Cambridge University researcher found multiple Android devices have vulnerability in restoring original factory settings
I thought that your Android device can be sold with "Restore original factory settings" without any worry. That's wrong. According to a study by the University of Cambridge, it is possible that devices with versions ranging from Android 0.5 billion to Android 2.3 may not be properly cleared after the original factory settings are restored, 0.63 billion department will not correctly clear the SD card information after replying to the factory settings. They tested 21 devices from five vendors and those running the Android version earlier, and found that even after the reset, they can still use some methods to initiate some important materials, including installed apps, contacts, SMS and email records, multimedia files (including photos and videos taken by cameras), browser cookies, Wi-Fi passwords, there are more than 8 chances to get the Google master token.
Researchers have pointed out that there are many reasons for the problem, including the vendor's failure to install the necessary data purge driver for the device and the difficulty of completely clearing the memory data itself. Finally, they did not announce the phone model or vendor name to be tested, nor did they know if Google would respond, but you may think twice before selling your phone.
Reference: Cambridge University