Hello, fellow hackers and pen testers. Welcome to Programming Thursdays! Today, we’re diving into microcode—a crucial but often overlooked component at the foundation of the modern computing stack.

Microcode is the lowest firmware layer in a computer system, below the operating system AND below the BIOS/UEFI. It lives in the gray area between “Software” and “Hardware.” It effectively turns the complex physics of electrons and gates into the logical instructions (x86_64) that we write.

1. How Microcode Works: The Abstract Machine

At a high level, microcode is an interpreter.

The Problem: CISC Complexity

Modern CPUs use a CISC (Complex Instruction Set Computer) architecture like x86. Instructions like REP MOVSB (Repeat Move String Byte) are massively complex. They involve loops, memory reads, memory writes, and counter updates—all in one instruction.

Creating a hard-wired physical circuit for every possible x86 instruction would be impossible (and incredibly expensive).

The Solution: The RISC Core

Inside deeply, an Intel or AMD processor behaves like a RISC (Reduced Instruction Set Computer) machine. It understands simple uOps (Micro-operations) like “Load”, “Store”, “Add”.

Microcode controls the Instruction Decoder. When the CPU sees REP MOVSB, the microcode sequencer looks up a routine in the internal ROM that translates this single complex instruction into a stream of simple uOps.

2. The Microcode Update Process: A Volatile Patch

One of the most critical things for a red teamer or reverse engineer to understand is that microcode updates are NOT persistent.

Modern CPUs have a small area of high-speed SRAM (Static RAM) dedicated to holding microcode updates. Because it’s RAM, the data is lost when the power is cut.

The Boot Chain

  1. Hard Reset: CPU loads basic microcode from internal Read-Only Memory (ROM) baked into the silicon.
  2. BIOS/UEFI: The motherboard firmware contains a later microcode blob. As the BIOS initializes, it uploads this blob to the CPU via MSR write.
  3. Operating System: The OS (Windows or Linux) carries its own microcode library (usually newer than the BIOS). Early in the boot process, it uploads its version.

Implication: If you can downgrade the OS or block the OS-level microcode loader, you can re-expose the system to vulnerabilities like Spectre/Meltdown or Downfall, even if the silicon is theoretically patched.

3. Security: Signatures, Encryption, and Trust

If microcode controls the CPU, what stops an attacker from uploading malicious microcode to create a hardware backdoor?

Cryptography.

The Integrity Check

Microcode updates are signed by the vendor (Intel/AMD) using RSA (usually 2048-bit or 3072-bit).

  1. The update blob contains the microcode data + a signature.
  2. The CPU has a hash of the Public Key burned into its hardware fuses.
  3. When an update is written to the special Model Specific Register (MSR 0x79 on Intel), the CPU’s hardware logic calculates the hash, verifies the signature, and only applies it if it matches.

The Weakness: Red Unlock

While we (currently) cannot forge the signature, researchers have found bugs in the parser. In 2020, the CTBurts vulnerability (CVE-2020-8696) and “Red Unlock” techniques showed that on some Intel Goldmont CPUs, you could bypass the signature check and execute unsigned microcode. This allows for:

  • Dumping internal CPU ROM.
  • Implementing hardware trojans.
  • Bypassing security fences like SGX.

4. Hardware Security Features and Failures

Microcode isn’t just for execution; it’s the enforcement layer for modern security features.

Intel SGX (Software Guard Extensions)

SGX relies on microcode to ensure that even the Kernel or Hypervisor cannot read the memory of an “Enclave.” Vulnerabilities like LVI (Load Value Injection) or SGAxe exploited side-channels in the microcode logic to leak these secrets.

AMD SEV (Secure Encrypted Virtualization)

Similar to SGX, SEV relies on the firmware (PSP) and microcode to handle encryption keys.

5. Microarchitectural Attacks: The New Normal

The last few years have seen a surge in attacks that target the CPU’s optimization logic.

1. Speculative Execution (Meltdown/Spectre)

These attacks exploit the CPU’s attempt to guess the future.

  • The Flaw: The CPU executes code speculatively. If the guess is wrong, it rolls back the architectural state (registers), but leaves traces in the microarchitectural state (Cache).
  • The Fix: Microcode updates introduced “Fences” (like IBPB: Indirect Branch Prediction Barrier) that software developers can use to tell the CPU “Stop guessing here.”

2. Downfall (CVE-2022-40982) [2023]

Targeting Intel CPUs (Skylake through Rocket Lake), Downfall exploits the GATHER instruction. By using the gather buffer, an attacker can leak data from other processes or SGX enclaves.

  • Mitigation: A microcode update that disables the optimization, causing significant performance hits (up to 50% in AVX workloads).

3. Zenbleed (CVE-2023-20593) [2023]

A devastating bug in AMD Zen 2 processors. It allows an unprivileged user to steal data (like RSA keys) from other processes, virtual machines, or the kernel at 30kb/s.

  • Cause: Incorrect handling of the vzeroupper instruction in the microcode.

6. Forensics: Checking the Version

As a red teamer, verifying the target’s patch level tells you if you can use a proof-of-concept like ZombieLoad, Downfall, or Zenbleed.

Linux

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# Check current microcode version
grep "microcode" /proc/cpuinfo | head -n 1

# Check vulnerability status (Kernel sysfs)
grep . /sys/devices/system/cpu/vulnerabilities/*

Windows (PowerShell)

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# Get the Revision (Microcode version)
# The value is hex. 0xBA = 186
reg query "HKLM\HARDWARE\DESCRIPTION\System\CentralProcessor\0" /v "Update Revision"

Conclusion

Microcode is the “Ghost in the Silicon.” It’s the layer that defines how our hardware behaves, yet it remains largely invisible. For the elite red teamer, understanding microcode is about understanding the ultimate boundary of security.

As we move toward more hardware-enforced security, the battle for control will shift from Ring 0 (Kernel) down to the microcode.

Happy hacking!

References