XSS from cookie

Steps to reproduce

Step 1. Log onto minecraft.net

Step 2. find 'session_username' cookie

Step 3. Edit the cookie to any Xss lets say "<marquee>script inside</marquee>"

Step 4. Click back to minecraft.net or if you are not on the profile page refresh

Step 5. The script is executed

will look like this:
https://gyazo.com/4689685db73acbb7cec646457f3e4a3d