Apple have released macOS Sierra 10.12.3. This update patches vulnerabilities in apache_mod_php, Bluetooth, Graphics Drivers, Help Viewer, IOAudioFamily, Kernel, libarchive, and Vim.
The latest version of macOS can be installed by opening the App Store, clicking Updates, and then clicking Update All. You will then be prompted to Download and Restart.
It is highly recommended you install the latest version of any operating system you are running.
Help Viewer can be exploited by an attacker who tricks a user into visiting a maliciously crafted website. The attacker would then have the ability to execute code on the users device.
libarchive can also be exploited by an attacker who tricks a user into extracting a malicious archive file, and again leads to arbitrary code execution.
Many of the other vulnerabilities can be exploited to execute arbitrary code – This means an attacker is able to run anything he desires on your device.
More information regarding the security patches can be found on the Apple website.