The data that developers can gather -- which can then be stored on the developer's own servers -- is said to help monitor how often users blink, smile, or even raise an eyebrow. Although this data can't unlock the iPhone X, according to documents about Face ID sent to security researchers, the "relative ease" with which developers can gain access to parts of a user's facial data and add it to their own servers has led to the new concerns raised by the ACLU and CDT today.
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Apple has strict policies against developers using face data for advertising and marketing, but those concerned groups cited worry about the company's "inability to control what app developers do with face data once it leaves the iPhone X." Stanley said that "the hard part" for Apple will come from having to find and catch the apps that might be violating these policies, meaning that the big household names probably won't be of concern to Apple, "but there's still a lot of room for bottom feeders."