Software-Defined Networking (SDN) and Network Function Virtualization (NFV) are two evolutionary networking paradigms, which introduce significant granularity, visibility, flexibility and elasticity to networking. However, they both introduce new security challenges as well. The 2021 ACM International Workshop on Software Defined Networks & Network Function Virtualization (SDN-NFV Sec 2021) will take place virtually on April 28, 2021. The target audience will be university researchers, scientists, and industry professionals who need to become acquainted with new theories and technologies related to security challenges and solutions in SDN and NFV.