AC-4(13): Decomposition into Policy-relevant Subcomponents

AC-4(13) requires you to break information into policy-relevant subcomponents before moving it between security domains, so your policy enforcement points can make correct allow/deny and sanitization decisions. Operationally, you define what “subcomponents” mean for your data types, implement decomposition at domain boundaries, and retain evidence that the mechanism consistently enforces policy. ¹

Key takeaways:

• Define “policy-relevant subcomponents” for each cross-domain transfer path and data type, then enforce that definition at the boundary. ¹
• Implement decomposition where enforcement happens (gateways, guards, proxies, brokers), not in ad hoc user workflows. ¹
• Keep assessor-ready artifacts: data-flow diagrams, policy rules, configuration baselines, and test results that prove decomposition feeds enforcement. ¹

AC-4(13): decomposition into policy-relevant subcomponents requirement is a boundary control for organizations that transfer information between security domains (for example, from a higher-trust network segment to a lower-trust segment, or from an internal enclave to a partner environment). The control’s intent is practical: policy enforcement mechanisms cannot reliably evaluate a “blob” of information. They need structured components (fields, labels, objects, attachments, metadata, portions of a message) that map to policy conditions.

For a CCO, Compliance Officer, or GRC lead, the fast path to operationalizing AC-4(13) is to treat it as an engineering requirement for each domain-crossing interface: identify the interface, define the decomposition rules that matter to policy, implement technical enforcement at the boundary, and then prove it with repeatable tests and retained evidence. This is less about writing a new policy document and more about making sure your enforcement points can see and act on the right parts of what’s being transferred.

This page gives requirement-level implementation guidance you can hand to control owners and auditors, with specific steps, artifacts, and common failure modes tied directly to the NIST text. ²

Regulatory text

NIST requirement (excerpt): “When transferring information between different security domains, decompose information into {{ insert: param, ac-04.13_odp }} for submission to policy enforcement mechanisms.” ¹

Operator interpretation: For every transfer between security domains, you must ensure the transferred content is broken into components that your enforcement mechanisms can evaluate against policy. In practice, that means your gateway/guard/proxy/broker must parse, separate, tag, or extract the parts of the content that your policy cares about (for example: headers vs body, fields within a record, attachments, embedded objects, labels/markings, or metadata), then apply rules to those parts before allowing the transfer. ¹

Plain-English interpretation (what AC-4(13) really demands)

“Decompose” is not a documentation exercise. It is a technical requirement: your policy enforcement point must receive information in a form it can evaluate. If your policy says “deny export of controlled data labels” or “strip scripts from documents,” the enforcement point needs the label, the scripts, and the document structure exposed as distinct elements, not an opaque encrypted payload or unparsed file. ¹

A useful way to explain this internally:

• Domain crossing exists (a boundary between different trust levels or security policies).
• Policy exists (rules about what can move across).
• Decomposition exists so the policy engine can actually apply those rules to the relevant parts of the content. ¹

Who it applies to (entity and operational context)

Applies to:

• Federal information systems and contractor systems handling federal data that implement NIST SP 800-53 controls. ¹

Operational contexts where AC-4(13) typically shows up:

• Transfers between network segments with different security policies (production to corporate, OT to IT, restricted enclave to general user VLAN).
• Cross-domain solutions (CDS) or guarded data transfers between classification-like domains or high/low environments.
• Data egress paths to third parties (partners, processors, managed service providers) where outbound policy must be enforced at a gateway.
• Messaging and file transfer services that bridge domains (email gateways, managed file transfer, API gateways, service buses). ¹

What triggers audit focus: any place the organization claims “policy enforcement” at a boundary but cannot show that the enforcement mechanism can inspect and act on meaningful parts of the transferred information. ¹

What you actually need to do (step-by-step)

1) Inventory and classify domain crossings (the scope list)

Create an authoritative list of “security domain transfer paths.” Include:

• Source domain, destination domain
• Transfer mechanism (gateway, proxy, API, file transfer tool, message bus)
• Data types moved (records, documents, logs, images)
• Primary enforcement mechanism (DLP engine, content filter, labeling engine, rules engine)
• Owner for the interface (system owner) and owner for the policy engine (security engineering) ¹

Output artifact: Domain Crossing Register (spreadsheet is fine if controlled).

