SC-31(1): Test Covert Channels for Exploitability

To meet the sc-31(1): test covert channels for exploitability requirement, you must take the covert channels you’ve already identified for the system and actively test a defensible subset to confirm which ones are realistically exploitable in your environment. Operationally, this means selecting high-risk channels, running repeatable tests, documenting results, and feeding outcomes into your mitigation plan and authorization evidence. ¹

Key takeaways:

• SC-31(1) is evidence-driven: “we identified channels” is not enough; you must test a subset for exploitability. ¹
• Your subset selection must be risk-based and documented, or assessors will treat it as arbitrary sampling.
• Retain test plans, results, and remediation decisions as authorization-ready artifacts mapped to a named control owner.

SC-31(1) sits in the System and Communications Protection family and focuses on covert channels: unintended communication paths that can be abused to move information in ways your controls do not monitor or intend. The requirement is narrow but operationally tricky because it expects hands-on validation, not theory. You are not asked to test every possible channel; you are asked to test a subset of the identified channels to determine which are exploitable. ¹

For a Compliance Officer, CCO, or GRC lead, the fastest path to “ready for assessment” is to treat this like a scoped security test with three deliverables: (1) a recorded inventory of identified covert channels, (2) a documented rationale for which subset you selected to test, and (3) test evidence that supports a clear conclusion for each tested channel (exploitable vs. not exploitable under stated assumptions). The output then has to connect to risk treatment: mitigations, compensating controls, or acceptance. Done well, SC-31(1) becomes a repeatable playbook you can run after major architecture changes, new high-side/low-side interconnects, or changes to virtualization and networking layers.

Regulatory text

Requirement excerpt: “Test a subset of the identified covert channels to determine the channels that are exploitable.” ¹

What the operator must do:

1. Start from a list of covert channels you have already identified for the system boundary and its components.
2. Choose a subset to test based on risk, feasibility, and coverage objectives.
3. Execute tests designed to show whether each selected channel can be used in practice to transmit information or signals in a way that violates policy or crosses trust boundaries.
4. Record conclusions and feed them into your mitigation and risk decisions. ¹

Plain-English interpretation

SC-31(1) asks a simple question: “Of the covert channels we think exist here, which ones can an attacker actually use?” Your job is to replace speculation with evidence.

A covert channel can be “real” in theory but non-exploitable in your deployed configuration because of segmentation, timing noise, strict resource controls, or monitoring. The reverse also happens: a channel can look low-risk until you test it and discover it works across a boundary you assumed was safe. SC-31(1) is the forcing function to validate those assumptions and document the results. ¹

Who it applies to (entity and operational context)

This requirement most commonly applies to:

• Federal information systems assessed against NIST SP 800-53 controls. ²
• Contractor systems handling federal data where NIST SP 800-53 is the controlling framework in contracts, ATO packages, or inherited control models. ²

Operational contexts where SC-31(1) becomes “real work”:

• Multi-tenant environments (shared compute, shared network fabrics)
• High-to-low or cross-domain architectures (even without a formal cross-domain solution)
• Virtualized and containerized platforms with shared kernel, shared storage, or shared orchestration planes
• Systems with strict separation requirements (regulated workloads, mission enclaves, or sensitive datasets)

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

Step 1: Assign ownership and define scope

• Name a control owner (often Security Architecture or Platform Security) and a test executor (internal security testing team, red team, or qualified third party).
• Define the system boundary and the “interesting” trust boundaries (e.g., tenant-to-tenant, workload-to-management plane, prod-to-dev).
  Output: SC-31(1) implementation procedure with owners and cadence.

Step 2: Build/confirm the covert channel inventory (prerequisite)

SC-31(1) assumes you have “identified covert channels.” If you don’t, you’ll fail on sequencing. Build an inventory that is specific to your architecture. Track at least:

• Channel name and type (e.g., timing channel, storage channel, shared resource contention)
• Where it could exist (component(s), boundary, protocol, hypervisor layer, shared cache, shared queues)
• Threat scenario (who can send/receive, what data could move, what policy is violated)
• Preconditions (privileges, co-residency, network adjacency)

Output: Covert channel register tied to the system architecture.

Step 3: Select a defensible subset to test

Auditors will ask why you tested what you tested. Make the selection rule explicit. Common selection criteria:

• Crosses a high-impact trust boundary
• Requires low attacker capability (easy to reproduce)
• Has high potential data sensitivity in the scenario
• Has history of architectural change (new platform, new hypervisor version, new network overlay)
• Provides coverage across channel classes (don’t test five variants of the same thing and call it “subset coverage”)

Practical way to document this: create a one-page “subset selection memo” that lists candidates, ranks them, and states why each was in or out.

Step 4: Write a test plan that produces binary-enough conclusions

For each selected channel, define:

• Hypothesis: what you are trying to prove/disprove
• Test method: tools, scripts, configuration, and how you will simulate sender/receiver
• Success criteria: what counts as “exploitable” in your environment (bandwidth threshold is optional; reproducibility and boundary crossing matter more)
• Constraints and assumptions: what you did not test (and why)

Keep the plan reproducible. If it can’t be rerun after a platform change, it won’t support ongoing compliance.

Step 5: Execute tests and capture raw evidence

During execution, capture:

• Test timestamps, target assets, and environment versioning (AMI/container image IDs, hypervisor versions, network policy revisions)
• Commands, scripts, and configuration snippets
• Logs/pcaps/telemetry showing attempted signal transfer and whether it was received
• Analyst notes describing anomalies and retest decisions

Step 6: Classify results and decide risk treatment

For each tested channel, produce a short result record:

• Exploitable / not exploitable / inconclusive
• Conditions required (e.g., co-residency, privileged access)
• Impact narrative (what could move, which boundary is crossed)
• Decision: mitigate, monitor, accept, or redesign

Tie mitigations to existing controls where possible (segmentation, resource quotas, scheduler isolation, disabling shared components, monitoring for contention patterns), and record any planned engineering work as tracked remediation.

