Android has become the most popular operating system for mobile devices, which makes it a prominent target for malicious software. The security concept of Android is based on app isolation and access control for critical system resources. However, users can only review and accept permission requests at install time, or else they cannot install an app at all. Android neither supports permission revocation after the installation of an app, nor dynamic permission assignment. Additionally, the current permission system is too coarse for many tasks and cannot easily be refined.