Apple has released Security Update 2018-001, patching vulnerabilities in macOS 10.13.4 High Sierra. The Apple support page lists only two items — a memory corruption issue with Crash Reporter and a problem that could result from a maliciously crafted text message.
However, on the Eclectic Light Company blog, Howard Oakley notes that the weighty download for Security Update 2018-001 includes many more changes, including updates to Apple RAID Utility, System Image Utility, and SetupAssistant, and changes to a large number of kernel extensions. It’s likely that these updates are ripple effects from small upstream changes in the source code. Nevertheless, we recommend holding off on this update for a few days to make sure it has no unwanted side effects. (Free from the Mac App Store or as a direct download, 970 MB, macOS 10.13.4)