SC-46: Cross Domain Policy Enforcement

Compliance teams often inherit “cross-domain” risk without owning the network. SC-46: cross domain policy enforcement requirement is the control that forces clarity: where do security domains connect, what is allowed to cross, and what technical mechanism blocks everything else. The control text is short, but the operational expectation is not. You need a documented policy for each boundary and a mechanism that enforces it between physical and/or network interfaces for the connected domains.

In practice, SC-46 shows up anywhere you have segmentation claims (e.g., production vs. corporate, regulated vs. non-regulated, classified vs. unclassified, high impact vs. moderate) and any path exists for data to move across that segmentation. It also appears when third parties connect into your environment and you treat that connection as a distinct domain.

This page gives requirement-level implementation guidance you can hand to network/security engineering and then audit against. It prioritizes what to build, how to prove it works, and what evidence to keep so you can answer assessor questions quickly and consistently.

Regulatory text

Requirement (verbatim): “Implement a policy enforcement mechanism {{ insert: param, sc-46_odp }} between the physical and/or network interfaces for the connecting security domains.” (NIST SP 800-53 Rev. 5 OSCAL JSON)

Operator interpretation:
You must implement a technical “gate” at every connection point between two security domains that enforces your cross-domain rules. The gate sits at the boundary interface (physical and/or network). It is not enough to (a) document segmentation, (b) rely on user training, or (c) assume applications will behave. The mechanism must enforce policy in-line with the cross-domain traffic path. (NIST SP 800-53 Rev. 5)

What “policy enforcement mechanism” means in real systems

A policy enforcement mechanism is any technical control that can:

• Identify the boundary (what is “Domain A” vs. “Domain B”).
• Inspect and evaluate traffic/data movement against explicit rules.
• Allow only what meets rules; block, quarantine, or sanitize everything else.
• Produce evidence (logs/events/config state) that enforcement is active.

Common patterns you can use (choose based on your architecture and risk):

• Cross Domain Solution (CDS) for higher-assurance environments where content inspection, sanitization, and strict workflows are required.
• Application-layer proxy / API gateway where cross-domain traffic is primarily HTTP/API and you can enforce authN/authZ, schema validation, and payload controls at the edge.
• Email/web security gateways if the cross-domain transfer is human-driven messaging or file movement through sanctioned channels.
• Network firewall + content inspection controls when the policy can be enforced at L3–L7 with explicit allowlists and inspection.
• Data diode / one-way transfer when policy is “only one direction, no exceptions.”

The control does not prescribe a single product. It does require that enforcement exists “between” interfaces, meaning it must be positioned so bypass is not possible through an alternate route.

Plain-English interpretation (what you must ensure is true)

If two security domains are connected, you must be able to say:

1. Where the boundary is (interfaces, routes, VLANs/VPCs/tenants, physical ports).
2. What is allowed to cross (protocols, endpoints, identities, data types).
3. What enforces the rules (the gate) and how it blocks/filters.
4. How you know it works (tests + logs + monitoring).
5. How changes are controlled (no silent rule drift).

Who it applies to

SC-46 commonly applies to:

• Federal information systems and contractor systems handling federal data implementing NIST SP 800-53 controls. (NIST SP 800-53 Rev. 5)
• Any environment with multiple security domains connected by physical links, network peering, VPNs, interconnects, or shared services.

Operational contexts where assessors expect SC-46-level rigor:

• Corporate IT network connected to production or regulated enclaves.
• Separate cloud accounts/subscriptions/projects that exchange data.
• M&A or joint venture networks with interconnects.
• Third-party connectivity treated as a separate domain (managed service providers, support tunnels, partner APIs).
• Boundary crossings for backup, logging, SIEM forwarding, or data analytics pipelines.

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

Use this as an execution checklist.

Step 1: Define your security domains and boundary owners

• List each domain with a name and purpose (e.g., “Corp IT,” “Prod,” “Dev,” “PCI segment,” “FedRAMP boundary”).
• Assign a domain owner (accountable) and boundary control owner (responsible).
• Document domain criteria: identity plane, network plane, data classification expectations, and trust assumptions.

Output: Domain map + owner register.

