AC-19(1): Use of Writable and Portable Storage Devices

AC-19(1): use of writable and portable storage devices requirement means you must tightly control when USB drives and other writable portable media can connect to your systems, and you must enforce those rules with technical controls plus auditable evidence. Operationalize it by defining allowed devices and use-cases, blocking everything else by default, and keeping proof of enforcement and exceptions. ¹

Key takeaways:

• Treat writable portable storage as a default-deny surface; allow only managed, approved devices for defined business needs.
• Pair policy with enforcement (endpoint controls, encryption, logging, and exception workflow) and keep recurring evidence.
• Auditors look for proof that the control works in production, not just a written standard. ²

Writable portable media (USB flash drives, external HDD/SSD, SD cards, and similar) sits at the intersection of two hard problems: data exfiltration and malware introduction. AC-19(1) is where NIST expects you to turn “we have a policy” into “the environment enforces it.” If you handle federal data (or you align your program to NIST SP 800-53 Rev. 5 for customer requirements), assessors will test whether users can copy sensitive data to removable media, whether unapproved devices are blocked, and whether exceptions are controlled and time-bounded. ¹

This page is written for a Compliance Officer, CCO, or GRC lead who needs to implement the requirement quickly without turning it into a multi-quarter engineering project. You’ll get: a plain-English interpretation, an operator-ready checklist, evidence to retain, typical audit hangups, and a practical execution plan. Where teams stumble, it’s usually on scope (what counts as “portable storage”), enforcement ownership (IT vs Security vs Endpoint Engineering), and exception creep (temporary approvals that become permanent). AC-19(1) gives you a clean way to set guardrails and prove they are working. ¹

Regulatory text

Requirement excerpt: “NIST SP 800-53 control AC-19.1.” ²

What the operator must do: Implement organization-defined controls for the use of writable and portable storage devices connected to organizational systems. In practice, you need (1) explicit rules for when portable writable media is allowed, (2) technical enforcement at endpoints and relevant systems, and (3) evidence that blocking/allow-listing and exception handling operate as designed. ¹

Plain-English interpretation (what AC-19(1) is really asking)

You must prevent “anyone can plug in any storage device and copy data” from being true in your environment. The control expects you to:

• Decide what’s allowed (device types, users/roles, systems, and data types).
• Block the rest by default using technical controls, not just a memo.
• Control exceptions (approval, justification, compensating controls, expiry).
• Prove operation with logs, configuration evidence, and review records. ¹

If your environment includes regulated data, treat writable portable media as a sanctioned pathway only for narrow operational needs (forensics, field work, controlled data transfer), not a general convenience.

Who it applies to (entity and operational context)

Entities:

• Federal information systems and contractor systems handling federal data that align to NIST SP 800-53. ¹

Operational scope (where this shows up):

• End-user endpoints (Windows/macOS/Linux laptops and desktops)
• Privileged/admin workstations
• Servers where removable media is possible (including jump boxes)
• Specialized environments (OT/ICS engineering workstations, lab devices, kiosk systems)
• Third-party or contractor-managed endpoints that connect to your systems or handle your data (contract language and technical controls both matter)

Media types you should explicitly address:

• USB mass storage, external drives, SD/microSD
• Mobile devices presenting as storage (MTP/PTP, tethered storage modes)
• Write-capable optical media where relevant
• Virtual “removable” media (USB passthrough to VDI; treat as in-scope if users can write data out)

What you actually need to do (step-by-step)

1) Set the policy decision: default-deny and explicit allow

Create a short “Portable Writable Media Standard” that answers:

• Are writable portable storage devices allowed at all? If yes, for which business cases.
• Which roles can request access (IT, SOC, incident response, engineering).
• Which data types may be written (e.g., prohibit regulated datasets unless explicitly approved).
• Required protections (encryption, labeling, secure handling, return/destruction).
• Approval and expiry requirements for exceptions.

Make the default posture easy to defend: “Unapproved writable portable storage is blocked on managed endpoints.”

2) Define “approved device” criteria

Write down what qualifies as approved:

• Corporate-owned, inventoried device with a unique identifier (serial)
• Encrypted with organization-approved encryption and recovery process
• Assigned to a custodian (named owner)
• Logged and monitored when connected (at least on managed endpoints)

This is the hinge: auditors accept restricted use if you can show you know which devices are in play.

3) Implement endpoint enforcement (the control must work)

Partner with endpoint engineering to implement one of these enforceable patterns:

• Block all removable storage writes; allow read-only if needed.
• Allow-list by device ID (only approved encrypted drives work).
• Role-based control (only specific groups can use approved devices; everyone else blocked).
• Conditional controls for high-risk systems (privileged workstations: stricter).

Document the enforcement mechanism and the scope (which device groups, OS versions, and network segments). Assessors will test a sample endpoint.

4) Build an exception workflow that does not sprawl

Create a ticket-driven process with required fields:

• Business justification and data classification
• Device identifier and custodian
• Systems where it will be used
• Compensating controls (encryption, DLP monitoring, limited duration)
• Explicit expiry date and review trigger
• Approval by Security and data owner (as applicable)

Keep exceptions rare and time-bounded. “Temporary” approvals that never expire are a common audit finding.

5) Add monitoring and periodic review

Minimum operational monitoring expectations:

• Log device connection events on endpoints in scope.
• Alert on blocked attempts (signal of user friction or malicious activity).
• Review exception list and device inventory for drift.

Tie this to your existing access review cadence. If you use Daydream for control operations, track each exception as a control task with an owner, approval evidence, expiry, and recurring review evidence.

