SI-4(4): Inbound and Outbound Communications Traffic

SI-4(4) requires you to define clear, testable criteria for what counts as unusual or unauthorized inbound and outbound communications traffic, then operationalize those criteria in monitoring, alerting, and response workflows. The fastest path is to document traffic “normal,” define detection thresholds and allow/deny rules, route alerts to an owner, and retain evidence that the criteria are reviewed and enforced. ¹

Key takeaways:

• You must determine criteria (not just “monitor traffic”) for inbound and outbound communications anomalies. ¹
• Criteria must be operational: measurable signals, thresholds, owners, and actions tied to your tooling and network boundaries.
• Audit readiness depends on repeatable evidence: criteria, baselines, tuned detections, alert triage, and periodic review records.

Compliance teams often “have monitoring” but cannot answer a basic assessor question: “What, specifically, is unusual or unauthorized inbound and outbound traffic in your environment, and how do you know?” SI-4(4) closes that gap by forcing you to define the criteria for suspicious traffic conditions so monitoring is purposeful, measurable, and defensible. ¹

For a CCO, GRC lead, or security compliance owner, the operational challenge is coordination: networking defines the boundaries, security operations runs detection, application teams own expected flows, and third parties introduce new egress destinations and inbound integrations. Without written criteria and change control, detections either alert on everything (alert fatigue) or miss obvious exfiltration patterns (blind spots). SI-4(4) is “medium” severity for a reason: it is frequently treated as implied, yet it’s a common failure point in assessments because teams cannot show the criteria were deliberately set, approved, and maintained. ¹

This page gives requirement-level implementation guidance you can execute quickly: how to define criteria, where to instrument, what evidence to retain, and how to survive exams without hand-waving.

Regulatory text

Requirement (excerpt): “Determine criteria for unusual or unauthorized activities or conditions for inbound and outbound communications traffic;” ¹

Operator meaning: You must decide, document, and maintain explicit rules for what “unusual” or “unauthorized” looks like for traffic entering and leaving your environment. That determination must be concrete enough that your monitoring stack (firewalls, proxies, IDS/IPS, EDR/NDR, SIEM, cloud logs) can detect it and your responders can act on it. ¹

---

Plain-English interpretation (what SI-4(4) is really asking)

You are expected to answer four questions, in writing, with evidence:

1. What are your monitored network boundaries and choke points? (Ingress/egress, cloud VPC/VNet edges, SaaS egress, remote access paths.)
2. What is “authorized” traffic? (Approved destinations, approved protocols/ports, approved authentication methods, approved third-party connections.)
3. What is “unusual” traffic? (Behavior that deviates from a baseline: new destinations, abnormal volumes, rare protocols, atypical geographies, unexpected data transfer patterns.)
4. What happens when criteria are met? (Alert routing, triage steps, escalation, containment, ticketing, and closure documentation.)

If you cannot show a controlled list of criteria plus evidence they are enforced in tooling and reviewed over time, you have not met SI-4(4). ¹

---

Who it applies to (entity and operational context)

Typical in-scope entities

• Federal information systems implementing NIST SP 800-53 controls. ²
• Contractor systems handling federal data (including many regulated service providers supporting federal customers). ²

Operational contexts where SI-4(4) becomes “make or break”

• Environments with significant outbound connectivity (SaaS usage, CI/CD downloads, container pulls, third-party APIs).
• Hybrid and cloud networks where “perimeter” is distributed across cloud gateways, endpoints, and identity.
• Organizations with many third-party integrations (SFTP, VPN, private links, webhook endpoints, API gateways).
• Teams relying on “default alerts” from tools without tailoring them to approved traffic patterns.

---

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

Use this sequence to get to an assessable implementation.

Step 1: Establish scope and traffic control points

1. Document network boundaries: internet egress points, inbound entry points, interconnects, remote access, and cloud edge services.
2. Identify where logs are generated for traffic: firewall, WAF, proxy, VPN, DNS, load balancers, cloud flow logs, endpoint telemetry.
3. Confirm log aggregation into a central platform (SIEM/log lake) with timestamps and consistent identifiers (source, destination, port, protocol, account/host).

Deliverable: “Inbound/Outbound Traffic Monitoring Coverage Map” (diagram + log sources list).

