Get-Doppelgangers.ps1

function Get-Doppelgangers
{
    <# 
    .SYNOPSIS 
    
    Detects use of NTFS transactions for stealth/evasion, aka 'Process Doppelganging'
     
    Author: Joe Desimone (@dez_) 
    License: BSD 3-Clause 
    
    .DESCRIPTION 
    
    Scans all processes for memory sections that are MEM_IMAGE but return ERROR_TRANSACTION_NOT_ACTIVE
    when querying the associated section file name object. This is typical for sections that are
    associated with rolled back transactions.
    
    .EXAMPLE 
    
    Get-Doppelgangers -verbose
    #>

    [CmdletBinding()]
    Param
    (
        [Parameter(Position = 0)]
        [ValidateNotNullOrEmpty()]
        [String]
        $OutPath = $PWD.Path
    )
    
    $Mod = New-InMemoryModule -ModuleName Win32 

    $MemoryBasicInformation = struct $Mod Mem.MEMORY_BASIC_INFORMATION @{
        BaseAddress = field 0 IntPtr
        AllocationBase = field 1 IntPtr
        AllocationProtect = field 2 UInt32
        RegionSize = field 3 IntPtr
        State = field 4 UInt32
        Protect = field 5 UInt32
        Type = field 6 UInt32
    }

    $FunctionDefinitions = @( 
      (func kernel32 OpenProcess ([UInt32]) @([UInt32], [UInt32], [UInt32]) -SetLastError), 
      (func kernel32 CloseHandle ([UInt32]) @([UInt32]) -SetLastError), 
      (func kernel32 VirtualQueryEx ([IntPtr]) @([IntPtr], [IntPtr], $MemoryBasicInformation.MakeByRefType(), [IntPtr]) -SetLastError),
      (func kernel32 ReadProcessMemory ([UInt32]) @([UInt32], [IntPtr], [Byte[]], [IntPtr], [IntPtr].MakeByRefType()) -SetLastError),
      (func kernel32 K32GetMappedFileName ([UInt32]) @([UInt32], [IntPtr], [Byte[]], [UInt32]) -SetLastError)
    ) 
     
    $Types = $FunctionDefinitions | Add-Win32Type -Module $Mod -Namespace 'Win32' 
    $Kernel32 = $Types['kernel32'] 

    $processList = Get-Process
    foreach ($process in $processList)
    {
        $id = $process | select id
        $processId = $id.id
        $processHandle = $Kernel32::OpenProcess(0x0400 -bxor 0x0010, 0, $processId)
        if ($processHandle -ne 0)
        {
            #"PID: $processId, Handle: $processHandle"
            $mbi = [Activator]::CreateInstance($MemoryBasicInformation)
            
            while(1)
            {
                $retSz = 0
                $val = [System.IntPtr]::Size
                
                $pAddr = [Int64]$mbi.BaseAddress + $mbi.RegionSize
                $pAddr = [IntPtr]$pAddr

                $retSz = $Kernel32::VirtualQueryEx($processHandle, $pAddr, [ref]$mbi, $MemoryBasicInformation::GetSize())
                if ($retSz -eq 0)
                {
                    break
                }

                if($mbi.Type -eq 0x01000000 -and $mbi.State -eq 0x1000)
                {
                    $fileBuf = New-Object byte[](1024)
                    $retSz = $Kernel32::K32GetMappedFileName($processHandle, $mbi.BaseAddress, $fileBuf, $fileBuf.Length);$LastError = [ComponentModel.Win32Exception][Runtime.InteropServices.Marshal]::GetLastWin32Error()
                    $fileName = [System.Text.Encoding]::UNICODE.GetString($fileBuf)
                                       
                    if($retSz -eq 0 -and $LastError.NativeErrorCode -eq 6701)
                    {
                        Write-Verbose ("Hit: {0} [pid {1:d}], 0x{2:x} - 0x{3:x} (size: 0x{4:x})" -f $process.ProcessName, 
                                $processId, [Int64]$mbi.BaseAddress, ([Int64]$mbi.BaseAddress+$mbi.RegionSize), [Int64]$mbi.RegionSize)
                                
