Apple has just released iOS 6.1.3, available on applicable iOS devices by tapping General > Software Update.
According to the update notes for this release, iOS 6.1.3 fixes a bug that could allow someone to bypass the passcode and access the Phone app. It also provides improvements to Maps in Japan.
The download is 17.0 MB in size. If the update is not immediately available on your device, it should be shortly.