I found suspicious application (with name like ePlayService or eVideoService, don't remember exactly) in app list, then connected using ADB and examined the fil

I found suspicious application (with name like ePlayService or eVideoService, don't remember exactly) in app list, then connected using ADB and examined the filesystem, and decompiled suspicious packages. Then I downloaded ROM from manufacturer's site to make sure that the backdoor was there too. Also I checked signatures on applications to exclude unmodified third party apps like Youtube and check only those with unknown signatures. This is where I found that the manufacturer signed their system applications using publicly available Android test key (which means device is vulnerable because anyone can make an app signed with that test key and gain high privileges on device). Also, some chipsets (like MediaTek) allow downloading storage contents to PC using proprietary Windows application.