Apple has just released a new download for users on OS X Mavericks to address the recently-discovered “Shellshock” bug. Apple previously noted that that only a few Macs were actually impacted by the bug and that most users were protected by default. The company promised to release an update shortly to address those who had manually configured their computers in a way that left them exposed.
For users on older versions of OS X, the Mavericks fix will not work. To secure those systems, there are separate downloads for Lion and Mountain Lion. The patch will likely be available through the built-in OS X Software Update mechanism soon. There is currently no patch for machines running the public or developer builds of OS X Yosemite.