Does Defender scan USB drives?

No, not by default. But this isn’t as bad as it sounds! Here is Microsoft’s explanation from ~four years ago:

“Historically, antivirus products had a function to scan all files when a removable device was mounted. However, with the increase in device storage capacity, full scans of removable devices can noticeably and severely impact performance. Today, Windows Defender Antivirus performs quick scans on the contents of removable devices (such as USB drives), before the contents are copied, or executed. This approach both mitigates the risk that a malicious threat can infect the host through a removable device, while maintaining host performance. (A dormant file on a removable drive cannot infect a host). However, if needed, Windows Defender Antivirus can be configured to perform a custom scan on all files when removable devices are mounted. Below is a sample script for achieving this scenario Reference: TechNet Custom scan a USB drive (microsoft.com)

And a more recent, albeit abbreviated explanation from November 2020:

“You can optionally run a PowerShell script to perform a custom scan of a USB drive after it is mounted, so that Microsoft Defender Antivirus starts scanning all files on a removable device once the removable device is attached. However, we recommend enabling real-time protection for improved scanning performance, especially for large storage devices. Reference: How to control USB devices and other removable media using Intune (Windows 10) – Windows security | Microsoft Docs

I tested the “scanusb.ps1” script and it failed to detect the Eicar.com sample malware file on a USB Drive.

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Also, the CPU spiked to 13% for the duration of the scan on the large drive.

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But as soon as you attempt to interact with the file then its immediately caught by the AV engine:

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Also, be aware that The default behavior for scheduled scans is to not scan removable media. You can enable it with Group Policy or running this confusing double-negative PowerShell command: set-MpPreference -disableRemovabledrivescanning $false

Therefore, I agree with Microsoft with this design decision and I will be guiding my clients to stick with the defaults which protect the machine from malware while avoiding costly CPU hits.