CVE-2024-36347: Ubuntu Linux Kernel EntrySign Flaw (USN-8185-2) — Detection and Remediation Guide

USN-8185-2 details a significant set of security updates for the Linux kernel, specifically highlighting the EntrySign vulnerability (CVE-2024-36347). This flaw allows a privileged attacker to bypass signature verification on CPU microcode for certain AMD Zen processors. This is not just a standard kernel bug; it represents a subversion of the hardware trust boundary. If an attacker gains root access—perhaps through a container escape or a misconfiguration—EntrySign allows them to inject malicious microcode, potentially achieving persistence and evading traditional operating system-based security controls. Given the broad subsystem impact listed (ACPI, Bluetooth, Crypto, TPM), this patch cycle is urgent for defenders managing Linux environments, particularly high-performance computing or workstation fleets running Ubuntu's Low Latency NVIDIA kernel.

Technical Analysis

Affected Products & Platforms:

• OS: Ubuntu Linux (specifically the Low Latency NVIDIA kernel variants)
• Architecture: Primarily AMD Zen processors (for CVE-2024-36347), with additional fixes impacting MIPS, PowerPC, and x86 architectures generally.
• Subsystems: Block layer, Cryptographic API, ACPI drivers, Network block device, Bluetooth, TPM, and Clock subsystems.

CVE-2024-36347 (EntrySign):

• Vulnerability: AMD Zen processors fail to properly verify the signature of CPU microcode updates.
• Attack Vector: Local/Privileged. An attacker with root privileges can load malicious microcode.
• Impact: Loss of integrity and confidentiality. Malicious microcode can theoretically bypass OS-level defenses, hide malware from the kernel, or siphon data invisible to the OS.
• Exploitation Status: While CVE-2024-36347 is a hardware/firmware interaction flaw that requires root access to trigger, it serves as a powerful persistence mechanism for attackers who have already achieved initial access. The additional kernel subsystem flaws (Block layer, Netlink, etc.) could potentially offer paths to that initial root escalation if exploited in-chains.

Detection & Response

Detecting the successful loading of malicious microcode is extremely difficult from within the OS, as the infection occurs below the kernel. However, we can detect the mechanisms used to update microcode and signs of potential exploitation attempts targeting the kernel subsystems mentioned in the advisory.

The following rules and scripts focus on identifying unauthorized microcode update attempts and monitoring for suspicious kernel module or binary interactions related to the affected subsystems (TPM, Bluetooth, Crypto).

---
title: Potential EntrySign Exploitation - Unauthorized Microcode Update Tool
id: 8825a1b0-1c0e-4f8d-9b3e-0a6b4f1c9e8d
status: experimental
description: Detects execution of tools often used to update CPU microcode (e.g., iucode_tool) or direct writes to firmware paths, which could indicate an attempt to exploit CVE-2024-36347 or similar low-level persistence mechanisms.
references:
  - https://ubuntu.com/security/notices/USN-8185-2
author: Security Arsenal
date: 2024/06/18
tags:
  - attack.persistence
  - attack.privilege_escalation
  - attack.t1542.001
logsource:
  category: process_creation
  product: linux
detection:
  selection_tools:
    Image|endswith:
      - '/iucode_tool'
      - '/microcode_ctl'
  selection_suspicious_write:
    CommandLine|contains:
      - '/lib/firmware/intel-ucode'
      - '/lib/firmware/amd-ucode'
    Image|endswith:
      - '/cp'
      - '/dd'
  condition: 1 of selection_*
falsepositives:
  - Legitimate system administration updates performed by package managers (apt/yum).
  - Verified firmware updates performed by sysadmin.
level: high
---
title: Suspicious Kernel Module Loading Related to Vulnerable Subsystems
id: 9c4f2e10-5d6a-4b8c-8e1f-9a2b3c4d5e6f
status: experimental
description: Detects manual insertion of kernel modules or interaction with device files associated with the vulnerable subsystems (Bluetooth, TPM, Netlink) which could indicate probing for the flaws fixed in USN-8185-2.
references:
  - https://ubuntu.com/security/notices/USN-8185-2
author: Security Arsenal
date: 2024/06/18
tags:
  - attack.execution
  - attack.defense_evasion
  - attack.t1014
logsource:
  category: process_creation
  product: linux
detection:
  selection_insmod:
    CommandLine|contains:
      - 'insmod'
      - 'modprobe'
  selection_modules:
    CommandLine|contains:
      - 'bluetooth'
      - 'tpm'
      - 'nbd'  # Network block device
      - 'cryptd'
  condition: all of selection_*
falsepositives:
  - Legitimate hardware driver loading by udev or administrators.
level: medium


