[REQ-END-090] ASR: Block Office communication application from creating child processes

Target Scope

• Applicable Systems: Tier 2 client workstations and member servers.
• Operating Systems: Windows 10 (and above) Enterprise/Professional, Windows Server 2016 (and above).

Implementation Details

• Priority: High
• GPO Path / Registry Location:
  • GPO Path: Computer Configuration\Administrative Templates\Windows Components\Windows Defender Antivirus\Windows Defender Exploit Guard\Attack Surface Reduction
  • Registry Location:
    • HKLM\SOFTWARE\Policies\Microsoft\Windows Defender\Windows Defender Exploit Guard\ASR\Rules
      • 26190899-1602-49e8-8b27-eb1d0a1ce869 = 1 (REG_SZ)

Rationale

Blocks Microsoft Outlook or other Office communication applications (e.g., Teams, Skype) from creating child processes. This prevents malware payloads delivered through emails, chats, or calendar invites from spawning command-line utilities or scripting environments.

Legacy Impact & Compatibility

Outlook integrations or custom add-ins that legitimately spawn helper utilities, such as starting external browsers or custom document handlers directly, may be blocked.

Implementation Steps

Option A: Group Policy Object (GPO) Configuration (Preferred)

1. Navigate to: Computer Configuration\Administrative Templates\Windows Components\Windows Defender Antivirus\Windows Defender Exploit Guard\Attack Surface Reduction
2. Set 'Configure Attack Surface Reduction rules' to 'Enabled'
3. Click 'Show...' and add the rule GUID 26190899-1602-49e8-8b27-eb1d0a1ce869 as Value Name, with Value set to 1 (Block).

Option B: PowerShell & Registry Configuration (Remediation / Non-GPO)

Download Script: Configure-AsrOutlookChild.ps1

# Configure-AsrOutlookChild.ps1
$AsrRulesPath = "HKLM:\SOFTWARE\Policies\Microsoft\Windows Defender\Windows Defender Exploit Guard\ASR\Rules"
if (-not (Test-Path $AsrRulesPath)) { New-Item -Path $AsrRulesPath -Force | Out-Null }
Set-ItemProperty -Path $AsrRulesPath -Name "26190899-1602-49e8-8b27-eb1d0a1ce869" -Value "1" -Type String -Force

To audit the hardening status: Download Script: Get-AsrOutlookChildStatus.ps1

# Get-AsrOutlookChildStatus.ps1
$AsrRulesPath = "HKLM:\SOFTWARE\Policies\Microsoft\Windows Defender\Windows Defender Exploit Guard\ASR\Rules"
$Value = Get-ItemProperty -Path $AsrRulesPath -Name "26190899-1602-49e8-8b27-eb1d0a1ce869" -ErrorAction SilentlyContinue
if ($Value -and ($Value."26190899-1602-49e8-8b27-eb1d0a1ce869" -eq "1" -or $Value."26190899-1602-49e8-8b27-eb1d0a1ce869" -eq 1)) {
    Write-Output "Compliant"
    exit 0
} else {
    Write-Output "Non-Compliant"
    exit 1
}

Sources & Compliance References

• CIS Microsoft Windows 10 Benchmark: Section 18.9 (ASR rules config)
• ANSSI Active Directory Hardening Guide: Protective controls baselines on client workstations