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Primary-Backup service replication does not constrain that the service be built as a deterministic state machine. It is meant to tolerate crashes, not intrusions. We consider an approach, called FORTRESS, for adding intrusion-resilience capability to a primary-backup server system. It involves using proxies that block clients from directly accessing servers, and periodically randomizing the executables of proxies and servers. We argue that proxies and proactive randomization can offer sound defense against attacks including de-randomization attacks. Using simulations, we then compare the attack resilience that FORTRESS adds to a primary-backup server system with that attainable through state machine replication (SMR) that is fit only for deterministic services. A significant observation is that FORTRESS emerges to be more resilient than an SMR system of four server replicas that are diversely randomized at the start and are subject to proactive recovery throughout.