This quick compliance checklist shows how to verify and control external connections to meet FAR 52.204-21 and CMMC 2.0 Level 1 control AC.L1-B.1.III — focusing on practical steps a small business can implement today to reduce risk, produce assessor evidence, and keep contractors' covered information secure.
Understanding the requirement (Compliance Framework / AC.L1-B.1.III)
AC.L1-B.1.III requires organizations to verify and control external connections so that systems interacting with controlled unclassified information (CUI) or contractor information are known, authorized, and constrained to approved services and endpoints. For compliance frameworks this means: maintain an inventory of external connections, document authorization for each connection, and implement technical controls to limit and log connections. For FAR 52.204-21 this aligns with the obligation to safeguard contractor information systems and, where applicable, restrict unnecessary external access.
Practical implementation steps (high level)
1) Inventory and data-flow mapping
Start by discovering all external connections and the data they carry. Produce a simple spreadsheet or CMDB entries listing: connection owner, source system, destination (IP / DNS / cloud service), ports/protocols, data classification (e.g., CUI/no-CUI), purpose, and authorization date. Use passive discovery tools (DHCP logs, router flow data, and proxy logs) and active scans (nmap -sT -p 1-65535
2) Implement network and egress controls
Control how systems reach the Internet and other external networks: deploy egress filtering on the firewall or edge router, deny-by-default and allow-only-for-approved-destinations. Example firewall ACL snippet (conceptual): deny ip any any log; permit tcp any host 198.51.100.25 eq 443; permit udp any host 203.0.113.10 eq 53; — where the listed hosts are documented in your inventory. Disable split-tunnel VPNs for machines that handle CUI or route all traffic through a corporate gateway where you can inspect TLS and DNS. Use TLS inspection selectively if policy and legality allow; at minimum enforce TLS 1.2+ and certificate validation (openssl s_client -connect example.com:443 -servername example.com to check cert chains).
3) Access controls, authentication and least privilege
Restrict who can create or approve external connections. Require change tickets and manager approval for firewall rule changes, and use MFA for any remote or vendor connections (e.g., VPN with certificate plus MFA). For administrative access use a bastion host or jump server with session logging; consider time-limited credentials or just-in-time (JIT) access. For IoT or OT devices, use network segmentation (VLANs, ACLs) and 802.1X/NAC to prevent lateral movement from an externally-exposed device.
Monitoring, logging, and evidence for assessors
Log all external connection events and retain enough history to demonstrate continuous control — common practice is 90 days minimum, longer if contract requires. Centralize logs (Syslog, Splunk, ELK) and collect: firewall allow/deny logs, VPN connection logs, proxy/Web Gateway access, DNS logs (to detect exfiltration via DNS), and authentication logs. Produce artifacts for assessors: current inventory, approved firewall rule exports (e.g., show run | include access-list), change tickets approving rules, VPN connection lists, and sample logs showing allowed/blocked connections.
Real-world examples and small-business scenarios
Example 1: A small engineering subcontractor uses Office 365 and a third-party simulation SaaS. Action: document connections to office365.com and vendor IP ranges, restrict outbound only to those FQDNs via DNS allow-list and firewall rules, require SSO with MFA. Evidence: Excel inventory, firewall rule export, screenshots of Azure AD Conditional Access policy.
Example 2: A two-person IT shop uses a managed service provider (MSP) for patching. Action: require MSP access via a dedicated VPN account bound to a jump host, enforce MFA, log sessions, and create a 30-minute access ticket for maintenance windows. Evidence: change ticket, VPN session log, screenshot of jump host session recording.
Risk of non-compliance
Not verifying and controlling external connections increases the risk of unauthorized data access, exfiltration, and supply-chain compromise — all of which can lead to loss of CUI, contract penalties, delisting from government contracts, and reputational damage. Technical risks include exposed management ports, unnoticed API tokens in the wild, and lateral movement from an inward-facing compromise to sensitive systems.
Practical tips and best practices
Keep the checklist lean and repeatable: (1) quarterly review of external-connection inventory, (2) firewall rule audits after every change, (3) monthly logs review for anomalous outbound flows, (4) enforce least-privilege and MFA for all external access, and (5) maintain an evidence folder with configuration exports, signed approvals, and screenshots. Automate where possible: schedule config backups, use scripts to export ACL lists (e.g., show access-lists on Cisco and save), and use cloud provider access logs (CloudTrail, Azure AD sign-ins) to detect suspicious remote access.
Summary: Implement a simple, documented process to discover, authorize, limit, and monitor external connections — keep an inventory, enforce deny-by-default egress controls, require documented approvals and MFA for remote access, and retain logs and change records as evidence. Following this checklist will help a small business meet FAR 52.204-21 and CMMC 2.0 Level 1 (AC.L1-B.1.III) expectations while reducing real operational risk.
