Shared/Get-BlockRulesMeta.psm1

Function Get-BlockRulesMeta {
    <#
    .SYNOPSIS
        Gets the latest Microsoft Recommended block rules, removes its allow all rules, removes the audit mode policy rule option and sets HVCI to strict
        It generates a XML file compliant with CI Policies Schema
    .INPUTS
        None. You cannot pipe objects to this function.
    .OUTPUTS
        System.String
    #>

    [CmdletBinding()]
    [OutputType([System.String])]
    param ()

    Begin {
        # Importing the $PSDefaultParameterValues to the current session, prior to everything else
        . "$ModuleRootPath\CoreExt\PSDefaultParameterValues.ps1"

        # Importing the required sub-modules
        Import-Module -FullyQualifiedName "$ModuleRootPath\Shared\Write-ColorfulText.psm1" -Force
    }

    Process {
        # Download the markdown page from GitHub containing the latest Microsoft recommended block rules
        [System.String]$MSFTRecommendedBlockRulesAsString = (Invoke-WebRequest -Uri $MSFTRecommendedBlockRulesURL -ProgressAction SilentlyContinue).Content

        # Load the Block Rules as XML into a variable after extracting them from the markdown string
        [System.Xml.XmlDocument]$BlockRulesXML = ($MSFTRecommendedBlockRulesAsString -replace "(?s).*``````xml(.*)``````.*", '$1').Trim()

        # Get the SiPolicy node
        [System.Xml.XmlElement]$SiPolicyNode = $BlockRulesXML.SiPolicy

        # Declare the namespace manager and add the default namespace with a prefix
        [System.Xml.XmlNamespaceManager]$NameSpace = New-Object -TypeName System.Xml.XmlNamespaceManager -ArgumentList $BlockRulesXML.NameTable
        $NameSpace.AddNamespace('ns', 'urn:schemas-microsoft-com:sipolicy')

        # Select the FileRuleRef nodes that have a RuleID attribute that starts with ID_ALLOW_
        [System.Object[]]$NodesToRemove = $SiPolicyNode.FileRules.SelectNodes("//ns:FileRuleRef[starts-with(@RuleID, 'ID_ALLOW_')]", $NameSpace)

        # Append the Allow nodes that have an ID attribute that starts with ID_ALLOW_ to the array
        $NodesToRemove += $SiPolicyNode.FileRules.SelectNodes("//ns:Allow[starts-with(@ID, 'ID_ALLOW_')]", $NameSpace)

        # Loop through the nodes to remove
        foreach ($Node in $NodesToRemove) {
            # Get the parent node of the node to remove
            [System.Xml.XmlElement]$ParentNode = $Node.ParentNode

            # Check if the parent node has more than one child node, if it does then only remove the child node
            if ($ParentNode.ChildNodes.Count -gt 1) {
                # Remove the node from the parent node
                $ParentNode.RemoveChild($Node) | Out-Null
            }

            # If the parent node only has one child node then replace the parent node with an empty node
            else {
                # Create a new node with the same name and namespace as the parent node
                [System.Xml.XmlElement]$NewNode = $BlockRulesXML.CreateElement($ParentNode.Name, $ParentNode.NamespaceURI)
                # Replace the parent node with the new node
                $ParentNode.ParentNode.ReplaceChild($NewNode, $ParentNode) | Out-Null

                # Check if the new node has any sibling nodes, if not then replace its parent node with an empty node
                # We do this because the built-in PowerShell cmdlets would throw errors if empty <FileRulesRef /> exists inside <ProductSigners> node
                if ($null -eq $NewNode.PreviousSibling -and $null -eq $NewNode.NextSibling) {

                    # Get the grandparent node of the new node
                    [System.Xml.XmlElement]$GrandParentNode = $NewNode.ParentNode

                    # Create a new node with the same name and namespace as the grandparent node
                    [System.Xml.XmlElement]$NewGrandNode = $BlockRulesXML.CreateElement($GrandParentNode.Name, $GrandParentNode.NamespaceURI)

                    # Replace the grandparent node with the new node
                    $GrandParentNode.ParentNode.ReplaceChild($NewGrandNode, $GrandParentNode) | Out-Null
                }
            }
        }

        # Save the modified XML content to a file - The Save method requires full file path
        $BlockRulesXML.Save("$((Get-Location).path)\Microsoft recommended block rules.xml")

        # Remove the audit mode rule option
        Set-RuleOption -FilePath '.\Microsoft recommended block rules.xml' -Option 3 -Delete

        # Set HVCI to Strict
        Set-HVCIOptions -Strict -FilePath '.\Microsoft recommended block rules.xml'
    }

    End {
        # Display the result
        Write-ColorfulText -Color MintGreen -InputText 'PolicyFile = Microsoft recommended block rules.xml'
    }
}

# Export external facing functions only, prevent internal functions from getting exported
Export-ModuleMember -Function 'Get-BlockRulesMeta'

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