                        [IntPtr]$szRead = 0
                        $buf = New-Object byte[]($mbi.RegionSize)
                        $bRet = $Kernel32::ReadProcessMemory($processHandle, $mbi.BaseAddress, $buf, $buf.Length, [ref]$szRead)
                        if($bRet -eq 1)
                        {       
                            Save-Data $process.ProcessName $buf $mbi.AllocationBase $OutPath
                        }
                    }
                    
                }
            }

            $bRet = $Kernel32::CloseHandle($processHandle)
        }
    }
    Write-Verbose ("Scanned {0:d} processes" -f $processList.Length)
}

function Save-Data
{
    Param
    (
        [Parameter(Position = 0, Mandatory = $True)]
        [String]
        $ProcessName,

        [Parameter(Position = 1, Mandatory = $True)]
        [Byte[]]
        $buffer,
        
        [Parameter(Position = 2, Mandatory = $True)]
        [IntPtr]
        $base,
        
        [Parameter(Position = 3, Mandatory = $True)]
        [String]
        $Path
        
    )

    New-Item -ItemType directory -force -Path $Path
    $file_path = "{0}\{1}_{2:x}" -f $Path, $ProcessName, [Int64]$base
    if (Test-Path $file_path)
    {
        # =(
        $existing = [io.file]::ReadAllBytes($file_path)
        $all = New-Object byte[]($buffer.length + $existing.length)
        $all = $existing + $buffer
        [io.file]::WriteAllBytes($file_path,$all)
    }
    else
    {
        [io.file]::WriteAllBytes($file_path,$buffer)
    }
    
    Get-Item $file_path
}



#region PSReflect
function New-InMemoryModule
{
<# 
.SYNOPSIS 
 
Creates an in-memory assembly and module 
 
Author: Matthew Graeber (@mattifestation) 
License: BSD 3-Clause 
Required Dependencies: None 
Optional Dependencies: None 
  
.DESCRIPTION 
 
When defining custom enums, structs, and unmanaged functions, it is 
necessary to associate to an assembly module. This helper function 
creates an in-memory module that can be passed to the 'enum', 
'struct', and Add-Win32Type functions. 
 
.PARAMETER ModuleName 
 
Specifies the desired name for the in-memory assembly and module. If 
ModuleName is not provided, it will default to a GUID. 
 
.EXAMPLE 
 
$Module = New-InMemoryModule -ModuleName Win32 
#>

    Param
    (
        [Parameter(Position = 0)]
        [ValidateNotNullOrEmpty()]
        [String]
        $ModuleName = [Guid]::NewGuid().ToString()
    )

    $AppDomain = [Reflection.Assembly].Assembly.GetType('System.AppDomain').GetProperty('CurrentDomain').GetValue($null, @())
    $LoadedAssemblies = $AppDomain.GetAssemblies()

    foreach ($Assembly in $LoadedAssemblies) {
        if ($Assembly.FullName -and ($Assembly.FullName.Split(',')[0] -eq $ModuleName)) {
            return $Assembly
        }
    }

    $DynAssembly = New-Object Reflection.AssemblyName($ModuleName)
    $Domain = $AppDomain
    $AssemblyBuilder = $Domain.DefineDynamicAssembly($DynAssembly, 'Run')
    $ModuleBuilder = $AssemblyBuilder.DefineDynamicModule($ModuleName, $False)

    return $ModuleBuilder
}

# A helper function used to reduce typing while defining function
# prototypes for Add-Win32Type.
function func
{
    Param
    (
        [Parameter(Position = 0, Mandatory = $True)]
        [String]
        $DllName,

        [Parameter(Position = 1, Mandatory = $True)]
        [string]
        $FunctionName,

        [Parameter(Position = 2, Mandatory = $True)]
        [Type]
        $ReturnType,

        [Parameter(Position = 3)]
        [Type[]]
        $ParameterTypes,

        [Parameter(Position = 4)]
        [Runtime.InteropServices.CallingConvention]
        $NativeCallingConvention,

        [Parameter(Position = 5)]
        [Runtime.InteropServices.CharSet]
        $Charset,

        [String]
        $EntryPoint,

        [Switch]
        $SetLastError
    )

