This post explains how to implement AC.L1-B.1.III — the FAR 52.204-21 / CMMC 2.0 Level 1 requirement to verify and limit external information system connections — with a compact, actionable plan for small businesses that need to protect controlled information while keeping operations practical and auditable.
What AC.L1-B.1.III Requires (plain language)
At a practical level this control requires you to know what external systems your environment connects to, to authorize those connections based on business need, and to apply simple technical limits so only approved external systems can be used. For small organizations that typically means inventorying SaaS apps, vendor remote-access endpoints, and employee-use services (cloud storage, collaboration tools), specifying why each connection is needed, and enforcing allowed connections with network and endpoint controls plus logging.
Step-by-step implementation (Compliance Framework specific)
1) Discover and document external connections, then authorize
Begin with a discovery sweep: collect data from firewall NAT/UTM logs, DNS query logs, proxy logs, and endpoint process lists. Commands and tools that help: on Linux use sudo ss -tunap or sudo lsof -i -n -P; on Windows use Get-NetTCPConnection or netstat -ano; scan your egress flows with NetFlow/sFlow or a packet capture. Produce an "External Connection Inventory" spreadsheet that records (a) destination FQDN/IP, (b) purpose, (c) owner/requester, (d) business justification, (e) authorization date, and (f) renewal date. Create a simple connection authorization form (digital ticket) and require an approver (IT/security lead) before marking a connection as "approved" in the inventory.
2) Apply technical limits to only allow approved external systems
Use egress filtering, proxy whitelisting, and endpoint controls to limit reachability. Examples: on a Linux perimeter host use iptables/ nftables rules that allow outbound TLS only to known IPs—e.g., iptables -A OUTPUT -p tcp -d 198.51.100.10 --dport 443 -j ACCEPT; iptables -A OUTPUT -p tcp --dport 443 -j DROP. On Windows, use PowerShell: New-NetFirewallRule -DisplayName "AllowVendorVPN" -Direction Outbound -RemoteAddress 198.51.100.10 -Protocol TCP -RemotePort 443 -Action Allow. If you run a web proxy (Squid), add ACLs like acl allowed_sites dstdomain .trustedvendor.com http_access allow allowed_sites and deny all other outbound web traffic. For cloud-hosted environments, configure security groups or firewall rules to permit outbound to specific CIDR blocks and known FQDNs. Disable split-tunnel VPNs unless you can enforce endpoint security because split-tunnel permits uncontrolled external connections. Where possible, use DNS allowlists (Pi-hole or managed DNS) to block known risky domains and enforce DNS over TLS to prevent evasion.
3) Verify, monitor and maintain the control
Verification = continuous and periodic checks. Send firewall, proxy and VPN logs to a log collector (syslog/SIEM) and configure alerts for unapproved egress destinations. Example alert: any outbound SSL/TLS connection to an IP/FQDN not present in the approved inventory triggers an email/Slack incident. Run weekly automated checks: a script that compares current outbound destinations (via netstat/ss or firewall connection table) against the approved list and generates a report. Keep configuration snapshots (firewall rules, proxy ACLs) and save screenshots / export of authorizations as audit evidence. Reauthorize connection entries at a scheduled cadence — quarterly for higher-risk external access, annually for low-risk SaaS — and remove stale entries immediately.
Small-business scenarios and real-world examples
Example 1: A 20-person engineering contractor uses a vendor for remote server maintenance. Implement a vendor access workflow: vendor requests access via a ticket, you add vendor IP(s) to a temporary firewall rule with start/end times, require the vendor to connect only via the company-managed VPN with MFA, enable monitoring on the target server (Windows Event Logs / syslog), and remove the firewall rule automatically after the approved window. Example 2: Employees want to use Dropbox/Google Drive. Add these SaaS apps to your inventory, enforce company-approved cloud storage via DLP/managed clients and proxy whitelisting, and block consumer cloud storage sites at the perimeter for endpoints that aren't registered. For a tiny company without a SIEM, you can forward firewall logs to a low-cost cloud log service or even a dedicated syslog VM and set up basic daily grep-based alerts to highlight new external destinations.
Compliance tips and best practices
Document every exception and tie it to business justification; enforce least privilege for network and account access; automate enforcement where feasible so approvals create and revoke firewall rules automatically (via API integration with your firewall or cloud provider). Maintain evidence: the authorization ticket, the firewall configuration snapshot, and logs showing the approved connection was used as authorized. Use simple automation (scripts, infrastructure-as-code) to keep firewall/proxy configs reproducible and auditable. Train staff on acceptable external services and require IT approval for new tools; even a short internal policy and a one-page connection request form increase compliance and reduce shadow IT.
Risks of not implementing this control
Failing to verify and limit external connections leaves your environment open to data exfiltration, accidental CUI (controlled unclassified information) disclosure via unmanaged cloud services, and vendor-mediated compromise. Attackers frequently use approved external services as conduits for command-and-control or stealth data transfers; unmanaged connections also increase the chance of supply chain breaches and can lead to contract loss, penalties, or failed audits under FAR / CMMC. For small businesses, a single compromised remote access or unmonitored SaaS connection can cause outsized damage to reputation and future government contracting opportunities.
Summary: Make discovery, authorization, technical enforcement, and monitoring the cornerstones of your implementation: maintain a simple inventory and approval workflow, enforce allowed external connections with egress filtering and proxy rules (with concrete firewall/host commands), log and alert on deviations, and schedule regular reauthorization and evidence collection. These steps satisfy AC.L1-B.1.III while remaining practical for small organizations operating under the Compliance Framework.
