Oracle VM VirtualBox now is at version 4.3.6 which brings various fixes and improvements. This tutorial shows how to install / upgrade to Virtualbox 4.3.6 in Ubuntu 13.10, Ubuntu 13.04, Ubuntu 12.04, Linux Mint and their derivatives.
Changes in VB 4.3.6:

• VMM: fixed a Guru Meditation VINF_EM_TRIPLE_FAULT caused by VMCB caching with nested paging on certain AMD CPUs
• VMM: fixed a Guru Meditation VERR_VMX_UNEXPECTED_INTERRUPTION_EXIT_TYPE while intercepting debug exceptions (VT-x only)
• VMM: fixed a Guru Meditation VERR_SVM_UNEXPECTED_EXIT while intercepting debug register accesses (AMD-V only)
• VMM: fixed a VERR_SSM_STRUCTURE_MAGIC error when trying to load a saved state made with VBox 4.3.4 when VT-x/AMD-V is disabled. Unfortunately, VBox 4.3.4 produced broken saved states for this configuration so you have to discard these states
• VMM: added a few more MSRs to the whitelist required by certain guests
• GUI: fixed deleting of inaccessible VMs (4.3 regression)
• GUI: fixed warnings in VM settings / number of guest processors
• Main: don’t automatically enable 64-bit guests on 64-bit hosts if VT-x/AMD-V is not available
• Main: always expose the DMI memory information to Windows 2012 guests
• Main: fixed occasional crashes on guest display resolution change
• Main: fixed reporting back temporary name when calling IGuestSession::DirectoryCreateTemp()
• API: fix for a hang when launching a GUI VM through the API, which crashes due to GUI unavailability
• Storage: fix for BLKCACHE_IOERR runtime errors under rare circumstances
• Network: allow to start more than 5 PCNet instances
• E1000: if the cable was disconnected before the guest initialized the device, the link status was not properly set to ‘down’ after the initialization completed despite the fact that there was no connection
• 3D support: fixed offset of guest 3D image elements (Mac OS X Retina hosts only)
• Solaris hosts: fixed accessing the host driver from non-global zones (4.3 regression)

Install or Upgrade Virtualbox 4.3.6:

Debian / Ubuntu DEBs and Fedora / SUSE RPMs are available in official download page.

Ubuntu users can also run the commands below in terminal (Ctrl+Alt+T) to install / upgrade from official repository:

sudo sh -c 'echo "deb http://download.virtualbox.org/virtualbox/debian $(lsb_release -sc) contrib" >> /etc/apt/sources.list'

wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox.asc -O- | sudo apt-key add -

sudo apt-get update

sudo apt-get install virtualbox-4.3