VirtualBox Extension pack fails to install due to missing privileges

[rbu]

I have tried upgrading the VirtualBox Extension Pack, but both removing the existing extension as well as adding a new one failed with my user privileges. The UI reported an error related to the SHELL variable instead of missing privileges and did not attempt to raise privileges (by asking for the password) to write to a directory.

See log for details:

00:03:16.930642 ExtPackInst ExtPack: Executing '/usr/lib64/virtualbox/VBoxExtPackHelperApp''install' '--base-dir' '/usr/lib64/virtualbox/ExtensionPacks' '--cert-dir' '/usr/lib64/virtualbox/ExtPackCertificates' '--name' 'Oracle VM VirtualBox Extension Pack' '--tarball' '/home/rbu/Downloads/Oracle_VM_VirtualBox_Extension_Pack-4.3.16-95972.vbox-extpack' '--sha-256' '93b01ac2c575388ea6ae994450907c24e30a788c271ae9ff18512a06f28d9abd'
00:03:16.955177 ExtPackInst ExtPack: enmReason=1 iStatus=127 stderr='The value for the SHELL variable was not found the /etc/shells file
00:03:16.955191 ExtPackInst 
00:03:16.955192 ExtPackInst This incident has been reported.
00:03:16.955194 ExtPackInst '
00:03:16.955231 ExtPackInst ERROR [COM]: aRC=NS_ERROR_FAILURE (0x80004005) aIID={3295e6ce-b051-47b2-9514-2c588bfe7554} aComponent={ExtPackManager} aText={The installer failed with exit code 127: The value for the SHELL variable was not found the /etc/shells file
00:03:16.955246 ExtPackInst 
00:03:16.955248 ExtPackInst This incident has been reported.}, preserve=false

[Asif]

The Problem

This seems to be a recurring problem when updating to a new version of VirtualBox by first uninstalling the older version of VirtualBox and then installing the newer version of VirtualBox. I ran into this on my Ubuntu (Trusty) 14.04 LTS, when I:
1) Uninstalled VirtualBox-4.3.30 (without first uninstalling the extension pack for 4.3.30).
2) Installed VirtualBox-5.0.0 and then noticed extension pack 4.3.30 and attempted to uninstall it (which caused the failure.)
3) I further reinstalled 4.3.30, and was still unsuccessful at removing the 4.3.30 extension pack.

Possible Resolution(s)

1) I found out that I had to be logged in as a user with root privileges to be able to uninstall the extension pack of any version. Whenever I attempted to uninstall the extension pack while logged in as a non-privileged user, I would not get the password prompt to complete the installation of the extension pack. Why? Because I was not logged in as a user with root privileges.

Related Bug(s)

1) Extension pack installation on Ubuntu no longer prompts for root password and fails.
https://www.virtualbox.org/ticket/13864