[Update: This article was updated to clarify that it is outdated versions of Flash Player that are blocked. - Editor]
Apple updated the Web plugin blocking system in OS X on Thursday to prevent outdated versions of Adobe Flash Player from launching. The change means you'll need Adobe Flash Player 18.104.22.168 to view Flash content in Safari on OS X.
Apple blocked older versions of the plugin from running because of security vulnerabilities. Users with those versions will be shown a dialog with a link to Adobe's downloader site so they can install an up to date version of the multimedia player.
Apple blocks outdated Flash Player versions again over security flaws
Apple routinely blocks Flash Player over security issues and displays a link in Web browsers alerting users their plugin version is out of date. OS X also automatically stops the plugin if it hasn't run in the past 30 days.
If you aren't sure if the Flash warning dialog you're seeing is legit, just go to Adobe's Flash download page to download the latest version of the plugin.