It's no secret that iOS is a secure operating system, but Apple's latest firmware update adds yet another level of protection for consumers. The popular Find My iPhone (also known as Find My iPad) service is getting an update, and one that can turn your device into an expensive brick in the wrong hands.

While this has widely been viewed as a very good thing, users purchasing a used iPhone running iOS 7 now face another concern: purchasing a device they can never use. Here's how Apple's new activation lock works, and what to look for when buying or selling an iOS 7 device.

What Is Activation Lock?

Find My iPhone was introduced when Apple unveiled iCloud along with iOS 5. The feature got off to a flying start, allowing users to track and send messages to their device from the web-based iCloud control panel or iOS apps, and in serious situations a remote-wipe could be initiated. Our own author James Bruce took the feature for a spin when he successfully used it to recover his wife's stolen phone a while back.

Unfortunately, the feature requires an active Internet connection in order to work, which means an in-range mobile data connection or recognised Wi-Fi network. Once your phone goes "off the grid", the thief could plug it into a Mac or PC, restore using iTunes and begin using it again as if it were new. This would remove any trace of you as the phone's owner, and thus render Find My iPhone useless.

This all changes with a new feature called activation lock, which prevents your phone or tablet from being tampered with while the Find My iPhone feature is enabled. Disabling the feature requires authentication in the form of a password, and once the rightful owner has noticed their iPhone is missing they can trigger Lost Mode from iCloud.com to display a custom message like this, indefinitely:

lost-mode

Even if you lose your device and it has no Internet connection, a thief cannot (in theory) restore the firmware within iTunes without seeing this message:

Assuming the thief is a little smarter than your average "gimme your wallet and phone" punks that patrol your local mean streets, they may try to put the iPhone into DFU mode and restore using this method. Well, they'll be out of luck when the phone finally reboots when they see this screen (which also works if you initiate a remote wipe):

In essence, Apple has done their best to render any stolen iPhone running iOS 7 completely useless. Like any security feature, it's only a matter of time before someone breaks it, but here's hoping Apple can stay one-step ahead and keep thieves at bay. In the grand scheme of things, this is more of a deterrent than it is tracking feature. If someone steals your iPhone, you're still missing an iPhone. If they turn it off or the battery dies, you can't track it any more either.

However you can take solace in the fact that the thief can't use your phone. That, however, does raise a new issue for users thinking of picking up a used iPhone.

Used iPhones & iOS 7

Even if a thief can't use your phone they can certainly try and sell it to an unsuspecting third party. Luckily, you're already reading this article so you're one-step ahead, and there are a number of things to look out for when picking up a used device.

Craigslist or similar face-to-face sales can be relatively safe provided you ask to see the phone first. Don't just settle for the box, or an iPhone that isn't powered-up. Don't let the seller tell you that "the battery just died, but it works fine!" – in fact if you do hear that line, run for the hills and scream bloody murder.

Should the device be on, and working, the first sign something is amiss will be the "This iPhone Has Been Lost" message you can see in the screenshot above. If that's not present, ask the owner to unlock the phone using their passcode (if they can't, run). Head to Settings > iCloud and check whether the device is connected to an iCloud account. It will say at the top of the menu, like in the screenshot below:

You can then also scroll to the bottom of the screen and check whether Find My iPhone/iPad is on. If the iCloud account is still connected and the feature hasn't been disabled, you will want to head over to Settings > General > Reset and get the seller to Erase All Content and Settings before you hand over any money. This requires the linked Apple ID password be entered, and you should explain that without the phone in an "as new" condition from a software point of view, it's useless to you (or anyone).

If your seller cannot enter this password, or refuses to erase the phone and its contents for you before you hand over the money then you should walk away from the sale altogether and look elsewhere.

Internet Sales

Internet sales carry a considerably higher risk factor as you cannot see the device first. A user might even ask for more money to unlink their Apple ID, if they decide to play a game of blackmail. Worst case scenario they don't even have the password to the linked account, so you will just end up losing more money with no gain.

There's actually no way of ensuring the used item you're buying isn't stolen, and the only thing you'll have to go on are the standard practices when making potentially risky Internet purchases: gut instincts and seller reliability. Unfortunately, these are not foolproof and people will get burned. That said, avoiding sellers with little to no feedback, particularly vague item descriptions or unusually low Buy It Now prices will help you avoid a costly mistake.

If you are selling an iPhone that's been upgraded to iOS 7, it's worth noting that you should Erase All Content and Settings under Settings > General > Reset which will remove all traces of your Apple ID, photos and other sensitive data as well as removing the Find My iPhone link.

Be Careful

You don't need to do anything to enable activation lock, it's a feature built into Find My iPhone. As long as that feature is enabled under iCloud settings, your iPhone will automatically be linked to your Apple ID. When selling, you should delete all your personal information using the above instructions. When buying, you should be especially vigilant.

If you do buy a device that's linked to another Apple ID that you cannot remove, Apple will not help you recover it. Food for thought.

What do you think of Apple's improved security in iOS 7? An effective deterrent? A second-hand nightmare? Add your thoughts in the comments below.