A new update is rolling out for the Xperia Z5 and Z5 Compact. The update appears to offer a fix for the "Stagefright" exploit. The update pushes the software for the phones to build number 32.0.A.6.152.
In addition to a fix for "Stagefright," users are also reporting that the update brings better heat management and improvements to the fingerprint sensor. The update OTA for the update weighs in at around 281MB, and is appearing on some phones now. If it isn't showing up for you just yet, you should be able to downloaded it manually by heading to your phone's Settings and checking for the update.
Source: Xperia Blog