So what is Secure Boot, and why would it say "Image failed to verify with *ACCESS DENIED*." after an Ubuntu kernel update?

@davidbaron Secure boot checks that the bootloader was signed by a recognized authority in the UEFI.

@davidbaron This is to prevent Rootkits to install and run before the OS so that antivirus cannot detect them.

@FremyCompany That still doesn't answer whether my system is compromised, or whether the latest Ubuntu kernel update broke something...