NIST 800-171 Control 3.1.21: Portable Storage Restrictions
Portable storage devices are a prime attack vector for data exfiltration and malware introduction. Control 3.1.21 requires you to limit when and how external systems can use USB drives, external hard drives, and similar removable media. This control directly protects your organization's most sensitive CUI assets.
What this means
Your organization must establish and enforce policies that restrict the use of portable storage devices (USB drives, external drives, memory cards, etc.) on systems that access or process controlled unclassified information (CUI). This includes both company-issued and personally-owned devices on external or uncontrolled systems. The goal is to prevent unauthorized data transfer, reduce malware infection vectors, and maintain audit trails of information movement.
How to comply
- 1.Inventory all portable storage device types used in your environment (USB drives, SD cards, external hard drives, etc.)
- 2.Define which systems require portable storage access and which ones should prohibit it entirely
- 3.Implement technical controls such as USB port disabling, device driver blacklisting, or mobile device management (MDM) policies
- 4.Create a portable storage use policy that specifies approved devices, approved use cases, and prohibited scenarios
- 5.Establish approval workflows for exceptions to portable storage restrictions
- 6.Configure logging and monitoring to detect unauthorized portable storage connections
- 7.Conduct security awareness training for all personnel on portable storage risks and approved practices
- 8.Audit portable storage usage quarterly and review logs for suspicious activity
Evidence auditors look for
- Portable storage device policy document with defined restrictions and exceptions
- Group Policy Objects (GPOs) or MDM configurations disabling USB ports on protected systems
- Device driver blacklist/whitelist configurations
- Endpoint protection logs showing blocked portable device connections
- Approved portable storage device inventory with serial numbers and ownership records
- Security awareness training completion records specific to portable storage risks
- Change management tickets documenting approved exceptions and their business justification
- Quarterly audit reports of portable storage access logs
Frequently asked questions
When will FAQs be available?
The FAQ for this control is currently being prepared.
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