CVE-2024-38433: Nuvoton NPCM750R Auth Bypass Vulnerability

CVE-2024-38433 Overview

CVE-2024-38433 is an authentication bypass vulnerability (CWE-305) affecting Nuvoton NPCM7xx Baseboard Management Controller (BMC) subsystems. An attacker with write access to the SPI-Flash can modify the u-boot image header on flash that is parsed by the BootBlock reference code, potentially leading to arbitrary code execution at the firmware level.

This vulnerability targets the boot process of BMC systems, which are critical components used for remote server management in data centers and enterprise environments. The BootBlock's failure to properly authenticate the u-boot image header before parsing allows attackers to inject malicious code that executes during the boot sequence.

Critical Impact
Attackers with SPI-Flash write access can achieve arbitrary code execution on Nuvoton NPCM7xx BMC subsystems by manipulating the u-boot image header, potentially compromising server management infrastructure.

Affected Products

Nuvoton NPCM750R Firmware
Nuvoton NPCM730R Firmware
Nuvoton NPCM710R Firmware
Nuvoton NPCM705R Firmware
Nuvoton NPCM750R Hardware
Nuvoton NPCM730R Hardware
Nuvoton NPCM710R Hardware
Nuvoton NPCM705R Hardware

Discovery Timeline

July 11, 2024 - CVE-2024-38433 published to NVD
November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2024-38433

Vulnerability Analysis

CVE-2024-38433 represents an authentication bypass vulnerability in the boot chain of Nuvoton NPCM7xx BMC processors. The vulnerability exists in the BootBlock reference code, which fails to properly validate the integrity and authenticity of the u-boot image header stored on SPI-Flash memory before parsing it.

BMC (Baseboard Management Controller) systems are embedded controllers used for out-of-band server management, providing administrators with remote access to servers regardless of the operating system state. The NPCM7xx series is Nuvoton's second-generation BMC solution designed for server platforms.

During the boot process, the BootBlock reads and parses the u-boot image header from SPI-Flash to determine how to load and execute the u-boot bootloader. The lack of proper authentication on this header allows an attacker who has gained write access to the SPI-Flash to modify critical fields, redirecting execution to attacker-controlled code.

Root Cause

The root cause of this vulnerability is the absence of proper authentication mechanisms (CWE-305: Authentication Bypass by Primary Weakness) in the BootBlock reference code. The code trusts the u-boot image header data read from SPI-Flash without verifying its cryptographic signature or integrity. This primary weakness in authentication allows the boot chain to be compromised.

Additionally, the vulnerability is classified under CWE-287 (Improper Authentication), as the system fails to prove that a claimed entity or data is authentic before proceeding with critical boot operations.

Attack Vector

The attack requires local access with high privileges to write to the SPI-Flash storage on the target system. An attacker must first obtain write access to the SPI-Flash, which could be achieved through:

Physical access to the hardware to directly interface with the SPI-Flash chip
Exploiting another vulnerability that grants flash write permissions
Compromising administrative credentials for the BMC interface
Supply chain attacks during manufacturing or maintenance

Once write access is obtained, the attacker can modify the u-boot image header fields that control the boot process. The BootBlock parses these header fields without authentication, allowing the attacker to redirect code execution to malicious payloads stored elsewhere on the flash or to corrupt the boot process in ways that facilitate further exploitation.

The attack mechanism involves manipulating header fields such as entry point addresses, image sizes, or load addresses to cause the BootBlock to load and execute arbitrary code. Since this occurs early in the boot sequence, the attacker gains control before higher-level security mechanisms are initialized.

Detection Methods for CVE-2024-38433

Indicators of Compromise

Unexpected modifications to SPI-Flash contents, particularly in the u-boot image header region
Changes to firmware checksums or hash values during integrity verification
Anomalous BMC behavior during boot sequences or unexpected boot failures
Unauthorized access attempts to flash programming interfaces

Detection Strategies

Implement firmware integrity monitoring to detect unauthorized modifications to SPI-Flash contents
Enable and monitor secure boot logs for authentication failures or unexpected boot paths
Deploy hardware security modules that can verify flash contents before boot
Monitor BMC access logs for unauthorized administrative access that could precede flash modification

Monitoring Recommendations

Establish baseline firmware hashes and regularly verify SPI-Flash contents against known-good values
Configure alerts for any access to flash programming interfaces or utilities
Monitor physical access to server hardware that could indicate tampering attempts
Implement continuous monitoring of BMC health and configuration status

How to Mitigate CVE-2024-38433

Immediate Actions Required

Review and restrict access to SPI-Flash write capabilities on affected NPCM7xx systems
Implement hardware write protection for SPI-Flash where possible
Audit BMC administrative access and credentials to prevent unauthorized flash modifications
Monitor affected systems for signs of firmware tampering

Patch Information

Organizations using affected Nuvoton NPCM7xx BMC subsystems should consult the Israeli Government CVE Advisories for official guidance and updates. Contact Nuvoton directly or through your server OEM vendor for updated BootBlock reference code that includes proper u-boot image header authentication.

When patching, ensure that:

Firmware updates are obtained from verified vendor sources
Updates are cryptographically verified before installation
Post-update verification confirms the patch was applied correctly

Workarounds

Enable hardware write-protect jumpers or pins on SPI-Flash chips if available on your hardware platform
Restrict physical access to servers containing affected BMC hardware
Implement network segmentation to isolate BMC management interfaces from untrusted networks
Disable unused BMC interfaces and features to reduce attack surface
Consider implementing an external trusted platform module (TPM) to verify boot integrity

bash

# Example: Check SPI-Flash write protection status (platform-specific)
# Verify your hardware documentation for specific commands
flashrom -p internal --wp-status

# Enable write protection if supported (example only - verify for your platform)
flashrom -p internal --wp-enable