CA.L2-3.12.4 System Security Plan

Discussion [NIST SP 800-171 R2]
System security plans relate security requirements to a set of security controls. System security plans also describe, at a high level, how the security controls meet those security requirements, but do not provide detailed, technical descriptions of the design or implementation of the controls. System security plans contain sufficient information to enable a design and implementation that is unambiguously compliant with the intent of the plans and subsequent determinations of risk if the plan is implemented as intended. Security plans need not be single documents; the plans can be a collection of various documents including documents that already exist. Effective security plans make extensive use of references to policies, procedures, and additional documents (e.g., design and implementation specifications) where more detailed information can be obtained. This reduces the documentation requirements associated with security programs and maintains security-related information in other established management/operational areas related to enterprise architecture, system development life cycle, systems engineering, and acquisition.
Federal agencies may consider the submitted system security plans and plans of action as critical inputs to an overall risk management decision to process, store, or transmit CUI on a system hosted by a nonfederal organization and whether it is advisable to pursue an agreement or contract with the nonfederal organization.
NIST SP 800-18 provides guidance on developing security plans

Further Discussion
A system security plan (SSP) is a document that outlines how an organization implements its security requirements. At a minimum, an SSP must include:

• Description of the CMMC Assessment Scope as discussed in Error! Reference source not found.;
• CMMC Assessment Scope Description: high-level description of the assets within the assessment scope;
• Description of the Environment of Operation: physical surroundings in which an information system processes, stores, and transmits information;
• Identified and Approved Security Requirements: requirements levied on an information system that are derived from applicable laws, Executive Orders, directives, policies, standards, instructions, regulations, procedures, or organizational mission/business case needs to ensure the confidentiality, integrity, and availability of the information being processed, stored, or transmitted;
• Implementation Method for Security Requirements: description of how the identified and approved security requirements are implemented with the system or environment;
• Connections and Relationships to Other Systems and Networks: description of related, dependent, and interconnected systems; and
• Defined Frequency of Updates: typically at least annually.

In addition to the requirements above, an SSP often includes:

• general information system description: technical and functional description;
• design philosophies: defense-in-depth strategies and allowed interfaces and network protocols; and
• roles and responsibilities: description of the roles and responsibilities for key personnel, which may include the system owner, system custodian, authorizing officials, and other stakeholders

This practice, CA.L2-3.12.4, which requires developing, documenting, and updating system security plans, promotes effective information security within organizational systems required by SC.L2-3.13.2, as well as other system and communications protection practices.