What evidence do auditors commonly ask for on AC-20(4) Use of External Systems | Network Accessible Storage Devices: Prohibited Use?

Access control policy; Procedures addressing use of network-accessible storage devices in external systems; System design documentation; System configuration settings and associated documentation; System connection or processing agreements; List of network-accessible storage devices prohibited from use in external systems.

What is the most common gap auditors find on AC-20(4)?

Defined network accessible storage devices are used in external systems despite the prohibition

NIST SP 800-53 Rev 5 · AC - Access Control

`AC-20(4)` Use of External Systems | Network Accessible Storage Devices: Prohibited Use

Prohibit the use of organization-defined network accessible storage devices in external systems.

Source: NIST SP 800-53 Rev 5, AC-20(4). The statement above is our statement of the clause, read from the published text and cited so you can check it against the official text, not the instrument verbatim. Open NIST SP 800-53 Rev 5 in the compliance library.

Base controls and their enhancements. Which apply follows the baseline you are assessed against (for example FedRAMP Moderate or High, or the low, moderate and high baselines of SP 800-53B). Text in square brackets is a parameter your organisation sets.

artefacts

held by a system

at each review

hard

to go live

Policy repository / GRC workspace

where the evidence lives

teal = a system already holds it · olive = produced at each review. To go live: easy when half or more of the artefacts are already held by a system, moderate from a quarter to a half, hard below a quarter.

These are the artefacts auditors commonly ask for on this requirement, our guidance rather than text from the standard. Your assessor decides what is sufficient in your context, and asks for samples across the period when a report covers one.

system holds itEvidence a system already holds

List of network-accessible storage devices prohibited from use in external systems · Endpoint management (MDM / EDR)

periodic reviewEvidence produced at each review

System audit records · Identity provider / directory
Test of Mechanisms prohibiting the use of network-accessible storage devices in external systems · Endpoint management (MDM / EDR)

governing documentDocuments that govern the control

Access control policy · Policy repository / GRC workspace
Procedures addressing use of network-accessible storage devices in external systems · Policy repository / GRC workspace
System design documentation · Policy repository / GRC workspace
System configuration settings and associated documentation · Policy repository / GRC workspace
System connection or processing agreements · Policy repository / GRC workspace
System security plan · Policy repository / GRC workspace

First move

Mostly documents and reviews. Pull the 1 system-held artefact from your Endpoint management (MDM / EDR) on a schedule; put the documents under version control with an owner and review date, and log each review as a dated record with a named reviewer.

Common gaps auditors find

Defined network accessible storage devices are used in external systems despite the prohibition
The network accessible storage devices subject to prohibition were never identified

Do this for your whole sheet

Paste the rows you run your controls from and get this mapping for every control at once, with the periodic-review ones flagged and a first move per row. No account for the first run.

Build my evidence sheet

AC-20(3) Use of External Systems | Non-organizationally Owned Systems: Restricted Use · AC-20(5) Use of External Systems | Portable Storage Devices: Prohibited Use