pyjail

pickle games 1

Description:

Tags: medium

I filtered everything in my python interpreter, Can you get a shell ?

Author : 1m4D

ncat -v --ssl pyjail.challs.shellmates.club 443

Files : challenge.py

First thoughts:

The challenge is a python jail.

Note: I wasn't able to solve the challenge because I didn't neither read the documentations nor test all the options I had. I included the challenge because it had a new thing that I learned from it.

Reading the script:

Reading the script, we find that the input we provide is tested for:

• each ord character must be less then 127, so only ascii.
• the input must not contain any character from BLACKLIST.
• the input must not include: input, eval or exec

So basically we are left with the alphabet and (, ).

TRYING to solving the challenge:

I opened a separate python interpreter and checked the builtin list using __builtins__, and I tried most of the functions I found. The approach I was trying to follow sa getting at least basic operations like addition or subtraction so I could craft a payload that tries to get a shell for me, but I wasn't able to do it.

After the CTF ended, I asked the creator of the challenge, and he asked me about the single function that I didn't try: breakpoint().

For context, breakpoint() opens a PDB Python Debugger, which is an interpreter that's used to debug the current state of the main interpreter.

Solution:

From here, we only call breakpoint(), import os or any thing we need, and read the flag.

>>> breakpoint()
--Return--
> <string>(1)<module>()->None
(Pdb) import os
(Pdb) os.system('ls')
challenge.py
entrypoint.sh
flag.txt
0
(Pdb) os.system('cat flag.txt')
shellmates{BR3kP01nT_BuiLT1N_D0_M4g1C_98765}

and the flag is:

shellmates{BR3kP01nT_BuiLT1N_D0_M4g1C_98765}