Goodbye Tracking? Impact of iOS App Tracking Transparency and Privacy Labels.

Tracking is a highly privacy-invasive data collection practice that has been ubiquitous in mobile apps for many years due to its role in supporting advertising-based revenue models. In defence of user privacy, Apple introduced two significant changes with iOS 14: App Tracking Transparency (ATT), a mandatory opt-in system for enabling tracking on iOS, and Privacy Nutrition Labels, which disclose what kinds of data each app processes. This paper studies two versions of 1,759 iOS apps from the UK App Store: one version from before iOS 14 and one that has been updated to comply with the new rules. We find that Apple's new policies, as promised, prevent the collection of the Identifier for Advertisers (IDFA), an identifier used to facilitate cross-app user tracking. However, many apps still collect device information that can be used to track users at a group level (cohort tracking) or identify individuals probabilistically (fingerprinting). We find real-world evidence of apps computing and agreeing on a fingerprinting-derived identifier through the use of server-side code, thereby violating Apple's policies and exposing the limits of what ATT can do against tracking on iOS. This is especially concerning because we explicitly refused opt-in to tracking in our study, and consent is a legal requirement for tracking under EU and UK data protection law. We find that Apple itself engages in some forms of tracking and exempts invasive data practices like first-party tracking and credit scoring from its new rules, and that the new Privacy Nutrition Labels were often inaccurate. Overall, our findings suggest that, while tracking individual users is more difficult now, the changes reinforce existing market power of gatekeeper companies with access to large troves of first-party data.