Everyone on Wikimedia wikis will shortly be logged out and will have to log back in again.
We are resetting all sessions because we believe that, due to a configuration error, session cookies may have been sent in cacheable responses. Some users reported that they saw the site as if they were logged in as someone else. We believe that the number of affected users was very small. However, we believe that resetting all sessions is a prudent measure to ensure that the impact is limited.
There are several layers of protection against something like this happening, and we don't yet know how all of them failed, but we have made a configuration change which should be sufficient to prevent it from happening again.
-- Tim Starling