The hacker is alledgedly known as "MaddoxX" and posted the following to an anti-steam website:

• Valve's private financial information
• Credit card information of customers
• Screenshots from internal Valve web pages
• Error logs
• Some of the Valve Cafe directory

The security breach is yet to be confirmed as real, but some of the information above has already been posted to an anti-steam website, the hacker obviously wanting to prove that Steam has been compromised. Screenshots can be seen here. The hacker is alledgedly holding Valve at ransom, proclaiming:

"If you want me to remove these files you can e-mail me at (address removed) and I prefer you come with something good unless you want me to expose ALL of the customers their information."

If this turns out to be the real deal, Valve could have a public relations nightmare on their hands, as they'd have to reveal to credit card holders that the system has been compromised, thanks to California Senate Bill 1386.

Update 4/19/07: Doug Lombardi, director of marketing at Valve, contacted DailyTech with the following statement:

"There has been no security breach of Steam. The alleged hacker gained access to a third-party site that Valve uses to manage the commercial partners in its Cyber Café program. This Cyber Café billing system is not connected to Steam. We are working with law enforcement agencies on this matter, and encourage anyone with more information to e-mail us at Catch_A_Thief@valvesoftware.com."