6) Validate with a simple test script (auditor-ready)

Maintain a short validation procedure:

• On a representative endpoint, attempt to mount an unapproved USB drive and write a file.
• Capture proof of block (system message, EDR event, or OS log).
• On an approved device, confirm allowed behavior matches policy.
• Retain the results as part of recurring control evidence. ¹

Required evidence and artifacts to retain

Keep evidence that proves design and operation:

Policy and design

• Portable Writable Media Policy/Standard (versioned, approved)
• Scope statement (systems/endpoints covered; exclusions with rationale)
• Roles and responsibilities (control owner, endpoint engineering owner)

Technical enforcement evidence

• Endpoint configuration baselines (screenshots or exported settings)
• Allow-list configuration (device IDs, groups, applied policies)
• Encryption configuration requirements for approved devices

Operational evidence

• Device inventory list (approved portable storage with custodian)
• Exception tickets (approval, justification, expiry, closure)
• Logs showing enforcement (blocked/allowed events) for sampled periods
• Periodic review records (exception review sign-off; inventory reconciliation)

A common gap is “we block USB” with no proof. Your evidence must be reproducible.

Common exam/audit questions and hangups

Assessors and auditors tend to focus on:

• “Show me it’s enforced.” Demonstrate a block event on a real endpoint.
• “What counts as portable storage here?” If you ignore SD cards or USB passthrough, justify why.
• “How do you control exceptions?” They will sample exceptions and check expiry and approval.
• “Do contractors follow the same rules?” If third parties use endpoints with your data, show contract requirements and enforcement approach.
• “How do you prevent data leaving the environment?” If you allow devices, show encryption and monitoring.

Hangup: teams often over-scope initially (“ban everything everywhere”) and then create too many exceptions. Start strict on high-risk systems, then expand with clear criteria.

Frequent implementation mistakes (and how to avoid them)

1. Policy-only controls

• Fix: Pair the policy with endpoint enforcement evidence and a test record.

2. Allowing “any encrypted USB”

• Fix: Require inventory + assigned custodian + allow-listing by device identifier.

3. No expiry on exceptions

• Fix: Make expiry mandatory in the ticket template and auto-remind owners before expiry.

4. Ignoring non-laptop endpoints

• Fix: Include admin workstations, jump hosts, and lab systems in scope decisions.

5. Unclear ownership

• Fix: Name one control owner (GRC/Security) and one technical owner (endpoint engineering). Document both.

Enforcement context and risk implications

No public enforcement cases were provided in the source catalog for this requirement, so you should treat this as a risk-driven control rather than a “case law” requirement.

Risk to explain to stakeholders:

• Writable portable media enables fast data removal outside normal monitoring paths.
• It also creates a malware ingress path that bypasses network-based controls.
• Control failure is usually easy for an assessor to demonstrate with a physical device, which makes it a high-visibility operational control during audits. ¹

Practical execution plan (30/60/90-day)

Use time-boxed phases to drive closure while avoiding fragile shortcuts.

First 30 days (baseline + decisions)

• Assign control owner and technical owner.
• Publish the Portable Writable Media Standard (default-deny + allowed use-cases).
• Identify in-scope endpoint populations (standard users, privileged users, kiosks, jump boxes).
• Choose enforcement approach (block all; allow-list; role-based).
• Stand up exception workflow in your ticketing system with mandatory fields and approvals.

Days 31–60 (enforce + inventory)

• Deploy endpoint policy to a pilot group; confirm block/allow behavior.
• Create approved device inventory process (procurement/asset mgmt tie-in).
• Turn on logging to your SIEM/EDR for device control events where available.
• Train IT support on “what happens when a user plugs in a USB drive.”

Days 61–90 (prove operation + stabilize)

• Expand deployment to broader endpoint populations based on risk.
• Run a validation test and retain evidence (screenshots/log exports/tickets).
• Perform the first exception and inventory review; remediate drift.
• Build an assessor-ready evidence packet in Daydream: policy, config proof, sample logs, exception samples, and review sign-offs. ¹

Frequently Asked Questions

Does AC-19(1) mean we must ban USB drives completely?

No. It requires controlled use. A defensible posture is “blocked by default, allowed only for approved devices and approved business cases,” backed by technical enforcement and evidence. ¹

What counts as a “writable and portable storage device” in practice?

Treat USB mass storage, external drives, and SD cards as in-scope by default. Also evaluate mobile devices and virtual USB passthrough if users can write files out of your environment.

We have developers who need to move build artifacts offline. How do we handle that?

Require an exception with a defined use-case, approved device inventory, encryption, and an expiry. If possible, replace the workflow with a managed transfer method (approved repository or secure file transfer) and keep portable media as the fallback.

How do auditors typically test this control?

They ask for the policy, then attempt (or ask you to demonstrate) plugging in an unapproved device to confirm blocking. They also sample exception tickets and verify approvals and expiry.

If a third party brings their own laptop onsite, does AC-19(1) apply?

If that third party endpoint connects to your systems or handles your data, you need contractual and technical guardrails appropriate to the access. At minimum, define whether portable media use is permitted during that engagement and how you enforce it.

What evidence is most persuasive if we can’t export detailed endpoint configs?

Provide a combination: policy, a screen-recorded or witnessed test showing block behavior, logs of device control events, and exception records. Evidence variety helps when one source is hard to extract.

Footnotes

1. NIST SP 800-53 Rev. 5

2. NIST SP 800-53 Rev. 5 OSCAL JSON