Network functions virtualization (NFV) and software-defined networking (SDN) improve network capabilities by enabling the deployment and control of network functions using software- instead of hardware-specific middleboxes. This programmability enables the development of new software services designed to meet a growing list of network requirements. Nevertheless, this same flexibility and programmability can facilitate malicious behavior by attackers with partial access to the SDN and NFV management infrastructure. In this chapter, we discuss security challenges that emerge in an SDN/NFV environment and analyze the main proposals aimed to secure SDN and NFV platforms. In addition, we illustrate the similarities with the requirements that secure operating systems address. Inspired by some of the best practices and lessons learned in the design of these systems, like reference monitors, mandatory access control, and policy verification, we argue that such principles can be used to define a standard SDN/NFV security architecture to facilitate the design and management of SDN/NFV applications.