AbstractSoftware‐Defined Networks (SDNs) are emerging as one of the most promising new era network technologies with its centralized and easily programmable nature. Many security issues with legacy networks could easily be resolved using SDNs central management and control; at the same time, security vulnerabilities of this technology are still the biggest concern of researchers and industries for adapting this technology.In this paper, a comprehensive review on security aspects of SDN is presented, considering how researchers are utilizing its features for providing security as well as their concerns about its security. Further, a critical comparison of existing countermeasures against SDN security including distributed denial of service (DDoS) is presented. This analysis includes the research works that have been done since the origin of OpenFlow protocol till 2015, including the contributions made by industries as well as universities towards securing the SDN framework. We have also compared our survey with existing surveys.The motivation behind this survey is to identify security concerns of SDN and ongoing research in this field. We have focused on DDoS in SDN mainly, which has not been much targeted by research surveys. By highlighting the current state of the art in this domain, we could facilitate researchers with a broad overview of advancement of research in SDN security and DDoS. Copyright © 2017 John Wiley & Sons, Ltd.