http://www.techradar.com/news/apple-has-patched-older-versions-of-macos-to-protect-against-meltdown

Apple has pushed out a patch to protect macOS users running older versions of the desktop OS from the major Meltdown flaw in Intel’s CPUs, following the original fix aimed at High Sierra (10.13).

This patch is for Sierra and El Capitan, the previous two incarnations of macOS (versions 10.12 and 10.11 respectively). As CNET spotted, Apple posted about the security updates on its support site. To get protected, check for new updates in the App Store.