    $Properties = @{
        DllName = $DllName
        FunctionName = $FunctionName
        ReturnType = $ReturnType
    }

    if ($ParameterTypes) { $Properties['ParameterTypes'] = $ParameterTypes }
    if ($NativeCallingConvention) { $Properties['NativeCallingConvention'] = $NativeCallingConvention }
    if ($Charset) { $Properties['Charset'] = $Charset }
    if ($SetLastError) { $Properties['SetLastError'] = $SetLastError }
    if ($EntryPoint) { $Properties['EntryPoint'] = $EntryPoint }

    New-Object PSObject -Property $Properties
}

function Add-Win32Type
{
<# 
.SYNOPSIS 
 
Creates a .NET type for an unmanaged Win32 function. 
 
Author: Matthew Graeber (@mattifestation) 
License: BSD 3-Clause 
Required Dependencies: None 
Optional Dependencies: func 
  
.DESCRIPTION 
 
Add-Win32Type enables you to easily interact with unmanaged (i.e. 
Win32 unmanaged) functions in PowerShell. After providing 
Add-Win32Type with a function signature, a .NET type is created 
using reflection (i.e. csc.exe is never called like with Add-Type). 
 
The 'func' helper function can be used to reduce typing when defining 
multiple function definitions. 
 
.PARAMETER DllName 
 
The name of the DLL. 
 
.PARAMETER FunctionName 
 
The name of the target function. 
 
.PARAMETER EntryPoint 
 
The DLL export function name. This argument should be specified if the 
specified function name is different than the name of the exported 
function. 
 
.PARAMETER ReturnType 
 
The return type of the function. 
 
.PARAMETER ParameterTypes 
 
The function parameters. 
 
.PARAMETER NativeCallingConvention 
 
Specifies the native calling convention of the function. Defaults to 
stdcall. 
 
.PARAMETER Charset 
 
If you need to explicitly call an 'A' or 'W' Win32 function, you can 
specify the character set. 
 
.PARAMETER SetLastError 
 
Indicates whether the callee calls the SetLastError Win32 API 
function before returning from the attributed method. 
 
.PARAMETER Module 
 
The in-memory module that will host the functions. Use 
New-InMemoryModule to define an in-memory module. 
 
.PARAMETER Namespace 
 
An optional namespace to prepend to the type. Add-Win32Type defaults 
to a namespace consisting only of the name of the DLL. 
 
.EXAMPLE 
 
$Mod = New-InMemoryModule -ModuleName Win32 
 
$FunctionDefinitions = @( 
  (func kernel32 GetProcAddress ([IntPtr]) @([IntPtr], [String]) -Charset Ansi -SetLastError), 
  (func kernel32 GetModuleHandle ([Intptr]) @([String]) -SetLastError), 
  (func ntdll RtlGetCurrentPeb ([IntPtr]) @()) 
) 
 
$Types = $FunctionDefinitions | Add-Win32Type -Module $Mod -Namespace 'Win32' 
$Kernel32 = $Types['kernel32'] 
$Ntdll = $Types['ntdll'] 
$Ntdll::RtlGetCurrentPeb() 
$ntdllbase = $Kernel32::GetModuleHandle('ntdll') 
$Kernel32::GetProcAddress($ntdllbase, 'RtlGetCurrentPeb') 
 
.NOTES 
 
Inspired by Lee Holmes' Invoke-WindowsApi http://poshcode.org/2189 
 
When defining multiple function prototypes, it is ideal to provide 
Add-Win32Type with an array of function signatures. That way, they 
are all incorporated into the same in-memory module. 
#>

    [OutputType([Hashtable])]
    Param(
        [Parameter(Mandatory = $True, ValueFromPipelineByPropertyName = $True)]
        [String]
        $DllName,

        [Parameter(Mandatory = $True, ValueFromPipelineByPropertyName = $True)]
        [String]
        $FunctionName,

        [Parameter(ValueFromPipelineByPropertyName = $True)]
        [String]
        $EntryPoint,

        [Parameter(Mandatory = $True, ValueFromPipelineByPropertyName = $True)]
        [Type]
        $ReturnType,

        [Parameter(ValueFromPipelineByPropertyName = $True)]
        [Type[]]
        $ParameterTypes,

