* https://askubuntu.com/questions/914997/install-virtualbox-while-keeping-secure-boot
* https://stegard.net/2016/10/virtualbox-secure-boot-ubuntu-fail/
* Assuming you disable secure boot due to a package installation, even when you re-enable secure boot in your system configuration, you may still see [https://askubuntu.com/questions/726052/ubuntu-booting-in-insecure-mode-with-secureboot-enabled Booting in insecure mode with SecureBoot enabled]
You'll always find a newer version of VirtualBox [https://www.virtualbox.org/wiki/Linux_Downloads using the Oracle ppa], so use that rather than the packaged version that comes with your distro. Warning: if you've already got VirtualBox installed from packages, you should remove it (but not 'completely' meaning don't destroy your existing images). Also, under packaged Ubuntu, you can simply <code>apt-get install virtualbox-guest-additions-iso</code>; however with the PPA, you'll need to download it yourself.