Step 2: Define “authorized” communications (allow criteria)

Create an “Authorized Communications Register” that answers:

• Approved inbound services (e.g., HTTPS to WAF, VPN endpoints) and the business owner.
• Approved outbound destinations categories (e.g., approved SaaS domains, update repositories, third-party APIs) and the approving authority.
• Approved protocols/ports and exceptions (with expiration date or review trigger).
• Approved third-party connections (VPN peers, IP allowlists, private links) tied to a third-party risk record.

Practical tip: Start with “default deny + explicit allow” where feasible for inbound. For outbound, many orgs start with “monitor + progressively restrict” because egress is messy. SI-4(4) does not force a specific architecture, but it does require criteria that distinguish authorized from unauthorized. ¹

Step 3: Define “unusual” and “unauthorized” detection criteria (your SI-4(4) criteria set)

Write criteria as IF/THEN statements your tools can implement. Examples you can tailor:

Inbound criteria (examples)

• IF inbound traffic targets a non-approved listening service/port, THEN alert and block (or quarantine) per policy.
• IF inbound requests show scanning patterns (high connection attempts across ports or hosts), THEN generate an incident ticket.
• IF inbound traffic originates from unexpected geographies for an internet-facing admin service, THEN require step-up auth and alert.

Outbound criteria (examples)

• IF a workload initiates outbound connections to a new domain category not in the Authorized Communications Register, THEN alert and route to app owner for validation.
• IF outbound traffic volume from a host deviates materially from its baseline, THEN investigate for exfiltration or malware beaconing.
• IF DNS queries indicate algorithmically generated domains or unusual NXDOMAIN patterns, THEN triage as potential command-and-control.

Make each criterion include:

• Data source(s) (proxy logs, DNS logs, VPC flow logs, firewall events).
• Detection logic (query, rule name, threshold approach).
• Owner for tuning and response (SecOps, NetOps, CloudSec).
• Expected response action (block, contain, investigate, document as false positive with rationale).

Deliverable: “SI-4(4) Traffic Criteria Standard” (controlled document).

Step 4: Implement criteria in tooling and workflows

1. Convert the written criteria into detections (SIEM correlation rules, NDR detections, firewall alerts, CASB alerts).
2. Create alert routing: severity mapping, on-call group, ticket templates, escalation.
3. Add tuning rules: documented suppression with approvals, expiration, and periodic review.
4. Validate with tabletop tests or controlled simulations (e.g., test host connecting to blocked destination, inbound port scan against a canary host).

Deliverables: rule inventory export, screenshots/config snapshots, ticket workflow configuration.

Step 5: Operationalize ongoing review and change control

You need a cadence that proves criteria stay accurate as the business changes:

• Trigger reviews when a new third party integration goes live, new SaaS is approved, or network topology changes.
• Record updates to the Authorized Communications Register and detection rules.
• Track metrics qualitatively in governance: top alert types, tuning actions, and recurring exceptions.

Deliverable: meeting notes or change tickets showing criteria updates and approvals.

---

Required evidence and artifacts to retain

Assessors typically want proof of determination and proof of operation. Keep:

Design evidence

• SI-4(4) Traffic Criteria Standard (version controlled). ¹
• Authorized Communications Register (including third-party connections and owners).
• Network boundary diagram and logging coverage map.

Operational evidence

• SIEM/NDR rule inventory showing inbound/outbound criteria implemented (rule names, last modified date, enabled status).
• Sample alerts and incident tickets showing triage steps and closure notes (include at least one inbound and one outbound example).
• Change records for additions/removals of authorized endpoints or ports (especially third-party related).
• Periodic review record (agenda, attendees, decisions, follow-up actions).

Daydream fit (practical): many teams fail SI-4(4) on evidence hygiene, not tooling. Daydream helps you map SI-4(4) to a named owner, a written procedure, and a recurring evidence pack so reviews produce the artifacts assessors ask for. ¹

---

Common exam/audit questions and hangups

Use these as a readiness checklist:

1. “Show me your criteria.” Not “we monitor traffic,” but the actual documented criteria for unusual/unauthorized inbound and outbound traffic. ¹
2. “Where is this implemented?” Which tools enforce/detect it, and how do you know logs are complete for all egress/ingress paths.
3. “How do you handle exceptions?” Third-party integrations, break-glass rules, temporary allowlists, developer testing.
4. “How do you review criteria?” Evidence of updates tied to changes in architecture and third-party connectivity.
5. “Prove it works.” A recent alert with investigation notes and disposition (true positive/false positive) and tuning rationale.

