Its not unusual for test builds of upcoming firmware updates to be leaked. A test build of Android 4.3 for Galaxy S3 GT-I9300 has been leaked online, making it the first such leak for last year’s flagship. Even though this Android 3.3 build I9300XXUGMJ9 is said to be stable, keeping in mind that this is after all a test build, there are bound to be a few bugs lurking around. Since flashing this firmware will wipe your device, it is recommended that you create a backup of all your data prior to proceeding with the installation.
The Android 4.3 update is a big deal for Galaxy S3 owners, seeing as how Samsung decided to disregard Android 4.2 altogether and upgrade directly to version 4.3 instead. Since its a test build, there exists the possibility that Samsung might add or remove features in the final build, which might take a few weeks to be released. One of the major differences this update brings is the core UI, which is now more in line with that found on Galaxy Note 3 and the Galaxy S4. Launcher and widgets remain the same though, at least as far as this build is concerned. Samsung Knox is missing in this particular build as well, but it is expected that Knox might make an appearance in the final build. The leaked Android 4.3 build for Galaxy S3 can be downloaded from the leak’s source.