In order to prevent attacks against smartphones and tablets, dedicated security applications can be deployed on the mobile devices themselves. However, these applications may have a significant impact on the device resources. Users may be tempted to uninstall or disable them with the objective of increasing battery lifetime and avoiding configuration operations and updates. In this paper, we propose a new approach for outsourcing mobile security functions and building transparent in-path security compositions for mobile devices. The outsourced functions are dynamically activated, configured and composed using software-defined networking and virtualization capabilities. We present a mathematical model to formalize the security com-positions, and describe the functional architecture. We provide an implementation prototype and evaluate the solution through an extensive set of experiments.