The new version of Fedora 23 has been released a while ago. If you are installing Fedora 23 on Virtualbox, you may install the guest additions but it still returned a critical error. The guest additions installer cannot install the X.org driver on Fedora 23. This is because Fedora 23 uses pre-release X.org driver and Virtualbox does not support yet.
If you encounter the following problem, you will need to downgrade the X.org driver and reinstall Guest Additions. The following result shown after installation of Guest Additions on my Virtualbox. I am using the latest Virtualbox 5.0.8 on Windows 7.
Open Terminal and execute the following command:
dnf –showduplicates –allowerasing –releasever=22 downgrade xorg-x11-server-Xorg
Wait until the process completed and reinstall Guest Additions. Please note that before you attempt to install Guest Additions, make sure you install kernel-devel, dkms and update your kernel. Use this command to do this
dnf update kernel*
dnf install dkms kernel-devel