Implementing RA.L2-3.11.2 (periodic vulnerability scanning) for NIST SP 800-171 Rev.2 / CMMC 2.0 Level 2 means establishing a repeatable, auditable vulnerability management process — and Nessus is a practical, widely-used tool to do that across servers, desktops, laptops, VMs, containers, firewalls, switches, and printers. This post gives a step-by-step implementation plan, technical configuration examples, small-business scenarios, and compliance tips to produce evidence for assessors and to reduce real risk.
Why RA.L2-3.11.2 matters and the risk of not implementing it
RA.L2-3.11.2 requires organizations to regularly scan for vulnerabilities and take corrective actions; without it, CUI-bearing environments remain exposed to known exploits (e.g., unpatched RDP, outdated firmware on firewalls/switches, vulnerable web servers or outdated printer firmware). For a small business this can mean a ransomware infection, data exfiltration, loss of contracts, and failing a CMMC assessment. Practical evidence (scan reports, remediation tickets, retest artifacts) is commonly requested during audits — so scanning + measurable remediation is essential.
High-level implementation approach
Use this workflow: 1) Asset discovery and classification, 2) Choose scan modality (credentialed network scan, agent-based, image/registry scanning), 3) Build Nessus policies per asset class, 4) Test in a non-production window, 5) Schedule production scans, 6) Triage and remediate, 7) Retest and produce evidence. For small organizations (20–200 endpoints) a mix of Nessus Professional + Nessus Agents (or Tenable.io) gives the best balance: agent coverage for laptops/offline systems and credentialed scans for servers and network gear.
Step 1 — Asset inventory and scoping (practical)
Start with an authoritative asset list that includes OS, role (web server, domain controller), IP ranges, VLANs, virtualization hosts, container orchestration (e.g., Kubernetes), and manufacturer/model/firmware level for printers, switches, and firewalls. Example: a 50-employee shop has 10 servers (VMs on ESXi), 60 laptops/desktops (mix of Windows 10/11 and macOS), a small Cisco ASA/Firewall, 4 managed switches, and 10 network printers. Map these to CUI-scope systems so scans focus on in-scope assets first.
Step 2 — Choose scanning methods per asset type
Define scanning modes using Nessus features: - Servers/VMs: Credentialed scans (SSH for Linux with key or password, WinRM/WMI or SMB for Windows) for full patch audit and configuration checks. - Desktops/Laptops: Nessus Agents for mobile/offline devices or scheduled credentialed scans when online; use agent policies to run during off-hours. - Containers: For container images, use image scanning (Tenable.io Container Security) or scan the registry/CI pipeline; for running containers, scan the host with credentialed checks and use container-aware plugins (check for kernel exploits, runtime CVEs). - Firewalls/Switches: Use SSH credentials (CLI) or SNMPv3 read-only with correct community strings; avoid intrusive checks during business hours. - Printers: Use SNMP (v2c or v3) and web/HTTP checks; disable aggressive checks that may disrupt print jobs. - Network devices with limited CLI: run read-only configuration checks and firmware version tests rather than deep intrusive scans. Make a table (internal) mapping asset class → Nessus policy type → credentials required → safe-check settings.
Step 3 — Configure Nessus scan policies (technical)
Create separate Nessus policies to minimize false positives and avoid service disruption: - "Servers — Credentialed Audit": Enable safe checks OFF for non-critical servers only after testing; enable credentialed patch audit plugins, SMB/WinRM creds, SSH keys for Linux, and set "Max. simultaneous checks" to 50 for on-premise networks. - "Endpoints — Agent Policy": Configure agent scheduling (e.g., daily at 02:00), enable local remediation reporting, and apply lightweight checks for notebooks. - "Network Devices — SNMP/SSH": Use SNMPv3 where possible, set timeouts higher (10s), limit concurrent ports scanned, and disable banner grabbing if devices are brittle. - "Printers — Light Discovery": Limit plugin families to "Printer" and "Web Servers", run safely. Example credential settings: Windows domain account with local admin for WinRM + SMB, Linux account with sudo (no password required for Nessus via SSH key or enabling "sudo -n"). Document credential vaulting and never embed credentials in exported reports.
Step 4 — Test scans and safe scheduling
Test each policy in a staging VLAN first. For small businesses, schedule full credentialed server scans weekly and endpoint/agent scans daily or nightly. Network device scans can be weekly or monthly depending on change frequency. For sensitive devices (medical devices, VOIP, SCADA) perform passive or agent-based checks only and gain owner approval. Keep a "blast radius" test plan: scan small groups first, monitor for dropped services, and gradually expand. Use Nessus "safe checks" and throttle settings to reduce risk (increase "Max checks per host" conservatively).
Step 5 — Triage, remediation workflow, and evidence for compliance
Scan results must feed a tracked remediation workflow. Integrate Nessus with ticketing systems (Jira, ServiceNow, GitHub Issues) or export CSV/HTML/PDF for evidence. Define SLA tiers: Critical (CVSS ≥ 9) remediate within 3 business days, High (7–8.9) within 14 days, Medium (4–6.9) within 30 days — align timelines to organizational risk tolerance and contractual requirements. Document exception/risk acceptance approvals for items that cannot be remediated (e.g., legacy printers needing replacement). Capture screenshots of scan configuration, scan UUIDs, remediation tickets, and retest reports as audit artifacts.
Step 6 — Container and VM specifics
For VMs, treat each guest as a host: install agents where possible or schedule credentialed scans. For containers, implement image scanning in CI/CD so vulnerabilities are caught before deployment; if using hosted container registries, scan images at rest (Tenable.io or integrated SCA tools). Also scan Kubernetes nodes with host checks and use kubeconfig read-only credentials to enumerate pods/namespaces. Example: build a workflow that fails a pipeline if base image CVEs exceed a threshold, and schedule nightly host scans for node-level CVEs.
Compliance tips and best practices
Maintain an evidence binder (digital): initial asset inventory, Nessus policy templates, credential handling policy, scan schedules, raw scan output, remediation tickets, and retest scans. Use versioned scan policies and keep change logs. Validate that scans cover CUI storages and communication paths. Use CVSS+context (exploit maturity, presence of mitigations) during prioritization and document the rationales. Regularly review and tune plugins to reduce noise and false positives. For external-facing systems, complement Nessus with authenticated web app scanning or external penetration testing annually.
Failing to implement this control leaves known vulnerabilities unaddressed and increases the chance of compromise, incident response costs, and lost government or defense contracts. Properly implemented scanning + remediation reduces attack surface, speeds detection of new exposures, and provides demonstrable artifacts for CMMC assessors.
In summary, implement RA.L2-3.11.2 with an asset-driven approach: inventory assets, select appropriate Nessus scanning methods (credentialed, agent-based, image scanning), build tuned policies per asset class, test safely, schedule regular scans, integrate results into a tracked remediation workflow, and retain evidence for audits. For a small business, this combination is practical, cost-effective, and will materially reduce risk while creating the documentation needed to satisfy NIST SP 800-171 Rev.2 / CMMC 2.0 Level 2 assessors.
