If the version of Adobe Flash plug-in you are using is out of date, you may see the message, “Blocked plug-in,” or “Flash Security Alert” or “Flash out-of-date” when attempting to view Flash content in Safari.
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According to Apple’s product security team, Apple blocked the Flash Player plug-in because outdated Adobe Flash versions are vulnerable to the Rosetta Flash attack scenario, which could expose a victims sensitive data:
Due to security issues in older versions, Apple has updated the web plug-in blocking mechanism to disable all versions prior to Flash Player 14.0.0145 and 13.0.0.231.
Mac, Windows and Linux users can find additional security information about the Adobe Flash Player 14.0.0.145 update here.