Traditional risk assessments rely on manual audits and system scans, often causing operational disruptions and leaving security gaps. To address these challenges, this work presents Security Digital Twin-as-a-Service (SDT-aaS), a novel approach that leverages Digital Twin (DT) technology for automated, non-intrusive security compliance. SDT-aaS enables real-time security assessments by mirroring real-world assets, collecting compliance artifacts, and creating machine-readable evidence. The proposed work is a scalable and interoperable solution that supports open standards like CycloneDX and Web of Things (WoT), facilitating seamless integration and efficient compliance management. Empirical results from a moderate-scale infrastructure use case demonstrate its feasibility and performance, paving the way for efficient, on-demand cybersecurity governance with minimal operational impact.