The unique IDs that Apple bakes into iOS mobile devices, such as iPhones and iPads, have long been the subject of criticism by privacy experts. In contrast to the cookies used to track consumers on the web, which can be deleted (at least by those consumers tech-savvy enough to navigate to obscure browser settings), UDIDs cannot be deleted or removed. As long as the consumer uses a particular iPhone, the UDID will stay the same. Unsurprisingly, advertising companies embraced the UDID as a way to effectively track and target users of mobile Apps.

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