The virtualization platform is an essential component of a secure Whonix ™ system. A vulnerable virtualizer may provide opportunities for attackers to perform a breakout from a virtual machine in order to undo the security by isolation features that Whonix ™ provides. The decision to install an alternative virtualizer should not be taken lightly.
The two methods described below to install VirtualBox are safer than downloading, verifying, and installing binaries manually.
Recommended VirtualBox Version
Moved to recommended VirtualBox version.
Newer VirtualBox Version
Install VirtualBox from Debian Unstable
Currently not possible. 
Install from VirtualBox.org Repository
The Free Support Principle applies to this procedure. The latest Oracle VirtualBox package information can be found here [archive].
Note: after upgrading the VirtualBox host version, this can cause issues with the guest VMs such as broken VM size adjustment (full screen)  unless the VirtualBox guest additions are also upgraded inside your virtual machine (which is not covered in these instructions).
1. Add the Oracle apt sources list.
Make sure to change
buster to the current name of your stable distribution.
sudo su -c "echo -e 'deb https://download.virtualbox.org/virtualbox/debian buster contrib' > /etc/apt/sources.list.d/oracle.list"
2. Add Oracle's signing key to apt-get keyring.
Securely download the key.
curl --tlsv1.2 --proto =https --location --remote-name-all --remote-header-name https://www.virtualbox.org/download/oracle_vbox_2016.asc
Display the key's fingerprint.
gpg --keyid-format long --import --import-options show-only --with-fingerprint oracle_vbox_2016.asc
Verify the fingerprint. Should show.
pub rsa4096/A2F683C52980AECF 2016-04-22 [SC]
Key fingerprint = B9F8 D658 297A F3EF C18D 5CDF A2F6 83C5 2980 AECF
uid Oracle Corporation (VirtualBox archive signing key) <firstname.lastname@example.org>
sub rsa4096/AD18C79D920E471F 2016-04-22 [E]
Do not continue if the fingerprint does not match -- this risks using infected or erroneous files! The whole point of verification is to confirm file integrity.
Add the signing key.
sudo apt-key --keyring /etc/apt/trusted.gpg.d/oracle.gpg add oracle_vbox_2016.asc
3. Update the package lists.
4. Install VirtualBox and Linux Kernel Headers, which are a dependency.
sudo apt-get install virtualbox-6.1 linux-headers-$(uname -r)
5. Mount the ISO in the guest virtual machine to install.
The VirtualBox Guest Additions ISO is included in the package. Instructions can be found here [archive].
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