CVE-2024-36347: Linux Kernel Azure FIPS EntrySign Vulnerabilities — Detection and Remediation

Ubuntu has released USN-8260-1, addressing critical security vulnerabilities in the Linux kernel, specifically impacting the Azure FIPS kernel implementation. The most significant issue in this batch is CVE-2024-36347, dubbed "EntrySign." This flaw affects AMD Zen processors and allows a privileged attacker to bypass signature verification for CPU microcode.

For defenders, this represents a high-risk scenario. While exploitation requires root privileges, the payload—malicious microcode—executes at a hardware level below the operating system. Successful exploitation compromises the integrity and confidentiality of the entire system, effectively rendering OS-based security controls (EDR, SIEM agents) blind to malicious activity at the CPU instruction level. Immediate patching is mandated to maintain the security posture of Linux infrastructure, particularly in cloud environments utilizing Azure FIPS images.

Technical Analysis

Affected Products and Platforms:

• OS: Ubuntu Linux (specifically Azure FIPS kernels)
• Architecture: AMD Zen processors (EntrySign), MIPS, PowerPC, x86
• Subsystems: Block layer, Cryptographic API, ACPI drivers, Network block device, Bluetooth, TPM, Character device, Clock.

CVE-2024-36347 (EntrySign) Details:

• Vulnerability: AMD Zen processors fail to properly verify the signature of CPU microcode updates.
• Attack Vector: Local. An attacker with root privileges can load unsigned, malicious microcode.
• Impact: Total compromise of system integrity and confidentiality. Malicious microcode can create a persistent backdoor that survives OS reinstalls and is invisible to the OS.
• CVSS: High (Est. ~7.0+ due to high impact on integrity/confidentiality, though requiring high privileges).

Additional Kernel Flaws: The update also rectifies issues in the Cryptographic API and TPM drivers, which could potentially lead to information disclosure or denial of service.

Exploitation Status: Currently, exploitation is considered theoretical but technically feasible for adversaries who have already achieved root access (e.g., via a container escape or application vulnerability). Once root is obtained, CVE-2024-36347 provides a mechanism for persistence and elevation that is nearly impossible to detect with standard software tools.

Detection & Response

Because the exploitation of CVE-2024-36347 occurs at the microcode/hardware level after a userland process gains root, direct detection of the microcode modification is extremely difficult from within the OS. However, we can detect the prerequisite activity: unauthorized access to the Model Specific Registers (MSRs) used to load microcode, or the presence of tools attempting to interact with CPU microcode.

Sigma Rules

The following Sigma rule detects suspicious access to /dev/cpu/*/msr devices. While legitimate microcode updates occur via standard daemons (like microcode_ctl), manual interaction or access by non-standard binaries is a strong indicator of an attempt to manipulate CPU state.

---
title: Potential CPU Microcode Manipulation - MSR Device Access
id: a4b2c8d1-9e3f-4a7b-8c5d-1e2f3a4b5c6d
status: experimental
description: Detects processes opening /dev/cpu/*/msr devices, which is required for loading microcode. While legitimate updates use specific tools, arbitrary access by other processes may indicate an attempt to exploit vulnerabilities like EntrySign.
references:
  - https://ubuntu.com/security/notices/USN-8260-1
  - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2024-36347
author: Security Arsenal
date: 2025/05/22
tags:
  - attack.privilege_escalation
  - attack.t1068
logsource:
  product: linux
  category: file_access
detection:
  selection:
    TargetFile|contains: '/dev/cpu/'
    TargetFile|endswith: '/msr'
  filter_legit_tools:
    Image|endswith:
      - '/microcode_ctl'
      - '/systemd-udevd'
  condition: selection and not filter_legit_tools
falsepositives:
  - Legitimate system administration tools checking CPU MSR stats (rare)
level: high
---
title: Linux Kernel Vulnerability - USN-8260-1 Check
id: b5c3d9e2-0f4a-5b8c-9d6e-2f3a4b5c6d7e
status: experimental
description: Identifies systems running vulnerable kernel versions prior to the USN-8260-1 update. Note: Specific kernel versions must be verified against the Ubuntu security notice.
references:
  - https://ubuntu.com/security/notices/USN-8260-1
author: Security Arsenal
date: 2025/05/22
tags:
  - attack.vulnerability_scanning
logsource:
  product: linux
  category: system
detection:
  selection:
    KernelVersion:
      # Example vulnerable signatures - adjust based on specific versions in USN-8260-1
      - '*5.15.0-*'
      - '*6.5.0-*'
  filter_patched:
    # Generic placeholder for patched versions; update with actual secure versions post-update
    KernelVersion|contains: '-generic'
  condition: selection and not filter_patched
falsepositives:
  - Systems running non-standard or custom kernels not affected by this USN
level: medium

