Valve has finally apologized for last week’s Steam Christmas disaster, explaining in a lengthy statement today that the issues stemmed from a Denial of Service attack and wound up exposing the information of around 34,000 users.

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Valve said they’re still working to identify the affected users and will contact them once the process is complete.

On Friday afternoon, December 25, users on Steam found that the digital network had malfunctioned, causing other people’s account information to show up in the store and settings. This lasted around an hour before Valve shut down Steam and fixed the problem. But issues with Steam have lingered, and as of yesterday Valve had remained mum on exactly what happened and how many people were affected.

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Today, Valve broke everything down, explaining that they’d been hit by multiple DoS attacks that have caused a wide variety of issues. In an attempt to thwart some of the attacks, Valve inadvertently caused a caching malfunction that exposed some users’ personal information.

“In response to this specific attack, caching rules managed by a Steam web caching partner were deployed in order to both minimize the impact on Steam Store servers and continue to route legitimate user traffic,” the company wrote. “During the second wave of this attack, a second caching configuration was deployed that incorrectly cached web traffic for authenticated users. This configuration error resulted in some users seeing Steam Store responses which were generated for other users.”

Here’s the full statement, via Steam:

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We’d like to follow up with more information regarding Steam’s troubled Christmas.

What happened

On December 25th, a configuration error resulted in some users seeing Steam Store pages generated for other users. Between 11:50 PST and 13:20 PST store page requests for about 34k users, which contained sensitive personal information, may have been returned and seen by other users.

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.

If you did not browse a Steam Store page with your personal information (such as your account page or a checkout page) in this time frame, that information could not have been shown to another user.

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.

How it happened

Early Christmas morning (Pacific Standard Time), the Steam Store was the target of a DoS attack which prevented the serving of store pages to users. Attacks against the Steam Store, and Steam in general, are a regular occurrence that Valve handles both directly and with the help of partner companies, and typically do not impact Steam users. During the Christmas attack, traffic to the Steam store increased 2000% over the average traffic during the Steam Sale.

In response to this specific attack, caching rules managed by a Steam web caching partner were deployed in order to both minimize the impact on Steam Store servers and continue to route legitimate user traffic. During the second wave of this attack, a second caching configuration was deployed that incorrectly cached web traffic for authenticated users. This configuration error resulted in some users seeing Steam Store responses which were generated for other users. Incorrect Store responses varied from users seeing the front page of the Store displayed in the wrong language, to seeing the account page of another user.

Once this error was identified, the Steam Store was shut down and a new caching configuration was deployed. The Steam Store remained down until we had reviewed all caching configurations, and we received confirmation that the latest configurations had been deployed to all partner servers and that all cached data on edge servers had been purged.

We will continue to work with our web caching partner to identify affected users and to improve the process used to set caching rules going forward. We apologize to everyone whose personal information was exposed by this error, and for interruption of Steam Store service.

You can reach the author of this post at jason@kotaku.com or on Twitter at @jasonschreier.