SIEM Requirements for CMMC Compliance in 2025

Do You Really Need a SIEM for CMMC? Breaking Down the Requirement

• CMMC does not explicitly mandate a SIEM, but it requires strong logging, monitoring, and incident response controls that often necessitate SIEM-like capabilities.

• Assessors expect evidence of centralized log collection, correlation, alerting, and timely incident response, not necessarily the use of a SIEM product.

• For many defense contractors, especially those with multiple systems or limited staff, deploying a SIEM or managed service is the most practical way to meet CMMC requirements.

Introduction

If you are a small or mid-sized defense contractor working toward CMMC 2.0 compliance, you have likely encountered the advice that a SIEM (Security Information and Event Management) system is essential. But is it really a firm requirement? The answer is nuanced.

While CMMC does not explicitly require a SIEM, the logging and monitoring requirements derived from NIST SP 800-171 Revision 2 push companies toward centralized, continuous monitoring with alerting capabilities. Assessors look for concrete evidence of these controls functioning effectively — something a well-implemented SIEM naturally provides.

This article clarifies exactly what CMMC and related regulations require, what inspectors typically expect to see, and how you can determine if a SIEM is right for your operations or if alternatives might meet your needs.

What CMMC Actually Requires

Understanding the Source of Logging and Monitoring Requirements

CMMC verifies compliance with FAR 52.204-21 (Level 1), NIST SP 800-171 Rev.2 (Level 2), and a selected subset of NIST SP 800-172 (Level 3); DFARS 252.204-7012 applies separately (e.g., 72-hour reporting, 90-day evidence preservation). For detailed insights on CMMC certification timing and contract award requirements, see CMMC Certification Timing for DoD Contract Awards in 2025. Fulfillment of all 110 NIST 800-171 controls is mandatory.

Several control families emphasize log management and incident response, notably:

• Audit and Accountability (AU)

• System and Information Integrity (SI)

• Incident Response (IR)

Specific Requirements Highlighting Logging and Monitoring

The essence here is continuous monitoring with actionable insights, not mere log storage.

What Assessors Expect During Evaluation

During a CMMC assessment by a C3PAO or DCMA DIBCAC, auditors typically focus on whether you can demonstrate:

• Centralized log visibility: Logs are systematically collected in one or few accessible locations.

• Alerting mechanisms: Automated or manual processes that identify and flag suspicious events.

• Documented retention sufficient for monitoring, analysis, and investigations: preserve images and relevant monitoring/packet-capture data for at least 90 days after reporting a DFARS-reportable incident.

• Incident response documentation: Evidence such as tickets, reports, or workflows tied to analyzed logs.

The form of the tool you use matters less than whether your controls consistently perform as required.

What a SIEM Provides

A SIEM system addresses many CMMC expectations in an integrated fashion by offering:

• Centralized Logging Collects and normalizes logs from diverse sources like servers, endpoints, firewalls, applications, and cloud services, removing silos and simplifying access.

• Real-Time Alerts Detects unusual behaviors (e.g., repeated login failures or abnormal network traffic), instantly notifying the security team and often integrating with ticketing systems.

• Event Correlation Links seemingly unrelated events into potential incident chains—such as a phishing email receipt followed by unusual account activity and possible data access.

• Incident Response Support Provides dashboards, investigative tools, and report generation that help security teams meet timely response and regulatory reporting requirements (such as DFARS 72-hour notification).

• Compliance Evidence Generates audit trails that prove logging, monitoring, and incident handling are operational and align with AU, SI, and IR controls.

Do You Need a SIEM to Comply?

When a SIEM Is Often the Most Practical Answer

• Multiple and varied log sources: Running several servers, endpoints, networking devices, and cloud services makes manual collection unfeasible and error-prone.

• Limited internal IT or security staff: Managed SIEM or managed detection and response (MDR) services provide expertise and 24/7 monitoring.

• Handling high-risk contracts: Primes or subcontractors with sensitive CUI have heightened expectations for continuous monitoring.

• Desiring higher maturity levels (e.g., CMMC Level 3): Advanced requirements under NIST SP 800-172 call for sophisticated monitoring that typically requires SIEM-class tools.

