Define the CUI system boundary before claiming NIST SP 800-171 coverage

Bottom line: an organization cannot evaluate CUI safeguards accurately until it knows what information is CUI, which agreement governs it, where it flows, and which system components process, store, transmit, or protect it. Scope is an evidence problem, not a label attached to an entire company or one server.

Source fact: what NIST SP 800-171 covers

NIST SP 800-171 Revision 3 provides federal agencies with recommended security requirements for protecting the confidentiality of Controlled Unclassified Information when it resides in nonfederal systems and organizations. NIST states that the requirements apply to nonfederal system components that process, store, or transmit CUI or that provide protection for those components. The publication is intended for use by federal agencies in contracts or other agreements with nonfederal organizations.

NIST identifies SP 800-171A as the companion assessment-procedure publication. Requirements and assessment evidence are therefore related but distinct.

What the source does not establish

SP 800-171 does not determine by itself whether a particular file, email, drawing, recording, ticket, or database is CUI. It does not create a contract requirement for every private organization, certify an environment, or prove compliance through a self-applied label.

This draft is not legal or contracting advice. The responsible contracting and information authorities must resolve classification, marking, clause, flow-down, version, and assessment obligations. Other rules may apply in addition to SP 800-171.

Applicability questions

Which contract, agreement, agency instruction, or authorized source establishes that the information is CUI and identifies the applicable requirements?

Where is CUI created, received, viewed, transformed, transmitted, stored, backed up, logged, supported, and destroyed?

Which identity, network, endpoint, cloud, security, monitoring, backup, support, and recovery components provide protection to those flows?

Which suppliers or subprocessors can access or protect the information, and what obligations flow to them?

How are changes to data flow or system architecture reviewed before they alter the boundary?

DSE recommendation: build a defensible boundary record

The following steps are DSE recommendations based on the cited source.

Obtain the governing contract or agreement, applicable clauses, authorized CUI category and marking direction, and responsible customer contacts. Resolve ambiguity with the appropriate authority.

Trace representative information from receipt or creation through all processing, storage, transmission, protection, backup, support, and disposal paths.

Identify components that handle CUI and components that protect them. Document included and excluded services, trust relationships, administrative paths, and shared dependencies with rationale.

Validate the boundary against actual configuration, logs, user workflows, integrations, and supplier access rather than diagrams alone.

Map the applicable SP 800-171 revision’s requirements to responsible owners and evidence within the verified boundary. Record gaps and planned actions honestly.

Put boundary review into change, onboarding, new integration, recovery, and supplier-management processes.

Verification and evidence

Retain the authoritative scope documents, data-flow diagrams, system and service inventory, sample workflow traces, configuration and log evidence, supplier records, boundary decisions, requirement mapping, gaps, approvals, and periodic review. Ensure sensitive evidence itself is stored and shared appropriately.

Official references

NIST SP 800-171 Rev. 3 — Protecting Controlled Unclassified Information in Nonfederal Systems and Organizations — National Institute of Standards and Technology; published May 2024

NIST SP 800-171A Rev. 3 — Assessing Security Requirements for CUI — National Institute of Standards and Technology

CUI Registry — National Archives and Records Administration; authoritative program registry