Top NIST 800-171 Compliance Mistakes Contractors Make—and How to Avoid Them

Many defense contractors assume they’re compliant with NIST 800-171—until an assessment proves otherwise. In reality, even the most well-intentioned teams overlook critical controls that can jeopardize their SPRS score, delay their CMMC certification, or put government contracts at risk.

In this article, we break down the top compliance mistakes contractors make, based on real-world experience and current assessment trends. Whether you're prepping for a CMMC Level 2 audit or just starting your compliance journey, use this guide to check your blind spots and strengthen your cybersecurity posture.

1. Misunderstanding the Scope of Compliance

Why Scoping Errors Are So Common

Scoping is the foundation of NIST 800-171 compliance—and it’s often misunderstood. Many contractors either over-scope and include systems unnecessarily, or under-scope and miss areas where CUI (Controlled Unclassified Information) flows. Either mistake can result in data leakage or audit failure.

How to Scope Correctly

Start with one question: What kind of CUI are we protecting? Then create a data flow diagram that traces the CUI from its source, through your systems, and out to any subcontractors or cloud services. Include all the following:

• Customer portals (e.g., Lockheed, Boeing)

• Email systems

• On-prem file shares and servers

• Cloud platforms (e.g., M365, SharePoint)

• ERP/MRP systems

• Subcontractor access or handoff points

Brooke recommends drawing this by hand first, then digitizing in tools like Visio or PowerPoint for clarity.

2. Missing Key Technical Controls

Multi-Factor Authentication (MFA)

Contractors often apply MFA only to remote access (VPNs) and admin logins—but forget about internal network access to CUI. If CUI is accessed across your local network, MFA must be enforced there as well.

Risk Assessments

Risk assessments are not optional or one-time tasks. NIST 800-171 requires:

• A formal risk management framework

• Annual updates

• Full documentation of methods and results

Failure to follow through consistently is a red flag for assessors.

Logging and SIEM Misunderstandings

Many assume that simply having logs is enough. But you must:

• Define what’s being logged

• Protect the logs

• Review them regularly

• Use a SIEM (Security Information and Event Management) tool if possible (though not required)

Cloud SIEMs, like those integrated with Microsoft GCC High, can simplify this process and help meet multiple control objectives.

3. Ignoring Flow-Down to Subcontractors

What Is Flow-Down Responsibility?

If you pass CUI to subcontractors or vendors, they must meet the same CMMC level as you. It’s your job to verify and document that compliance—not theirs.

Even if your supplier only handles a portion of a document or works on CUI-adjacent processes, they must be held accountable.

4. Undervaluing Documentation Requirements

Documentation: The Backbone of Compliance

Beyond your System Security Plan (SSP), you must maintain:

• Written policies and procedures

• Proof of implementation (e.g., GPO settings, screenshots, logs)

• A full Plan of Action and Milestones (POAM)

• Regular updates to reflect system or process changes

Assessors expect these documents to be clear, current, and accessible—ideally housed in a GRC (Governance, Risk, and Compliance) tool like FutureFeed.

5. Treating Compliance as a One-Time Project

Ongoing Maintenance Is Mandatory

NIST 800-171 and CMMC are management systems, not point-in-time certifications. Contractors must demonstrate:

• Regular patching and updates

• Periodic reviews of authorized users and devices

• Consistent backups and validation

• System monitoring and log review

Your organization must designate someone—internal or external—to own and operate compliance tasks on an ongoing basis.

How to Get Started: Actionable Takeaways

1. Identify Your CUI

Understand exactly what data you're protecting and where it comes from.

2. Map Your Data Flow

Draw a comprehensive data flow diagram showing how CUI enters, moves through, and exits your systems.

3. Conduct a Real Gap Assessment

Use the NIST 800-171A Assessment Guide and go control-by-control, objective-by-objective. Mark each as met or unmet with justification.

4. Build a POAM

Group your compliance gaps into actionable projects. Prioritize based on impact, complexity, and SPRS score improvement.

5. Involve Decision Makers

Compliance projects require leadership buy-in. Without it, you’ll face delays and budgeting roadblocks.

Compliance is complex—but avoidable mistakes are often the easiest to fix once you know what to look for. By addressing these common gaps, you can significantly reduce your risk, improve your audit readiness, and gain a competitive edge in the defense contracting space.