AC-4(14): Security or Privacy Policy Filter Constraints

AC-4(14): security or privacy policy filter constraints requirement means you must control cross-domain data transfers by forcing information into fully enumerated, tightly constrained formats, then filtering against those constraints before the data crosses into another security domain. Operationally, this is a “constrain-then-validate” pattern enforced at boundary controls, not a general data loss prevention aspiration. ¹

Key takeaways:

• Require fully enumerated data formats for cross-domain transfers, then enforce strict validation at the boundary. ¹
• Apply the control where information moves between different security domains (networks, enclaves, tenants, classifications, or trust zones). ¹
• Prove it with documented allowed formats, filter rules, test results, and change records tied to each cross-domain data flow. ²

AC-4(14) is easy to misunderstand because many programs treat “filtering” as a generic firewall or DLP capability. This enhancement is narrower and more demanding: when information crosses between security domains, you need policy filters that are constrained by fully enumerated formats that restrict both structure and content. In practice, that means you define what “allowed” looks like down to the schema, field types, permitted values, and disallowed constructs, then you enforce that definition at the transfer point. ¹

This requirement shows up in high-scrutiny environments: federal information systems, contractor systems handling federal data, multi-tenant platforms that segment customers into different trust zones, and any architecture that uses separate enclaves for regulated data. It also applies to third-party provided boundary components (for example, a managed file transfer gateway) if they mediate the domain crossing. ²

The operator goal is straightforward: reduce the attack surface and data spillage risk that comes from permissive formats (free-form text, unbounded JSON blobs, arbitrary attachments) and from ambiguous parsing. You operationalize it by inventorying cross-domain flows, forcing approved formats, implementing boundary validators, and retaining evidence that filters are defined, tested, and governed.

What AC-4(14) requires (plain-English interpretation)

AC-4(14): security or privacy policy filter constraints requirement says: if data moves between different security domains, you must implement security or privacy policy filters that only allow data in fully enumerated formats, and those formats must restrict the data’s structure and content. ¹

Translate that into implementation language:

• “Different security domains”: any boundary where trust changes. Examples: public to private network, user subnet to admin subnet, lower to higher classification network, regulated enclave to corporate IT, one cloud tenant/account to another, or production to analytics. The defining property is that policy differs across the boundary. ²
• “Fully enumerated formats”: formats you can specify completely. Think: strict schemas, fixed field sets, fixed file types with restricted features, fixed message types, and explicit allowlists. ¹
• “Restrict data structure and content”: you constrain fields, types, lengths, encodings, and permitted values; you also constrain what content can appear (for example, no macros, no executable content, no embedded objects, no unexpected keys). ¹

Regulatory text

Excerpt (verbatim): “When transferring information between different security domains, implement {{ insert: param, ac-4.14_prm_1 }} requiring fully enumerated formats that restrict data structure and content.” ¹

Operator meaning: you need a documented, enforced rule that cross-domain transfers must use formats that can be exhaustively specified, and the boundary control must reject anything that fails validation. “We scan it for malware” or “we have a firewall” does not satisfy the “fully enumerated formats” constraint by itself. ¹

Who it applies to (entity and operational context)

Applies to:

• Federal information systems implementing NIST SP 800-53 controls. ²
• Contractor systems handling federal data where NIST SP 800-53 is part of contractual or program requirements. ²

Operational contexts where auditors expect to see AC-4(14) mechanics:

• Cross-domain solutions (CDS), data diodes, guard systems, transfer gateways.
• API gateways brokering calls from a lower-trust zone to a higher-trust zone.
• Managed file transfer between segmented enclaves.
• ETL pipelines pulling from regulated stores into general analytics.

If a third party operates the boundary control (hosted gateway, managed CDS, SaaS integration), you still own the requirement outcome. Your due diligence must confirm the format constraints and validation evidence exist for your specific flows.

