While I was asleep, apparently the site was hacked. Luckily, (big) part of the lemmy.world team is in US, and some early birds in EU also helped mitigate this.
As I am told, this was the issue:
There is an vulnerability which was exploited
Several people had their JWT cookies leaked, including at least one admin
Attackers started changing site settings and posting fake announcements etc
Our mitigations:
We removed the vulnerability
Deleted all comments and private messages that contained the exploit
Rotated JWT secret which invalidated all existing cookies
The vulnerability will be fixed by the Lemmy devs.
Many thanks for all that helped, and sorry for any inconvenience caused!
Update
While we believe the admins accounts were what they were after, it could be that other users accounts were compromised. Your cookie could have been 'stolen' and the hacker could have had access to your account, creating posts and comments under your name, and accessing/changing your settings (which shows your e-mail).
For this, you would have had to be using lemmy.world at that time, and load a page that had the vulnerability in it.
Do we have any details on how Michelle's account was compromised? Right now in the GitHub issue about the vulnerability they're clueless about how the custom emoji exploit could be performed without first an already compromised admin account.