>>>A freshly installed Ubuntu as guest under VirtualBox allows only 640*480 resolution - nothing you can really work with today.
>>>The normal hint to get smoother graphics, including higher resolution, is to install guest additions for VirtualBox.
>>>As normal install of guest additions also allows for shared folders to be created,
>>>which is something I want to avoid for security reasons at least in mostly web targeted VMs, I did not install them.
>>>I have found a way to enable 800*600 and 1024*768 via enabling debugging options in xDiagnostics, which is half-way to ok.
>>>
>>>I have NOT RTFM on guest addition installation options to see if it is possible to select which part of the additions will be installed.
>>>At least one post I found seems to hint that way. Avideo pointing to a combination of "sudo cvt" and "sudo xRandr" did not work out.
>>>Another hint:
>>>"Go To the 'File' menu, and activate 'Environment setting (P)'. Select 'Display', and change the setting for 'Maximum guest screen size' to 'hint', which allows you to set an arbitrary size for both width and height (eg., 1920 and 1200)."
>>>did not work for me, as under 14.04.4 I did not find a environment setting under the "File" menu called up from second icon and did not find another "File" menu call up...
>>>
>>>Has anybody here
>>>found a way to enable other screen resolutions under Virtualbox without installing guest additions or
>>>is certain that there is a way to disable the installation of shared folders when installing guest additions ?
>>>
>>
>>I never run VB without guest additions, so won't be of any help regarding the first part of your question, but for the second part: doesn't the host have to create a shared folder before the guest can access it? That is, even if Guest Additions enable you to create shared folders you really don't have to, and if is that so what risks are involved in this?
>
>The main risk I worry about is a folder on my main machine where a lot of portable programs and a couple of .Iso are bundled. Sometimes this is copied into physical or virtual machines, sometimes only a mapping to a disc letter is established, depending on the risc the machine incurs - most of the VMs created are for internal use and do not access the internet, allowing me to skip anti-virus and to map to such folders.
>
>My fear is of a malicious site contacted in one of the internet-enabled VMs when that VM is running on the main machine where the above mentioned folder sits: somehow such a site finding a weakness in the linux browser, sidestepping any sudo needs for establishing shared folders and infecting my Win portables. Seems far-fetched, but a few weaknesses I found roboting IE4 or IE5 in pre Y2K were later defined as vulnerabilities. The higher the fences are around my sandboxes, the better: medieval castles had walls plus moat ;-)
>
>And the effort put into getting ransomware running is considerable: sometimes target mails are delivered "personalized", requiring perhaps 3-5 min to create. Creating scripts able to jump OS if disk access is available might take more time but will hit a couple of targets without further personalization ;-(
>
>I have found the correct syntax for xrandr calls to put other resolutions into the dropdown list for screen resolution - but selecting it does only result in an error as somehow inital available monitor max setting comes into play... Probably some .conf read at start :-(

My understanding is what Oracle gives the user-friendly name "guest additions" is actually a set of paravirtualized (PV) drivers and virtual adapters. These are much more capable than the generic drivers/adapters and my understanding is they perform better as well.

As far as a VM is concerned, a given virtual adapter *is* the hardware. The basic, non-PV adapters/drivers have limited capabilities, in the case of virtual video adapters only a limited range of resolutions is supported, as you've found (maybe only 1024x768 or 1280x1024). There is no trick possible in the guest OS on that VM that will magically give that guest higher resolution than is supported in the virtual adapter.

Bottom line, if you want higher resolution in a guest VM than is supported by the generic/standard adapters/drivers then you must install at least the PV video support in the guest.

So far I've only used VirtualBox with Windows VMs and like Antonio I always install the guest additions because I need full screen 1920x1080, sound, shared Clipboard and much more seamless support for keyboard/mouse. With a Windows host, Windows guest additions are part of the VirtualBox installation and can be installed from the VM's menu bar while it's running. Basically, you're mounting an ISO as a virtual CD drive.

Ubuntu guest additions can be installed e.g. http://askubuntu.com/questions/22743/how-do-i-install-guest-additions-in-a-virtualbox-vm but I imagine you've already found similar resources.

I never use shared folders. I don't know if they're disabled or enabled by default, I'd like to think disabled but maybe VBox's defaults are aimed more at functionality than security. I treat VMs like separate computers and network to them as required.

I suppose in a perfect world you'd like to install just the video guest PV for your Ubuntu VMs. I have no idea how to do that but it's the sort of thing Ubuntu might support. If you can't break it out separately I suppose you'll have to decide if security is more important than functionality.