This paper investigates the existence of strong Nash equilibria in a cybersecurity investment supply chain game theory model. We consider a supply chain network consisting of retailers and consumers at demand markets with each retailer being faced with nonlinear budget constraints on his security investments. We also assume that the demand for the product at each demand market is known and fixed and, hence, the conservation law of each demand market must be fulfilled. The model is a Generalized Nash equilibrium model for which we define a variational equilibrium, that allows us to give a variational inequality formulation. Our aim is to give a necessary condition to be a strong Nash equilibrium of the model in terms of a system of variational inequalities.