Sony's Xperia X, Xperia XZ, and Xperia X Performance are all starting to have updates rolled out to them which contain the August 1, 2017 Android security bulletin. The Xperia X Compact is reportedly going to be next in line to get this update, which makes sense, given that it truly is a compact version of its namesake. Other current Sony Xperia devices, such as the Xperia XZ Premium and Xperia XZs, are likely to see this update come their way in the near future. As far as anybody who has reported receiving the update on their device thus far can tell, the new security bulletin is the only thing in the update package, and there are no other changes to report.
Owners of the Xperia XZ and Xperia X Performance seem to have received the same update; users with these devices are reporting that their new build number after applying the update is 41.2.A.7.53. These two devices both use a Qualcomm Snapdragon 820 processor alongside 4GB of RAM, so the similarly-specced Xperia XZs is quite likely to end up getting the same update. Xperia X users, meanwhile, have jumped to firmware version 34.3.A.0.217, and the same update will probably be coming to the handset's Compact cousin. Sony has not officially announced these updates, but with no changes aside from a security patch, owners of other modern Xperia phones can probably expect the updates very soon.
These are relatively small security updates, so the risk of having your device rendered completely inoperable due to the update failing is lower than with something like an Android version update. Still, it's a good idea to ensure that your device is either plugged in or has a good battery in it with at least 50% left when you apply the update. If you have one of the three listed devices and you're not seeing the update yet, try heading into your settings menu and checking for the update manually via the About Device menu. If that doesn't get you the update, then a reboot may do the trick. It should be live for all users with the listed devices.