Control statement
Isolate security functions from nonsecurity functions.
Discussion
Security functions are isolated from nonsecurity functions by means of an isolation boundary implemented within a system via partitions and domains. The isolation boundary controls access to and protects the integrity of the hardware, software, and firmware that perform system security functions. Systems implement code separation in many ways, such as through the provision of security kernels via processor rings or processor modes. For non-kernel code, security function isolation is often achieved through file system protections that protect the code on disk and address space protections that protect executing code. Systems can restrict access to security functions using access control mechanisms and by implementing least privilege capabilities. While the ideal is for all code within the defined security function isolation boundary to only contain security-relevant code, it is sometimes necessary to include nonsecurity functions as an exception. The isolation of security functions from nonsecurity functions can be achieved by applying the systems security engineering design principles in [SA-8](#sa-8) , including [SA-8(1)](#sa-8.1), [SA-8(3)](#sa-8.3), [SA-8(4)](#sa-8.4), [SA-8(10)](#sa-8.10), [SA-8(12)](#sa-8.12), [SA-8(13)](#sa-8.13), [SA-8(14)](#sa-8.14) , and [SA-8(18)](#sa-8.18).
From control text to operational evidence
Use Security Function Isolation as a testable risk decision. Translate the official statement into accountable people, repeatable processes, configured technology, and evidence that demonstrates the outcome over time. In this family, pay particular attention to secure architecture, boundary protection, communications protection, cryptography, and system isolation.
Implementation workflow
- Define the control boundary, responsible owner, inherited portions, and systems or processes in scope.
- Resolve each organization-defined parameter before declaring the control implemented.
- Document how the implementation satisfies every clause of the official control statement.
- Collect evidence as a normal byproduct of operation rather than only before an assessment.
- Review exceptions, changes, and monitoring results on a risk-based cadence.
Evidence examples
- network and trust-boundary diagrams
- firewall and gateway configurations
- cryptographic configuration and key records
- segmentation and isolation test results
Common failure patterns
- diagrams omit cloud and third-party paths
- encryption enabled without key governance
- flat trust zones allow unnecessary lateral movement
- boundary rules accumulate without owner review
Questions practitioners should ask
- What risk decision is this control intended to support in this system?
- Which parts are implemented locally, inherited, shared, or not applicable—and what evidence supports that decision?
- Do the documented narrative, deployed configuration, operating process, and collected evidence agree?
- What event or threshold requires the implementation to be reviewed or changed?
Assessment objectives and methods
Show the assessment objective
security functions are isolated from non-security functions.
Examine
- System and communications protection policy
- procedures addressing security function isolation
- list of security functions to be isolated from non-security functions
- system design documentation
- system configuration settings and associated documentation
- system audit records
- system security plan
- other relevant documents or records
Interview
- System/network administrators
- organizational personnel with information security responsibilities
- system developer
Test
- Separation of security functions from non-security functions within the system
Related controls
These relationships come from the official OSCAL catalog. They indicate useful dependencies or context, not automatic inheritance or equivalence.
Related defensive techniques
D3FEND maps this base control or one of its enhancements to the following defensive techniques. The ontology relation label is preserved and does not by itself prove implementation or effectiveness.
MITRE D3FEND™ and the D3FEND logo are trademarks of The MITRE Corporation. Bare Metal Cyber is not affiliated with or endorsed by MITRE.
Control enhancements
Enhancements add specificity, strength, or scope to the base control. Baseline badges show explicit selections in the official SP 800-53B OSCAL profiles.
SC-3(1) — Hardware Separation
Employ hardware separation mechanisms to implement security function isolation.
Official discussion
Hardware separation mechanisms include hardware ring architectures that are implemented within microprocessors and hardware-enforced address segmentation used to support logically distinct storage objects with separate attributes (i.e., readable, writeable).
Assessment objectives and methods
hardware separation mechanisms are employed to implement security function isolation.
Examine
- System and communications protection policy
- procedures addressing security function isolation
- system design documentation
- hardware separation mechanisms
- system configuration settings and associated documentation
- system audit records
- system security plan
- other relevant documents or records
Interview
- System/network administrators
- organizational personnel with information security responsibilities
- system developer
Test
- Separation of security functions from non-security functions within the system
SC-3(2) — Access and Flow Control Functions
Isolate security functions enforcing access and information flow control from nonsecurity functions and from other security functions.
