virtualbox.org

After you download the new Virtualbox Extension pack onto your host system and install the Extension pack into Virtualbox Manager then you should do the following to install the guest additions onto your debian based guest.

open terminal in guest os
apt-get update && apt-get install -y linux-headers-$(uname -r)
hit cancel when autorun window pops up.
cp /media/cdrom/VBoxLinuxAdditions.run /Documents/
or (check version is latest)
cp /media/cdrom0/VBoxLinuxAdditions.run /Documents/
cd Documents
chmod 755 VBoxLinuxAdditions.run
./VBoxLinuxAdditions.run
shutdown guest os
shutdown reboot host os
start guest os
sudo apt-get autoremove

Actually there is no connection between :-

• installing the extension pack (a host activity you perform once - mandatory if you need the extended VirtualBox features such as USB2/USB3 support), and...
• installing the Guest Additions (a guest activity that may be performed separately in every VM, but is not mandatory).

The GAs mainly improve graphics performance, graphics capabilities, and integration with the host OS, but none of these is absolutely necessary. Indeed the Guest Additions are not even available for all guests (e.g. there are no GAs for OS X or Android) - though GAs are of course available for popular Linux guests such as Debian.