AC-4(26): Audit Filtering Actions

To meet the ac-4(26): audit filtering actions requirement, you must log and regularly review the filtering decisions and outcomes made when information crosses security domain boundaries (for example, via a cross-domain solution, gateway, proxy, or DLP filter). Capture what was inspected, what rule fired, what action was taken, and the result, then audit those events for correctness and abuse. ¹

Key takeaways:

• You need auditable records of content filtering actions and results specifically tied to cross-domain transfers. ¹
• “Logs exist” is not enough; someone must review filtering actions to detect misconfiguration, bypass, or data spillage.
• Operationalize by standardizing event fields, log routing, retention, review cadence, and escalation for filter exceptions.

AC-4(26) is one of those requirements that fails in practice for a simple reason: teams implement content filtering at boundaries (email gateways, web proxies, API gateways, DLP, cross-domain solutions), but the audit trail is fragmented, incomplete, or not reviewed. The requirement is narrow and very operational: if your environment transfers information between different security domains, you must record the filtering action taken on that information and the result, then audit those actions. ¹

For a CCO, Compliance Officer, or GRC lead, the fastest path is to treat this as a “control over boundary decisioning.” Identify every place content is filtered at a domain boundary, define what “domain” means in your environment (classification, network segment, tenant, enclave, or trust zone), and require uniform logging that proves the filter did what you claim it does. Then prove ongoing oversight by retaining review evidence and exception handling.

This page gives you requirement-level implementation guidance you can hand to security engineering and audit/logging teams, then validate quickly through artifacts and interviews.

Regulatory text

Requirement (verbatim): “When transferring information between different security domains, record and audit content filtering actions and results for the information being filtered.” ¹

Operator interpretation: If content passes through a mechanism that inspects and filters it because it is moving from one security domain to another, you must (1) generate logs that capture what the filter did and (2) periodically audit those logs to verify the filter is working as intended and not being bypassed or misused. ¹

Plain-English interpretation (what this really means)

• “Different security domains” means two zones with different trust levels or policy constraints (for example, “public internet” to “internal,” “low side” to “high side,” “tenant A” to “tenant B,” or “restricted enclave” to “general corporate”). Your job is to define these domains explicitly so engineering can map where the requirement applies.
• “Content filtering actions” means allow, block, quarantine, redact, strip attachments, rewrite URLs, downgrade file types, sanitize, or require human review.
• “Results” means whether the filter succeeded, failed open/closed, produced a sanitized output, detected malware, matched a DLP rule, or triggered an exception path.

The audit objective is accountability: you should be able to reconstruct, for any cross-domain transfer, what decision the filter made and why.

Who it applies to

Entity scope

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

Operational scope (where you will see this control in real systems)

AC-4(26) applies wherever you have boundary filtering tied to domain separation, including:

• Cross-domain solutions (CDS) or guards between enclaves
• Secure web gateways / proxies enforcing content rules across internet-to-corporate boundaries
• Email security gateways performing attachment stripping, sandboxing, or DLP decisions
• DLP systems enforcing policy when moving data to external SaaS, removable media, or partner networks
• API gateways performing schema validation, content inspection, or request/response filtering between trust zones
• File transfer gateways (SFTP, MFT) applying content controls when sending to third parties

If there is no domain boundary and no filtering decision, AC-4(26) is usually not the right enhancement to test. The trigger is “transfer between domains” plus “content filtering.”

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

Step 1: Define your “security domains” and boundary points

Create (or update) a short register that answers:

• What are the domains (names, owners, trust level, data classification constraints)?
• What are the approved transfer paths between them?
• Which systems enforce filtering on those paths?

Tip: If engineering debates what a “domain” is, pick an operational definition that matches your architecture: network segment, enclave, tenant boundary, or data classification boundary. Document it and apply it consistently.

Step 2: Inventory filtering mechanisms tied to cross-domain transfers

For each boundary mechanism, capture:

• System name and owner
• Inbound/outbound direction (Domain A → Domain B)
• What content is inspected (HTTP/S payload, email body, attachments, file content, API request/response)
• What policies/rules are enforced (malware, file type allowlist, DLP patterns, classification tags)
• Where logs are generated and where they are stored

This inventory becomes your AC-4(26) applicability list and stops the common failure mode of “we forgot the MFT gateway” or “the proxy logs are retained somewhere else.”

