While Adobe has already fixed the flaws being exploited, Apple instituted the plug-in-blocking feature in its Safari web browser to safeguard users who may not have downloaded the latest patches. The change affects Macs running OS X Mountain Lion, Lion, and Snow Leopard.
From Apple's support document:
To help protect users from a recent vulnerability, Apple has updated the web plug-in-blocking mechanism to disable older versions of the web plug-in: Adobe Flash Player.
Users who have not yet downloaded the most recent version of Flash, designated as version 11.6.602.171, will see a "Blocked Plug-in" alert in Safari. Selecting the prompt will bring up a pop-up window containing a link to download and install the most up-to-date version of Adobe's software.
Adobe on Tuesday acknowledged the existence of three separate vulnerabilities being exploited in the wild, including one targeting the Firefox browser, and recommended users update to the latest Flash version.
With Flash being a popular form of content delivery on the web, nefarious programmers are constantly developing malicious software to take advantage of the player's many flaws. Most recently, Apple blocked Flash in early February to protect against a similar exploit.