2) Define “policy-relevant subcomponents” per path (the missing definition)

AC-4(13) contains an organization-defined parameter. You must define what “policy-relevant subcomponents” means in your environment. ¹

For each transfer path, document:

• Policy decisions enforced (allow/deny, redact, strip, transform, route, quarantine)
• Attributes needed to decide (labels, fields, metadata, content signatures)
• Subcomponents to extract/parse (examples below)

Practical examples of subcomponents (choose what matches your policies):

• Email: envelope headers, subject, body, attachments, URLs.
• API/JSON: specific fields (PII fields, tokens, classification tags), headers, claims.
• Files: document metadata, embedded objects/macros, OCR-extracted text for images/PDFs.
• Records: columns in exports, row-level tags, data lineage metadata. ¹

Output artifact: “AC-4(13) Decomposition Specification” per interface (short template).

3) Implement decomposition at the policy enforcement point (technical control)

Work with engineering to implement decomposition where enforcement occurs. Common implementation patterns:

• Application-layer gateways that parse protocol objects and pass components to a policy engine.
• Content disarm and reconstruction or sanitization tooling that separates active content from inert content before transfer.
• Structured DLP that inspects fields/metadata rather than only raw streams.
• Message brokers with schema validation and field-level policies. ¹

Non-negotiable design rule: if the enforcement mechanism cannot parse the relevant components, you are not meeting the control intent for that path. ¹

4) Bind decomposition to explicit policy rules (show the “so what”)

Auditors will ask: “Decomposed into what, and which rules act on those parts?”

Maintain:

• A mapping from each subcomponent to policy logic (e.g., “Attachment content -> malware scan + file type allowlist”; “Field X -> redact on egress”; “Label -> block if Restricted”). ¹

5) Test with realistic samples and keep results (prove it works)

Build a small test suite per transfer path:

• Positive tests (allowed transfers)
• Negative tests (blocked transfers)
• Transformation tests (redaction/sanitization occurs)

Tests should demonstrate the policy engine’s decision depends on the decomposed component, not on manual review. ¹

6) Operationalize change control (keep it from drifting)

Decomposition breaks when schemas, file formats, or gateways change.

Minimum operational requirements:

• Version-controlled policy rules and parsing configs
• Change review that includes security engineering sign-off for domain boundary changes
• Logging that indicates what components were inspected and which rule fired, to the extent supported by the tooling. ¹

Where Daydream fits naturally: AC-4(13) assessments often fail on “we think it happens” versus “we can prove it per boundary.” Daydream can act as the system of record that maps AC-4(13) to the control owner, the interface-level procedure, and the recurring evidence set, so you can answer assessor questions without rebuilding context every audit cycle. ¹

Required evidence and artifacts to retain

Keep artifacts per domain crossing, not just one global policy:

1. Domain Crossing Register (authoritative list, owners, mechanisms).
2. Decomposition Specification (policy-relevant subcomponents definition for that path). ¹
3. Policy rules/config exports from the enforcement mechanism (sanitized if needed).
4. Architecture diagrams and data-flow diagrams showing where decomposition and enforcement occur.
5. Test plan + test results demonstrating block/allow/redact based on subcomponents.
6. Change records for material updates (gateway upgrades, schema changes, new destinations).
7. Operational logs or reports showing enforcement activity (what was inspected, actions taken), consistent with your logging policy. ¹

Common exam/audit questions and hangups

Expect these, and prep the evidence mapping:

• “Show me every place data crosses a security domain. How do you know the list is complete?”
• “Define your ‘policy-relevant subcomponents.’ Where is that definition approved and maintained?” ¹
• “For this interface, what exactly is decomposed? What tool does it? Show configuration.”
• “Demonstrate a transfer where the policy decision depends on inspecting a component (field/attachment/metadata).”
• “How do you prevent bypass (alternate route, direct internet, unmanaged file transfer)?”
• “How do you revalidate decomposition after schema/file-format changes?”

