Apple updated the built-in security safeguards in OS X to block versions of Flash prior to 18.104.22.168 from running following the discovery of a flaw in the multimedia platform that could let attackers steal login credentials for thousands of websites. Adobe released an update for Flash on Mac, Windows, and Linux earlier this week to address the issue, too.
Apple disables older Flash versions on OS X over security issues
The flaw could let an attacker intercept login cookies for vulnerable websites, and use them to pose as the victim. Many sites have already been updated to help block anyone from exploiting the flaw.
Apple can disable Flash in OS X based on version numbers and routinely does so when major security risks are exposed. Anyone with a version of Flash installed that's included on Apple's blocked list will see a dialog telling them Flash Player is out of date when they try to play content such as videos that rely on Adobe's platform.