Security Update 2019-002 (Mojave) and 2019-007 (High Sierra)
Apple has released Security Update 2019-002 for macOS 10.14 Mojave and Security Update 2019-007 for 10.13 High Sierra, patching just two security vulnerabilities in the older operating systems. The updates address a memory corruption that could allow an application to execute arbitrary code with system privileges and patch a memory corruption issue that could allow arbitrary code execution with kernel privileges. (Free. For 10.14 Mojave, 1.58 GB; for 10.13 High Sierra, 1.92 GB; security content release notes)
After installing the security update on my new iMac running High Sierra, my USB keyboard would not work. I had to get out my bluetooth keyboard and mouse (which came with the iMac) so I could log back in. (I normally use a USB trackball plugged into my keyboard.) After I logged on I realized the that all my other USB devices were working normally, so I unplugged my keyboard and plugged it into a USB hub. The keyboard and trackball worked fine. (This is a USB-C to USB-A hub adapter plugged into a Thunderbolt port.) I then tried switching it with other USB-A devices to see if the port was bad. Everything else worked in that port. However, the keyboard would not work unless it was plugged into the USB-C to USB-A hub.