You can find the full statement here, but the gist of it It’s exactly what everyone thought happened, a.k.a. when hit with a DDOS attack on Christmas morning, someone accidentally pushed some bad code that caused a caching error of private info.
Or, as Valve puts it:
According to Valve, 34,000 users were affected by the caching error—a paltry amount compared to Steam’s 125 million users, but still a sizable breach of personal information. Still, it’s nowhere near the size of the PlayStation Network breach that happened in 2011 which exposed data on 77 million users.
As for what was exposed Valve said: “The content of these requests varied by page, but some pages included a Steam user’s billing address, the last four digits of their Steam Guard phone number, their purchase history, the last two digits of their credit card number, and/or their email address. These cached requests did not include full credit card numbers, user passwords, or enough data to allow logging in as or completing a transaction as another user.”
You don’t need to panic yet. First of all, if you didn’t go on Steam on Christmas, your account details are safe by default. Other than that, “Valve is currently working with our web caching partner to identify users whose information was served to other users, and will be contacting those affected once they have been identified. As no unauthorized actions were allowed on accounts beyond the viewing of cached page information, no additional action is required by users.”
The presence of the phrase “web caching partner” seemingly indicates the screw-up arose from a third-party outside Valve—although that, of course, doesn’t absolve Valve of blame. Best you can do is hope you’re not one of the unlucky few.