Software-Defined Networking (SDN) is a relatively new networking paradigm that proposes to separate the control and the data logic in networks. The control logic is centralized in a controller, which allows for a programmable network. SDN is promising but also intro- duces some critical security vulnerabilities to networks. This work proposes a survey of state-of-the-art research into attacks and state-of-the-art defences arising from controller place- ment, controller failure and the northbound interface. Furthermore, it proposes a comparison and analysis of the limitations of that research. Finally, it proposes future research directions to improve SDN security focused on network con- sistency and on the interoperability of different defences.