Software-Defined Networking (SDN) promises to introduce flexibility and programmability into networks by offering a northbound interface (NBI) for developers to create SDN applications. However, current designs and implementations have several drawbacks, including the lack of extended security features. In this paper, we present a secure northbound interface, through which an SDN controller can offer network resources, such as statistics, flow information or topology data, via a REST-like API to registered SDN applications. A trust manager ensures that only authenticated and trusted applications can utilize the interface. Furthermore, a permission system allows for fine-grained authorization and access control to the aforementioned resources. We present a prototypical implementation of our interface and developed example applications using our interface, including an SDN management dashboard.