tl;dr

• Intended: Append ; secure; samesite=none to cookie. Now, <script src="https://jason.2021.chall.actf.co/flags?callback=load"></script> would retrieve the flag.
• Unintended: Append .actf.co as domain to cookie using CSRF -> Setup a xss payload in reaction.py challenge -> Log in to this using CSRF -> Payload in Reaction.py exfiltrates document.cookie