CA-3(5): Restrictions on External System Connections

CA-3(5): Restrictions on External System Connections requirement means you must explicitly restrict which external systems can connect to your system, under defined conditions, with documented approval and technical enforcement. Operationalize it by maintaining an authoritative inventory of external connections, requiring risk-based authorization before enabling any connection, and continuously monitoring for unauthorized paths. ¹

Key takeaways:

• Keep a governed allowlist of external system connections, not an informal “known integrations” list.
• Require formal authorization and boundary protections before any external connection goes live.
• Retain evidence that restrictions are defined, enforced, and monitored over time. ²

External system connections are where your boundary breaks first: cloud-to-cloud peering, API integrations, B2B file transfers, contractor access paths, and “temporary” tunnels that never get removed. CA-3(5) targets that reality by forcing you to restrict external connections to what you have approved, what you can secure, and what you can monitor. ¹

For a CCO, Compliance Officer, or GRC lead, the fastest path to operational compliance is to turn CA-3(5) into a controlled workflow and a set of technical guardrails: an external connection inventory; a standard authorization packet; a decision point that security and system owners must sign; and continuous detection for anything that bypasses the process. This is not a “write a policy” control. Auditors will look for proof that connections are restricted in practice, especially when business teams can spin up integrations without security involvement.

This page gives you requirement-level implementation guidance you can apply immediately, including the artifacts to retain and the questions assessors ask when they suspect external connections are unmanaged. ²

Regulatory text

Control: “NIST SP 800-53 control CA-3.5.” ¹

Operator interpretation of what the control is asking for: CA-3(5): Restrictions on External System Connections requires you to define restrictions for external system connections and then enforce those restrictions. In practice, that means external connectivity is allowed only when it is authorized, bounded, and monitored under explicit conditions (who, what, where, how, and for how long). ²

Plain-English interpretation

You need a repeatable way to answer: “Which external systems are allowed to connect to us, through what interfaces, with what security conditions, and who approved it?” If you cannot produce a current allowlist and show technical enforcement (not just documentation), you will struggle to defend CA-3(5). ¹

Who it applies to

Entities

• Federal information systems.
• Contractor systems handling federal data (including systems supporting federal programs or contracts). ¹

Operational context (where this control shows up in real life)

• Cloud environments: VPC/VNet peering, private endpoints, cross-account roles, shared services networks.
• Application integrations: inbound/outbound APIs, webhook callbacks, SFTP endpoints, message queues.
• Remote access paths: third-party support access, managed service provider administrative access, jump hosts.
• Data exchange: ETL pipelines to third-party analytics, data clean rooms, file drops to partners.

If a third party system can send traffic into your environment or receive traffic/data from it, treat it as an external system connection in scope for CA-3(5). ²

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

1) Name an owner and define “external connection” for your environment

• Assign a control owner (often Network Security or Security Architecture) and a process owner (often GRC or Change Management).
• Write a one-page definition that includes examples specific to your stack (peering, VPN, API, SFTP, cross-tenant identity trust).
  Output: “External System Connection Standard” (short) that engineering teams can follow.

This prevents the common dodge: “That’s not a connection, it’s just an integration.” ²

2) Build the authoritative inventory (your allowlist baseline)

Create a register of every external system connection, including:

• External system name and owner (third party and internal sponsor)
• Business purpose and data types exchanged
• Connection type (API, VPN, peering, SFTP, IAM trust, etc.)
• Network paths (source/destination, ports, protocols) and DNS endpoints
• Identity/auth method (mTLS, OAuth, keys, IP allowlists, federation)
• Environments in scope (prod, staging, dev)
• Start date, review date, end date (if time-bound)
• Link to approval ticket and risk acceptance (if any)

Tip: If you do not have a clean source of truth, start by pulling from firewall rules, cloud security groups, API gateways, and SSO federation trusts, then reconcile with system owners.

Output: External Connections Register (spreadsheet, CMDB table, or GRC system record).