        [Parameter(ValueFromPipelineByPropertyName = $True)]
        [Runtime.InteropServices.CallingConvention]
        $NativeCallingConvention = [Runtime.InteropServices.CallingConvention]::StdCall,

        [Parameter(ValueFromPipelineByPropertyName = $True)]
        [Runtime.InteropServices.CharSet]
        $Charset = [Runtime.InteropServices.CharSet]::Auto,

        [Parameter(ValueFromPipelineByPropertyName = $True)]
        [Switch]
        $SetLastError,

        [Parameter(Mandatory = $True)]
        [ValidateScript({($_ -is [Reflection.Emit.ModuleBuilder]) -or ($_ -is [Reflection.Assembly])})]
        $Module,

        [ValidateNotNull()]
        [String]
        $Namespace = ''
    )

    BEGIN
    {
        $TypeHash = @{}
    }

    PROCESS
    {
        if ($Module -is [Reflection.Assembly])
        {
            if ($Namespace)
            {
                $TypeHash[$DllName] = $Module.GetType("$Namespace.$DllName")
            }
            else
            {
                $TypeHash[$DllName] = $Module.GetType($DllName)
            }
        }
        else
        {
            # Define one type for each DLL
            if (!$TypeHash.ContainsKey($DllName))
            {
                if ($Namespace)
                {
                    $TypeHash[$DllName] = $Module.DefineType("$Namespace.$DllName", 'Public,BeforeFieldInit')
                }
                else
                {
                    $TypeHash[$DllName] = $Module.DefineType($DllName, 'Public,BeforeFieldInit')
                }
            }

            $Method = $TypeHash[$DllName].DefineMethod(
                $FunctionName,
                'Public,Static,PinvokeImpl',
                $ReturnType,
                $ParameterTypes)

            # Make each ByRef parameter an Out parameter
            $i = 1
            foreach($Parameter in $ParameterTypes)
            {
                if ($Parameter.IsByRef)
                {
                    [void] $Method.DefineParameter($i, 'Out', $null)
                }

                $i++
            }

            $DllImport = [Runtime.InteropServices.DllImportAttribute]
            $SetLastErrorField = $DllImport.GetField('SetLastError')
            $CallingConventionField = $DllImport.GetField('CallingConvention')
            $CharsetField = $DllImport.GetField('CharSet')
            $EntryPointField = $DllImport.GetField('EntryPoint')
            if ($SetLastError) { $SLEValue = $True } else { $SLEValue = $False }

            if ($PSBoundParameters['EntryPoint']) { $ExportedFuncName = $EntryPoint } else { $ExportedFuncName = $FunctionName }

            # Equivalent to C# version of [DllImport(DllName)]
            $Constructor = [Runtime.InteropServices.DllImportAttribute].GetConstructor([String])
            $DllImportAttribute = New-Object Reflection.Emit.CustomAttributeBuilder($Constructor,
                $DllName, [Reflection.PropertyInfo[]] @(), [Object[]] @(),
                [Reflection.FieldInfo[]] @($SetLastErrorField,
                                           $CallingConventionField,
                                           $CharsetField,
                                           $EntryPointField),
                [Object[]] @($SLEValue,
                             ([Runtime.InteropServices.CallingConvention] $NativeCallingConvention),
                             ([Runtime.InteropServices.CharSet] $Charset),
                             $ExportedFuncName))

            $Method.SetCustomAttribute($DllImportAttribute)
        }
    }

    END
    {
        if ($Module -is [Reflection.Assembly])
        {
            return $TypeHash
        }

        $ReturnTypes = @{}

        foreach ($Key in $TypeHash.Keys)
        {
            $Type = $TypeHash[$Key].CreateType()
            
            $ReturnTypes[$Key] = $Type
        }

        return $ReturnTypes
    }
}

function psenum
{
<# 
.SYNOPSIS 
 
Creates an in-memory enumeration for use in your PowerShell session. 
 
Author: Matthew Graeber (@mattifestation) 
License: BSD 3-Clause 
Required Dependencies: None 
Optional Dependencies: None 
  
.DESCRIPTION 
 
The 'psenum' function facilitates the creation of enums entirely in 
memory using as close to a "C style" as PowerShell will allow. 
 
