i am not sure what "main code" is supposed to mean (i am no native speaker). but there is a reference implementation by intel and according to matthew garret most real-world implementations are only slightly changed.
"We've seen similar bugs in Intel's reference code in the past, but they were all fixed early last year"
So, this might actually be a bug written by Intel.
Problem is these 'slight changes' sometimes are what is wrong.
And this is not only about secure boot, but the whole UEFI infrastructure, which, as far as I know, is very complex.
From Garrets linuxconf.au talk, year ago. As it is reference implementation, it is pretty much untested in the wild. So you can expect those "slight changes" to be mostly bugfixes.
Punchline is that Intel DOES provide pretty much all code for UEFI. And it is still bad.