The Network Functions Virtualization (NFV) paradigm is a novel networking technology which, by means of a decoupling between the network functions and the hardware appliances, allows software processes to be installed as service functions on general-purpose servers. Among the consequent benefits, this principle entails further agility and flexibility in the creation of a Service Graph, which is a generalization of the Service Function Chain (SFC) concept, describing how the single network functions must be organized and connected. A problem which, however, arises in the creation of a Service Graph in this scenario is that this task is typically performed by a service designer; instead, the security manager is separately in charge of the allocation and configuration of the Network Security Functions (NSFs) - such as firewalls and anti-spam filters - needed to protect the network from cybersecurity attacks.