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.
What could a hacker even do with your Lemmy info? Unless you are posting critical information about yourself I don't see this as a huge issue. More of a concern for the mod and admin teams imo
The most I see is that they could see your IP address. Not good if you are being stalked. Again I don't think this is a very big deal. If you are worried turn on a VPN.
If you opened an infected page they were able to use your session. That means acting as your account, including seeing and changing your settings. (Which could have allowed them to change your account email or password.)
If information is complete, attackers gained access to active user sessions by leaking cookies via script injection, access would have been terminated when the cookies were invalidated. They likely did not have access to your password (changing it never hurts though).