[REQ-PAW-108] Configure User Rights: Modify firmware environment values for PAWs

Target Scope

  • Applicable Systems: Privileged Access Workstations (PAWs) dedicated to Tier 0 and critical administrative functions. (For Tier 2 Client Workstations and Member Servers, refer to standard baseline REQ-END-116). (For Domain Controllers, refer to REQ-DC-130).
  • Operating Systems: Windows 10 Enterprise (1809 and above) and Windows 11 Enterprise.

Implementation Details

  • Priority: Medium
  • GPO Path / Registry Location:
    • Policy Display Name: Modify firmware environment values
    • Privilege Constant: SeSystemEnvironmentPrivilege
    • GPO Path: Computer Configuration\Policies\Windows Settings\Security Settings\Local Policies\User Rights Assignment\Modify firmware environment values
    • Registry Location: Stored inside local security database under privilege SeSystemEnvironmentPrivilege set to *S-1-5-32-544 (Administrators).

Rationale

The SeSystemEnvironmentPrivilege allows a process to query and modify Non-Volatile RAM (NVRAM) firmware environment variables via Win32 APIs GetFirmwareEnvironmentVariable and SetFirmwareEnvironmentVariable. NVRAM variables govern UEFI boot sequences, Secure Boot policies, boot configuration data (BCD) handoffs, and hardware configuration flags.

1. Technical Threat Vector & Abuse Mechanics

Adversaries exploit SeSystemEnvironmentPrivilege to install firmware bootkits and undermine OS integrity: (1) Secure Boot Bypasses: Attackers alter UEFI NVRAM variables to invalidate Secure Boot validation, allowing unsigned bootloaders or malicious early-launch payloads (e.g., BlackLotus UEFI bootkit) to execute before the Windows kernel loads; (2) Hypervisor Tampering: Modifying virtualization parameters in NVRAM can weaken Virtualization-Based Security (VBS) and Credential Guard; (3) Persistence: Firmware modifications persist across complete OS re-installations and drive replacements.

2. Architectural Defense & Least Privilege Enforcement

Privileged Access Workstations (PAWs) serve as the clean-source platform for managing Tier 0 Active Directory and cloud infrastructure. Because administrative credentials exist in memory on these devices, strict isolation must be maintained at the operating system level. Restricting this user right strictly prevents lower-tier sessions, third-party software, or interactive users from interfering with administrative operations, upholding the Clean Source Principle and preventing token kidnapping or session hijacking.

This privilege must be restricted strictly to Administrators (S-1-5-32-544). Standard users, interactive accounts, and standard applications must never be permitted to modify firmware environment variables.

3. MITRE ATT&CK Mapping

  • T1542.001 - Pre-OS Boot: System Firmware
  • T1562.001 - Impair Defenses: Disable or Modify Tools
  • T1068 - Exploitation for Privilege Escalation

Legacy Impact & Compatibility

  • Operational Impact: Restricting SeSystemEnvironmentPrivilege to Administrators protects UEFI firmware integrity. Firmware update tools (e.g., OEM BIOS flashers) running under administrative credentials operate normally. Auditing is captured under Security Event ID 4672.

Implementation Steps

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

  1. Open the Group Policy Management Console (gpmc.msc).
  2. Navigate to the targeted GPO linked to Tier 0 PAW systems (e.g., GPO_Hardening_PAW).
  3. In the console tree, browse to: Computer Configuration\Policies\Windows Settings\Security Settings\Local Policies\User Rights Assignment
  4. Open the policy Modify firmware environment values.
  5. Select the Define these policy settings check box.
  6. Configure the security principal allocation to: *S-1-5-32-544 (Administrators).
  7. Click Apply and OK.
  8. Apply and verify policy enforcement across target hosts using gpupdate /force and inspect with secedit /export /cfg C:\Windows\Temp\sec_audit.cfg.

