# NIST SP 800-53 Rev. 5 AC-20(4) — Network Accessible Storage Devices -- Prohibited Use - Source: https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-20-4-network-accessible-storage-devices-prohibited-use - Section: [Compliance](https://support.icompaas.com/compliance) › [NIST SP 800-53 Rev. 5 Controls](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5) › AC — Access Control - Last updated: 2026-06-05 - Publisher: iCompaas, Inc. (https://www.icompaas.com) **NIST SP 800-53 Rev. 5**·Control `AC-20(4)`·high·Component: **Access Control** ## What this control requires > Prohibits use of network-accessible storage devices (e.g., cloud storage) in external systems to prevent unauthorized access or data exposure in public or hybrid environments. This control supports the Network Access Control System capability within the Access Control family. > > — NIST SP 800-53 Rev. 5, Control AC-20(4) ### In plain English Prohibits use of network-accessible storage devices (e.g., cloud storage) in external systems to prevent unauthorized access or data exposure in public or hybrid environments. In practice, this means you can prove you access management-level controls on a defined cadence. ## Why it matters SP 800-53 is the catalogue every U.S. federal authorisation (FedRAMP, FISMA, CMMC) and most regulated industries point at. A missing control surfaces as a Plan of Action & Milestones (POA&M) item that an Authorizing Official must approve before the Authority To Operate is granted or renewed. This control is rated **high**. Auditors expect a named owner, a remediation SLA, and a tooling artefact that evidences continuous operation. ## What an auditor looks for A 3PAO (FedRAMP) or Independent Assessor (NIST RMF) tests against NIST SP 800-53A assessment procedures. For each control they expect a documented implementation that addresses every assessment objective (Examine, Interview, Test) and an implementation summary in the System Security Plan. Within **Access Control**, the assessor will trace your implementation back to the catalogue reference (Control AC-20(4)) and look for objective evidence on the latest two assessment cycles. ## How to implement #### Cloud Posture **CSPM.** Run Wiz / Prisma / Defender for Cloud / Security Command Center over every account; auto-ticket findings above your accepted-risk threshold. #### Network Security **AWS.** VPC Flow Logs to S3, default-deny security groups, AWS Network Firewall for north-south, and Transit Gateway for hub-and-spoke segmentation. **Azure.** NSGs with deny-by-default, Azure Firewall Premium with TLS inspection, Private Endpoints for PaaS, and Microsoft Defender for Cloud's secure score for drift. **GCP.** VPC Service Controls perimeters around sensitive APIs, Cloud NGFW Enterprise for L7 inspection, Private Service Connect for managed services. #### Configuration Management **Policy-as-code.** Codify the control as Service Control Policies (AWS), Azure Policy initiatives, or Organisation Policy (GCP) so the platform enforces what the document promises. **IaC.** Gate Terraform / Pulumi / CloudFormation changes with Checkov / tfsec / OPA so misconfigurations never reach production. **Drift.** Run a CSPM (Wiz / Prisma / Defender for Cloud / Security Command Center) over every account and route findings to a single risk register. ### Operational checklist - **Access.** Document the trigger, the owner and the cadence. - **Security.** Document the trigger, the owner and the cadence. - **Configuration.** Document the trigger, the owner and the cadence. - **Data.** Document the trigger, the owner and the cadence. - **Network.** Document the trigger, the owner and the cadence. - **Scope:** management, access control, system, e, external systems, configuration, environments, capability. ## Evidence to collect - **Infra Scan.** Continuous infrastructure-posture telemetry — AWS Config, Azure Policy compliance, GCP Organisation Policy, Wiz / Prisma findings. Infrastructure posture tooling (AWS Config, Azure Policy, Nessus) verifies platform hardening. Focus: Network Accessible Storage Devices -- Prohibited Use. ## Common pitfalls - Treating CSPM findings as a backlog instead of a hot queue: findings older than the policy SLA accumulate until the auditor asks why. - Default-allow egress rules; East-West traffic with no segmentation; jump hosts on the public internet with password auth. - Manual cloud-console changes that never make it back to Terraform — drift accumulates until the next outage forces a reconciliation. ## How it maps to other frameworks Computed from shared tags, control component and key-operation subjects — handy when you are reconciling controls across an audit. - [**NIST SP 800-171 Rev. 3 3.1.20** — Use of External Systems](https://support.icompaas.com/compliance/nist-sp-800-171-rev-3/3-1-20-use-of-external-systems) - [**BSI C5:2020 COS-06** — Segregation of data traffic in jointly used network environments](https://support.icompaas.com/compliance/bsi-c5-2020/cos-06-segregation-of-data-traffic-in-jointly-used-network-environments) - [**ISO/IEC 27001:2022 A.5.34** — Privacy and protection of personal identifiable information (PII)](https://support.icompaas.com/compliance/iso-27001-2022/a-5-34-privacy-and-protection-of-personal-identifiable-information-pii) ## Related controls in NIST SP 800-53 Rev. 5 - [**Control AC-18(4)** — Restrict Configurations By Users](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-18-4-restrict-configurations-by-users) - [**Control AC-17(3)** — Managed Access Control Points](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-17-3-managed-access-control-points) - [**Control AC-18(3)** — Disable Wireless Networking](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-18-3-disable-wireless-networking) - [**Control SA-9(7)** — Organization-Controlled Integrity Checking](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/sa-9-7-organization-controlled-integrity-checking) - [**Control AC-23** — Data Mining Protection](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-23-data-mining-protection) - [**Control AC-19** — Access Control For Mobile Devices](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-19-access-control-for-mobile-devices) ## References - [NIST Computer Security Resource Center — NIST SP 800-53 Rev. 5](https://csrc.nist.gov/Projects/risk-management/sp800-53-controls) - Catalogue identifier: `Control AC-20(4)` Topics: network securitycloud infrastructurecloud postureconfiguration management ## Related compliance guidance - [NIST SP 800-53 Rev. 5: a practical implementation guide for cloud teams](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5): Start with the full NIST SP 800-53 implementation guide — scope, roadmap, evidence and how the assessment runs. - [NIST SP 800-171 and CMMC: a practical guide to protecting CUI in the cloud](https://support.icompaas.com/compliance/nist-sp-800-171-rev-3): If your obligation is protecting CUI in your own systems for a federal or defence contract, this derived subset — not the full catalogue — is the requirement. - [SOC 2 compliance: a practical implementation guide for cloud-native SaaS teams](https://support.icompaas.com/compliance/soc-2-trust-services-criteria): Most commercial SaaS already hold a SOC 2 Type 2, and FedRAMP 20x Class A is designed for providers that have one.