Step 3: Standardize what must be logged (minimum event fields)

Engineering can implement the fields differently by platform, but you need consistency. Require logs to capture, at minimum:

Filtering event content (minimum):

• Timestamp and timezone
• Boundary system identifier (device/service name)
• Source domain and destination domain (or source/destination network zone tags)
• Source and destination identity where possible (user, service account, workload identity)
• Content/object identifier (message ID, file hash, transaction ID, URL, API route)
• Rule/policy identifier that triggered (rule name/ID)
• Action taken (allow/block/quarantine/redact/sanitize/rewrite)
• Result/outcome (success, failure, partial, bypassed, error)
• Exception path indicator (approved exception, manual release, override used)

Be careful with logging actual sensitive content. In many environments you should log identifiers, hashes, and rule hits rather than full payloads.

Step 4: Route logs to a centralized, tamper-resistant store

AC-4(26) does not prescribe your tooling, but auditors will expect:

• Logs from boundary filters reach your centralized logging/SIEM (or equivalent)
• Access is restricted
• Integrity is protected (at least through access controls and change tracking)

If your organization uses Daydream to manage control ownership and evidence collection, map each boundary system to the control owner and attach the recurring artifacts described below. This prevents the “we have logs but can’t prove review” outcome.

Step 5: Establish an audit/review procedure for filtering actions

“Audit” here means more than log generation. Write a short operating procedure that covers:

• Who reviews (role, team, primary and backup)
• What they review (specific dashboards/queries and which boundary systems)
• What constitutes a finding (unexpected allow, frequent failures, disabled rules, excessive overrides, consistent false negatives)
• How findings are documented, escalated, and tracked to closure

Focus reviews on:

• Blocked vs allowed anomalies: sudden drops in blocks, spikes in allows, or rule changes correlating with behavior shifts
• Failure modes: filter engine errors, timeouts, “fail open” behavior, or bypass routing
• Overrides and exceptions: manual releases from quarantine, temporary bypass rules, emergency changes

Step 6: Control changes to filtering rules and prove traceability

Filtering actions are driven by configuration. Your audit story improves if you can show:

• Change tickets for rule changes
• Approvals (especially for exceptions)
• Testing evidence (what was validated before production)
• Rollback plan

Tie rule changes to observed log outcomes when possible (for example, change implemented → expected shift in events).

Step 7: Test the control with realistic transfer scenarios

Run a small set of scripted tests per boundary:

• A known-benign file that should pass
• A disallowed file type that should be blocked
• A DLP-pattern sample that should trigger quarantine/redaction (use safe test strings)
• A malformed payload that should be rejected

Your goal is to confirm two things: the filter enforces policy, and the logs record the action and result in a way you can later audit.

Required evidence and artifacts to retain

Keep artifacts that prove design, operation, and review:

1. Security domain and boundary register (domains, transfer paths, boundary systems, owners)
2. Filtering policy/rule documentation (high-level control intent plus system-specific rule sets or exported configs)
3. Logging specification for boundary filtering events (required fields, where stored)
4. Sample logs showing actions and results for cross-domain transfers (redacted as needed)
5. Audit/review records (tickets, review checklists, annotated dashboards, SIEM reports, sign-offs)
6. Exception handling records (approved bypasses, quarantine releases, compensating controls)
7. Change management evidence for filtering rules (approvals, testing notes, deployment records)
8. Access control evidence for who can change rules and who can access logs (role mappings, screenshots, IAM exports)

A common audit gap is missing linkage between “filter exists” and “we audit filtering actions.” Your evidence should connect: boundary system → log events → review artifacts → findings → remediation.

Common exam/audit questions and hangups

Auditors and assessors usually probe these areas:

• “Show me where domain boundaries exist.” If your domains are not defined, the control becomes untestable.
• “Which systems filter content between those domains?” They expect a complete inventory, not a single flagship gateway.
• “Prove the filter’s actions and results are recorded.” They will ask for logs that clearly show action + outcome, not generic traffic logs.
• “Who reviews these logs, and how often?” If you cannot show review records, expect a finding even if logs exist.
• “How do you detect bypass or fail-open behavior?” They may ask for error events, health monitoring, or alerting tied to filter failures.
• “How are exceptions governed?” Manual releases and bypasses often drive the highest residual risk.