.PARAMETER Module 
 
The in-memory module that will host the enum. Use 
New-InMemoryModule to define an in-memory module. 
 
.PARAMETER FullName 
 
The fully-qualified name of the enum. 
 
.PARAMETER Type 
 
The type of each enum element. 
 
.PARAMETER EnumElements 
 
A hashtable of enum elements. 
 
.PARAMETER Bitfield 
 
Specifies that the enum should be treated as a bitfield. 
 
.EXAMPLE 
 
$Mod = New-InMemoryModule -ModuleName Win32 
 
$ImageSubsystem = psenum $Mod PE.IMAGE_SUBSYSTEM UInt16 @{ 
    UNKNOWN = 0 
    NATIVE = 1 # Image doesn't require a subsystem. 
    WINDOWS_GUI = 2 # Image runs in the Windows GUI subsystem. 
    WINDOWS_CUI = 3 # Image runs in the Windows character subsystem. 
    OS2_CUI = 5 # Image runs in the OS/2 character subsystem. 
    POSIX_CUI = 7 # Image runs in the Posix character subsystem. 
    NATIVE_WINDOWS = 8 # Image is a native Win9x driver. 
    WINDOWS_CE_GUI = 9 # Image runs in the Windows CE subsystem. 
    EFI_APPLICATION = 10 
    EFI_BOOT_SERVICE_DRIVER = 11 
    EFI_RUNTIME_DRIVER = 12 
    EFI_ROM = 13 
    XBOX = 14 
    WINDOWS_BOOT_APPLICATION = 16 
} 
 
.NOTES 
 
PowerShell purists may disagree with the naming of this function but 
again, this was developed in such a way so as to emulate a "C style" 
definition as closely as possible. Sorry, I'm not going to name it 
New-Enum. :P 
#>

    [OutputType([Type])]
    Param
    (
        [Parameter(Position = 0, Mandatory = $True)]
        [ValidateScript({($_ -is [Reflection.Emit.ModuleBuilder]) -or ($_ -is [Reflection.Assembly])})]
        $Module,

        [Parameter(Position = 1, Mandatory = $True)]
        [ValidateNotNullOrEmpty()]
        [String]
        $FullName,

        [Parameter(Position = 2, Mandatory = $True)]
        [Type]
        $Type,

        [Parameter(Position = 3, Mandatory = $True)]
        [ValidateNotNullOrEmpty()]
        [Hashtable]
        $EnumElements,

        [Switch]
        $Bitfield
    )

    if ($Module -is [Reflection.Assembly])
    {
        return ($Module.GetType($FullName))
    }

    $EnumType = $Type -as [Type]

    $EnumBuilder = $Module.DefineEnum($FullName, 'Public', $EnumType)

    if ($Bitfield)
    {
        $FlagsConstructor = [FlagsAttribute].GetConstructor(@())
        $FlagsCustomAttribute = New-Object Reflection.Emit.CustomAttributeBuilder($FlagsConstructor, @())
        $EnumBuilder.SetCustomAttribute($FlagsCustomAttribute)
    }

    foreach ($Key in $EnumElements.Keys)
    {
        # Apply the specified enum type to each element
        $null = $EnumBuilder.DefineLiteral($Key, $EnumElements[$Key] -as $EnumType)
    }

    $EnumBuilder.CreateType()
}

# A helper function used to reduce typing while defining struct
# fields.
function field
{
    Param
    (
        [Parameter(Position = 0, Mandatory = $True)]
        [UInt16]
        $Position,
        
        [Parameter(Position = 1, Mandatory = $True)]
        [Type]
        $Type,
        
        [Parameter(Position = 2)]
        [UInt16]
        $Offset,
        
        [Object[]]
        $MarshalAs
    )

    @{
        Position = $Position
        Type = $Type -as [Type]
        Offset = $Offset
        MarshalAs = $MarshalAs
    }
}

function struct
{
<# 
.SYNOPSIS 
 
Creates an in-memory struct for use in your PowerShell session. 
 
