Not too long ago we read about malware that supposedly was taking over older Amazon Fire TV devices. That wasn't exactly the case, but headlines are gonna headline. The thing was if you enabled the installation of "unknown sources" — which allows applications to be loaded from outside of whatever app store is tied to the device, and is something that doesn't happen on its own — and then got tricked into loading up this malware, it could spread to other Fire TV devices that happened to have ADB access enabled (which would open up a wireless port for wireless ADB access), and you'd never know it happened.
Let's be clear: this sort of thing spreads by people sideloading hacky stuff on to their devices and then leaving them vulnerable. Sometimes on purpose, so things "work," and sometimes because they forgot to close the front door. This malware didn't just happen, and isn't something most folks would run into.
Anyway. A new update to the Fire TV software — Fire TV OS 5.2.6.6, which apparently is starting to roll out — throws one more safeguard into the mix. Now, for an app to be installed from outside the Amazon Appstore over ADB, you'll first have to enable ADB access — but then you'll also have to manually approve a connection to another device in addition.
It's called RSA Key Fingerprinting — basically saying "Yes, it's cool to connect to this other device, thanks for asking." And it's something that was introduced in Android 4.2.2 way the hell back in 2013. (Fire TVs run a version of Android, of course.) The original Fire TV box was released in 2014, but as we all know the Fire TV OS doesn't line up with mainline releases of Android. To its own peril, sometimes. (And the older Fire TV OS 5 is based on Android 5.1, so there really is no excuse here.)
If you have a newer Fire TV — the 4K dongle or the newer Amazon Fire TV Cube — this was never a problem. And unless you were poking around places without really knowing what you were doing (or were tricked into doing so), it probably wasn't a problem in the first place.
All this is to say that older Fire TV devices are getting a safety feature that's 5 years old — and that they really should have had all this time.
