[REQ-PAW-078] ASR: Block all Office applications from creating child processes for PAWs

Target Scope

• Applicable Systems: Privileged Access Workstations (PAWs) used for Tier 0 directory administration.
• Operating Systems: Windows 10 Enterprise (1607+) and Windows 11 Enterprise.

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
      • d4f940ab-401b-4efc-aadc-ad5f3c50688a = 1 (REG_SZ)

Rationale

Blocks Microsoft Office applications (Word, Excel, PowerPoint) from creating child processes. This prevents malicious files containing embedded VBA macros or exploiting unpatched vulnerabilities (such as CVE-2021-40444) from launching scripting environments or system commands to download and execute code.

Legacy Impact & Compatibility

Office macros that legitimately launch external scripts, shell scripts, or third-party executable helpers will be blocked. Business processes relying on advanced inter-process macros must be rewritten to use modern API alternatives.

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 d4f940ab-401b-4efc-aadc-ad5f3c50688a as Value Name, with Value set to 1 (Block).

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

Download Script: Configure-PawAsrOfficeChild.ps1

# Configure-PawAsrOfficeChild.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 "d4f940ab-401b-4efc-aadc-ad5f3c50688a" -Value "1" -Type String -Force

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

# Get-PawAsrOfficeChildStatus.ps1
$AsrRulesPath = "HKLM:\SOFTWARE\Policies\Microsoft\Windows Defender\Windows Defender Exploit Guard\ASR\Rules"
$Value = Get-ItemProperty -Path $AsrRulesPath -Name "d4f940ab-401b-4efc-aadc-ad5f3c50688a" -ErrorAction SilentlyContinue
if ($Value -and ($Value."d4f940ab-401b-4efc-aadc-ad5f3c50688a" -eq "1" -or $Value."d4f940ab-401b-4efc-aadc-ad5f3c50688a" -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 Privileged Access Workstations