We develop a theory of technological resiliency of financial system architecture. Financial platforms compete with services that play critical functions along various stages of financial trade, and make investments in technological resiliency. Network effects relax competition on security, which are exploited by cyber adversaries. Vulnerabilities evolve over time, but generically reach a tipping point at which technological resiliency is too low and creates technological drag on the financial system. We find support in tri-party repo settlement: the exit of a duopolist resulted in a significant drop in IT-related investment by the sole provider, even as peer firms ramp up investment.