Limiting external connections is a core requirement of FAR 52.204-21 and CMMC 2.0 Level 1 (AC.L1-B.1.III): it reduces attack surface and helps protect Federal Contract Information (FCI) by ensuring systems only communicate with approved external entities and services.
What the requirement means in practice
At its core, AC.L1-B.1.III requires organizations to ensure that system connections to external networks or external devices are limited to only those necessary for business operations. For a small business, this translates to: document allowed external connections, enforce those rules with technical controls, and maintain evidence that controls are in place and reviewed periodically.
Practical policy elements to create
Start with a short, actionable policy document that covers scope, definitions, approval workflow, and enforcement. Example policy elements: a) "Approved External Connections" — an inventory defining allowed cloud services, IP ranges, and remote support vendors; b) "Connection Approval Process" — a standard request and risk assessment template requiring manager and ISSO approval; c) "Temporary Exceptions" — time-bound exceptions with automated expiration and logging; d) "Enforcement and Monitoring" — where technical controls and log collection requirements are specified.
Sample policy language (brief)
"All outbound connections from CUI and FCI systems are denied by default. Exceptions must be requested using the External Connection Request form, include a business justification, and be approved by the Information Security Officer. Approved connections will be limited by egress filters and logged for 90 days."
Technical controls you can implement now
Implement a layered set of technical controls: perimeter egress filtering, host-based outbound controls, DNS filtering, network segmentation, VPN/Zero Trust, and logging. For example, configure your perimeter firewall or cloud security group to deny all outbound traffic by default and then explicitly allow only required destinations and ports. In AWS, an egress security group could be restricted to TCP/443 to known service IP ranges; in a physical firewall, create egress rules that allow only ports 80/443 and necessary management ports to specific IPs.
Concrete examples and commands
Small business examples: 1) Home-office employee accessing cloud-based CRM — allow outbound TCP/443 to crm.example.com and block all SMB (TCP/445) egress; 2) Remote support using TeamViewer — create a time-bound firewall rule permitting TeamViewer IP ranges for the duration of the support window; 3) IoT printer — move to a VLAN with restricted outbound DNS and HTTP only. Sample iptables egress block (test before production):
iptables -P OUTPUT DROP
iptables -A OUTPUT -p tcp --dport 443 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p udp --dport 53 -j ACCEPT
iptables -A OUTPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
For Windows endpoints managed by Group Policy, create outbound firewall rules restricting applications to communicate only with specific FQDNs or IPs; for cloud identities use Conditional Access (e.g., Azure AD) to block logins from unmanaged or risky endpoints.
Monitoring, evidence, and audit readiness
Logging is essential for compliance evidence. Enable firewall, proxy, DNS, and cloud flow logs (e.g., AWS VPC Flow Logs, Azure NSG flow logs) and forward to a central log repository or lightweight SIEM. Maintain a change log of approved external connections and screenshots or exports of firewall rules. Schedule quarterly reviews of the approved list and automated scans to detect unauthorized outbound connections (use tools like Zeek, Suricata, or cloud-native services such as GuardDuty).
Risks of not implementing these controls
Failing to limit external connections increases the risk of credential theft, data exfiltration, malware callbacks, and lateral movement. A single unmanaged external connection (e.g., an employee syncing FCI to a personal cloud storage account) can lead to contract non-compliance, mandatory breach reporting, loss of contracts, and regulatory fines. From a practical perspective, it also makes incident response slower because unknown external hosts complicate containment.
Compliance tips and best practices
Practical tips: 1) Inventory first — map what systems and services need external connectivity; 2) Least privilege — only allow the minimum protocols, ports, and destinations; 3) Automate expiry — make exceptions expire automatically; 4) Use allowlists rather than blacklists; 5) Segment guest and IoT networks; 6) Apply MFA on all external access and use VPNs or ZTNA for admin connections; 7) Keep evidence — exports of firewall rules, tickets for approvals, and log retention policies aligned with audit timelines (90–365 days as required).
Implementing these policies and technical controls does not require a large security team: small businesses can use managed firewall services, cloud-native security controls, MDM (Intune, JAMF) for device posture, and simple ticket-based approvals to meet FAR 52.204-21 and CMMC 2.0 Level 1 requirements while reducing operational risk.
Summary: create a concise policy that defines allowed external connections and an approval workflow, enforce it with egress filtering, segmentation, host controls, DNS filtering, and logging, and maintain evidence through regular reviews and automated controls — these steps deliver practical, auditable protection for FCI and help satisfy AC.L1-B.1.III.
