03.01.18: Access Control for Mobile Devices

Mobile devices are an assessor’s favorite place to find gaps because they blend identity, endpoint security, network access, and user behavior in a single control surface. The 03.01.18: access control for mobile devices requirement exists to prevent uncontrolled devices from becoming an unmonitored bridge into systems that store, process, or transmit CUI. In practice, this requirement forces a clear decision: either you manage mobile devices to an approved baseline and allow access under controlled conditions, or you block mobile access to CUI entirely.

For a CCO or GRC lead, operationalizing 03.01.18 is mostly about two deliverables: (1) a policy decision that defines the allowed mobile access patterns for CUI, and (2) a set of technical enforcement points that make the policy real (identity provider conditional access, MDM/MAM compliance, and data-layer controls such as containerization and DLP). Assessors will look for consistency between what you say (policy), what you configured (systems), and what you can prove (evidence). The good news: once you standardize the “mobile access path,” ongoing compliance becomes a repeatable evidence cycle rather than a recurring fire drill.

Regulatory text

Requirement: “NIST SP 800-171 Rev. 3 requirement 03.01.18 (Access Control for Mobile Devices).” (NIST SP 800-171 Rev. 3)

Operator interpretation (what you must do):
You must control which mobile devices can access organizational systems and CUI, and you must implement enforceable access restrictions for those devices. “Mobile devices” should be interpreted broadly in your environment: smartphones, tablets, and other portable endpoints capable of storing data or authenticating to resources. For assessment readiness, you need three things aligned:

1. Rules: documented access rules for mobile (allowed devices, allowed apps, allowed networks, allowed resources).
2. Enforcement: technical controls that block noncompliant mobile access.
3. Proof: evidence that mobile access is managed as designed.

Plain-English interpretation

Mobile devices can’t be a free pass into CUI. If a device is going to access CUI (email, files, Teams/Slack equivalents, VDI, web apps, ticketing systems, SFTP, etc.), it must be approved, configured to your baseline, and continuously checked for compliance. If you can’t manage it, you block it.

Who it applies to

Entity scope

• Federal contractors and other nonfederal organizations that handle CUI in nonfederal systems, and have adopted NIST SP 800-171 Rev. 3 as a requirement baseline (NIST SP 800-171 Rev. 3).

Operational scope (where this becomes real)

• Users who access CUI from mobile mail clients, mobile browsers, or mobile apps.
• Any mobile device that authenticates to your identity provider, VPN, VDI, or SaaS where CUI is present.
• BYOD programs and contractor-owned devices, if you allow them to access CUI.
• Third parties (support providers, consultants) who use mobile devices for MFA and device-based authentication into your environment. Treat them under your third-party access governance, but enforce the same mobile access gates.

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

1) Decide your allowable mobile access patterns (policy-level)

Create a short “mobile access decision” that you can defend in an assessment:

• Allow mobile access to CUI only from managed devices, or
• Block mobile access to CUI entirely (and document compensating workflow like VDI-only access from managed desktops).

Write this into your access control policy and your mobile/BYOD standard. Keep the language enforceable: “must,” “must not,” and “exceptions require approval.”

2) Define what counts as a “managed/approved mobile device”

Minimum attributes to define:

• Ownership types: corporate-owned, personally owned (BYOD), contractor-owned.
• Enrollment requirement: MDM/MAM enrollment required before access.
• Supported OS versions and patch posture expectation.
• Required security posture: device encryption, screen lock, no jailbroken/rooted devices.

Your baseline must be concrete enough that you can produce screenshots or exported configuration profiles as evidence.

3) Build an inventory of mobile devices with access to CUI

You need an inventory that answers two assessor questions quickly:

• Which mobile devices can access CUI systems?
• Which users have used mobile devices to access those systems?

Practical method:

• Export enrolled device lists from MDM.
• Export sign-in logs from your identity provider filtered to mobile user agents and OS types.
• Reconcile the two lists. Investigate “sign-ins from unmanaged mobile.”

