Apple today has confirmed that is updating its web plug-in blocking mechanism in OS X to disable all versions of Adobe Flash Player prior to the most recent, which is version 126.96.36.199. On older systems, all versions of Flash prior to 188.8.131.521 are blocked.
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Apple notes on its Support page that if you must run an older version of Flash in Safari, you can do so by using the Internet plug-in management option in the browser. With this option, you can choose to run the Flash Player plug-in in unsafe mode on websites that you trust.
Apple cites security issues for its reasoning to block old versions of Adobe Flash:
Due to security issues in older versions, Apple has updated the web plug-in blocking mechanism to disable all versions prior to Flash Player 184.108.40.206 and 220.127.116.111.
This isn’t the first time Apple has taken steps to block potentially insecure versions of Adobe Flash on its machines. Last summer, Apple pulled a similar move and blocked all outdated versions of Adobe Flash in Safari due to similar vulnerabilities.
The most recent version of Flash Player is available on Adobe’s website.