My phone "thinks" it has been rooted

[Android Central Question]

Hi guys, I have Samsung Note 3. When I checked for software update it says: "The Operating system on your device been modified in unauthorised way. try downloading software updates using Samsung Kies on your Pc or visit a customer service center."
Well, the thing is, I bought my phone brand new from the store, I NEVER rooted that phone. I dud 2x times official OTA update, everything was ok, I always keep checking for new updates and it just says no new updates. Until now.
I am afraid it might virus or any other suggestions??

Thanks a lot for all answers.

 

[Rukbat]

"modified in unauthorised way" doesn'r necessarily mean rooting. It could be a virus installed as a system app, it could be a bad update the last time.

You;ll need to back the phone up - everything on the phone - see Backing up an Android Device. Then you'll need to flash a stock ROM - see [Samsung] How to flash Stock ROM via ODIN. (The reason you need the backup is because flashing a ROM wipes the phone.)

I can't see any updates, though. The last legitimate update was Marshmallow, and I thought Samsung had stopped issuing security updates for the Note 3. (I haven't gotten any in the US in so long I can't remember how long it's been.)