Defacement?

[Crayo]

What was the vulnerability here?
0day exploit?

Not in MyBB, else they would of got Hack Forums too, a much bigger target. All of those sites I think were under the same host. There is an exploit you can upload onto shared servers that will give you root access, and quite a lot of shared hosting providers haven't patched it yet. HostGator was even vulnerable to this, as 2 of their servers were compromised and hundreds of databases were posted on a black hat site.

So basically, this isn't a MyBB issue.

That's odd, Omniscient explained it was due to an outdated MyBB forum he was running, which was still vulnerable to an old exploit.

His other forums were on older versions, but where did he say that? He posted on mybb.com saying it wasn't a MyBB issue, but did stress users to use the latest versions of MyBB at all times so perhaps it was. I'm pretty sure it was just his host that was fudged over, and since Omni doesn't manually secure those the sites were hacked. The HF server is manually secured by Omni, and whatever exploit they used, it clearly wasn't good enough to fudge HF over.

People, you need to stop making such a fuss. No need to give these guys more publicity than they want.