While the researchers claim that they've yet to find evidence of the vulnerability being used by hackers, the extent of the security risk led them to call Stagefright one of the worst security vulnerabilities ever discovered. At the time the report was published, Google had already sent out fixes for the vulnerability, but no carriers had gotten around to actually sending out the patches.
Sprint recently became the first carrier in the US to send out patches for the Stagefright exploit, at least for some of its carrier-specific smartphone versions. A couple of days ago, the carrier started sending out fixes for the Samsung Galaxy Note 4 (bundled with the Android 5.1.1 Lollipop update), and today Sprint started rolling out Stagefright patches for a bunch of other smartphones, such as the Galaxy S6, the Galaxy S6 edge, the Galaxy Note Edge, last year's Galaxy S5, as well as the Google Nexus 5 and Google Nexus 6.
The updates will be rolled out in batches, so don't worry if your handset hasn't been updated yet. You can always manually check for the availability of the Stagefright fixes by heading over to the settings menu.
If your handset is not included in this list, or if you can't wait for the fix to reach your device, make sure to read our previous article on how to guard your phone against the Stagefright exploit.