4) Implement technical enforcement at the identity layer (your primary choke point)

Make the identity provider the default enforcement point:

• Require MFA and bind access to device compliance (only compliant, enrolled devices can authenticate to CUI-bearing apps).
• Block legacy authentication and unmanaged app access paths where possible.
• Restrict high-risk actions from mobile (downloads, offline sync) if you can’t protect local storage.

Assessors generally care less about the brand of tool and more about whether access is actually blocked for noncompliant devices.

5) Implement device controls with MDM/MAM (the mobile baseline)

Configure policies that directly support access control outcomes:

• Device encryption enabled.
• Screen lock + inactivity timeout.
• Remote wipe capability for lost/stolen devices.
• App control: require managed apps for email/files; restrict copy/paste or “open in” to unmanaged apps if your use case warrants it.
• Detect and block rooted/jailbroken devices.

Where BYOD is allowed, prefer application management (MAM) or containerization to avoid collecting unnecessary personal device data. Document the tradeoff: less device visibility, but stronger separation of corporate data.

6) Control data paths to CUI from mobile

Map the CUI “data paths” and close the obvious gaps:

• Email: restrict attachments and local downloads to managed containers.
• File storage: prevent offline sync to unmanaged apps.
• Collaboration tools: restrict guest access and enforce device compliance for mobile clients.
• Browser access: require compliant device posture; consider app protection policies to control downloads.

If you allow mobile browser access to CUI web apps, make sure session controls, token lifetimes, and device compliance checks match your risk.

7) Define and run an exception process (and treat it as risk acceptance)

Mobile always produces edge cases (executives, field teams, subcontractors). Build an exception workflow:

• Business justification
• Compensating controls
• Time-bound approval
• Named owner
• Evidence of periodic review

Exceptions without an end date are a common audit failure mode.

8) Operationalize ongoing monitoring and recurring evidence collection

Turn this control into a repeatable monthly/quarterly evidence packet:

• Device compliance reports (enrolled vs noncompliant)
• Conditional access policy export or screenshots
• Sign-in logs showing blocks for noncompliant devices
• Exception register updates
• Incident tickets for lost/stolen devices and wipe actions (as applicable)

If you use Daydream to manage control ownership and evidence requests, map 03.01.18 to your mobile access policy, the identity enforcement configuration, and the recurring evidence schedule so evidence collection is predictable rather than ad hoc.

Required evidence and artifacts to retain

Keep evidence in a form an assessor can review without “trust me” meetings.

Policy and governance

• Access control policy section covering mobile access to CUI (NIST SP 800-171 Rev. 3).
• Mobile device/BYOD standard with baseline requirements.
• Exception procedure + current exception register.

Technical configuration evidence

• Conditional access policies (export or screenshots) showing device compliance enforcement for CUI apps.
• MDM/MAM policy settings: encryption, lock, wipe, jailbreak/root detection, managed app requirements.
• List of approved CUI applications and how mobile access is restricted.

Operational evidence

• Inventory of enrolled devices and assigned users.
• Identity provider sign-in logs demonstrating enforcement (blocked attempts, compliant access).
• Records of lost/stolen device handling, including wipe confirmation where applicable.

Common exam/audit questions and hangups

• “Show me how you prevent an unmanaged iPhone from accessing the CUI SharePoint/Drive/Email tenant.”
• “Which apps are considered in-scope for CUI, and how do you enforce mobile restrictions for each?”
• “Do you allow BYOD? If yes, prove that BYOD devices meet your baseline or are constrained via MAM.”
• “How do you detect and respond to rooted/jailbroken devices?”
• “Where is the device inventory, and how do you reconcile it to sign-in activity?”
• “Show exceptions and who approved them.”

Frequent implementation mistakes (and how to avoid them)

1. Policy says ‘MDM required,’ but conditional access doesn’t enforce it.
   Fix: make device compliance a hard gate for the specific apps where CUI exists.

