unique device ID (Apple) (UDID)

The Unique Device Identifier (UDID) is a unique device identifier of Apple devices.

The UDID identifier is a 40- digit string of numbers and letters that is assigned to specific Apple devices such as the iPhone, iPad, and iPod Touch and from which a relationship to the real user can be established.

The UDID enables app providers to track the behavior of users and draw conclusions for advertising offers. Since the disclosure of user IDs violated data protection, app providers were already asked by Apple in 2012 to end this practice.