Plug your iPhone or iPad into a USB port and you may be asked whether you want to “Trust This Computer.” You may even see this message when you plug your iPhone or iPad into a USB charger. This prompt helps protect your iPhone from malicious chargers. It also ensures no one can snatch your iPhone and access its data from a computer without your passcode.
A Computer or Device is Trying to Access Your Files
You’ll see this prompt when a computer or other device you’ve plugged your iPhone into is trying to access your files. For example, the first time you plug your iPhone into a Mac or PC running iTunes, you need to “trust this computer” before you can access your files and otherwise manage your device from iTunes.
This alert message only pops up when your iPhone or iPad is unlocked. This means that someone can’t grab your iPhone and plug it into their PC to get at your files — it will have to be unlocked first.
Avoiding Juice Jacking
If you plug your iPhone or iPad into a USB charger and see this prompt, don’t agree to it. This would give whatever device you’re plugged into access to your files. For example, if you ever plug it into a USB charger in a public place and see this alert, just say no.
“Juice jacking” is an attack that uses compromised USB chargers to access files on devices. Apple added this prompt in iOS 7 to prevent such attacks, ensuring your phone or tablet’s files are protected from devices you plug it into unless you specifically agree. Because the charging cable is the same as the data transfer cable, this provides an additional layer of security that allows you to charge your device without giving a malicious charger access to your stuff.
What if You Don’t Trust The Computer?
If you don’t trust a computer or device you’re plugged into, it won’t be able to access your files. Your iPhone or iPad will still charge from it normally, so there’s no real risk in plugging your phone or tablet into a USB charging port in a public place or someone else’s laptop if that’s all you have available to you. Just be sure to say “Don’t Trust” if you see the popup. If you leave your phone locked the whole time, it will just not trust the device it’s plugged into by default.
If You Change Your Mind Later
If you accidentally tapped “Don’t Trust” and you actually do want to trust the computer, don’t worry. You’ll see this alert each time you connect your iPhone or iPad to the device. Just unplug your iPhone or iPad and connect it again. You’ll see the prompt message again and you can agree to trust the computer. If you don’t see the message for some reason, try rebooting your iPhone or iPad.
If you accidentally tapped “Trust” and trusted a device you don’t want to trust, your iPhone or iPad will normally remember this choice and trust the computer every time you plug into it. You’ll probably want to unplug your iPhone or iPad first if you don’t trust the device it’s plugged into. Next, you’ll need to wipe the list of all trusted computers on your iPhone or iPad.
As of iOS 8, you can do this in Settings > General > Reset. Tap “Reset Network Settings,” which will wipe the list of trusted computers along with your network settings, or “Reset Location & Privacy,” which will wipe the list of trusted computers along with your location and privacy settings. After you do, you’ll get the “Trust This Computer?” prompt every time you plug your phone back into a device until you trust it again.
What Exactly Does Trusting This Computer Expose?
Trusting a computer exposes your iPhone or iPad’s data to it — everything you can access via iTunes. This includes your photos, files, contacts, media files, settings, and more. A trusted computer can pull files from your device and push files to it. Basically, everything you can do from within iTunes can be performed by a trusted computer.
This is no big deal if you actually trust a computer and are sitting down in front of it. The prompt just ensures you’re in control of which devices have this access, and that random charges aren’t given unrestricted access to your phone or tablet.