Google's Android platform includes a permission model thatprotects access to sensitive capabilities, such as Internet ac-cess, GPS use, and telephony. We have found that Android'scurrent permissions are often overly broad, providing appswith more access than they truly require. This deviationfrom least privilege increases the threat from vulnerabili-ties and malware. To address this issue, we present a novelsystem that can replace existing platform permissions withfiner-grained ones. A key property of our approach is thatit runs today, on stock Android devices, requiring no plat-form modifications. Our solution is composed of two parts: Mr. Hide, which runs in a separate process on a device andprovides access to sensitive data as a service; and Dr. An-droid (Dalvik Rewriter for Android), a tool that transformsexisting Android apps to access sensitive resources via Mr.Hide rather than directly through the system. Together, Dr. Android and Mr. Hide can completely remove several ofan app's existing permissions and replace them with finer-grained ones, leveraging the platform to provide completemediation for protected resources. We evaluated our ideason several popular, free Android apps. We found that we canreplace many commonly used “dangerous” permissions withfiner-grained permissions. Moreover, apps transformed touse these finer-grained permissions run largely as expected,with reasonable performance overhead.