EDIT: 28/03/14 – Virtualbox 4.3.10 is now out – it is recommended to download this now together with 4.3.10 virtualbox guest additions and the issue below is resolved 🙂
However – it is also likely that if you just download the 4.3.7 or later guest additions and install in your v4.2 or 4.3 series this should work – but remember this is an unsupported configuration.
EDIT: 8/09/14 – as one commenter mentioned some people have been also had luck with the following package install:
sudo apt-get install virtualbox-guest-dkms
Restart the VM afterwards.
The latest updates of Ubuntu 14.04 is broken in virtualbox 4.3.6 – the screen no longer resizes and is fixed at a very low resolution. It also appears all previous versions of virtualbox that is available in the repositories are also affected.
Attempts to reinstall the virtualbox guest additions does not fix the issue. Instead in the terminal you see this error:
Building the VirtualBox Guest Additions kernel modules The headers for the current running kernel were not found. If the following module compilation fails then this could be the reason. Building the main Guest Additions module ...done. Building the shared folder support module ...done. Building the OpenGL support module ...done. Doing non-kernel setup of the Guest Additions ...done. You should restart your guest to make sure the new modules are actually used Installing the Window System drivers Warning: unknown version of the X Window System installed. Not installing X Window System drivers. Installing modules ...done. Installing graphics libraries and desktop services components ...done. Press Return to close this window...
It appears the the latest version of Xorg breaks the virtualbox graphics driver.
EDIT: there is also this bug (https://bugs.launchpad.net/ubuntu/+source/virtualbox/+bug/1282758)
Fortunately this is fixed in a test build of the virtualbox guest additions – version 4.3.7 (and later) together with installing the latest gcc library
sudo apt-get install libgcc-4.8-dev
Grab a copy of the ISO from this virtualbox ticket. Connect the ISO to your virtualbox VM – and the guest additions including the graphics driver will update correctly. Enjoy!