SDN can have a positive impact on security. SDN makes it easier to segment parts of your network so that sensitive aspects are kept away from less critical ones. These decisions can be made on a workload-by-workload basis, letting you safely try out new technologies without putting your existing infrastructure at risk.
Centralization also brings consistency benefits. You can ensure all your networking hardware is running the same set of policies, preventing situations where individual switches run outdated configurations. SDN gives you an overall view of your networking position, improving transparency for administrators.
But there are trade-offs. SDN's controller-centric model means a breach of the controller grants attackers access to your entire network. SDN has a greater attack surface because it has more moving parts. You need to audit, patch, and secure the controller, your devices, and the communication APIs between them. This can create additional challenges beyond those that network administrators are typically familiar with.