Step 2: Inventory every cross-domain connection (including “indirect” paths)

Build a table of all connections that enable data movement:

• Network interconnects (VPNs, VPC/VNet peering, MPLS, SD-WAN).
• Shared services (AD/IdP, DNS, NTP, logging, CI/CD runners).
• Application paths (APIs, message buses, SFTP, ETL pipelines).
• Admin paths (jump hosts, remote support tools).
• Third-party connections (support tunnels, partner endpoints).

Output: Cross-domain connection register (authoritative list).

Step 3: Specify the cross-domain policy per connection

For each connection, define rules in enforceable terms:

• Directionality (A→B, B→A, both).
• Allowed protocols/ports and endpoints.
• Required identity (service accounts, mTLS, OAuth scopes).
• Allowed data types (file types, schemas, size limits) and required transformations (sanitization, tokenization).
• Required security checks (malware scanning, DLP patterns, content disarm and reconstruction).
• Default action for nonconforming traffic (block/quarantine/alert).

Write the policy so engineering can implement it as a ruleset, not prose.

Output: Boundary policy statements mapped 1:1 to connections.

Step 4: Implement an enforcement mechanism “between the interfaces”

Place the gate so there is no practical bypass:

• Terminate cross-domain traffic at the enforcement point (proxy/gateway/CDS).
• Remove any direct routing that would allow lateral paths around the gate.
• Use deny-by-default on the boundary and allowlist only defined flows.

Engineering acceptance criteria (what you should require):

• Rules are version-controlled.
• Administrative access to the enforcement mechanism is restricted and logged.
• Alerts exist for rule changes and policy violations.

Output: Implemented boundary controls + rule artifacts.

Step 5: Validate enforcement with tests you can show an assessor

Run repeatable tests per boundary:

• Positive tests: allowed traffic passes and is logged.
• Negative tests: disallowed ports/protocols blocked.
• Content tests: prohibited file types or patterns blocked/quarantined if applicable.
• Bypass tests: confirm no alternate route exists (routing tables, security group rules, firewall policies).

Output: Test plan + test results tied to each connection.

Step 6: Operationalize monitoring and change control

• Centralize logs from the enforcement point and generate alerts for policy violations.
• Require change tickets for rule edits, with approvals by domain/boundary owners.
• Schedule periodic reviews of:
  • Connection register accuracy.
  • Ruleset alignment to documented policy.
  • Exceptions and compensating controls.

Output: Monitoring runbooks + change records + periodic review evidence.

Required evidence and artifacts to retain

Keep evidence aligned to “domain connection → policy → enforcement → proof.”

Artifact	What it proves	Good enough for audit
Domain diagram + domain definitions	Domains are defined	Labeled boundaries, owners
Cross-domain connection register	You know all crossings	Unique IDs, endpoints, direction
Policy per connection	Policy exists and is specific	Allowlist rules, default deny
Enforcement configs/rulesets	Policy is implemented	Exported config snapshots, rule diffs
Change tickets + approvals	Governance for changes	Who approved, what changed, when
Test plans + results	Control works	Positive/negative/bypass results
Logging/alert samples	Ongoing operation	Example events and triage notes
Exception register	Deviations are managed	Risk acceptance and time bounds

A common SC-46 failure is “we have a firewall” without a clean mapping to each domain connection and without test evidence that the firewall is the enforced chokepoint.

Common exam/audit questions and hangups

Expect these and pre-build answers:

1. “Show me all cross-domain interfaces.”
   Hangup: teams only list VPNs and forget shared services or cloud peering.

2. “Where is the policy written, and where is it enforced?”
   Hangup: policy exists in a standard but is not mapped to rules in the enforcement device.

3. “How do you prevent bypass?”
   Hangup: a proxy exists, but direct security group rules also allow the same traffic.

4. “Prove the control is operating.”
   Hangup: logs exist but are not tied to specific boundary flows or are not retained consistently.

5. “Who approves changes?”
   Hangup: rule edits are made under a generic admin account or via emergency changes with no follow-up.

Frequent implementation mistakes (and how to avoid them)

• Mistake: Treating SC-46 as documentation-only.
  Fix: Require a ruleset export and a test record per boundary.

• Mistake: One global policy statement for many different crossings.
  Fix: Create per-connection policies; reuse templates, not vague language.