2. Inventory exists in MDM, but unmanaged mobile sign-ins still happen.
   Fix: review identity sign-in logs for mobile user agents and block unknown device states.

3. BYOD allowed with no data-layer controls.
   Fix: implement MAM/container rules for email and files; restrict downloads and “open in” behavior.

4. Exceptions are informal (email approvals, no expiry).
   Fix: central exception register with time bounds and compensating controls.

5. Shared accounts or ambiguous device ownership.
   Fix: require per-user enrollment and tie devices to named identities.

Enforcement context and risk implications

No public enforcement cases were provided in the supplied source catalog for this requirement. Practically, the risk is straightforward: unmanaged mobile devices increase the likelihood of CUI exposure through loss/theft, insecure apps, or uncontrolled syncing to personal cloud accounts. Your assessment risk is equally direct: if you can’t prove enforcement and monitoring, assessors will treat mobile access as an uncontrolled pathway into CUI systems.

Practical execution plan (30/60/90)

Use this as an operational cadence, not a promise of elapsed time.

First 30 days (Immediate stabilization)

• Decide: block mobile access to CUI, or allow only managed devices.
• Identify CUI-bearing apps and data paths (email, file storage, collaboration, VDI).
• Export current mobile device inventory and identity sign-in logs; find unmanaged access.
• Draft/update mobile access policy language and exception workflow.

By 60 days (Enforcement in place)

• Configure conditional access rules that require compliant/enrolled devices for CUI apps.
• Deploy MDM/MAM baselines (encryption, lock, wipe, jailbreak/root detection, managed apps).
• Implement BYOD approach (MAM/container) or formally prohibit BYOD for CUI.
• Stand up reporting: compliance status, blocked attempts, exception tracking.

By 90 days (Evidence-ready operations)

• Run a full evidence cycle: collect policy, configurations, inventories, logs, and exception reviews.
• Test key scenarios: unmanaged device blocked, noncompliant device blocked, lost device wipe executed.
• Formalize ownership: who administers MDM, who administers conditional access, who reviews exceptions.
• If you use Daydream, schedule recurring evidence requests and map artifacts to 03.01.18 so audits pull from a consistent repository.

Frequently Asked Questions

Do I have to allow mobile access to CUI to meet 03.01.18?

No. You can block mobile access to CUI entirely and document that decision, then enforce it through identity and application controls. If you do allow mobile access, you must manage and restrict it per 03.01.18 (NIST SP 800-171 Rev. 3).

Does 03.01.18 cover BYOD phones?

If BYOD devices can access systems that store, process, or transmit CUI, they are in scope for mobile access control. If you can’t enforce baseline controls on BYOD, block BYOD access to CUI apps and document the restriction (NIST SP 800-171 Rev. 3).

What evidence is most persuasive to an assessor?

Exports or screenshots of conditional access rules that require device compliance, plus MDM policy configurations and sign-in logs showing blocked unmanaged devices. Pair that with an inventory and an exception register tied to named approvals (NIST SP 800-171 Rev. 3).

Is MDM required, or can we rely on policy statements and user training?

Policy and training help, but assessors typically expect technical enforcement for access control. If you permit mobile access to CUI, you need a control point that can block noncompliant devices, and MDM/MAM plus conditional access is the common pattern (NIST SP 800-171 Rev. 3).

How do we handle contractors or other third parties who want mobile access?

Treat third-party mobile access like workforce mobile access: require managed device posture and enforce it through conditional access, or deny mobile access and provide an alternate path (for example, VDI from managed endpoints). Document the access decision in the third-party access approval and your exception register (NIST SP 800-171 Rev. 3).

What if a mobile device is used only for MFA prompts and never opens CUI?

Scope it based on what the device can access and enable. If the device is only a second factor and never accesses CUI apps or stores CUI, your primary control is strong authentication and account protection; still document your rationale and confirm no CUI apps are accessible from unmanaged mobile (NIST SP 800-171 Rev. 3).