This paper presents a quantitative threat modeling method, the Threat Modeling method based on Attack Path Analysis (T-MAP), which quantifies security threats by calculating the total severity weights of relevant Attack Paths for Commercial Off The Shelf (COTS) systems. Compared to existing approaches, TMAP is sensitive to an organization’s business value priorities and IT environment. It distills the technical details of thousands of relevant software vulnerabilities into management-friendly numbers at a high-level. T-MAP can help system designers evaluate the security performance of COTS systems and analyze the effectiveness of security practices. In the case study, we demonstrate the steps of using T-MAP to analyze the cost-effectiveness of how system patching and upgrades can improve security. In addition, we introduce a software tool that automates the T-MAP.