Defense in Depth is a security strategy that involves implementing multiple layers of protection throughout an information technology (IT) infrastructure, covering hardware, software, and human aspects. This principle implies that if one security control fails, other layers will still provide protection. By diversifying security measures and ensuring redundancy, organizations can minimize the likelihood of a single point of failure, safeguard business-critical data, and maintain system availability. Defense in Depth typically consists of preventive, detective, and corrective controls that work together to create a resilient security posture.

Least Privilege is a security principle that restricts user access rights and permissions to only those resources and functions absolutely necessary for them to perform their job duties. This minimizes the potential damage caused by unauthorized access, human error, or the exploitation of compromised accounts. By limiting access to sensitive information and critical system functionalities, organizations can reduce the risk of data leakage or system breaches. To implement the least privilege principle, administrators should regularly review user permissions, implement role-based access controls, and segregate duties where appropriate.

Secure Development Lifecycle (SDLC) is a systematic approach to software development that integrates security considerations from start to finish. By incorporating security practices and policies throughout the development process, organizations can minimize the risk of vulnerabilities in the code and establish a consistent set of security requirements. Key stages of the SDLC include defining security objectives, design, implementation, verification, and maintenance phases. Through a proactive and iterative approach to security, an SDLC helps to ensure that security best practices are consistently applied, reducing the likelihood of software-related incidents and the associated costs.

Fail-safe defaults refer to the practice of designing a system to operate securely by default, ensuring that if a failure or error occurs, the system reverts to a secure state. This means that permissions and access controls are set to the most restrictive settings by default, requiring explicit authorization for any deviations. Restrictive settings prevent unauthorized access, protect sensitive data, and maintain system integrity. By designing a system with fail-safe defaults, security is implemented at the core of the system, rather than added on as an afterthought, making it more difficult for attackers to exploit vulnerabilities.

Separation of duties is a principle in secure system design that involves dividing critical functions among different individuals or groups to prevent conflicts of interest, fraud, and abuse of power. By ensuring that no single individual or group has complete control over a process, security risks are mitigated. This separation is typically implemented through role-based access controls, assigning duties to different users based on their responsibilities. By distributing duties across different individuals or groups, the potential for an unauthorized action or a security compromise is reduced, as it would require collusion among multiple parties.

Layered security, also known as having multiple layers of defense, is a system design principle that involves implementing various security measures at different levels in a system. This approach is based on the premise that no single security mechanism can provide complete protection against all threats. By having multiple layers of protection, the failure of one layer can be compensated by the effectiveness of another, reducing the system's overall vulnerability. Layered security can include a combination of hardware, software, administrative, and physical controls, making it difficult for an attacker to penetrate the system or cause harm.

Information hiding is a key principle in secure system design that aims to minimize the exposure of sensitive data and system components. By obscuring internal mechanisms and restricting access to certain information, the system reduces potential attack vectors and makes it harder for adversaries to exploit vulnerabilities. This can be achieved through encapsulation, where the internal workings of a component are hidden from the rest of the system, and only accessible through a controlled interface. Additionally, sensitive data should be encrypted or otherwise protected, so that even if an attacker gains access, they cannot easily exfiltrate or use it.

The Principle of Least Astonishment is a secure system design guideline that suggests that a system's behavior should be consistent, predictable, and intuitive, minimizing unexpected actions or results. By designing a system that is easy to understand and use correctly, the likelihood of users causing security breaches, either intentionally or accidentally, decreases. This principle can be applied to system interfaces, access controls, and process requirements, among other areas. Ensuring that security mechanisms are transparent and intuitive also encourages user adoption, as they do not feel overwhelmed or burdened by implementing security practices.

Secure baselines are the minimum set of security configurations applied to systems, applications, and networks to maintain an acceptable level of security. These baselines provide a strong foundation for the protection of information assets and help reduce the attack surface. Secure baselines can be developed through collaboration between security experts, system administrators, and industry best practices. They are typically enforced through Group Policy Objects (GPOs) in a Microsoft Windows environment or through configuration management tools in other environments. Regular review and updates of these baselines are required to accommodate changes in the threat landscape, technology updates, and organizational requirements. A key aspect of secure baselines is the principle of least functionality, which ensures that software, hardware, and configurations only provide necessary functions and nothing more. This minimizes potential vulnerabilities and reduces the attack surface available to threat actors.

Domain Separation is the concept of separating resources, processes, and users into different security domains. This is done to limit the exposure of sensitive information to those with a legitimate need to access it. By isolating resources in separate domains, it becomes much harder for an attacker to move laterally across the network, thus reducing the risk of system compromise. This concept is particularly important in multi-user or multi-tenant environments, where multiple users or organizations share resources in a single system. Examples of domain separation in practice include splitting network traffic into separate VLANs, isolating file storage areas, or implementing separate database schemas.

Configuration management is the process of maintaining and controlling the configuration of hardware, software, and infrastructure to ensure that systems are consistent with their intended designs, meet security standards, and perform optimally. In Secure System Design Principles, proper configuration management helps prevent misconfiguration that could lead to security vulnerabilities or expose the system to potential exploits. This process includes documenting system settings, monitoring configurations for changes, and implementing secure baselines or guidelines to ensure that system configurations remain secure, reliable, and efficient.