3) Define restriction rules (your “conditions to connect”)

Document enforceable restrictions, such as:

• Allowed connection types (for example: “no site-to-site VPN to production unless approved by Security Architecture”)
• Minimum security conditions (encryption in transit, strong auth, key rotation ownership, logging)
• Data handling constraints (least data necessary; prohibited datasets)
• Network constraints (allowlisted IP ranges, specific ports, no “any/any” rules)
• Change control requirements (ticket, approver roles, testing evidence)
• Expiration rules for temporary access (time-bound and auto-disabled)

Output: External Connection Control Requirements (a checklist used for approvals).
Goal: Make approvals consistent and auditable. ¹

4) Implement an authorization workflow that blocks “shadow connections”

You need a gating mechanism. Pick the one that matches how your org ships changes:

• Change management gate: No firewall/security group/peering change without an approved change ticket referencing the external connection record.
• Infrastructure-as-code gate: Pull requests require a control checklist and security approval before merging network/IAM trust changes.
• Integration intake gate: New APIs/SFTP endpoints require an intake form, threat review, and explicit authorization.

Minimum approval packet:

• Requester and system owner
• Third party name and contact
• Data classification and purpose
• Proposed architecture diagram (simple is fine)
• Security controls mapping (how restrictions are met)
• Monitoring plan (what logs, who reviews, alerting)

Output: External Connection Authorization (ticket template + required attachments). ²

5) Enforce restrictions technically (not just in paperwork)

Map restriction requirements to concrete controls:

• Firewalls / security groups with explicit allow rules
• API gateway policies, WAF rules, mTLS requirements
• IAM: scoped roles, conditional access, restricted trust relationships
• Network segmentation: isolate partner connections to dedicated subnets
• Egress controls: proxy, DNS controls, deny-by-default outbound where feasible
• Secrets management: managed keys, rotation ownership, no shared credentials

Auditor test: “Show me the rule set, and show me it matches the authorized design.” Be ready to walk from a record in your register to the actual configuration. ¹

6) Monitor continuously and reconcile against the allowlist

Operational tasks you need:

• Detect new external endpoints or trust relationships (cloud configuration monitoring, firewall change alerts).
• Review logs for connection establishment attempts from non-allowlisted sources.
• Reconcile the external connection register against technical configs on a recurring cadence.
• Terminate unused connections (decommission step is part of the control).

Output: Reconciliation report + list of exceptions + remediation tickets.

7) Prove it works: test and sample

Run a lightweight internal assessment:

• Sample a set of external connections.
• For each, verify: authorization exists, restrictions documented, configuration enforces restrictions, logging exists, and review occurred.
• Record gaps and corrective actions.

This is where teams often discover legacy connections with no owner. Make ownership assignment part of closure. ²

Required evidence and artifacts to retain

Keep artifacts in a place auditors can access without heroics:

Core artifacts (must-have)

• External Connections Register (current and version history)
• External Connection Standard / restriction checklist
• Approval records (tickets) for each connection
• Architecture diagrams or connection descriptions tied to each approval
• Configuration evidence (screenshots/exports of firewall rules, security groups, API gateway policies, IAM trust policies)
• Monitoring evidence (logging enabled, alert rules, and a sample of alerts or review notes)
• Reconciliation results and remediation tickets

Nice-to-have (helps in exams)

• Exception/risk acceptance records for nonstandard connections
• Decommission records for removed connections
• Periodic attestation from system owners that the register is accurate

Daydream can reduce friction by mapping CA-3(5) to a control owner, a standard implementation procedure, and a recurring evidence set so you do not rebuild the audit packet every cycle. ¹

Common exam/audit questions and hangups

Assessors tend to focus on these “prove it” points:

1. Show the allowlist. “Where is the authoritative list of external connections, and how do you know it’s complete?”
2. Show enforcement. “Where are restrictions enforced technically, and who can change them?”
3. Show authorization. “Pick a connection and show the approval, the conditions, and the implementation.”
4. Show monitoring. “How do you detect unauthorized connections or drift from approved configs?”
5. Show lifecycle management. “How do connections expire, get reviewed, or get terminated?” ²

Hangup to anticipate: teams present a third-party inventory (procurement list) and call it “external connections.” Auditors will reject that if it does not enumerate actual technical connectivity paths.

