Cyber Manufacturing Systems (CMS) brings improvements to the manufacturing system and offers an information-transparent environment. Although CMS’s enhanced system brings many advantages to its manufacturing process, it is at a higher risk of security breach ironically due to its accessibility and connectivity. Therefore, implementing CMS without adequate security measures will make the system vulnerable to intruders including insiders. To identify insider threats, a risk assessment-based framework has been explored by many researchers in the information security community. however, less work has been done to analyze CMS’s vulnerabilities. Therefore, an Insider Attack Scenario Assessment Framework (IASAF) is proposed in this paper. IASAF is developed to detect the system’s security vulnerabilities and evaluate the insider attack scenarios by using five domains including: actor, preparation, implementation, consequence, and recovery. To validate and demonstrate the effectiveness of the framework, a testbed was used to study a case.