There are various good reasons to perform a factory rest: fixing bugs following an Android update,
general housekeeping for maintaining Android performance and completely wiping data from your phone.
The problem is that Google’s built-in factory reset option can expose your data even after a reset.
Here’s why a factory reset doesn’t wipe all your data, and what you can do about it.
The factory reset problem was uncovered by some Cambridge University researchers in the first major study of this taken-for-granted Android security feature.
The factory reset, we’ve always been told, will delete all data, accounts, passwords and content from your Android device. The problem is, this is only partially true.
Why doesn't a factory reset work?
The researchers tested a range of second-hand Android devices running Android versions from Android 2.3 to Android 4.3 and found that in all cases they were able to recover account tokens – which are used to authenticate you once a password is entered the first time – from service providers such as Google, Facebook and WhatsApp. In a staggering 80 percent of cases, they were able to recover the master token.
The master token is essentially the key to the front door, the equivalent of installing a top-notch security system and then hiding the key under the doormat.
Once a master token is recovered, the user’s credential file can be restored and all your data re-synced to the device: that means emails, cloud-stored photos, contacts and calendars.
How could this happen?
There are a few reasons. Part of the blame is with the manufacturers who simply don’t provide the software required to fully wipe flash storage.
Likewise, flash storage is notoriously hard to wipe, and of course, Google is to blame for not providing a more fail-safe option for users.
The researchers went on to note that while security and antivirus companies may use these findings to promote their own tools and services that the only real solution was likely to come from the vendors themselves.
Unfortunately, even devices with built-in encryption are not safe from these weakness.
The decryption key is also left intact on a device once it has been factory reset. While that key is itself encrypted, gaining access to it would be a few days’ worth of work for most hackers, according to the researchers.
What can I do about it?
It must be noted that devices running Android 4.4 and above were not tested, so it is not clear whether devices running Android KitKat and Lollipop are also affected, although the researchers were quick to point out that it’s plausible that they could be.
The main things one can do to protect themselves is to encrypt their phone and use a strong, randomly-generated password that contains a mixture of upper- and lower-case letters, numbers and symbols and is at least 11 characters long.
The issue with this is that it is sufficiently awkward to do on a regular basis that most users simply won’t do it.
Alternatively, once a phone has been factory reset, the flash storage can be refilled with useless data to overwrite the tokens and crypto keys left in flash storage.
Of course, the app used to fill the phone would need to be installed outside of Google Play to avoid a Google token being registered on the device once again.
The only other solution the researchers came up with was to destroy the device.
This solution, however, raises issues for users that find themselves with a lost or stolen device, or for those devices that have been remotely wiped with Android Device Manager.
Until a legitimate solution can be found, just be careful who you sell your second-hand phone to.