We know an issue occurred on the site over an hour ago with someone using my account to redirect the site, make fake posts, and change other settings. The problem has been corrected.
We will continue to monitor the situation and keep you informed.
Basically it's Javascript that was injected in the main sidebar. That means that Lemmy doesn't escape HTML in the main sidebar (what about community sidebars?) and that Lemmy devs need to prioritize a security audit of the whole code base right now, I did this kind of shit when I was a kid and it's just insane that Lemmy has this vulnerability, this was not some sophisticated hack.
What it also means is that it's very unlikely that any personal data was compromised.
Yes that's what allowed them to modify the contents of the sidebar, but the more serious problem is that you can put HTML in the sidebar and it won't be escaped by the Lemmy backend. That's what allowed this JavaScript redirection.
I'm opening Pandora's box: what if all sidebars, not just the main one, have this vulnerability? An admin account being compromised will be the least of our worries if this is the case.
Cookies were likely obtained too, so they could have logged into your account using those cookies and gotten your email address or posted something with your account, but I think it's more likely they prioritized admin accounts as they were also sending a flag indicating whether the account is an admin or not. Ruud's has invalidated those cookies a while ago so they're worthless now and the hacker can't use them to log in. See ruud's announcement.
What personal data did you give Lemmy? Lol. And if you're going to question lemmy.world then head over to every other instance and take your stance there too.
I don't use other instances, I use lemmy.world. Questioning why and how a major instance got hacked isn't an outlandish line of questioning; I need to know whether this will continue to be a safe, reliable, and secure instance to use otherwise I and presumably others will go elsewhere. This was relatively minor in the grand scheme of things, so hopefully it'll be enough to prompt a fix for whatever caused it (seems like it was possibly insecure code not correctly sanitising HTML).
You're absolutely spot on, it will be important to hear what happened and how and why. If an admin account was compromised, that's not a good reflection on the security and related practices of this instance.