Strange applications are being installed sometimes I am using Android 5.1 and there is strange thing.Sometimes, when I am connected to wifi apps are being installed without permissions.When I go to settings and uncheck 'Allow installing apps from sources different to google play store' I have no idea how, but after around hour it is being marked as checked and new app is being installed.

I tryed installing anti virus programs like AVG's mobile version, Avast's mobile version, Malwarbytes and others..Do you have any ideas how to fix that, without reseting phone's fabric settings.

Thanks! :)
backup important data
do factory reset


if your device is rooted or the factory reset didn't work:


search for a complete factory image for this specific device model (also known as ROM)
flash it. (or if scared, find someone who is willing to do it)


Anything else would be a half measure, but if you really want to avoid resetting:


check if the attached Google account is compromised, because it's possible to install apps remotely via Google play.
Another possibility is some applications which got "device administrator" permission, so you can first find "device administrators" in settings and make sure that there are no device administrators.