After Rolling out iOS 11.3.1 update, Apple has also release a new security update to the macOS 10.13.4 that is designated as Security Update 2018-001. The update fixes a memory corruption issue that could give an application elevated privileges.
The update patches a spoofing issue that existed in the handling of URLs and could let maliciously crafted text messages to process UI spoofing.
To download the new security update on your macOS High Sierra 10.13.4 running machine, launch the App Store and then find the latest macOS update from the Updates section (or Download directly by Following this link from your Mac device). Click the Download button on the App Store listing and then enter your Apple ID credentials in the provided box.
Once downloaded, an information window will pop up on your screen. Now, click on Continue and wait until the update automatically installs on your device.