CMMC compliance is not a checklist or quick-fix project. It’s a company-wide transformation involving operational maturity, documentation, leadership commitment, and cross-functional execution.
At Sera Brynn, our certified assessors and compliance advisors have worked with many organizations to prepare for compliance certification. Throughout our experiences, we've noticed common misunderstandings and misconceptions about what is required to achieve CMMC compliance. We’ve compiled this list to help you avoid the most common (and most damaging) mistakes companies make during the journey.
This is the final article in our series The Top 15 Most Common CMMC Mistakes We See Businesses Make. Below you will find a summary of each of the 15 entries, and links to the respective articles to learn more.
What You’ll Learn:
- The 15 most common missteps that can derail CMMC readiness
- Real-world explanations, examples, and mitigation strategies
- How to strengthen your security posture and prepare for a smoother audit
#15: Incomplete Customer Responsibility Matrix (CRM) Documentation
When organizations work with external service providers, it's essential to clearly define who is responsible for which security objectives. Unfortunately, many organizations either overlook this requirement or stop at vague documentation. A CRM should map specific responsibilities at the objective level, including monitoring, access, and remediation responsibilities.
What else you’ll learn: What makes a CRM audit-ready, and how to structure your documentation around shared services.
Read the full article: Incomplete Customer Responsibility Matrix (CRM) Documentation
#14: Overlooking Legal Requirements from External Service Providers
CMMC requires that organizations not only have agreements in place with external providers but that those agreements include specific cybersecurity clauses aligned with the control objectives. Many businesses either skip these steps or rely on outdated, generic contracts. Proper documentation must include a Shared Responsibility Matrix and contract language covering each applicable requirement.
What else you’ll learn: What to ask vendors for, how to verify agreement coverage, and where most businesses fall short.
Read the full article: Overlooking Legal Requirements from External Service Providers
#13: Misunderstanding What Can Be Outsourced
Outsourcing can help with operational efficiency, but it doesn’t remove your responsibility for compliance. Many organizations mistakenly believe that using a third-party MSSP or cloud provider eliminates their need to implement certain controls. In reality, even when tasks are outsourced, the OSC must still prove oversight, documentation, and risk ownership.
What else you’ll learn: Examples of misunderstood responsibilities and what remains your obligation even with vendor support.
Read the full article: Misunderstanding What Can Be Outsourced
#12: Overlooking the Value of External Expertise in Compliance Efforts
First-time OSCs often attempt to handle compliance internally, relying on IT staff or general legal counsel. This usually leads to gaps in documentation, scope misunderstanding, or missed requirements. Working with a qualified consultant or advisor helps accelerate progress, validate readiness, and avoid wasted effort.
What else you’ll learn: When to bring in external help, and the most common areas where advisors add value.
Read the full article: Overlooking the Value of External Expertise in Compliance Efforts
#11: Skipping Internal Security Risk Assessments with a Consultant
Before your formal assessment, you should conduct a detailed internal review of your current state. This risk assessment helps identify control gaps and gives your team an opportunity to improve responses. Without it, you’re flying blind and auditors will see that immediately. It also gives key personnel a chance to practice speaking to auditors and demonstrating maturity.
What else you’ll learn: How to structure an internal risk assessment, and why simulated audits are a critical rehearsal.
Read the full article: Skipping Internal Security Risk Assessments with a Consultant
#10: Misunderstanding Continuous Monitoring: Past, Present, and Future
Too many businesses think continuous monitoring is something they start after certification. But CMMC requires that you demonstrate a history of operational maturity at the time of assessment. This includes showing at least 90 days of activity, audit logs, incident response tests, and evidence of detection and mitigation.
What else you’ll learn: A timeline of what monitoring evidence is required when, and how to structure it for audit review.
Read the full article: Misunderstanding Continuous Monitoring: Past, Present, and Future
#9: Underestimating the Company-Wide Impact of CMMC
CMMC isn’t just an IT project. In fact, IT typically accounts for only one-third of the required controls. Other departments like HR, operations, legal, facilities, and leadership play critical roles in access control, background screening, training, and organizational policy. Without full engagement from all functions, compliance is nearly impossible.
What else you’ll learn: A breakdown of departmental responsibilities and how to drive organization-wide buy-in.
Read the full article: Underestimating the Company-Wide Impact of CMMC
#8: Assuming an Enclave Solves Everything and Missing On-Premises Control
Some organizations try to isolate their CUI into an enclave and assume that’s enough. But physical security, process documentation, and access policies often exist outside of the enclave, and CMMC assessors will evaluate the entire picture. If your policy says all employees use the enclave but logs show otherwise, you have a problem.
