This goes for the iPad and iPod Touch as well. Why is this security issue not talked about more? The problem as I see it is clear:  Every iPhone (and iPad, iPod Touch) has a unique ID number called the 'UDID' that does not change even if you restore your device.  Many apps use this UDID to identify your device and provide access to your game scores, playlists, etc. In some cases personal contact information and authorization to other linked accounts is also provided. This is so users don't need to register or log on. The app servers already know who you are. Or to be more specific, they know your device.  Clueless user sells iPhone after restoring. Buyer downloads previously used app. App servers identify the UDID and gives access to previous user's information. What is frustrating to me is that there is no way of knowing what specific apps do this. It seems that some apps just use the UDID without informing the user while some apps require you to manually input your UDID. In any case, with UDID spoofing easily done through jailbreaking, authorization through UDID is obviously a severe security flaw. Why is this not talked about more? What is being done to identify and prevent these potentially harmful apps?