CVE-2024-36347 (EntrySign) & USN-8245-1: AMD Zen and Linux Kernel Defense Guide

Security teams must immediately prioritize the deployment of USN-8245-1. This update addresses a critical security flaw, CVE-2024-36347 (dubbed "EntrySign"), alongside several other subsystem vulnerabilities.

The standout issue in this release is the EntrySign vulnerability, discovered by prominent researchers including Tavis Ormandy and Matteo Rizzo. It affects AMD Zen processors and represents a significant breach in the trust boundary between the operating system and the CPU hardware. While exploitation requires local privileges, the impact—loading malicious CPU microcode—fundamentally undermines the integrity and confidentiality of the entire system.

This advisory also corrects flaws in the MIPS, PowerPC, and x86 architectures, as well as the Block layer, Cryptographic API, and Bluetooth drivers. For defenders, this is not a routine maintenance update; it is a mandatory integrity control.

Technical Analysis

Vulnerability: CVE-2024-36347 (EntrySign) Affected Component: Linux Kernel / AMD Zen Microcode Loading Attack Vector: Local Privileges Required: High (Root/Administrator) Impact: Integrity Loss, Confidentiality Loss, Execution of Arbitrary Code at CPU Level

Mechanics of the EntrySign Flaw

The EntrySign vulnerability stems from a failure in certain AMD Zen processors to properly verify the digital signature of CPU microcode updates. The Linux kernel facilitates the loading of this microcode. A privileged attacker capable of executing code on the target system can exploit this verification gap to inject and load malicious microcode onto the CPU.

Because microcode operates at a level below the operating system hypervisor and kernel, successful exploitation allows an attacker to bypass traditional OS-based security controls, potentially establishing a persistent, undetectable foothold within the hardware itself.

Other Subsystems Affected

In addition to EntrySign, USN-8245-1 patches vulnerabilities in:

• Architecture Support: MIPS, PowerPC, User-Mode Linux (UML), x86
• Core Subsystems: Block layer, Cryptographic API, ACPI drivers
• Drivers: Ublk userspace block driver, Bluetooth, Character device drivers

Exploitation Status

Currently, there is no indication of widespread, in-the-wild exploitation of CVE-2024-36347. However, the publication of the vulnerability details by high-profile researchers increases the probability of weaponization. Given the severity (privilege escalation to hardware level), organizations should assume active scanning for vulnerable kernels is imminent or ongoing.

Detection & Response

Detecting the exploitation of kernel-level microcode loading is challenging. Traditional EDR agents operate at the OS level and may miss or be subverted by malicious microcode. However, we can detect the prerequisites and the immediate side-effects of manipulation attempts.

The following detection logic focuses on identifying unusual access to Model Specific Registers (MSRs) via userspace tools (a common method for testing or exploiting CPU flaws) and monitoring kernel logs for microcode update failures.

---
title: Potential AMD Zen Hardware Manipulation via Userspace MSR Access
id: 9c8f5a21-7b3d-4f1a-9a10-e3d5c8f7b6a2
status: experimental
description: Detects userspace processes accessing /dev/cpu/*/msr. Legitimate microcode updates typically occur via kernel mechanisms during boot; direct userspace MSR access is often indicative of exploitation attempts, hardware debugging, or rootkit behavior targeting CPU registers.
references:
  - https://attack.mitre.org/techniques/T1068/
author: Security Arsenal
date: 2025/04/01
tags:
  - attack.privilege_escalation
  - attack.t1068
logsource:
  product: linux
  category: process_creation
detection:
  selection:
    CommandLine|contains:
      - '/dev/cpu/'
      - 'rdmsr'
      - 'wrmsr'
    Image|endswith:
      - 'msr-tools'
      - 'cpuid'
  condition: selection
falsepositives:
  - Legitimate system administration diagnostics (rare)
level: high
---
title: Linux Kernel Microcode Update Failure
id: 1a2b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects kernel logs indicating a failure in CPU microcode updates. While updates can fail due to corruption, a sudden series of failures on a healthy system may indicate an attacker attempting to load malformed microcode (e.g., testing CVE-2024-36347).
references:
  - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2024-36347
author: Security Arsenal
date: 2025/04/01
tags:
  - attack.defense_evasion
  - attack.t1068
logsource:
  product: linux
  service: kernel
detection:
  selection:
    message|contains:
      - 'microcode: update failed'
      - 'loading microcode failed'
      - 'bad microcode'
  condition: selection
falsepositives:
  - Hardware malfunction during legitimate update
level: medium

KQL (Microsoft Sentinel / Defender)

// Hunt for Microcode Update Failures or MSR Access
Syslog
| where ProcessName contains "kernel" or SyslogMessage contains "microcode"
| where SyslogMessage has_any ("failed", "error", "signature", "rejected")
| project TimeGenerated, Computer, SyslogMessage, FacilityLevel
| summarize count() by Computer, SyslogMessage
| order by count_ desc

Velociraptor VQL

-- Hunt for processes interacting with CPU MSRs
SELECT Pid, Name, Exe, CommandLine, Cwd
FROM pslist()
WHERE Exe =~ 'msr'
   OR CommandLine =~ '/dev/cpu'
   OR Name =~ 'cpuid'

-- Check Kernel Release Version
SELECT Fqdn, OSRelease, KernelVersion, Architecture
FROM info()

Remediation Script (Bash)

#!/bin/bash
# Remediation for USN-8245-1 (CVE-2024-36347)
# Applies to Ubuntu systems. Verify USN matches your specific release.

echo "[+] Initiating Remediation for CVE-2024-36347..."

# Update package lists
echo "[+] Updating package lists..."
apt-get update -y

# Install the security updates for the kernel
echo "[+] Applying Kernel Security Updates..."
apt-get install -y linux-image-generic linux-headers-generic

# Verify if a reboot is required (checks if /var/run/reboot-required exists)
if [ -f /var/run/reboot-required ]; then
    echo "[!] SUCCESS: Updates installed."
    echo "[!] CRITICAL: A system reboot is REQUIRED to load the new kernel and microcode."
    cat /var/run/reboot-required.pkgs
else
    echo "[-] System appears up to date or no reboot required for packages."
fi

Remediation

1. Patch Immediately: Apply the updates provided in USN-8245-1. This updates the Linux kernel to versions that properly handle microcode loading for AMD Zen processors and patch the other architectural flaws.
2. System Reboot: Kernel updates require a reboot to take effect. Simply running the update command is insufficient; the vulnerable code remains active in memory until the system restarts.
3. Verify Firmware Updates: While the OS patch mitigates the software-side verification failure, ensure your system BIOS/firmware is up to date. Hardware vendors often release microcode updates via firmware that cooperate with OS protections.
4. Audit Privileged Access: Since exploitation requires root privileges, reinforce your access controls. Ensure that local sudo access is minimized and strictly audited.

Official Vendor References:

• Ubuntu Security Notice USN-8245-1: https://ubuntu.com/security/notices/USN-8245-1
• NVD Entry for CVE-2024-36347: https://nvd.nist.gov/vuln/detail/CVE-2024-36347

Related Resources

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