The Adobe Flash Player browser plug-in is frequently updated to patch security vulnerabilities, and Apple has recently begun enforcing that Mac users keep Flash up to date.

In mid November, Apple began pushing down XProtect definitions that prevent outdated versions from running, resulting with either a “Flash out-of-date” dialog or the alert “The version of this plug-in on your computer does not include the latest security updates and it was blocked” alert. Both include a link to Adobe’s Flash player update site.

Updating Flash requires an admin user account. Sometimes the update fails, requiring a uninstall followed by a clean install of the Flash Player. Although instructions in the following URLs indicate that restarting the Mac is only necessary to verify the uninstall is complete, I’ve found that a restart is necessary or the clean install of Flash Player will not run.
http://helpx.adobe.com/flash-player/kb/uninstall-flash-player-mac-os.html
https://forums.adobe.com/thread/1267823

The Flash Player plug-in is used by most web browsers, including Safari and Firefox. Google Chrome however uses it’s own built-in version of the Flash Player, and Apple’s Flash version enforcement doesn’t apply to Chrome. Chrome’s Flash is sandboxed, adding a layer of security, theoretically allowing it to play Flash content safety even if out of date.

In the event that a non-admin Mac user is blocked from playing Flash content until their Flash plug-in is updated, they should be able to use Chrome.