Software-Defined Networking (SDN) is a novel network approach that has revolutionised existent network architectures by decoupling the control plane from the data plane. Researchers have shown that SDN networks are highly vulnerable to security attacks. For instance, adversaries can tamper with the controller's network topology view to hijack the hosts' location or create fake interswitch links. These attacks can be launched for various purposes, ranging from impersonating hosts to bypassing

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... dleboxes or intercepting network traffic. Several countermeasures have been proposed to mitigate topology attacks but to date there has been no comprehensive analysis of the level of security they offer. A critical analysis is thus an important step towards better understanding the possible limitations of the existing solutions and building stronger defences against topology attacks. In this paper, we evaluate the actual security of the existing mechanisms for network topology discovery in SDN. Our analysis reveals 6 vulnerabilities in the state-of-the-art countermeasures against topology attacks: TopoGuard, TopoGuard+, SPV and SecureBinder. We show that these vulnerabilities can be exploited in practice to manipulate the network topology view at the controller. Furthermore, we present 2 novel topology attacks, called Topology Freezing and Reverse Loop, that exploit vulnerabilities in the widely used Floodlight controller. We responsibly disclosed these vulnerabilities to Floodlight. While we show that it is difficult to fully eradicate these attacks, we propose fixes to mitigate them. In response to our findings, we conclude the paper by detailing practical ways of further improving the existing countermeasures.