KQL (Microsoft Sentinel)

Hunt for suspicious MSR device access or potential kernel modification attempts via Syslog.

// Hunt for processes accessing MSRs (Model Specific Registers)
Syslog
| where ProcessName != "" and SyslogMessage contains "/dev/cpu/"
| where SyslogMessage matches regex @"/dev/cpu/[\d]+/msr"
| project TimeGenerated, Computer, ProcessName, SyslogMessage
| summarize count() by Computer, ProcessName
| where count_ > 0

Velociraptor VQL

This artifact hunts for processes that have open file handles to the CPU MSR devices, indicating potential microcode manipulation attempts. It also checks the current kernel version.

-- Hunt for suspicious MSR device access
SELECT 
  Pid, 
  Name, 
  Username, 
  Exe,
  Cwd
FROM process_open_handles()
WHERE FullPath =~ "/dev/cpu/.*" AND FullPath =~ "msr"

-- Identify running kernel version for patch verification
SELECT 
  Fqdn,
  read_file(path="/proc/version") AS KernelVersion
FROM info()

Remediation Script (Bash)

Use the following script to check for the availability of the USN-8260-1 security update and apply it.

#!/bin/bash

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

# Check if linux-image-azure-fips security updates are available
# Note: USN-8260-1 specifically addresses the Azure FIPS kernel
# Adjust the package name if running standard Ubuntu generic images
KERNEL_PKG="linux-image-azure-fips"

if apt-cache policy "$KERNEL_PKG" | grep -q "Candidate:"; then
    echo "[+] Checking for security updates for $KERNEL_PKG..."
    # A more robust check involves checking apt-cache policy vs installed version
    # but for remediation purposes, we attempt the upgrade.
fi

echo "[+] Applying security updates (USN-8260-1)..."
# --only-upgrade ensures we don't install new kernels if not needed, saving space/time
sudo apt-get install --only-upgrade -y "$KERNEL_PKG" linux-headers-azure-fips

# Check reboot requirement
if [ -f /var/run/reboot-required ]; then
    echo "[!] System requires a REBOOT to complete the kernel patch."
    cat /var/run/reboot-required.pkgs
else
    echo "[+] No immediate reboot required (though recommended for kernel updates)."
fi

# Verify current kernel
echo "[+] Current running kernel:"
uname -r

Remediation

1. Patch Immediately: Apply the updates provided in USN-8260-1. For Azure FIPS instances, run sudo apt-get update && sudo apt-get install --only-upgrade linux-image-azure-fips.
2. System Reboot: Kernel updates require a reboot to load the patched version and the new microcode handling logic. Schedule downtime immediately.
3. Verify Post-Patch: After reboot, ensure the kernel version matches the version listed in the USN as fixed.
4. Audit Privileged Access: Since exploitation of CVE-2024-36347 requires root access, review logs for suspicious sudo usage or privilege escalation attempts that occurred prior to patching. If an attacker had root access, they may have already compromised the microcode, in which case a patch alone will not remove the malicious firmware (though it prevents future loads).

Related Resources

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