Valve has released a statement on Steam's Christmas bug that allegedly allowed users of the online gaming service, to access each other's account.

　　The bug is said to have been caused by a caching issue, and up to 34.000 users might have seen private information from other Steam users. According Valve, the content that could be accessed, varied by page. Some pages though, contained 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. The steam creator states that, full credit card numbers, user passwords, weren´t revealed.

　　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.

Steam Christmas Bug started with DOS Attack on Steam Store

　　The question remains how this bug occurred in the first place, and according Valve it all began with a DOS attack at the Steam Store, early Christmas morning.

　　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.

　　Valve´s official statement on the matter can be read here.