Frequent implementation mistakes and how to avoid them

Mistake	Why it fails CA-3(5)	Fix
No single inventory of external connections	You cannot show restrictions apply to all connections	Build the register from configs, then assign owners
Approval exists, but configs don’t match	Paper compliance; technical reality differs	Require config evidence as a closure step
Over-broad network rules (“any/any”, whole internet)	Restrictions are not meaningful	Default to least-privilege ports, sources, destinations
Temporary access that never expires	External paths accumulate silently	Add an expiry field and decommission workflow
No monitoring for drift/new connections	Unauthorized routes persist	Reconcile register to cloud/network configs regularly

Enforcement context and risk implications

No public enforcement cases were provided in the source catalog for this requirement, so you should treat CA-3(5) as an assessment-readiness control rather than a “named case law” control for this page.

Risk-wise, weak restrictions on external system connections increase the chance of:

• Unauthorized access paths into sensitive environments
• Data exfiltration channels that bypass application controls
• Third-party compromise becoming your compromise through trusted connectivity

Operationally, CA-3(5) is also a governance control: it forces the business to identify an internal sponsor and accept accountability for each external connection. ²

Practical 30/60/90-day execution plan

Day 0–30: Establish control and stop new uncontrolled connections

• Assign owners (control owner + process owner).
• Publish the definition and the minimum restriction checklist.
• Implement a “no new external connection without ticket” gate in change management or IaC.
• Start the external connection register with a first-pass discovery from network/cloud configurations.

Day 31–60: Backfill authorizations and align configs to restrictions

• Triage existing connections: high-risk first (production, privileged access, broad network exposure).
• Backfill missing approvals using a standard authorization packet.
• Close the largest technical gaps (overly permissive rules, missing encryption/auth, missing logs).
• Stand up monitoring alerts for new peering/VPN/IAM trust and high-risk firewall changes.

Day 61–90: Prove repeatability and audit readiness

• Run a formal reconciliation of the register versus live configurations.
• Sample connections and create an internal “audit packet” per connection.
• Add lifecycle reviews and decommission steps to your operational runbooks.
• In Daydream, map CA-3(5) to your owner, procedure, and recurring evidence so the next cycle is maintenance, not reinvention. ¹

Frequently Asked Questions

Does CA-3(5) apply to SaaS-to-SaaS integrations (like a CRM syncing to a marketing platform)?

Yes if there is a system-to-system connection that transfers data or establishes trust you rely on. Track the integration endpoints, auth method, and approved data scope in the external connections register. ²

What counts as “restricted” in practice?

“Restricted” means you have explicit conditions (allowed endpoints, protocols, auth, data scope) and the environment enforces them through configuration. A policy without technical controls usually will not satisfy an assessor. ¹

We have hundreds of outbound internet connections. Do we need to register all of them?

Focus the register on meaningful external system connections: integrations, trusted links, and defined data exchanges. For general outbound browsing/egress, document your egress control approach and monitoring, then treat named third-party endpoints as managed connections where feasible. ²

How do we handle a third party that demands broad network access for support?

Treat it as an exception path: document why broad access is required, narrow it as much as possible, add compensating controls (segmentation, MFA, jump hosts, session logging), and retain explicit risk acceptance tied to an approver. ²

What evidence is strongest for auditors?

A complete chain: register entry → approval ticket → diagram/requirements checklist → live configuration evidence → logging/monitoring proof. Auditors want traceability from authorization to enforcement. ¹

Who should approve external system connections?

Require approval from the internal system owner and a security authority who can enforce restrictions (security architecture/network security). Add privacy/legal approval when personal or regulated data is exchanged. ²

Footnotes

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

2. NIST SP 800-53 Rev. 5