HTML5 is quickly gaining traction for developing mobile apps. New W3C standards such as WebRTC, WebNFC, WebSockets or WebWorkers elevate HTML5 to a first class citizen for mobile development. However, a regular web app running inside a browser is sandboxed and has only limited access to a smartphone's data or sensors. E.g., it is not possible to access a users contact database or sensors. WebAppBooster is a free Android app that acts as a gateway for web apps to do just that. Particular care must be taken with regards to security. WebAppBooster is used by TouchDevelop and was developed as part of a MSR funded research project. The presentation will discuss motivation and challenges in designing and implementing WebAppBooster. http://webappbooster.