We present EntrySign, a cryptographic flaw in AMD’s microcode patch verification logic, including how we discovered the bug and how you can extend our results. EntrySign lets us execute arbitrary microcode on all AMD CPUs from Zen to Zen 5 and modify the behavior of x86 instructions. We will delve into the format of AMD microcode, how their patches are verified, how we were able to reverse engineer this process, and how we were able to access the key information required to defeat it.