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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.
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.