Security concerns that arise in safety-critical domains, such as air-traffic control and energy system management, might be analyzed using rigorous security cases. Such analysis has been explored minimally. We present a case study of the application of rigorous security arguments to a novel approach to thwarting command-injection attacks based on transformation of the binary form of a program (no source code). The case study illustrates an approach to security argument structure, the construction process for the argument, and organizes and reveals the defensive capabilities of the technique and its limitations thereby demonstrating the power of security argument.