You may have experienced some odd behavior on Valve’s Steam site this morning, and apparently it’s due to a hacking website targeting the platform. A hacking website altering the front page of the Steam Users’ Forum with a posted advertisement to their website, with reports of spam, seem to be the extent of the damage. For the time being, the Steam forums have been disabled and are offline.
Valve asks for your patience while they sort through the recent hack, but hasn’t (at the time of writing), offered any guidance on the severity of the recent breach.
It’s hard to say if any other information had been compromised in the attack. Though it appears that only the User Forums have been affected by the illicit activity.