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Update: It appears the Steam Store is back online and the issue appears to be resolved. We have yet to hear from Valve in an official capacity, and will update the blog post when we do.
Update 2: Valve has given a statement to GameSpot.
Update 3: Valve also sent us the same statement:
Steam is back up and running without any known issues. As a result of a configuration change earlier today, a caching issue allowed some users to randomly see pages generated for other users for a period of less than an hour. This issue has since been resolved. We believe no unauthorized actions were allowed on accounts beyond the viewing of cached page information and no additional action is required by users.
We'd like to follow up with more information regarding Steam's troubled Christmas.
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.
Original post: Earlier today, Steam users started seeing incorrect information on the Steam Store, as if they were signed into someone else's account.
There is no official confirmation from Valve yet, so we can only speculate as to why this issue happened. Valve is known to use Akamai as their CDN and Varnish for caching. Our theory is that a caching misconfiguration in one of these components has caused Steam to incorrectly serve rendered and cached pages intended for a single user only.
This issue means that users’ private information such as email address, billing address, and sometimes credit card details are at risk. As far as we know, this issue is read-only, and no one is able to perform any actions involving your account on your behalf.
To protect yourself, we strongly recommend completely avoiding visiting any Steam store links. This includes visiting the Steam store using the Steam client. At the time of this writing, the Steam store is inaccessible. We can only assume Valve is currently working on fixing the issue.
See updates on the top of this post.
This was not a hack or a DDoS attack. This was highly likely to be a misconfiguration in one of Valve’s caching layers.
If you used a PayPal account and had the details saved, you can unlink your account by logging on PayPal.com and going to Settings and Preapproved payments under the Payment options heading.
Going forward, we strongly encourage you not to store your billing information on the Steam store. Valve have proven multiple times that they’re unable to keep their security standards to a high level.