EntrySign: create your own x86 microcode for fun and profit

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Presented at OffensiveCon 2025 by

Eduardo Nava

Matteo Rizzo

Kristoffer Janke

Josh Eads

Microcode is a fundamental building block for x86 CPUs – implementing everything from complex x86 instructions, to privilege transitions and virtualization. Beyond ring 0 and SMM lies the microcode privilege level, with maximum control over the x86 core and the internal buffers within. Accessing and tinkering with microcode is a hacker’s dream, but cryptographic protections prevent all but Intel and AMD from doing so. Previous work has demonstrated methods to bypass microcode patch authentication on the Intel Atom and reverse engineer the microcode ISA enough to write sophisticated custom microcode patches (Intel Atom Goldmont and AMD K8/10). However, easily accessible arbitrary microcode execution was unavailable on modern high-performance CPUs until now. In this talk we present the full details about 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 4 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. Finally, we are releasing all of the tools necessary to reproduce our work and continue building upon our research – including a short demo showing how to write and install your own microcode patch.