Security is a critical concern in shared spectrum environments. In additional to degrading service, attacks can influence the market interactions between competing service providers (SPs). This paper investigates these interactions by considering two SPs engaged in Cournot competition while utilizing both proprietary and shared spectrum, with shared spectrum available in either licensed or open-access forms. Additionally, we assume the presence of an attacker whose objective is to deny service to one or more of the shared bands for a fraction of the time, consequently reducing the overall total revenue. We analyze the optimal forms of attacks under different attacker objectives and their repercussions on the resulting market equilibrium. Utilizing these analyses, we compare the impacts of various spectrum sharing approaches (licensed and open access) and differing amounts of spectrum holdings of the two providers.