the same: use an AC power outlet, and pack an AC adapter, a car charger, your own USB cable or a backup battery. It also suggests a charging-only cable from a trusted supplier, and choosing charge only if the phone asks about sharing data. The UK's NCSC, in its guidance for people at higher risk, also says to avoid public USB charging points and use a traditional power plug.

How much protection your phone already gives you

Phones do not hand over data just because a cable is plugged in. An iPhone shows a Trust This Computer alert the first time it connects to a computer or other device. Apple explains that a trusted computer can sync with the iPhone and access its photos, videos and contacts, while choosing Don't Trust blocks that access. On Android, a phone connected by USB charges by default, and you have to tap the Charging this device via USB notification and pick File Transfer before any files move.

Those prompts are good but not perfect. Researchers at Graz University of Technology presented an attack called ChoiceJacking, in which a malicious charger fakes user input to approve its own data connection. They reached photos, documents and app data on phones from eight vendors, and say Apple, Google, Samsung and Xiaomi acknowledged the problem and were adding mitigations. Keeping your phone updated is part of the answer.

Settings worth checking before you leave

• iPhone: Apple says that by default you have to unlock the iPhone before it connects to an accessory or computer, while it still charges as usual from a USB power adapter. The setting is in Settings, Privacy & Security, Wired Accessories. On USB-C models you can tighten it from Automatically Allow When Unlocked to Ask for New Accessories or Always Ask; avoid Always Allow.
• iPhone: if a charging point ever triggers Trust This Computer, tap Don't Trust. A charger has no reason to see your photos.
• iPhone: to forget every computer you trusted in the past, go to Settings, General, Transfer or Reset iPhone, Reset, Reset Location & Privacy.
• Android: leave the USB mode on charging and ignore any request to switch to file transfer at a public port.
• Both: keep the screen locked while it charges, as INCIBE recommends, and install system updates before the trip.

None of these settings replace your own plug. They are there for the day the only free socket in the departure lounge is a USB one.

A simple charging kit for the trip

Pack your own wall charger, a travel adapter for the sockets at your destination and your own cable. A power bank covers the hours when there is no socket at all, and INCIBE and the FCC both mention it as the alternative to public ports. A USB data blocker, a small adapter that lets power through and cuts the data lines, is a reasonable extra if you know you will rely on public ports.

Battery planning matters more on a trip than at home, because the phone is your map, your boarding pass and your translator at once. If it drains faster than usual abroad, our guide on why phones drain faster when travelling explains the usual causes, from weak signal to the screen staying on for maps.