Over the past few months, Sony has been making great strides in keeping their phones up to date with major Android releases as well as the monthly Android Security Update patches. This month it would seem is no exception. The Japanese company has rolled out the December security update to the Xperia XZ, Xperia XZs and the Xperia X Performance.
The update is firmware build 41.3.A.2.24 for all three devices and the update is hitting unlocked phones first as you likely suspect. The update is small, less than 200MB, so it is a quick install but does require a reboot of your phone once you have it downloaded and installed.