• Mistake: Allowing “temporary” any/any rules that never get removed.
  Fix: Track exceptions in an exception register and tie them to an expiry and re-approval.

• Mistake: Ignoring third-party connectivity as “out of scope.”
  Fix: Treat third-party connections as a separate domain; enforce at the boundary the same way.

• Mistake: No bypass analysis.
  Fix: Make bypass testing an explicit control activity: routing review + security policy review + negative tests.

Enforcement context and risk implications

No public enforcement cases were provided for this requirement in the source catalog, so this page does not cite specific enforcement actions.

Operational risk if SC-46 is weak:

• Data exfiltration routes across segmentation boundaries.
• Malware propagation between domains that were assumed isolated.
• Loss of integrity when “lower trust” domains can inject data into “higher trust” domains.
• Audit failures when you cannot prove where enforcement occurs and how it is governed.

Practical 30/60/90-day execution plan

Time-boxing helps, but durations are guidance. Adjust to your environment size and change windows.

First 30 days (stabilize and get to inventory accuracy)

• Assign owners for each domain and boundary enforcement stack.
• Build the cross-domain connection register.
• Identify top-risk crossings (internet-adjacent, third-party, regulated-to-unregulated).
• Standardize a policy template per connection.
• Capture current-state configs for existing boundary devices.

Deliverable: A complete, owned inventory plus documented policy for the highest-risk crossings.

By 60 days (implement enforcement consistency + testing)

• Close bypass routes for top-risk crossings (route and rule cleanup).
• Implement deny-by-default and allowlists where feasible.
• Stand up standard logging/alerting for boundary enforcement points.
• Run and document tests for the top-risk crossings.

Deliverable: Enforced chokepoints with test evidence for prioritized boundaries.

By 90 days (operationalize governance and prove repeatability)

• Expand policy enforcement and testing to remaining crossings.
• Implement change control standards: approvals, versioning, and emergency change follow-up.
• Establish periodic reviews of the connection register and ruleset-to-policy mapping.
• Prepare an assessor-ready evidence package per boundary.

Deliverable: Repeatable control operation with evidence that survives staff turnover.

Where Daydream fits (without adding tools to the path)

Most SC-46 friction is control mapping, evidence assembly, and staying current as networks change. Daydream helps you map SC-46 to a control owner, an implementation procedure, and recurring evidence artifacts, then keep those artifacts organized for assessments and internal reviews. This reduces scramble time when an auditor asks for “every cross-domain interface and proof of enforcement.”

Frequently Asked Questions

What counts as a “security domain” for SC-46?

A security domain is any environment where you assume a distinct trust level, identity boundary, or security policy. If you claim segmentation between two environments and data can cross between them, treat them as separate domains for SC-46 purposes. (NIST SP 800-53 Rev. 5)

Do firewalls alone satisfy sc-46: cross domain policy enforcement requirement?

Sometimes, if the firewall rules are the explicit policy enforcement point between the domains, are deny-by-default, and you can prove there is no bypass. If you need content inspection or sanitization, you may need a proxy/CDS pattern beyond basic L3/L4 filtering. (NIST SP 800-53 Rev. 5)

How do I prove “between the interfaces” to an auditor?

Show the path diagram and the routing/security policies that force cross-domain traffic through the enforcement device, plus negative testing that confirms bypass is blocked. Pair the diagram with exported rulesets and change records for that specific boundary. (NIST SP 800-53 Rev. 5 OSCAL JSON)

What evidence is most often missing in SC-46 assessments?

Teams usually miss (1) an authoritative inventory of cross-domain connections and (2) test results tied to each connection. Config screenshots without traceable ruleset exports and approvals also slow assessments.

How should we handle temporary exceptions for cross-domain transfers?

Track them in an exception register with a defined scope, compensating controls, and an expiry tied to re-approval. Remove the exception through standard change control and retain the closure evidence.

Does SC-46 apply to third-party connections like MSP support tunnels?

Yes if you treat the third party environment as a different security domain and data/admin actions can cross into your domain. Put the enforcement mechanism at the boundary (e.g., controlled jump access, proxying, strict allowlists) and keep logs and approvals for the connection.