[Reverse] SECCON - Powerful_Shell

Reverse obfuscation, powershell, python, seccon2017 Comments


Powerful_Shell
300 points
Powerful_Shell

Crack me.
powerful_shell.ps1-1fb3af91eafdbebf3b3efa3b84fcc10cfca21ab53db15c98797b500c739b0024

Windows powershell… We started by analysing the file:

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$ECCON="";
$ECCON+=[char](3783/291);
$ECCON+=[char](6690/669);
$ECCON+=[char](776-740);
$ECCON+=[char](381-312);
$ECCON+=[char](403-289);
$ECCON+=[char](-301+415);
$ECCON+=[char](143-32);
$ECCON+=[char](93594/821);
$ECCON+=[char](626-561);
$ECCON+=[char](86427/873);
$ECCON+=[char](112752/972);
$ECCON+=[char](43680/416);
$ECCON+=[char](95127/857);
$ECCON+=[char](-682+792);
$ECCON+=[char](-230+310);
$ECCON+=[char](-732+846);
$ECCON+=[char](1027-926);
$ECCON+=[char](94044/922);
$ECCON+=[char](898-797);
$ECCON+=[char](976-862);
$ECCON+=[char](52419/519);
$ECCON+=[char](1430/13);
$ECCON+=[char](18216/184);
$ECCON+=[char](21715/215);
$ECCON+=[char](12320/385);
$ECCON+=[char]([int][Math]::sqrt([Math]::pow(61,2)));
.....
Write-Progress -Activity "Extracting Script" -status "20040" -percentComplete 99;
$ECCON+=[char](520-510);
Write-Progress -Completed -Activity "Extracting Script";.([ScriptBlock]::Create($ECCON))

As we can see we have alot of operations, it looks like something is being hidden behind those arithmetic operations, I did a python script to parse all the operation using regex:

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import re
from math import *

script = ''
lines = open('powerful_shell','r').read().split('\n')
for line in lines:
	if 'sqrt' in line:
		op = line.replace('$ECCON+=[char]([int][Math]::','').replace('[Math]::','').replace(';','').replace(')','') + '))'
		script += chr(int(eval(op)))
	elif '$ECCON' in line and len(re.findall(r'\((-*[0-9]+.-*[0-9]+)\)', line)) > 0:
		op = re.findall(r'\((-*[0-9]+.-*[0-9]+)\)', line)[0]
		script += chr(int(eval(op)))
print script

After extracting it we got this:

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... truncanted ...

$keytone=@{'a'=261.63}
$pk='a'
ForEach($k in ('w','s','e','d','f','t','g','y','h','u','j','k')){
	$keytone+=@{$k=$keytone[$pk]*[math]::pow(2,1/12)};$pk=$k	
}
Write-Host -b 00 -f 15 "Play the secret melody."

Write-Host -b 15 -f 00 -n '   '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '   |   '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 '   |    ' 
Write-Host -b 15 -f 00 -n '   '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '   |   '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 '   |    ' 
Write-Host -b 15 -f 00 -n '   '
Write-Host -b 00 -f 15 -n ' w '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n ' e '
Write-Host -b 15 -f 00 -n '   |   '
Write-Host -b 00 -f 15 -n ' t '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n ' y '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n ' u '
Write-Host -b 15 -f 00 '   |    ' 
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00  '    '
Write-Host -b 15 -f 00 -n '  a |'
Write-Host -b 15 -f 00 -n '  s |'
Write-Host -b 15 -f 00 -n '  d |'
Write-Host -b 15 -f 00 -n '  f |'
Write-Host -b 15 -f 00 -n '  g |'
Write-Host -b 15 -f 00 -n '  h |'
Write-Host -b 15 -f 00 -n '  j |'
Write-Host -b 15 -f 00  '  k '
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00  '    '
Write-Host
$stage1=@();$f="";
While($stage1.length -lt 14){
	$key=(Get-Host).ui.RawUI.ReadKey("NoEcho,IncludeKeyDown")
	$k=[String]$key.Character
	$f+=$k;
	If($keytone.Contains($k)){
		$stage1+=[math]::floor($keytone[$k])
		[console]::beep($keytone[$k],500)
	}
}
$secret=@(440,440,493,440,440,493,440,493,523,493,440,493,440,349)
If($secret.length -eq $stage1.length){
	For ($i=1; $i -le $secret.length; $i++) {
		If($secret[$i] -ne $stage1[$i]){
			Exit
		}
	}
	x "Correct. Move to the next stage."
}
$text=@"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=
"@

$plain=@()
$byteString = [System.Convert]::FromBase64String($text)
$xordData = $(for ($i = 0; $i -lt $byteString.length; ) {
	for ($j = 0; $j -lt $f.length; $j++) {
		$plain+=$byteString[$i] -bxor $f[$j]
		$i++
		if ($i -ge $byteString.Length) {
			$j = $f.length
		}
	}
})
iex([System.Text.Encoding]::ASCII.GetString($plain))

I removed some of the beginning of the script, it was useless code and was just printing some things to the screen, continuing with the rest we have more two steps to conclude, the first one was to play the piano accordantly to the key in the code 440,440,493,440,440,493,440,493,523,493,440,493,440,349.

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While($stage1.length -lt 14){
	$key=(Get-Host).ui.RawUI.ReadKey("NoEcho,IncludeKeyDown")
	$k=[String]$key.Character
	$f+=$k;
	If($keytone.Contains($k)){
		$stage1+=[math]::floor($keytone[$k])
		Write-Host ([math]::floor($keytone[$k])).toString(),$k
		[console]::beep($keytone[$k],500)
	}
}

Each key of the piano corresponds to a number this is easy to discover without having to even look at the code, I just added a few prints in the loop and go through the executable and enter the correct key for the piano was hhjhhjhjkjhjhf

The last phase is to decrypt that big base64 string with the key we got from the piano, once again we don’t even need to check the code just print the variable and you will get the last powershell script of the challenge.

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$plain=@()
$byteString = [System.Convert]::FromBase64String($text)
$xordData = $(for ($i = 0; $i -lt $byteString.length; ) {
	for ($j = 0; $j -lt $f.length; $j++) {
		$plain+=$byteString[$i] -bxor $f[$j]
		$i++
		if ($i -ge $byteString.Length) {
			$j = $f.length
		}
	}
})
iex([System.Text.Encoding]::ASCII.GetString($plain))
Write-Host iex([System.Text.Encoding]::ASCII.GetString($plain))
Read-Host -Prompt "Press Enter to exit"

Now we get another powershell script but the obfuscation on this one is real asking for a password:

Well it wasn’t very hard to me to reverse it, if you look closely there are alot of semicolons separating each variable assignment, for instance I checked one bye one variable and the one who looked interesting was the variable {;}

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PS C:\Users\root> ${;}
iex

iex is an invocation, for example it can be used to run a .ps1 file, if we look closely at the end of the script we can see a very big string (in green) which is being piped into iex:

if we copy that big string into the terminal we can see alot of [char]number, to easily convert them into text we can assign it into a variable and then call it and get the password!

As we can see the flag was the password! SECCON{P0wEr$H311}

TeamRocketISTCTF Team

Portuguese Computer Science Students