Common hangup: teams show firewall rules but cannot show “unusual” criteria (behavioral or baseline-driven). SI-4(4) explicitly expects criteria for unusual conditions, not only static allow/deny lists. ¹

---

Frequent implementation mistakes and how to avoid them

Mistake	Why it fails	How to fix it
“We have a SIEM” as the control response	Tool presence does not equal defined criteria	Publish the criteria standard, then map each criterion to a detection rule ID and log source
Criteria exist only in analysts’ heads	Not assessable, not repeatable	Put criteria into a controlled document and link it to tickets/rule changes
Only inbound is covered	Outbound is where exfiltration and C2 often show	Require outbound criteria: new destinations, anomalous volume, rare protocols, DNS anomalies
No linkage to third-party approvals	New third-party connections create “unauthorized” ambiguity	Tie third-party connectivity to the Authorized Communications Register and TPDD workflow
Alerts fire but nobody owns them	Untriaged alerts are operationally equivalent to no control	Assign owner, on-call, and response SLAs in your procedure

---

Enforcement context and risk implications

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

Operationally, weak SI-4(4) implementation increases the chance you miss:

• Malware beaconing and command-and-control over outbound channels.
• Data exfiltration via approved ports to unapproved destinations.
• Shadow integrations with third parties that bypass security review.
• Misconfigurations exposing non-approved inbound services.

The risk is compounded in contractor environments handling federal data, where assessors expect clear, documented criteria aligned to your environment and boundary definitions. ²

---

A practical 30/60/90-day execution plan

Avoid calendar promises you cannot keep; use phased deliverables you can complete and evidence.

First 30 days (stabilize and document)

• Name a control owner (Security/GRC) and two operators: NetOps and SecOps.
• Produce the boundary/logging coverage map and identify any unmonitored egress paths.
• Draft the SI-4(4) Traffic Criteria Standard with an initial set of inbound and outbound criteria.
• Create the Authorized Communications Register template and populate high-risk entries (remote admin paths, third-party tunnels, critical SaaS).

Days 31–60 (implement and prove operation)

• Implement the top criteria as detections in SIEM/NDR/firewalls; document rule IDs and log sources.
• Stand up alert routing and ticket templates with required fields (asset, dest, justification, disposition).
• Run at least one controlled validation per direction (inbound test, outbound test) and retain results.

Days 61–90 (tune, govern, and scale)

• Add exception workflow for temporary allowlists with expiry and approval.
• Establish a recurring criteria review meeting tied to change management and third-party onboarding.
• Build an “evidence pack” folder structure that auto-collects: rule exports, sample tickets, review notes, register changes.

Daydream can serve as the system of record for the control narrative, owner assignment, and evidence collection schedule so your SI-4(4) artifacts stay current between assessments. ¹

Frequently Asked Questions

What counts as “criteria” under si-4(4): inbound and outbound communications traffic requirement?

Criteria are explicit conditions that distinguish normal/authorized traffic from unusual/unauthorized traffic, written so they can be implemented as rules or detections. If you cannot map a criterion to a log source and an alert or enforcement action, it is not operational. ¹

Do I need to block traffic to meet SI-4(4)?

The requirement is to determine criteria; it does not prescribe block vs alert. In practice, you should block clearly unauthorized inbound traffic where feasible and at least detect and triage suspicious outbound traffic. ¹

How do we handle approved third-party integrations that look “unusual”?

Put them in the Authorized Communications Register with an owner and approval record, then tune detections with documented suppressions or allowlists tied to that approval. Treat temporary exceptions as time-bound changes with review triggers.

What evidence is most persuasive to an assessor?

A version-controlled criteria document, proof the criteria are implemented (rule inventory/config snapshots), and real alert tickets showing triage and closure. Periodic review records that show criteria changes over time reduce “paper control” findings. ¹

We’re cloud-heavy. Where should we monitor inbound/outbound traffic?

Start at cloud edge and egress points: load balancers/WAF for inbound, NAT gateways/proxies/DNS resolvers for outbound, plus cloud flow logs for coverage validation. Your criteria should name the specific cloud log sources used for each detection.

How granular should the criteria be?

Granular enough to be testable and assignable to an owner. Start with a smaller set of high-signal criteria (non-approved inbound services, new outbound destinations, abnormal outbound volume, suspicious DNS) and expand as you tune.

Footnotes

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

2. NIST SP 800-53 Rev. 5