3.13.4 — No Data Leaks Through Shared Resources

The One-Liner

If one user’s CUI can leak to another through shared temp files or clipboard on the same system, you have a covert channel.

What It Says

Prevent unauthorized and unintended information transfer via shared system resources.

What It Actually Means

On multi-user systems (shared servers, VDI, terminal servers), data can leak between sessions through:

• Temp directories — if all users share the same temp folder, one user’s cached CUI documents are visible to another
• Clipboard — in VDI environments, clipboard sharing between sessions can move CUI outside the controlled environment
• Shared memory — processes running in shared memory space can potentially access each other’s data
• Page/swap files — CUI data written to disk by the OS as part of memory management

For most modern single-user workstations, the OS handles process isolation well. The risk is concentrated in shared environments: VDI, terminal servers, multi-user applications, and shared databases.

The assessor will focus on your shared environments. If you don’t have any multi-user systems, document that in your SSP and this may be straightforward to satisfy.

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Pass or Fail

Your assessor needs a “yes” to every row:

#	Question	What “yes” looks like
1	Is unauthorized information transfer via shared system resources prevented?	Process isolation, per-user temp dirs, clipboard restrictions in place
2	Are shared system resources configured to prevent unintended data exposure?	VDI clipboard policies, temp directory permissions, swap file encryption

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What to Have Ready on Assessment Day

Documents they’ll review: System and communications protection policy; system security plan; system design documentation; system configuration settings showing shared resource protections

People they’ll talk to: System or network administrators; personnel with information security responsibilities; system developers

Live demos they’ll ask for: Mechanisms preventing unauthorized information transfer through shared resources

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The Assessor’s Playbook

These are the actual questions. Have answers ready.

• “Do you have any multi-user systems — VDI, terminal servers, shared workstations?”
• “How are temp directories configured on shared systems? Per-user or shared?”
• “Is clipboard sharing enabled in your VDI environment?”
• “How do you prevent data leakage through page/swap files?”
• “Show me the temp directory permissions on a shared server.”

Real-World Example

A contractor uses Azure Virtual Desktop for CUI work. The AVD session hosts are configured with: per-user temp directories (each user’s temp folder is unique and inaccessible to others), clipboard redirection disabled between the AVD session and the local endpoint, drive redirection disabled (users can’t map local drives into the session), and BitLocker encrypting swap files on the hosts. The assessor checks the AVD host configuration and confirms clipboard copy-paste from the CUI session to the local machine is blocked.

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Where Companies Trip Up

Shared temp directories on servers. All users writing to C:\Temp. Configure per-user temp directories via Group Policy.

VDI clipboard sharing enabled. Users can copy CUI text from the VDI session and paste it on their personal device. Disable clipboard redirection for CUI sessions.

Drive mapping in VDI. Users can map their personal USB drive into the VDI session and copy CUI out. Disable drive redirection.

No swap file encryption. CUI data written to swap/page file in plain text. Enable BitLocker to encrypt the entire volume including swap.

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How to Talk About This

To a CEO or Board

Our shared systems prevent data from leaking between users through temp files, clipboard, or shared resources.

To an Engineering Team

VDI: clipboard redirect disabled, drive redirect disabled, per-user temp dirs. Shared servers: per-user temp via GPO. BitLocker encrypts swap files. Process isolation via modern OS defaults. Documented in SSP.

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Connected Requirements

Requirement	Why it matters here
3.1.3 — Where CUI Can Flow	Shared resources are a CUI flow path that must be controlled
3.13.16 — Encrypt CUI at Rest	Encryption protects CUI in swap/page files

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Implementation

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Step-by-step guides for Microsoft 365, AWS, Azure, and GCP are available to Ancitus clients.

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CMMC Practice ID: SC.L2-3.13.4 | SPRS Weight: 1 point | POA&M Eligible: Yes