This is a notice that on Tuesday, May 21st between 20:00-21:00 UTC
(1-2pm PDT) Wikimedia Foundation will release security updates for
current and supported branches of the MediaWiki software. Downloads
and patches will be available at that time, with the git repositories
updated later that afternoon. Although MediaWiki does not have the
vulnerable feature enabled by default, most wiki using common advanced
features will want to patch for this issue.