Frequent implementation mistakes (and how to avoid them)

Mistake	Why it fails AC-4(26)	Fix
Logging only “allowed/blocked” without rule IDs or outcomes	You can’t reconstruct why a decision happened or whether the filter succeeded	Include policy/rule identifier and explicit result (success/error/bypass)
Treating “SIEM ingestion” as “audit”	Collection is not review	Require documented reviews and retain proof of performance
Missing key transfer paths (MFT, API gateways, admin jump hosts)	Control scope is incomplete	Build the boundary register and validate with network/app architecture owners
Storing logs locally on appliances with short retention	Evidence disappears; integrity is weaker	Centralize logs with restricted access and defined retention
Exceptions handled in chat/email	No traceability; high spillage risk	Use tickets with approvals, time bounds, and post-implementation review

Enforcement context and risk implications

No public enforcement cases were provided in the source materials for this requirement, so don’t anchor your risk narrative on specific penalties or settlements.

Practically, AC-4(26) reduces two high-impact risks:

• Undetected data spillage across domains because filters were misconfigured, failing, or bypassed
• Inability to investigate cross-domain transfers after an incident because filtering decisions were not recorded with enough detail

For regulated programs, a control failure often becomes an assessment finding tied to boundary protection, incident response readiness, and audit logging maturity.

A practical 30/60/90-day execution plan

First 30 days (stabilize scope and logging)

• Define your security domains and enumerate approved cross-domain transfer paths.
• Inventory boundary filtering mechanisms and assign owners.
• Set minimum required log fields for filtering actions and results.
• Confirm logs are being produced and can be retrieved for each boundary system.

Next 60 days (operationalize audit and evidence)

• Route boundary filter logs to a centralized logging platform.
• Build standard queries/dashboards per boundary for: blocks, allows, failures, overrides.
• Publish an audit/review SOP and start collecting review records.
• Implement ticketed exception handling for bypasses and quarantine releases.

By 90 days (prove control effectiveness)

• Run test transfers for each boundary and archive the results (test plan + sample logs).
• Tighten change control for filtering rules and link changes to approvals and post-change validation.
• Perform an internal mini-assessment: sample events, trace to review, trace to remediation.
• In Daydream, map AC-4(26) to each boundary owner and schedule recurring evidence uploads (log samples, review sign-offs, exception reports).

Frequently Asked Questions

Do we need to log the full content being filtered to satisfy AC-4(26)?

The requirement is to record filtering actions and results, not to store full payloads. In practice, store identifiers (message IDs, hashes, transaction IDs) plus rule hits and outcomes, and protect sensitive data in logs. ¹

What counts as “different security domains” in a cloud/SaaS-heavy environment?

Treat domains as trust zones you can defend and define: separate tenants, restricted enclaves, production vs non-production, or environments with different data handling constraints. Document the definition and map the cross-domain transfer points to it. ¹

We already have web proxy logs. Does that meet the ac-4(26): audit filtering actions requirement?

Only if the logs show content filtering actions and results (rule triggered, action taken, outcome) for cross-domain transfers. Basic URL access logs usually lack the decisioning detail assessors want. ¹

Who should own this control: security engineering, SOC, or GRC?

Security engineering typically owns filter configuration and logging, while the SOC (or operations) owns ongoing review and escalation. GRC owns the requirement mapping, evidence expectations, and assessment readiness.

How do we prove we “audit” filtering actions without creating a lot of manual work?

Use repeatable SIEM dashboards with a lightweight review checklist and ticketing for findings. Retain the dashboard output (or an exported report) plus reviewer sign-off as your audit evidence.

How should we handle emergency bypasses for business continuity?

Allow bypass only through a documented exception workflow with approvals, time bounds, and post-event review. Your logs should clearly mark the bypass action and outcome so it can be audited later. ¹

Footnotes

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