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

1) Identify and classify “domain crossings”

Build a cross-domain transfer register:

• Source domain, destination domain, and why domains are different (policy/trust boundary).
• Transfer mechanism (API, SFTP, message bus, removable media, batch export).
• Data types (PII, CUI, logs, telemetry) and security objectives (confidentiality, integrity).
  Output: a list of in-scope flows where AC-4(14) must be enforced. ²

2) Define “fully enumerated formats” per flow

For each flow, define the approved format contract:

• Schema: JSON Schema / XML Schema / protobuf IDL / CSV column spec with strict typing.
• Constraints: allowed fields only, max lengths, regex patterns, enumerated values.
• Disallowed constructs: unexpected keys, nested objects beyond a set depth, external entity resolution, macros, embedded objects, scripts.
• File type restrictions: allowlist extensions plus magic-byte validation; restrict archive types or require controlled extraction rules. Output: versioned format specs, owned by a named control owner. ¹

Practical example: If you send “user export” data from a regulated enclave to corporate IT, define a CSV with fixed columns and strict value constraints. Reject any row with extra columns, invalid encodings, or fields outside allowed patterns.

3) Implement boundary enforcement (validate, then pass)

Put enforcement at the boundary component that mediates transfer:

• Positive validation: accept only known-good structures; reject by default.
• Normalization: canonicalize encodings; strip ambiguous representations.
• Policy decisions: map validation outcomes to allow/deny actions; log rejections with reason codes.
• Fail closed: parsing/validation errors deny transfer. Output: technical configuration showing validation is enforced for each cross-domain interface. ¹

Common patterns:

• API gateway with request/response schema validation.
• Message broker consumer that validates payload before forwarding into the higher-trust topic/queue.
• File transfer gateway that only accepts approved file types and runs schema validation on content before release.

4) Add governance: change control and exception handling

Because format contracts change, you need governance that survives staff turnover:

• Change requests for schema updates, with security review.
• Versioning strategy (v1/v2) and deprecation windows.
• Exception path for urgent transfers, with compensating controls and expiry. Output: change tickets, approvals, and an exception register tied to the transfer register. ²

5) Test the filters like an attacker would

Testing needs to prove that “restrict structure and content” is real:

• Negative tests: extra fields, oversized fields, invalid types, malicious encodings, nested payload bombs, malformed files.
• Parser hardening tests: ensure consistent behavior across libraries/versions.
• Regression tests: schema updates don’t widen acceptance unintentionally. Output: test cases, results, and evidence that failures block transfer. ²

6) Operational monitoring and logging

Minimum operational expectations:

• Logs for accepted and rejected transfers, tied to identity/service account and flow ID.
• Alerts on repeated validation failures (possible probing).
• Metrics that show the control is active (rejections happen for invalid payloads). Output: sample logs, SIEM rules, and runbooks. ²

Required evidence and artifacts to retain

Keep evidence organized by cross-domain flow. Auditors commonly sample a few flows and go deep.

Artifact	What it proves	What “good” looks like
Cross-domain transfer register	You know where AC-4(14) applies	Each flow has owner, boundary, data type, format
Approved format specifications	“Fully enumerated formats” are defined	Versioned schema, constraints, allowlists
Boundary configuration exports	Enforcement exists	Gateway/guard config shows schema validation + deny-by-default
Test plans and results	Filters work	Negative tests, rejection evidence, regression coverage
Change records	Governance	Tickets, approvals, rollout notes
Exception register	Controlled deviations	Time-bound, approved, compensating controls
Logging/monitoring evidence	Ongoing operation	Logs + alerts tied to flow IDs

Daydream (as a GRC system) fits cleanly here as the place to map AC-4(14) to the control owner, store the procedure, and collect recurring evidence on a schedule so you can answer assessors without rebuilding the story each cycle. ²

Common exam/audit questions and hangups

Expect these questions:

• “Show me all domain crossings for this system boundary.” If you cannot enumerate flows, the assessor will assume gaps.
• “What are your fully enumerated formats for this transfer?” “JSON” alone is not an enumerated format without a schema and constraints.
• “Where is validation enforced?” They will look for enforcement at the boundary, not in an upstream app that can be bypassed.
• “How do you prevent format drift?” They will ask how schema changes are reviewed, tested, and rolled out.
• “Prove it blocks bad inputs.” Have negative test evidence ready.

