The flaw resides on users who lock their devices with a traditional PIN code or password. The exploit has been tested successfully on iOS 7 when running on the iPhone 4S, 5, 5C, and 5S, and the most recent iPad model. Until an official fix is available from Apple, iOS 7 users can avoid this security hole by disabling the control center of the lock screen in their device settings. The flaw is demonstrated in the video below and it works as follows: Swipe up from the bottom of the Lock screen to open Control Center and Launch the Clock app.
Source: https://thehackernews.com/2013/09/iphones-ios-7-lockscreen-hack-allows-to.html