Scan systems and files when downloaded

Run periodic full scans on your systems and scan files in real time as they're downloaded or opened. Microsoft Defender's default settings (real-time protection + scheduled scans) satisfy this; same for macOS XProtect plus an EDR or AV product configured for on-access scanning.

Official text

“Perform periodic scans of the information system and real-time scans of files from external sources as files are downloaded, opened, or executed.”

, FAR 52.204-21(b)(1)(xv), CMMC Level 1 v2.13 Assessment Guide

What evidence satisfies this

Any one of these, by itself, won't satisfy the practice, but showing a few of them together is what an assessor or a prime contractor expects to see:

✓Defender configured for real-time scanning and a scheduled weekly quick scan (the default).
✓Email attachment scanning at the tenant (default in M365 / Workspace).
✓Browser safe-browsing on (default in Chrome / Edge / Safari).
✓EDR / AV product configured for on-access scanning at file open.
✓A monthly review confirming scheduled scans ran and didn't quarantine anything unexpected.

Common ways small shops fail this

✗Real-time protection disabled because a developer found it slow.
✗Scheduled scans cancelled by users without warning.
✗Downloads from the browser to a folder excluded from scanning.
✗USB drives plugged in without any auto-scan.
✗Email tenant scanning turned off after a deliverability dispute.

How to fix it in a weekend

1Leave real-time protection on. If a tool's slowness pushed someone to disable it, fix the tool, not the protection.
2Confirm a weekly scheduled scan exists on every endpoint.
3Re-enable email-attachment and link scanning at the tenant.
4Set USB / removable-media auto-scan on (Defender's default behavior).
5Add a 10-minute monthly review of the AV console to your routine.

FAQ

How often does "periodic" mean?+

FAR doesn't define a frequency. Industry practice is real-time on-access scanning continuously, plus a weekly full or quick scan. Pick a schedule you can defend, document it, and stick to it.

Related references

Clause

FAR 52.204-21, Basic Safeguarding of Covered Contractor Information Systems

Glossary

FAR 52.204-21

Doing all 15 yourself? Use the checklist.

Custodia's free CMMC Level 1 checklist walks the same 15 requirements with a self-assessment workflow, generates your SSP and affirmation memo, and posts your SPRS score for you.

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