According to one of the vuln posts a redirect and cookie stealing code was added as onload js (can even be seen in a screenshot).
Together with the JWT that are valid for a year and non revokable (https://github.com/LemmyNet/lemmy/issues/3364) that means if you logged in or browsed an affected instance while logged in to it the attacker got your account and the only way to get it back is not in your hands but in the instance admins (they have to delete all sessions from the DB).
Correct. We don't know for sure what the initial injection was, but they did manage to inject and all the accounts will need their sessions purged maybe force password reset as well.
EDIT: No longer correct -- the injection appears to have been through custom emojis in markdown, see github for details.
I hope everything will be resolved quickly. Saw a post on kbin about it, and I was just about to login my .world account and see what’s up. ~ Waiting on updates…
Tbf, I'm surprised this hasn't happened already. The software is not mature and is suddenly being exposed to a huge group of people. There are lots of eyes on it that weren't on the code before, and the big audience makes Lemmy a juicy target.
Other instances should be vigilant in case Lemmy could have an exploit until we know for sure what happened.
As of 2:45 UTC it's still hacked, modified main page to redirect to offensive materials. Discussion claims that other admins are aware of the hack, but the website is still contaminated.