Apple today released a new security update for macOS High Sierra 10.13.4, which comes nearly one month after the first version of macOS High Sierra 10.13.4 was released to the public.
The new macOS High Sierra security update can be downloaded directly from the Mac App Store or through the Software Update function in the Mac App Store on all compatible Macs that are already running macOS High Sierra.
According to Apple's release notes, Security Update 2018-001 is recommended for all users and is meant to improve the security of macOS. The update addresses two security vulnerabilities related to Crash Reporter and LinkPresentation, both of which could be used maliciously to gain access to a Mac. A Safari 11.1 update is also included, with fixes for WebKit vulnerabilities.
Additional information on what's included in the security update can be found in the accompanying support document.