Hot on the heels of news that a crowdfunded competition aims to figure out if the fingerprint security implementation on Apple's iPhone 5s can be exploited comes news of a proven security risk, squarely involving iOS 7. The exploit specifically involves the lockscreen, the most common piece of security that stops some unauthorized individual from gaining access to anything important on your phone.
The "hack", if you want to call it that, is simple: Swipe up on the lockscreen to enter the control center, and then open the alarm clock. From there, hold the phone's sleep button to bring up a prompt that will ask you if you wish to shut down, but instead of doing that, hit the cancel option, and then tap the home button to access the phone's multi-tasking screen.
With access to this multi-tasking screen, anyone could try opening up what you've already had open on your phone. If you had Twitter open, for example, this person might be able to pick up where you left off and post on your behalf. Or, they could access the camera - and of course, every single photo stored on the phone. This is definitely what I'd consider a significant security risk.
Around the Web, there's proof that this exploit does in fact exist, with many users backing that up. However, there do seem to be limitations to what can be done once access is granted. Some apps still might not accessible, for example, and so far, there doesn't seem to be much rhyme or reason to what's accessible. What is a certainty though is the fact that Apple is sure to be rather quick in patching this bug up. I'm not sure I'd go as far as to call the bug "critical", but when someone can access your photos and Twitter with just a couple quick swipes and taps on the screen, it sure isn't minor.