Should I delete locally synced data if the user can't log in?
Scenario 1 will only work if the attacker is stupid. If a guy manages to steal the password of somebody and sync a cloud folder on his machine, the next thing he should do would be to save the content of the stolen data elsewhere just to make sure that a sync problem could not change it.
In that case erasing the synced folder will just have no effect...
You should keep user in control. That said, you can do the following:
- Never delete user synced files automatically
- You can delete synced files on user request, similarly like erasing data on the phone remotely.
This way user is in control of his files, and can remove files from other devices upon failed logout.
This would have to be a feature, like user in his control panel can see various devices / locations syncing his files, and he can then choose which locations / devices should erase backup upon failed login.
This gives user good control, that he can first update his password on all devices, and then request to delete from everywhere else, after his sync worked with new password.
In simplest scenario, it would not be good to delete the backup, because this leaves user in the dust and keeps him out of control.