When Alternatives Can Suffice

For smaller contractors or limited scopes, lighter approaches may work:

• Centralized syslog with monitoring scripts: Tools like Graylog, Wazuh, or syslog-ng can centralize logs and provide basic alerting without expensive licenses.

• Cloud-native logging platforms: Systems such as Microsoft Defender for Business (powered by Microsoft Sentinel), AWS CloudWatch, or Google Cloud Logging, if properly configured, can meet core requirements.

• Manual review of logs: For organizations with few systems, documented daily or weekly log reviews with screenshots can sometimes pass assessments, though this is burdensome and fragile.

"Important: While no regulation explicitly states “you must have a SIEM,” assessors expect SIEM-like outcomes—centralization, alerting, monitoring, and evidence of response."

Common Pitfalls to Avoid

• Purchasing Overly Complex SIEMs Large enterprise-grade tools like Splunk Enterprise can overwhelm a small team and incur costs exceeding $100,000 annually.

• Inadequate Configuration SIEMs do not function optimally out-of-the-box; failing to set up correct log sources or tuning alerts leads to coverage gaps and compliance failures.

• Relying Solely on Technology Without Process A SIEM without documented review workflows, incident tickets, or report logs won’t satisfy assessors. Tools support processes but cannot replace them.

Tools and Scripts That Help

Popular Commercial SIEM Solutions

• Splunk: Feature-rich but costly and requires skilled staff.

• Microsoft Sentinel: Cloud-native, tightly integrated with Microsoft 365, scalable with pay-per-use pricing.

• Elastic Security: Open-source friendly, flexible platform with strong community support.

Lightweight and Open-Source Options

• Graylog: Free tiers available, good for centralized logging.

• Wazuh: Combines endpoint security with SIEM-like dashboards and alerts.

• Security Onion: Network monitoring and logging in a comprehensive open environment.

Useful PowerShell Scripts for Windows Log Collection and Review

# Configure Windows Event Collector (basic)
wecutil qc

# Query latest failed logon events (EventID 4625) - Last 5 entries
wevtutil qe Security /q:"*[System[(EventID=4625)]]" /f:text /c:5

# Export daily failed login report to CSV
Get-WinEvent -LogName Security -MaxEvents 1000 |
Where-Object { $_.Id -eq 4625 } |
Export-Csv C:\\Logs\\FailedLogins.csv -NoTypeInformation

Such scripts can supplement your monitoring, providing documented evidence of proactive log review for assessors if a SIEM is not feasible.

Practical Guidance: Decision Framework

Keep in mind that SIEM licensing is only part of the equation. Continuous tuning, alert triage, and incident response demand ongoing investment in skilled staff or trusted managed services. Many SMBs find managed SIEM/MDR providers more cost-effective and sustainable over time.

Conclusion

So, do you need a SIEM to comply with CMMC?

• No, the regulations do not explicitly require a SIEM.

• Yes, for most small and mid-sized defense contractors, a SIEM or a managed SIEM service provides the most practical path to meet rigorous logging, monitoring, and incident response controls.

Your decision should weigh your contract risk profile, IT resources, number of systems, and budget constraints. Avoid overspending on tools you cannot support, but also don’t underinvest and risk failing assessments.

For contractors aiming for higher levels of compliance, such as CMMC Level 3, understanding evidence requirements and how assessments are conducted is critical; refer to What Evidence is Needed for a CMMC Level 2 Assessment? and What to Expect During a C3PAO CMMC Assessment for deeper guidance.

Start by auditing your current logging and monitoring practices. Identify whether you can demonstrate centralized collection, active alerting, and thorough retention. Then select a solution, whether it is a lightweight SIEM, a managed service, or a structured manual log review—that best fits your compliance journey.

For comprehensive tools to help monitor, manage, and document your CMMC compliance journey including logging and incident response workflows, consider signing up at the CMMC Dashboard registration page. This platform simplifies continuous compliance management tailored to defense contractors of all sizes.

"👉 Next Step: Evaluate your current log collection, centralization, alerting, and retention. Identify gaps and carefully choose a tailored monitoring solution — lightweight SIEM, managed service, or manual review — to ensure a smooth CMMC compliance path."