Official discussion
Security function isolation occurs because of implementation. The functions can still be scanned and monitored. Security functions that are potentially isolated from access and flow control enforcement functions include auditing, intrusion detection, and malicious code protection functions.
Assessment objectives and methods
- SC-03(02)[01]security functions enforcing access control are isolated from non-security functions;
- SC-03(02)[02]security functions enforcing access control are isolated from other security functions;
- SC-03(02)[03]security functions enforcing information flow control are isolated from non-security functions;
- SC-03(02)[04]security functions enforcing information flow control are isolated from other security functions.
Examine
- System and communications protection policy
- procedures addressing security function isolation
- list of critical security functions
- system design documentation
- system configuration settings and associated documentation
- system audit records system security plan
- other relevant documents or records
Interview
- System/network administrators
- organizational personnel with information security responsibilities
- system developer
Test
- Isolation of security functions enforcing access and information flow control
SC-3(3) — Minimize Nonsecurity Functionality
Minimize the number of nonsecurity functions included within the isolation boundary containing security functions.
Official discussion
Where it is not feasible to achieve strict isolation of nonsecurity functions from security functions, it is necessary to take actions to minimize nonsecurity-relevant functions within the security function boundary. Nonsecurity functions contained within the isolation boundary are considered security-relevant because errors or malicious code in the software can directly impact the security functions of systems. The fundamental design objective is that the specific portions of systems that provide information security are of minimal size and complexity. Minimizing the number of nonsecurity functions in the security-relevant system components allows designers and implementers to focus only on those functions which are necessary to provide the desired security capability (typically access enforcement). By minimizing the nonsecurity functions within the isolation boundaries, the amount of code that is trusted to enforce security policies is significantly reduced, thus contributing to understandability.
Assessment objectives and methods
the number of non-security functions included within the isolation boundary containing security functions is minimized.
Examine
- System and communications protection policy
- procedures addressing security function isolation
- system design documentation
- system configuration settings and associated documentation
- system audit records
- system security plan
- other relevant documents or records
Interview
- System/network administrators
- organizational personnel with information security responsibilities
Test
- Mechanisms supporting and/or implementing an isolation boundary
SC-3(4) — Module Coupling and Cohesiveness
Implement security functions as largely independent modules that maximize internal cohesiveness within modules and minimize coupling between modules.
Official discussion
The reduction of inter-module interactions helps to constrain security functions and manage complexity. The concepts of coupling and cohesion are important with respect to modularity in software design. Coupling refers to the dependencies that one module has on other modules. Cohesion refers to the relationship between functions within a module. Best practices in software engineering and systems security engineering rely on layering, minimization, and modular decomposition to reduce and manage complexity. This produces software modules that are highly cohesive and loosely coupled.
Assessment objectives and methods
- SC-03(04)[01]security functions are implemented as largely independent modules that maximize internal cohesiveness within modules;
- SC-03(04)[02]security functions are implemented as largely independent modules that minimize coupling between modules.
Examine
- System and communications protection policy
- procedures addressing security function isolation
- system design documentation
- system configuration settings and associated documentation
- system audit records
- system security plan
- other relevant documents or records
Interview
- System/network administrators
- organizational personnel with information security responsibilities
Test
- Organizational processes for maximizing internal cohesiveness within modules and minimizing coupling between modules
- mechanisms supporting and/or implementing security functions as independent modules
SC-3(5) — Layered Structures
Implement security functions as a layered structure minimizing interactions between layers of the design and avoiding any dependence by lower layers on the functionality or correctness of higher layers.
Official discussion
The implementation of layered structures with minimized interactions among security functions and non-looping layers (i.e., lower-layer functions do not depend on higher-layer functions) enables the isolation of security functions and the management of complexity.
Assessment objectives and methods
security functions are implemented as a layered structure, minimizing interactions between layers of the design and avoiding any dependence by lower layers on the functionality or correctness of higher layers.
Examine
- System and communications protection policy
- procedures addressing security function isolation
- system design documentation
- system configuration settings and associated documentation
- system audit records
- system security plan
- other relevant documents or records
Interview
- System/network administrators
- organizational personnel with information security responsibilities
Test
- Organizational processes for implementing security functions as a layered structure that minimizes interactions between layers and avoids dependence by lower layers on functionality/correctness of higher layers
- mechanisms supporting and/or implementing security functions as a layered structure
Authoritative sources
Bare Metal Cyber is an independent educational publisher and is not affiliated with or endorsed by NIST. Official control requirements and interpretations remain with NIST and the responsible authorizing organization.