VirtualBox 5.1.8 installation didn't install kernel extensions. How do I fix this? [macOS 10.13]
High Sierra comes with a new security feature: Secure Kernel Extension Loading, which blocks kernel extension loading. The implementation appears to be questionable.

To allow the loading of some non-Apple kernel extension you have to either allow execution in Preferences > Security &amp; Privacy > General:



or disable the security feature with a Terminal command (not recommended):

sudo spctl --master-disable