Software-Defined Networking (SDN), controlling underlying network devices (i.e., data plane) in a logically centralized manner, is now actively adopted in many real world networking environments. It is clear that a network administrator can easily understand and manage his networking environments with the help of SDN. In SDN, a network operating system (NOS), also known as an SDN controller, is the most critical component because it should be involved in all transactions for controlling network devices, and thus the security of NOS cannot be highly exaggerated. However, in spite of its importance, no previous works have thoroughly investigated the security of NOS. In this work, to address this problem, we present the NOSArmor, which integrates several security mechanisms, named as security building block (SBB), into a consolidated SDN controller. NOSArmor consists of eight SBBs and each of them addresses different security principles of network assets. For example, while role-based authorization focuses on securing confidentiality of internal storage from malicious applications, OpenFlow protocol verifier protects availability of core service in the controller from malformed control messages received from switches. In addition, NOSArmor shows competitive performance compared to existing other controllers (i.e., ONOS, Floodlight) with secureness of network assets.