Beta version of iOS 7 has been rolled out by Apple. It’s intended for developers only of course, with various annoying bugs.
One of the bugs has been found is a hole in security side. The bug allows anyone that knows it to access iOS 7 devices even locked with security code and explore contents on the device without any restrictions.
By opening Control Center on lockscreen, then open Calculator on Control Center, continued with opening camera, a locked iOS 7 device can be unlocked. Apple should work to fix the bug before the final version of iOS 7 released a few months away.