Real-ish? Privacy Problem Regarding Buying Devices from Unofficial Source.

Someone told me that they once bought a device from a used device dealer. There are a lot of pornography content stored in the phone memory along with means of contact to those adult only websites/facilities.

To their surprise, some of those contents persists even after multiple factory reset (factory reset is able to remove like 90% while the rest 10% can't be removed). How this is possible? we know that Android has AVB secure boot feature so dealer or …

devices privacy problem