[REQ-DC-103] ASR: Use advanced protection against ransomware on Domain Controllers

Target Scope

• Applicable Systems: Domain Controllers.
• Operating Systems: 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
      • c1db55ab-c21a-4637-bb3f-a12568109d35 = 1 (REG_SZ)

Rationale

Enables advanced behavioral heuristics and cloud analytics checks on files that attempt to modify multiple user files, detect signature-less encryption behavior, and block rapid write activity to prevent ransomware from encrypting system and user documents.

Legacy Impact & Compatibility

Minor performance overhead when scanning active files during large batch edits or heavy database updates.

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 c1db55ab-c21a-4637-bb3f-a12568109d35 as Value Name, with Value set to 1 (Block).

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

Download Script: Configure-DcAsrRansomware.ps1

# Configure-DcAsrRansomware.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 "c1db55ab-c21a-4637-bb3f-a12568109d35" -Value "1" -Type String -Force

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

# Get-DcAsrRansomwareStatus.ps1
$AsrRulesPath = "HKLM:\SOFTWARE\Policies\Microsoft\Windows Defender\Windows Defender Exploit Guard\ASR\Rules"
$Value = Get-ItemProperty -Path $AsrRulesPath -Name "c1db55ab-c21a-4637-bb3f-a12568109d35" -ErrorAction SilentlyContinue
if ($Value -and ($Value."c1db55ab-c21a-4637-bb3f-a12568109d35" -eq "1" -or $Value."c1db55ab-c21a-4637-bb3f-a12568109d35" -eq 1)) {
    Write-Output "Compliant"
    exit 0
} else {
    Write-Output "Non-Compliant"
    exit 1
}

Sources & Compliance References

• CIS Microsoft Windows Server Benchmark: Section 18.9 (ASR rules config)
• ANSSI Active Directory Hardening Guide: Protective controls baselines on Domain Controllers