Me and some other students have conducted some various experiments on Secure Boot enabled machines. The main focus of the tests was to circumvent Secure Boot and load unsigned kernels or kernels that have been signed with other keys.
On your SecureBoot (https://wiki.ubuntu.com/SecurityTeam/SecureBoot) it is outlined that GRUB will boot unsigned kernels when the kernel is unsigned. During one of our experiments it seemed that this statement was true and that GRUB loads unsigned kernels as described on your page. We understand that for various reasons GRUB should still support the use-case when an unsigned kernel must be loaded, but with the current approach the user isn't aware if there is a whole chain of trust. For example, it could still be possible to load some malware before it boots the Operating System itself (bootkits). One of the many reasons that Secure Boot has been developed is to protect the user from these kind of attacks.
With the current approach the purpose of Secure Boot is somewhat defeated, and the user doesn't know if the whole chain has been verified or not. It could easily be the case that an unsigned kernel has been loaded by Ubuntu without the user noticing. From our point of view, a better approach would be to inform the user that an unsigned kernel will be loaded and that the user can make a choice if he/she wants to proceed. The default action could be to accept the option, remember the user's option and sometimes remind the user of the fact that it is loading an unsigned kernel.
This problem is of course related to GRUB itself and not to Ubuntu itself. The reason for filing this bug and informing the SecurityTeam of Ubuntu is to ask for their opinions and what your point of view is on the current approach and to see if other users classify this as a "bug".
GRUB2 versions: grub-2.02~beta2, 1.34.1+2.02~beta2-9ubuntu1
Ubuntu version: Trusty (will also affect newer and older versions, GRUB specific problem)