The security kernel is the part of an operating system (OS) that is responsible for providing security interfaces among the hardware, OS, and other parts of the computing system. The security kernel is a core component of the OS that implements the security policy and enforces the security rules. The security kernel mediates all access requests from the subjects (users or processes) to the objects (resources or data) and ensures that only authorized and valid requests are granted. The security kernel also isolates itself from the rest of the OS and the applications, and protects itself from unauthorized modification or tampering. The security kernel is designed to be as small and simple as possible, to reduce the complexity and the potential for errors or vulnerabilities. References: CISSP All-in-One Exam Guide, Chapter 3: Security Architecture and Engineering, Section: Operating System Security, pp. 297-298.