Valve explains 'Steam's troubled Christmas,' says 34,000 users affected

Valve issued a statement — and an apology — today about what happened to Steam on Christmas Day, when users of the platform were exposed to the personal account information of other users. At the root of the issue, which affected about 34,000 Steam customers, was a denial of service attack and a caching error, Valve said.

A configuration error was to blame, Valve said, for exposing "sensitive personal information."

"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," Valve said in a statement. "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 said it's working with its web caching partner to identify which Steam users were exposed during the 90-minute window on Dec. 25. The company said that "no unauthorized actions were allowed on accounts beyond the viewing of cached page information [and] no additional action is required by users."

The Steam store was the target of a denial of service attack on Christmas morning, Valve said. And while that kind of attack is "a regular occurrence that Valve handles both directly and with the help of partner companies," traffic to the Steam store increased 2,000 percent over the average traffic during the Steam Sale.

Valve's response to the attack resulted in the error that exposed its users' personal information, according to the company's statement:

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.