A critical vulnerability exists in Adobe Reader and Acrobat version 9.3.3 and earlier versions.
Critical is Adobe's highest vulnerability rating and could when exploited allow malicious native-code to execute, potentially without a user being aware.
This vulnerability was announced during the Black Hat conference in USA late July this year.
Adobe has announced an out-of-band security update available NOW.
How to update: Open Adobe Reader - select 'Check for updates' from the 'Help' Menu - your adobe reader should check for updates, download and install the update fixing this particular vulnerability... a system restart IS required.
Article: Link from Adobe
Update: This vulnerability affects users on Windows, Mac and Linux, so everyone needs to update.