Security breaches in large socio-technical systems cost billions. Many breaches can be attributed to the piecemeal security design, leaving parts of the system vulnerable while others are over-protected. The authors advocate holistic security design, and have introduced techniques to support security analysis across multiple layers. This paper presents MUSER, a prototype that assists security requirements analysts dealing with security requirements from a holistic viewpoint based on a three-layer framework. Their prototype analyzes security requirements and related security mechanisms in business layer, application layer, and physical layer.