ctf-writeup/2023/AmateursCTF 2023/Censorship Lite

Censorship Lite

There was clearly not enough censorship last time. This time it's lite™️. I'm afraid now you'll never get in to my system! Unfortunate for those pesky CTFers. Better social engineer an admin for the flag!!!!

About the Challenge

We got a python script called main.py. Here is the content of main.py

#!/usr/local/bin/python
from flag import flag

for _ in [flag]:
    while True:
        try:
            code = ascii(input("Give code: "))
            if any([i in code for i in "\lite0123456789"]):
                raise ValueError("invalid input")
            exec(eval(code))
        except Exception as err:
            print(err)

So, this script will execute our input, but we can't input \, l, i, t, e, and also a numbers (0-9). We cannot use unicode character because of ascii()

How to Solve?

To solve this problem, we don't need to execute an OS command. But how? As you can see the package flag was already imported into the code and also the value of flag was assigned to _ variable.

from flag import flag

for _ in [flag]:
    ...

Do you know there is a built-in function called vars()? This function returns a dictionary of an object. For example:

So, to print the value of variable flag or _, we can use this payload

vars()[flag]
vars()[_]

Because we can't input character l in the program, we can only use the second payload

amateursCTF{sh0uld'v3_r3strict3D_p4r3nTh3ticaLs_1nst3aD}