It is indeed a hack. I am working with our host to resolve this. Will post an update soon. Thanks for the report! I would have noticed it earlier if it wasn't that my ad blocker was blocking it already. =)
-- Missagh Alami, webmaster of Phantasy Star Cave.
I have noticed this too, this carries over to the actual site itself, not just the forum. While clicking on pretty much anything, it'll open up some page, sometimes asking to install something. Happens usually only once per new link clicked.
It was pure luck I noticed it. I have basic ad-blocking on Firefox but I have tons of other measures taken on my home server so that each connected device here will get ad-blocking too.
That said I haven't stumbled into any other odd thing here.