Author: Matthew Graeber (@mattifestation) 
License: BSD 3-Clause 
Required Dependencies: None 
Optional Dependencies: field 
  
.DESCRIPTION 
 
The 'struct' function facilitates the creation of structs entirely in 
memory using as close to a "C style" as PowerShell will allow. Struct 
fields are specified using a hashtable where each field of the struct 
is comprosed of the order in which it should be defined, its .NET 
type, and optionally, its offset and special marshaling attributes. 
 
One of the features of 'struct' is that after your struct is defined, 
it will come with a built-in GetSize method as well as an explicit 
converter so that you can easily cast an IntPtr to the struct without 
relying upon calling SizeOf and/or PtrToStructure in the Marshal 
class. 
 
.PARAMETER Module 
 
The in-memory module that will host the struct. Use 
New-InMemoryModule to define an in-memory module. 
 
.PARAMETER FullName 
 
The fully-qualified name of the struct. 
 
.PARAMETER StructFields 
 
A hashtable of fields. Use the 'field' helper function to ease 
defining each field. 
 
.PARAMETER PackingSize 
 
Specifies the memory alignment of fields. 
 
.PARAMETER ExplicitLayout 
 
Indicates that an explicit offset for each field will be specified. 
 
.EXAMPLE 
 
$Mod = New-InMemoryModule -ModuleName Win32 
 
$ImageDosSignature = psenum $Mod PE.IMAGE_DOS_SIGNATURE UInt16 @{ 
    DOS_SIGNATURE = 0x5A4D 
    OS2_SIGNATURE = 0x454E 
    OS2_SIGNATURE_LE = 0x454C 
    VXD_SIGNATURE = 0x454C 
} 
 
$ImageDosHeader = struct $Mod PE.IMAGE_DOS_HEADER @{ 
    e_magic = field 0 $ImageDosSignature 
    e_cblp = field 1 UInt16 
    e_cp = field 2 UInt16 
    e_crlc = field 3 UInt16 
    e_cparhdr = field 4 UInt16 
    e_minalloc = field 5 UInt16 
    e_maxalloc = field 6 UInt16 
    e_ss = field 7 UInt16 
    e_sp = field 8 UInt16 
    e_csum = field 9 UInt16 
    e_ip = field 10 UInt16 
    e_cs = field 11 UInt16 
    e_lfarlc = field 12 UInt16 
    e_ovno = field 13 UInt16 
    e_res = field 14 UInt16[] -MarshalAs @('ByValArray', 4) 
    e_oemid = field 15 UInt16 
    e_oeminfo = field 16 UInt16 
    e_res2 = field 17 UInt16[] -MarshalAs @('ByValArray', 10) 
    e_lfanew = field 18 Int32 
} 
 
# Example of using an explicit layout in order to create a union. 
$TestUnion = struct $Mod TestUnion @{ 
    field1 = field 0 UInt32 0 
    field2 = field 1 IntPtr 0 
} -ExplicitLayout 
 
.NOTES 
 
PowerShell purists may disagree with the naming of this function but 
again, this was developed in such a way so as to emulate a "C style" 
definition as closely as possible. Sorry, I'm not going to name it 
New-Struct. :P 
#>

    [OutputType([Type])]
    Param
    (
        [Parameter(Position = 1, Mandatory = $True)]
        [ValidateScript({($_ -is [Reflection.Emit.ModuleBuilder]) -or ($_ -is [Reflection.Assembly])})]
        $Module,

        [Parameter(Position = 2, Mandatory = $True)]
        [ValidateNotNullOrEmpty()]
        [String]
        $FullName,

        [Parameter(Position = 3, Mandatory = $True)]
        [ValidateNotNullOrEmpty()]
        [Hashtable]
        $StructFields,

        [Reflection.Emit.PackingSize]
        $PackingSize = [Reflection.Emit.PackingSize]::Unspecified,

        [Switch]
        $ExplicitLayout
    )

    if ($Module -is [Reflection.Assembly])
    {
        return ($Module.GetType($FullName))
    }

    [Reflection.TypeAttributes] $StructAttributes = 'AnsiClass, 
        Class, 
        Public, 
        Sealed, 
        BeforeFieldInit'

    if ($ExplicitLayout)
    {
        $StructAttributes = $StructAttributes -bor [Reflection.TypeAttributes]::ExplicitLayout
    }
    else
    {
        $StructAttributes = $StructAttributes -bor [Reflection.TypeAttributes]::SequentialLayout
    }

