Everyone on all Wikimedia wikis has been logged out, and will have to log back in again.
This was done out of an abundance of caution, after we received one (1) user report of being logged in as someone else.
Said report coincided with the deployment of a new MediaWiki release which caused other problems around User session objects; this is possibly related and under active investigation.
We believe the number of possibly-affected users was small, and that the time window in which the error was possible was short. However, we believe that resetting all sessions is a prudent measure to ensure that the impact is limited.
More details to follow, after technical investigation has determined a cause. https://phabricator.wikimedia.org/T264370
Apologies for the disruption,