In the current digitalized world, everything is interconnected and accessible from everywhere. Although traditional networks are widely adopted, their management is complicated. Therefore, they are not effective in providing services to the future Internet like a wide range of accessibility, high bandwidth, management, and security. On the other hand, Traditional network architecture relies on manual configurations of proprietary devices that are error-prone and inefficient to utilize the network devices properly. Software-defined Networking (SDN) has drawn massive changes in the traditional network paradigm by decoupling the network operations from the physical hardware and encouraging network control to be logically centralized. It provides network programmability and improves security by enabling a global view of the entire network and issues handled effectively by the centralized controller. As a result, SDN allows networks to monitor the traffic and detect vulnerabilities more effectively. It also simplifies the deployment of new services with more flexibility at a faster pace. On the other hand, the decoupling of control and the data planes introduces security threats such as Distributed Denial of Service (DDoS) attacks, Man in the Middle attacks, Saturation attacks, etc. As a result, SDN has attracted a lot of interest from both academics and industry. In this paper, we study security vulnerabilities on layers of SDN, the security frameworks that protect each layer, and many security methodologies for network-wide security.