What else you’ll learn: What enclaves do well, what they don’t, and how to use them without overlooking key requirements.
Read the full article: Assuming an Enclave Solves Everything and Missing On-Premises Control
#7: Poor Assessment Preparation
You can’t fake readiness in a CMMC assessment. Interviews must be consistent, documentation must be thorough, and evidence must show historical operation. Many organizations fail because SMEs don’t know what to expect, leadership isn’t involved, or control implementation hasn’t been validated.
What else you’ll learn: How to prepare your team, rehearse responses, and conduct readiness reviews with internal or third-party support.
Read the full article: Poor Assessment Preparation
#6: Expecting Immediate Compliance Without Buy-In or Budget Planning
CMMC success requires planning, funding, and leadership commitment. Many businesses underestimate how much change is needed or expect the process to be quick and inexpensive. A rushed implementation without senior sponsorship almost always fails to deliver the necessary documentation, controls, or staff training.
What else you’ll learn: Why budget and timeline planning should start on day one, and what stakeholders must be involved.
Read the full article: Expecting Immediate Compliance Without Buy-In or Budget Planning
#5: Overestimating Self-Assessment Scores in SPRS
The average OSC overestimates their NIST SP 800-171 control implementation in SPRS. Without a third-party assessment, it's easy to think you're compliant based on partial implementations or optimistic interpretations. But CMMC assessments are far more rigorous, and inflated scores create a false sense of readiness.
What else you’ll learn: Why most SPRS scores don’t hold up, and how to conduct a more honest self-assessment.
Read the full article: Overestimating Self-Assessment Scores in SPRS
#4: Treating CMMC Assessment Like a Snapshot in Time
CMMC assessors aren’t just looking at where you are today, they want proof you’ve operated securely for at least 90 days. Many organizations build their SSP or policies right before the audit and fail to show historical evidence. Everything from audit logs to onboarding processes needs to be documented and active for months in advance.
What else you’ll learn: What “evidence of maturity” looks like and how to build it into your daily operations.
Read the full article: Treating CMMC Assessment Like a Snapshot in Time
#3: Not Reading Contracts and Bids for All Cybersecurity Clauses
Cybersecurity obligations often come from contract language, not just regulations. Businesses miss key requirements hidden in Cybersecurity clauses or fail to understand which requirements override others. A cybersecurity expert can identify conflicts, clarify obligations, and ensure your implementation matches your contractual responsibilities.
What else you’ll learn: How to analyze contract language for security implications, and when to escalate for expert review.
Read the full article: Not Reading Contracts and Bids for All Cybersecurity Clauses
#2: Inadequate System Security Plan (SSP) Documentation
The SSP is the backbone of your CMMC compliance program. It should be detailed enough for a third party to operate your system if you walked away tomorrow. Instead, most SSPs are incomplete, out of date, or missing key references. A weak SSP signals immaturity and leads to extensive follow-up questioning during the audit.
What else you’ll learn: What to include in your SSP, how to structure it, and why it serves as the central story for your security program.
Read the full article: Inadequate System Security Plan (SSP) Documentation
#1: Improper Scoping of Information Flow and Asset Boundaries
The number one mistake we see is incorrect scoping. Organizations fail to map how CUI flows through systems and where control boundaries begin and end. This leads to missed assets, overexposure, or overlooked vulnerabilities. A properly scoped environment defines the entire compliance strategy.
What else you’ll learn: How to perform scoping exercises, define system boundaries, and build an audit-ready environment.
Read the full article: Improper Scoping of Information Flow and Asset Boundaries
The Three Most Important Key Takeaways from This Series
- Hire external experts early. Third-party consultants help with scoping, SSP development, policy alignment, audit readiness, and continuous monitoring. Their expertise helps avoid mistakes that internal teams might miss.
- CMMC compliance requires company-wide involvement. IT may be responsible for implementation, but operations, HR, legal, procurement, and leadership must all contribute to successful compliance.
- Start early and build evidence over time. Your assessment isn’t just about what you do today—it’s about proving you’ve operated securely for months. Operational maturity can’t be rushed.
In Conclusion: The Top 15 Most Common CMMC Compliance Mistakes
CMMC is one of the most demanding cybersecurity compliance programs ever introduced to the defense industrial base. But with the right preparation and guidance, success is entirely achievable.
By studying and avoiding these 15 common mistakes, your organization can reduce cost, avoid delays, and improve the likelihood of a successful assessment. Sera Brynn is here to help you every step of the way.
Whether you’re just beginning or preparing for a final assessment, we invite you to connect with our team for expert guidance.