Step 7: Operationalize recurrence (so it survives audits)

Define re-test triggers:

• Major architecture changes
• New virtualization layer, orchestration, or network overlay
• Boundary changes (new interconnects, new tenants, new data types)

Put SC-31(1) in your change management checklist so testing is event-driven, not aspirational.

Where Daydream fits (without adding bureaucracy)

Most SC-31(1) failures are evidence failures: no owner, no subset rationale, scattered test outputs, and no linkage to risk decisions. Daydream can act as the system of record that maps SC-31(1) to the control owner, the procedure, and recurring evidence artifacts so you can answer assessor questions quickly and consistently. ¹

Required evidence and artifacts to retain

Retain artifacts in an assessor-friendly folder structure (by system, then by test cycle):

• Covert channel register (identified channels)
• Subset selection memo (risk-based rationale)
• Test plan(s) and procedures (version-controlled)
• Raw evidence: logs, pcaps, screenshots, tool output, scripts
• Test report summarizing outcomes per channel (including “inconclusive” handling)
• Risk treatment records: POA&M items, risk acceptance sign-offs, or architecture change tickets
• Control mapping: where SC-31(1) is described in the SSP and where evidence is referenced

Common exam/audit questions and hangups

Expect these questions:

• “Show me the list of identified covert channels, then show me which subset you tested.” ¹
• “Why did you choose these channels and not those?”
• “Who executed the tests, and are they independent from system operators?”
• “What makes you confident a channel is not exploitable, versus ‘we didn’t reproduce it once’?”
• “How do you re-run this after changes?”

Hangups that stall assessments:

• No documented prerequisite inventory of identified channels
• “Subset” equals whatever was easiest to test, with no risk rationale
• Test evidence exists but can’t be tied to a specific system version or boundary

Frequent implementation mistakes and how to avoid them

Mistake	Why it fails	Fix
Treating covert channels as purely theoretical	Assessors want exploitability evidence, not a literature review	Run bounded, reproducible tests and record outcomes. 1
Testing only application-layer “channels”	Many covert channels are in shared infrastructure layers	Include at least one test that targets shared compute/storage/network behavior.
No “inconclusive” pathway	Teams overstate certainty or bury failed tests	Allow “inconclusive,” then define next steps (environment changes, deeper tooling, third-party testing).
Evidence scattered across chats and laptops	You can’t produce audit-ready artifacts quickly	Centralize artifacts with versioning and a clear naming scheme.

Enforcement context and risk implications

No public enforcement cases were provided for SC-31(1) in the source catalog, so you should treat this as an assessment and authorization readiness requirement rather than a control with a known enforcement pattern in this dataset.

Risk-wise, SC-31(1) matters most where data separation is a promise you make to an authorizing official, customer, or mission owner. If a covert channel is exploitable, it can undermine boundary controls you rely on for confidentiality and can invalidate assumptions in your SSP and risk assessment.

Practical 30/60/90-day execution plan

First 30 days (stand up the control)

• Assign control owner and tester; document roles and escalation.
• Confirm the covert channel register exists; if not, build the first version from architecture review workshops.
• Draft subset selection criteria and select the first test subset.
• Write test plans for the subset and get stakeholder sign-off (security + platform owners).

Days 31–60 (execute and document)

• Run tests in a controlled environment that matches production as closely as practical.
• Capture raw evidence and compile a results report per channel.
• Hold a technical readout to validate conclusions and label items exploitable/not/inconclusive.
• Open remediation tickets for exploitable channels and document interim compensating controls.

Days 61–90 (make it repeatable)

• Update SSP/control narratives to reference the procedure and evidence locations.
• Formalize re-test triggers in change management.
• Run a tabletop “assessor walkthrough” using your artifacts to ensure you can answer typical audit questions.
• If gaps remain, plan the next subset cycle and address tooling or access constraints.

Frequently Asked Questions

What counts as a “subset” for SC-31(1)?

NIST does not set a number in the requirement text; it requires that the subset come from your identified covert channels and that testing determines exploitability. Document your risk-based rationale and coverage approach. ¹

Do we need a red team to satisfy SC-31(1)?

The requirement is “test,” not “red team,” so you can use internal testing if it is competent and evidence-driven. Independence helps in assessments; if internal conflicts exist, a qualified third party can strengthen credibility. ¹

We identified covert channels but can’t reproduce them. Are we noncompliant?

If you attempted testing with a documented method and captured evidence, you can classify the outcome as inconclusive and define next steps. What fails audits is having no testing record or no plan to resolve inconclusive results. ¹

How do we connect SC-31(1) results to risk treatment?

For each exploitable channel, document a decision: mitigate, monitor with compensating controls, or accept with explicit approval. Link the decision to tickets, architecture changes, or risk acceptance artifacts so the trail is complete.

Does SC-31(1) apply to SaaS where we don’t control the infrastructure?

If the system boundary inherits infrastructure controls from a cloud service provider, you still need to address SC-31(1) in your boundary narrative. You may rely on third-party evidence where appropriate, but you still must show how exploitability is tested or justified for your deployment context. ²

What’s the minimum evidence set an assessor will expect?

Expect to show the identified covert channel list, the rationale for the tested subset, the test plan, and test results sufficient to support exploitability conclusions. Keep raw artifacts available for sampling. ¹

Footnotes

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

2. NIST SP 800-53 Rev. 5