NIST 800-171 is specific about threat protection: you have to defend against malicious code, find and fix flaws on a schedule, and harden the channel attackers use most, which is email. Microsoft Defender covers all three in GCC High.
Deploy Defender for Endpoint (3.14.2)
NIST 800-171 3.14.2 requires malicious-code protection, and Defender for Endpoint gives you real-time antivirus plus endpoint detection and response. Onboard devices through Intune, then turn on real-time protection, cloud-delivered protection and tamper protection.
Run vulnerability management and patching (3.11.2, 3.14.1)
NIST 800-171 3.11.2 and 3.14.1 require finding and remediating flaws on a regular cadence, not once a year. Enable Defender Vulnerability Management and review the exposure score weekly, and configure Windows Update for Business or Intune update rings so patches roll out on a predictable schedule.
Turn on anti-phishing protection (3.14.2, 3.13.13)
Email is the top attack vector, so Defender for Office 365 is doing real work here. In the Defender portal, under Email and collaboration then Policies, enable Safe Links and Safe Attachments, and apply the preset Standard or Strict security policy to all users.
A note on what "done" means
Configuring these settings hardens your Microsoft 365 GCC High tenant to the NIST 800-171 technical baseline. It is not a CMMC certification. A C3PAO assesses your documented System Security Plan, not your tenant directly, so the configuration above only counts once it is written down with evidence. Our free GCC High Setup Autopilot walks you through every step in order and hands the result to your control set when you move to Readyline Pro.
