A user's investment in security in an interdependent system is often viewed as a public good, as other users will also benefit from the positive externality of this expenditure. Consequently, users' expenditures at the equilibrium state of the system are far from the social-welfare-maximizing levels of investment. In this work, we present a mechanism that can incentivize the socially optimal solution using simple cyber insurance contracts, designed through a message exchange process among users, and backed by a monopolist, profit-neutral insurer. We show that as opposed to many existing solutions, the insurer does not need to monitor or audit users.