In case you're not aware, multiple Lemmy instances suffered hacks recently that allowed the hackers to gain admin privileges and deface the instances and/or redirect users to other sites. Luckily, midwest.social was not a victim of this from what I can tell. To mitigate any more issues I have deleted the single custom emoji that had been uploaded and rotated the JWT which means you will have to log in again on all your devices.
Update: The devs have released 0.18.2 with a security fix for this and I've upgraded to it.
If you log in and it doesn't show your username, you might have to clear your cookies for midwest.social and login again. I had to do that in Firefox anyway.
So basically, it sounds like the issue is insufficient input sanitation in the markdown editor allowing unexpected JS to execute on the site. Sounds like the front end can be compromised, but I don't see anyone saying the back end is compromised, although an admin on lemmy.world was compromised.