as of this writing, has not been merged into the official Debian repositories but is required if you want to run Windows 8, which the “guest additions” for, are still somewhat short of full OpenGL video support which prevents usage of some applications like Nik HDR Pro 2 and Color Efex Pro 4 photo software (which need to be able to identify specific video cards, unfortunately).
Not being available through the Debian repositories, there can be a problem with the virtualbox USB initialization for guest OS access. The script /usr/share/virtualbox/VBoxCreateUSBNode.sh cannot be found by udev on my LVM configuration because the /usr filesystem is not yet mounted. To circumvent this startup error so the shell script is visible from the root filesystem at boot..
cp /usr/share/virtualbox/VBoxCreateUSBNode.sh /root
mkdir -p /usr/share/virtualbox
mv /root/VBoxCreateUSBNode.sh /usr/share/virtualbox/
Another irritant is that the vboxdrv module appears not to load at boot which then requires the following to be executed prior to launching a guest OS for the first time..
To avoid this after every reboot..
echo >>/etc/modules # ensure a linefeed terminates file
echo vboxdrv >>/etc/modules