Software-defined networking (SDN) is an emerging architecture with a great potentialto foster the development of modern networks. By separating the controlplane from the network devices and centralizing it at a software-based controller,SDN provides network-wide visibility and flexible programmability to networkadministrators. However, the security aspects of SDN are not yet fully understood.For example, while SDN is resistant to some topology poisoning attacks inwhich the attacker misleads the routing algorithm about the network structure,similar attacks by compromised hosts and switches are still known to be possible.The goal of this thesis is to thoroughly analyze the topology poisoning attacksinitiated by compromised switches and to identify whether they are a threat toSDN. We identify three base cases of the topology poisoning attack, in which theattack that requires a single compromised switch is a new variant of topologypoisoning. We develop proof-of-concept implementations for these attacks inemulated networks based on OpenFlow, the most popular framework for SDN.We also evaluate the attacks in simulated networks by measuring how muchadditional traffic the attacker can divert to the compromised switches. A widerange of network topologies and routing algorithms are used in the simulations.The simulation results show that the discovered attacks are severe in many cases.Furthermore, the seriousness of the attacks increases according to the number oftunnels that the attacker can fabricate and also depends on the distance betweenthe tunnel endpoints. The simulations indicate that network design can help tomitigate the attacks by, for example, shortening the paths between switches in thenetwork, randomizing regular network structure, or increasing the load-balancingcapability of the routing strategy.