The original implementation of Android's face unlock feature was a mess. It took too long to unlock (when it worked), brought up a huge interface that overpowered the lock screen, and maybe most egregiously, was able to be "hacked" by someone holding up a picture of your mug.
For the most part, this feature has been improved in Android Lollipop, as long as you set up a decent "Trusted face." With proper lighting and framing, not only was the unlock recognition fast, it worked in the background.
But after flashing the official Lollipop build on my Nexus 5, one bug about this feature remains, and is likely the reason that Google themselves warn that this unlock method isn't exactly the most secure. Yes, your device can still be unlocked with a picture.
Now admittedly, the picture I used was taken in the same room and around the same time of day as the one that was used when I set up the "Trusted face" feature, but it does go to show you that if someone really wanted to, it wouldn't be too much trouble for them to break into your device.
Did you set up a "Trusted face"? Have you tried unlocking your device with a picture? Let us know in the comment section below.
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