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We examine two privacy controls for Android smartphones that empower users to run permission-hungry applications while protecting private data from being exfiltrated: (1) covertly substituting shadow data in place of data that the user wants to keep private, and (2) blocking network transmissions that contain data the user made available to the application for on-device use only. We retrofit the Android operating system to implement these two controls for use with unmodified applications. A key challenge of imposing shadowing and exfiltration blocking on existing applications is that these controls could cause side effects that interfere with user-desired functionality. To measure the impact of side effects, we develop an automated testing methodology that records screenshots of application executions both with and without privacy controls, then automatically highlights the visual differences between the different …