Hangup you can avoid: teams present a DLP or firewall screenshot, but cannot show parsing of the components that the policy references. ¹

Frequent implementation mistakes (and how to avoid them)

1. Treating decomposition as a documentation-only control.
   Fix: tie decomposition to a running enforcement mechanism and keep test evidence. ¹

2. One generic definition of subcomponents for all transfers.
   Fix: define subcomponents per domain crossing and data type, because policy decisions differ by interface. ¹

3. Inspecting only “containers” (file name, MIME type) and missing embedded content.
   Fix: confirm the tooling parses inside the object types you allow across the boundary (documents, archives, nested formats). ¹

4. Encrypting before enforcement, creating an opaque payload at the boundary.
   Fix: place enforcement where content is inspectable, then protect it in transit after the policy decision. ¹

5. No ownership model for each crossing.
   Fix: assign a technical owner for the interface and a policy owner for the ruleset, then document both in the register. ¹

Enforcement context and risk implications

No public enforcement cases were provided in the source catalog for this specific control enhancement, so do not expect to “defend by citation.” Your risk argument should be operational: if data is not decomposed into policy-relevant subcomponents at domain boundaries, policy engines can miss restricted elements (fields, labels, embedded content), leading to unauthorized disclosure across trust zones. That is a classic path to reportable incidents, contract noncompliance, and failed assessments under NIST-based programs. ¹

A practical 30/60/90-day execution plan

First 30 days (stabilize scope and definitions)

• Name the control owner and technical SMEs for domain boundary enforcement.
• Build the Domain Crossing Register from network diagrams, gateway inventories, and integration catalogs.
• Draft the Decomposition Specification template and fill it for the highest-risk crossings (egress to third parties, internet-facing APIs, cross-enclave file transfer). ¹

Days 31–60 (implement and prove for priority crossings)

• Configure or adjust enforcement mechanisms so they parse the defined subcomponents (protocol parsing, schema validation, content inspection).
• Write interface-level procedures: where enforcement runs, what gets inspected, what gets logged, and who reviews exceptions.
• Create a repeatable test set for each priority crossing; capture results and remediation tickets. ¹

Days 61–90 (scale coverage and lock operations)

• Extend decomposition specs and tests to remaining crossings.
• Add change-control gates: any new integration or gateway change must declare decomposition and show a passing test set.
• Centralize evidence collection (for example, in Daydream): register, specs, configs, tests, and change records mapped to AC-4(13) so audits become retrieval work, not archaeology. ¹

Frequently Asked Questions

What counts as a “security domain” for AC-4(13)?

Treat it as any environment boundary where security policy meaningfully differs (trust level, data handling rules, monitoring, user populations). If policy changes across the boundary, assume AC-4(13) applies to transfers across it. ¹

How do we define “policy-relevant subcomponents” without overengineering?

Start from your actual enforcement rules and work backward: list the attributes the rule needs to evaluate, then define those as subcomponents. Keep the definition per interface and tied to concrete policy outcomes (block, redact, sanitize). ¹

Does a firewall or network ACL satisfy AC-4(13)?

Usually no, because ACLs enforce flow control without decomposing content into inspectable subcomponents. AC-4(13) expects submission to policy enforcement mechanisms that can act on the relevant parts of the information being transferred. ¹

We already have DLP. Are we automatically compliant?

Only if the DLP inspection decomposes the transferred information into the specific components your policies reference (fields, metadata, attachments) at each domain crossing. You still need interface-level documentation and tests that prove the mechanism enforces policy on those components. ¹

What evidence is strongest for an assessor?

A complete crossing inventory, a written decomposition spec for the sampled interface, configuration exports showing parsing/inspection, and a test result where changing a single subcomponent changes the policy decision. Pair that with change records that show you keep it current. ¹

How do we handle SaaS-to-SaaS or cloud-managed transfer paths where we don’t control the gateway?

Document the boundary, identify what enforcement exists (native controls, broker/CASB, API gateway), and show how information is decomposed for policy evaluation in that stack. Where decomposition is not possible, record the risk decision and implement compensating controls aligned to your authorization boundary. ¹

Footnotes

1. NIST SP 800-53 Rev. 5 OSCAL JSON

2. NIST SP 800-53 Rev. 5; NIST SP 800-53 Rev. 5 OSCAL JSON