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

Download Script: Configure-PawUraSeSystemEnvironmentPrivilege.ps1

# Configure-PawUraSeSystemEnvironmentPrivilege.ps1
# Configure-PawUraSeSystemEnvironmentPrivilege.ps1
$SecTempDir = Join-Path $env:TEMP "SecurityTemplates"
if (-not (Test-Path $SecTempDir)) { New-Item -Path $SecTempDir -ItemType Directory -Force | Out-Null }
$CfgFile = Join-Path $SecTempDir "ura_sesystemenvironmentprivilege.cfg"
$DbFile = Join-Path $SecTempDir "secedit_sesystemenvironmentprivilege.sdb"
$LogFile = Join-Path $SecTempDir "secedit.log"

$Process = Start-Process secedit -ArgumentList "/export /cfg `"$CfgFile`"" -Wait -NoNewWindow -PassThru
if ($Process.ExitCode -ne 0) { Throw "Failed to export security template" }

$ConfigText = Get-Content -Path $CfgFile -Raw
if ($ConfigText -notmatch "\[Privilege Rights\]") {
    $ConfigText += "`r`n[Privilege Rights]`r`n"
}

$Lines = $ConfigText -split "`r?`n"
$NewLines = @()
$InPriv = $false
$KeyAdded = $false

foreach ($Line in $Lines) {
    if ($Line -match "^\[(.*)\]$") {
        if ($Matches[1] -eq "Privilege Rights") {
            $InPriv = $true
        } else {
            $InPriv = $false
        }
    }
    if ($InPriv -and $Line -match "^\s*SeSystemEnvironmentPrivilege\s*=") {
        $NewLines += "SeSystemEnvironmentPrivilege = *S-1-5-32-544"
        $KeyAdded = $true
    } else {
        $NewLines += $Line
    }
}

if (-not $KeyAdded) {
    # Find Privilege Rights section index and insert it right after
    $Idx = $NewLines.IndexOf("[Privilege Rights]")
    if ($Idx -ge 0) {
        $NewLines.Insert($Idx + 1, "SeSystemEnvironmentPrivilege = *S-1-5-32-544")
    } else {
        $NewLines += "SeSystemEnvironmentPrivilege = *S-1-5-32-544"
    }
}

$NewLines | Set-Content -Path $CfgFile -Force
$Proc = Start-Process secedit -ArgumentList "/configure /db `"$DbFile`" /cfg `"$CfgFile`" /areas USER_RIGHTS /log `"$LogFile`"" -Wait -NoNewWindow -PassThru
if ($Proc.ExitCode -ne 0) { Throw "Failed to configure secedit user rights" }
Remove-Item -Path $CfgFile, $DbFile -ErrorAction SilentlyContinue

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

# Get-PawUraSeSystemEnvironmentPrivilegeStatus.ps1
# Get-PawUraSeSystemEnvironmentPrivilegeStatus.ps1
$SecTempDir = Join-Path $env:TEMP "SecurityTemplates"
if (-not (Test-Path $SecTempDir)) { New-Item -Path $SecTempDir -ItemType Directory -Force | Out-Null }
$CfgFile = Join-Path $SecTempDir "ura_audit_sesystemenvironmentprivilege.cfg"

$Process = Start-Process secedit -ArgumentList "/export /cfg `"$CfgFile`"" -Wait -NoNewWindow -PassThru
if ($Process.ExitCode -ne 0) {
    Write-Output "Non-Compliant"
    exit 1
}

$ConfigText = Get-Content -Path $CfgFile -Raw
Remove-Item -Path $CfgFile -ErrorAction SilentlyContinue

$Match = $ConfigText -match "(?mi)^\s*SeSystemEnvironmentPrivilege\s*=\s*(.*)$"
$CurrentValue = ""
if ($Match) {
    $CurrentValue = $Matches[1].Trim()
}

$Expected = "*S-1-5-32-544"
if ($CurrentValue -eq $Expected) {
    Write-Output "Compliant"
    exit 0
} else {
    Write-Output "Non-Compliant"
    exit 1
}

Sources & Compliance References

  • ANSSI Active Directory Hardening Guide: ANSSI Active Directory Hardening Guide: R28 (User Rights Assignment)
  • CIS Benchmark: 2.2.39 (L1) Ensure 'Modify firmware environment values' is set to 'Administrators'
  • Microsoft Security Baseline: User Rights Configuration specifications
  • Microsoft Learn: User Rights Assignment Reference

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