A while ago it was discovered that access to phone app of an iPhone running iOS 6.1.2 could be gained by bypassing the lockscreen with the help of a careful series of steps. As this bypassing did not need any specific software or technical knowledge on the part of the person, it posed a security risk to the millions of iPhone users all over the world. But we all know how much seriously Apple takes security and they have just pushed iOS 6.1.3, to prove the point, for all iOS 6 compatible devices.
While Apple has focused its attention on solving the lockscreen bypass issue mainly, there are many minor security fixes all over the operating system. They have also made some improvements to maps for Japan. You can download this small 18 MB update through OTA or you can also click here to download the IPSW file for your iOS 6 device.
[Update 20.03.2013] = Because of this update, the exploit that evasi0n was relying on has been patched implying that you can not jailbreak an A5 or A6 based iDevice if you have this iOS version. So take care before you do the update.