The author of: Super-UEFIinSecureBoot-Disk < https://firmwaresecurity.com/2019/02/27/super-uefiinsecureboot-disk-is-a-bootable-image-with-grub2-bootloader-designed-to-be-used-as-a-base-for-recovery-usb-flash-drives/ > has a new post on Habr.com (in Russian) on UEFI Secure Boot security.
Excerpt of Google Translation: […]In this article we proved the existence of not enough reliable bootloaders signed by Microsoft key, which allows booting untrusted code in Secure Boot mode. Using signed Kaspersky Rescue Disk files, we achieved a silent boot of any untrusted .efi files with Secure Boot enabled, without the need to add a certificate to UEFI db or shim MOK. These files can be used both for good deeds (for booting from USB flash drives) and for evil ones (for installing bootkits without computer owner consent).[…]