Frequent implementation mistakes (and how to avoid them)

1. Allowing “generic JSON” across domains.
   Fix: require JSON Schema with additionalProperties: false, strict typing, bounds, and explicit allowlists per endpoint.

2. Relying on DLP/malware scanning as the control.
   Fix: keep scanning, but add structural/content validation. AC-4(14) is about constrained formats and policy filters at the domain crossing. ¹

3. Validating only at the producer, not at the boundary.
   Fix: enforce at the transfer control plane (gateway/guard/broker) so bypass paths still fail.

4. No evidence trail.
   Fix: store schemas, configs, and tests as artifacts per flow. If you cannot show it, you cannot pass it.

Enforcement context and risk implications

No public enforcement cases were provided in the source catalog for this requirement, so you should treat AC-4(14) as an assessment-driven control rather than one tied here to a specific published penalty narrative. ¹

Risk-wise, permissive formats across trust boundaries are a predictable path for:

• Injection and parser exploits.
• Data spillage (sensitive fields slipping into exports).
• Covert channels (unexpected fields embedded in “allowed” messages).
• Integrity loss (ambiguous parsing or inconsistent validation across components).

Practical 30/60/90-day execution plan

First 30 days (triage and scoping)

• Assign a control owner and technical owners for boundary components. ²
• Build the cross-domain transfer register and identify the highest-risk flows (regulated data, internet-facing boundaries).
• Pick standard schema technologies per transport (API, files, messages).
• Create an evidence checklist and repository structure (Daydream can track owners and recurring evidence requests).

Days 31–60 (implement enforceable constraints)

• Write enumerated format specs for the first set of sampled flows.
• Implement boundary validation with deny-by-default behavior.
• Add logging fields that tie events to a flow identifier and schema version.
• Establish change control for schema updates.

Days 61–90 (prove operation and harden)

• Run negative and regression tests; fix bypasses.
• Review exception paths; time-box and add compensating controls.
• Train operators on rejection handling and incident triage.
• Prepare an assessor-ready packet per sampled flow: register entry, schema, config export, tests, and last change record.

Frequently Asked Questions

Does AC-4(14) require a cross-domain solution (CDS)?

It requires constrained formats and boundary filtering for cross-domain transfers; a formal CDS is one way to implement that. The right mechanism depends on your architecture, but you still need enforceable validation at the crossing. ¹

What counts as a “fully enumerated format” for APIs?

A concrete contract like OpenAPI plus request/response schema validation, with explicit allowlists and strict constraints per endpoint. “JSON” without a schema and bounds is usually too permissive for this control. ¹

We already scan files for malware. Is that sufficient?

Malware scanning helps, but AC-4(14) is about restricting structure and content through enumerated formats and filter constraints at the domain boundary. Add format allowlists and content validation, then retain proof it blocks nonconforming inputs. ¹

How do we handle “business exceptions” for ad-hoc transfers?

Use a documented exception process with explicit approver, scope, and expiration, and require compensating controls like manual review and quarantined staging. Keep an exception register tied to the affected cross-domain flow. ²

What evidence is most likely to fail an audit?

Missing mapping from flows to enforced constraints. If you cannot show the schema/format spec, boundary config that enforces it, and negative test results for at least sampled transfers, the control often fails on evidence even if some filtering exists. ²

How does this apply when a third party runs the transfer gateway?

You remain accountable for the outcome, so require the third party to provide the format specifications, proof of enforcement (config screenshots/exports), and test results for your flows. Track those deliverables and renewal cadence in your GRC workflow. ²

Footnotes

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

2. NIST SP 800-53 Rev. 5