**KQL (Microsoft Sentinel / Defender for Linux)**

Hunt for syslog messages indicating microcode reloads or failures, which are the only telemetry signals available for CPU-level changes.

Syslog
| where ProcessName contains "kernel" or SyslogMessage contains "microcode"
| project TimeGenerated, Computer, HostName, ProcessName, SyslogMessage
| where SyslogMessage has "reload" or SyslogMessage has "updated" or SyslogMessage has "patch"
| summarize count() by SyslogMessage, Computer
| order by count_ desc


**Velociraptor VQL**

Hunt for modification times on CPU microcode binaries to identify unauthorized changes on the filesystem.

-- Hunt for recently modified microcode binaries
SELECT FullPath, Mtime, Atime, Size, Mode
FROM glob(globs='/lib/firmware/amd-ucode/*')
WHERE Mtime > now() - 7d  -- Look for changes in the last 7 days

-- Also check Intel paths if relevant hardware exists
SELECT FullPath, Mtime, Atime, Size, Mode
FROM glob(globs='/lib/firmware/intel-ucode/*')
WHERE Mtime > now() - 7d


**Remediation Script (Bash)**

Automate the verification of the installed kernel version against the USN-8185-2 advisory and apply the patch.

#!/bin/bash
# Remediation script for USN-8185-2 (CVE-2024-36347)
# Checks for kernel updates and applies them.

echo "[+] Starting remediation for USN-8185-2..."

# Update package lists
sudo apt-get update -y

# Check if linux-image-generic or linux-image-lowlatency needs updating
# Specifics depend on the Ubuntu version (20.04, 22.04, 24.04)
# This script ensures the latest available security patches are installed.

KERNEL_VERSION=$(uname -r)
echo "[+] Current running kernel: $KERNEL_VERSION"

# Upgrade security packages specifically
sudo apt-get upgrade -y linux-image-generic linux-image-lowlatency linux-headers-generic

# Check if a reboot is required (common with kernel updates)
if [ -f /var/run/reboot-required ]; then
    echo "[!!!] System reboot REQUIRED to apply kernel updates for USN-8185-2."
    cat /var/run/reboot-required.pkgs
else
    echo "[+] No immediate reboot required (or check pending)."
fi

echo "[+] Remediation script complete. Please review the output above."

Remediation

1. Immediate Patching: Apply the updates provided in USN-8185-2 immediately. This advisory covers the Linux kernel for Ubuntu Low Latency NVIDIA kernels.

• Command: sudo apt update && sudo apt upgrade
• Specific Packages: Focus on linux-image-lowlatency, linux-image-lowlatency-nvidia, and linux-headers-lowlatency.

2. System Reboot: Kernel updates require a system reboot to take effect. Merely updating the packages leaves the system vulnerable to EntrySign until the vulnerable kernel code is unloaded from memory.

3. Vendor Advisory: Refer to the official Ubuntu Security Notice for specific package versions matching your distribution release (Focal, Jammy, Noble, etc.):

• Source: https://ubuntu.com/security/notices/USN-8185-2

4. Workarounds: There are no practical workarounds for CVE-2024-36347 (EntrySign) other than updating the kernel to enforce proper microcode signature verification. Restricting root access and enforcing strict privilege access management (PAM) are the only preventative measures to stop the initial prerequisite of the attack.

Related Resources

Security Arsenal Penetration Testing Services AlertMonitor Platform Book a SOC Assessment vulnerability-management Intel Hub