    $StructBuilder = $Module.DefineType($FullName, $StructAttributes, [ValueType], $PackingSize)
    $ConstructorInfo = [Runtime.InteropServices.MarshalAsAttribute].GetConstructors()[0]
    $SizeConst = @([Runtime.InteropServices.MarshalAsAttribute].GetField('SizeConst'))

    $Fields = New-Object Hashtable[]($StructFields.Count)

    # Sort each field according to the orders specified
    # Unfortunately, PSv2 doesn't have the luxury of the
    # hashtable [Ordered] accelerator.
    foreach ($Field in $StructFields.Keys)
    {
        $Index = $StructFields[$Field]['Position']
        $Fields[$Index] = @{FieldName = $Field; Properties = $StructFields[$Field]}
    }

    foreach ($Field in $Fields)
    {
        $FieldName = $Field['FieldName']
        $FieldProp = $Field['Properties']

        $Offset = $FieldProp['Offset']
        $Type = $FieldProp['Type']
        $MarshalAs = $FieldProp['MarshalAs']

        $NewField = $StructBuilder.DefineField($FieldName, $Type, 'Public')

        if ($MarshalAs)
        {
            $UnmanagedType = $MarshalAs[0] -as ([Runtime.InteropServices.UnmanagedType])
            if ($MarshalAs[1])
            {
                $Size = $MarshalAs[1]
                $AttribBuilder = New-Object Reflection.Emit.CustomAttributeBuilder($ConstructorInfo,
                    $UnmanagedType, $SizeConst, @($Size))
            }
            else
            {
                $AttribBuilder = New-Object Reflection.Emit.CustomAttributeBuilder($ConstructorInfo, [Object[]] @($UnmanagedType))
            }
            
            $NewField.SetCustomAttribute($AttribBuilder)
        }

        if ($ExplicitLayout) { $NewField.SetOffset($Offset) }
    }

    # Make the struct aware of its own size.
    # No more having to call [Runtime.InteropServices.Marshal]::SizeOf!
    $SizeMethod = $StructBuilder.DefineMethod('GetSize',
        'Public, Static',
        [Int],
        [Type[]] @())
    $ILGenerator = $SizeMethod.GetILGenerator()
    # Thanks for the help, Jason Shirk!
    $ILGenerator.Emit([Reflection.Emit.OpCodes]::Ldtoken, $StructBuilder)
    $ILGenerator.Emit([Reflection.Emit.OpCodes]::Call,
        [Type].GetMethod('GetTypeFromHandle'))
    $ILGenerator.Emit([Reflection.Emit.OpCodes]::Call,
        [Runtime.InteropServices.Marshal].GetMethod('SizeOf', [Type[]] @([Type])))
    $ILGenerator.Emit([Reflection.Emit.OpCodes]::Ret)

    # Allow for explicit casting from an IntPtr
    # No more having to call [Runtime.InteropServices.Marshal]::PtrToStructure!
    $ImplicitConverter = $StructBuilder.DefineMethod('op_Implicit',
        'PrivateScope, Public, Static, HideBySig, SpecialName',
        $StructBuilder,
        [Type[]] @([IntPtr]))
    $ILGenerator2 = $ImplicitConverter.GetILGenerator()
    $ILGenerator2.Emit([Reflection.Emit.OpCodes]::Nop)
    $ILGenerator2.Emit([Reflection.Emit.OpCodes]::Ldarg_0)
    $ILGenerator2.Emit([Reflection.Emit.OpCodes]::Ldtoken, $StructBuilder)
    $ILGenerator2.Emit([Reflection.Emit.OpCodes]::Call,
        [Type].GetMethod('GetTypeFromHandle'))
    $ILGenerator2.Emit([Reflection.Emit.OpCodes]::Call,
        [Runtime.InteropServices.Marshal].GetMethod('PtrToStructure', [Type[]] @([IntPtr], [Type])))
    $ILGenerator2.Emit([Reflection.Emit.OpCodes]::Unbox_Any, $StructBuilder)
    $ILGenerator2.Emit([Reflection.Emit.OpCodes]::Ret)

    $StructBuilder.CreateType()
}
#endregion PSReflect