Vulnerability Discussion

Windows 10 allows Windows Update to obtain updates from additional sources instead of Microsoft. In addition to Microsoft, updates can be obtained from and sent to PCs on the local network as well as on the internet. This is part of the Windows Update trusted process; however, to minimize outside exposure, obtaining updates from or sending to systems on the internet must be prevented.

Check

If the following registry value does not exist or is not configured as specified, this is a finding.

Registry Hive: HKEY_LOCAL_MACHINE
Registry Path: \SOFTWARE\Policies\Microsoft\Windows\DeliveryOptimization\

Value Name: DODownloadMode

Value Type: REG_DWORD
Value: 0x00000000 (0) - No peering (HTTP Only)
0x00000001 (1) - Peers on same NAT only (LAN)
0x00000002 (2) - Local Network / Private group peering (Group)
0x00000063 (99) - Simple download mode, no peering (Simple)
0x00000064 (100) - Bypass mode, Delivery Optimization not used (Bypass)

A value of 0x00000003 (3), Internet, is a finding.

v1507 LTSB:
Domain joined systems:
Verify the registry value above.
If the value is not 0x00000000 (0) or 0x00000001 (1), this is a finding.

Standalone or nondomain-joined systems (configured in Settings):
If the following registry value does not exist or is not configured as specified, this is a finding:

Registry Hive: HKEY_LOCAL_MACHINE
Registry Path: \SOFTWARE\Microsoft\Windows\CurrentVersion\DeliveryOptimization\Config\

Value Name: DODownloadMode

Value Type: REG_DWORD
Value: 0x00000000 (0) - Off
0x00000001 (1) - LAN

Fix

Configure the policy value for Computer Configuration >> Administrative Templates >> Windows Components >> Delivery Optimization >> "Download Mode" to "Enabled" with any option except "Internet" selected.

Acceptable selections include:
Bypass (100)
Group (2)
HTTP only (0)
LAN (1)
Simple (99)

v1507 (LTSB) does not include this group policy setting locally. For domain-joined systems, configure through domain group policy as "HTTP only (0)" or "Lan (1)".

For standalone or nondomain-joined systems, configure using Settings >> Update & Security >> Windows Update >> Advanced Options >> "Choose how updates are delivered" with either "Off" or "PCs on my local network" selected.