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Many popular, free online services provide cross-platform interfaces via Web browsers as well as apps on iOS and Android. To monetize these services, many additionally include tracking and advertising libraries that gather information about users with significant privacy implications. Given that the Web-based and mobile-app-based ecosystems evolve independently, an important open question is how these platforms compare with respect to user privacy. In this paper, we conduct the first head-to-head study of 50 popular, free online services to understand which is better for privacy---Web or app? We conduct manual tests, extract personally identifiable information (PII) shared over plaintext and encrypted connections, and analyze the data to understand differences in user-data collection across platforms for the same service. While we find that all platforms expose users' data, there are still opportunities to significantly limit how much information is shared with other parties by selectively using the app or Web version of a service.
Leung et al. (Thu,) studied this question.