T-Mobile has updated its version of the Samsung Galaxy S5 to Android 5.1.1. It is believed to be the only version of the phone to be running on this build of Android. Besides receiving the new build, the update also includes a fix that prevents the phone from being open to the Stagefright exploit. The update is being sent out OTA or it can be downloaded and installed via Samsung Kies.
The Stagefright exploit is serious stuff. You could find that someone has sent you a malicious MMS which in some cases doesn't even need to be opened for you to lose control of your camera, microphones, and personal data. And because it is believed that Stagefright has a genesis from insecure code found in the Stagefright multimedia library in use since Froyo, as many as 95% of active Android devices could be vulnerable.