Cloud computing has transformed a large portion of the IT industry through its ability to provision infrastructure resources – computing, networking, storage, and software - as services. Software-Defined Networking (SDN) has transformed the physical underlying network infrastructure into programmable and virtualized networks. Network Functions Virtualization (NFV) has transformed physical telecommunication infrastructures and network functions into virtualised network functions and services. Cloud, SDN and NFV technologies and their associated software-defined infrastructures all rely on the virtualization technology to provision their virtual resources and offer them as services to users. These new technologies and infrastructures invariably bring with them traditional vulnerabilities and introduce new technology-specific security risks. In this paper, we discuss extensively cloud-, SDN-, and NFVspecific security challenges as well as approaches for addressing integrated infrastructural issues where cloud, SDN, and NFV all play their integral parts.