Steam's Christmas Day security lapse exposed "sensitive personal information" on 34,000 users, Valve has finally admitted.
Any user whose details may have been seen by others will now be contacted by Valve, the company has said.
In a new blog post addressing the security error, Valve revealed it had been the victim of a distributed denial of service (DDOS) attack at the time.
But it was not the attack itself which caused the personal information - stored on cached pages served to the wrong users - to become visible. Instead, it was Steam's own anti-DDOS countermeasures which slipped up.
Here's Valve's explanation in full:
"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 per cent 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 minimise 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 apologise to everyone whose personal information was exposed by this error, and for interruption of Steam Store service."
Steam's store quickly returned online, but it has been a five day wait for information on what exactly went wrong.
Dozens of Eurogamer readers reported being able to see details of other Steam users.
The type of personal information exposed by the security error varied from person to person, but included billing addresses, email addresses, purchase histories, the last four digits of a Steam Guard phone number and the last two digits of a credit card number.
Has Steam contacted you about your personal information being at risk? Let us know in the comments below or via email: contact[at]eurogamer.net