CVE-2024-6285

Executive Summary

CVE-2024-6285 is an integer underflow vulnerability in Renesas arm-trusted-firmware (ATF). The flaw resides in image range check calculations, where an integer underflow (wrap or wraparound) can occur. This allows an attacker to bypass address restrictions and load firmware images to unallowed memory regions. With a CVSS score of 7.5, the vulnerability is considered high severity, primarily impacting the integrity of the system by enabling unauthorized code execution in secure memory areas. No public exploit or patch is currently available, but the risk is significant for embedded systems using Renesas ATF.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It is a firmware-level issue in Renesas arm-trusted-firmware, which is typically distributed as source code or binary blobs by Renesas or integrated into board support packages (BSPs). Organizations using Renesas SoCs should verify the ATF version in their supply chain and monitor for vendor updates.

CVSS Breakdown & Threat Level

• CVSS Score: 7.5 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
• Attack Vector: Network (AV:N) – The vulnerability can be exploited remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: None (UI:N) – No user action required.
• Scope: Unchanged (S:U) – The exploit does not affect other components.
• Confidentiality Impact: None (C:N) – No data disclosure.
• Integrity Impact: High (I:H) – Unauthorized image loading can corrupt firmware.
• Availability Impact: None (A:N) – No denial of service.

Threat Level: High. The ability to load images to arbitrary memory regions can lead to secure boot bypass, privilege escalation, and persistent compromise of embedded devices.

Deep Dive & Mechanics

The vulnerability is an integer underflow (CWE-191) in the image range check logic of Renesas arm-trusted-firmware. When validating the memory region for loading a firmware image, the code performs subtraction operations on size or address values. If an attacker provides a crafted image header with a size value that causes an integer underflow (e.g., a very small or negative value when interpreted as unsigned), the resulting calculation wraps around to a large positive number. This bypasses the intended address restriction, allowing the image to be loaded into protected memory regions such as secure RAM or boot ROM.

Root Cause: The lack of proper bounds checking before arithmetic operations on image size or offset fields. The underflow occurs when subtracting a larger value from a smaller unsigned integer, producing a result that is unexpectedly large.

Attack Vector: An attacker with network access to the device (e.g., during firmware update or boot process) can send a malicious firmware image with manipulated header fields. The integer underflow in the range check allows the image to be placed in arbitrary memory, potentially overwriting critical secure firmware.

Impact: Successful exploitation can lead to:

• Secure boot bypass
• Execution of arbitrary code in the secure world (TrustZone)
• Persistent firmware backdoors
• Full compromise of the device’s security model

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

// Vulnerable: integer underflow in range check
uint32_t image_size = get_image_size(header);
uint32_t base_addr = get_load_address(header);
uint32_t end_addr = base_addr + image_size;

if (end_addr > MAX_ALLOWED_ADDR) {
    // This check can be bypassed if image_size underflows
    return ERROR;
}

Patched Code (Conceptual):

// Patched: validate before arithmetic
uint32_t image_size = get_image_size(header);
uint32_t base_addr = get_load_address(header);

if (image_size > MAX_ALLOWED_ADDR - base_addr) {
    // Prevent underflow by checking size against remaining space
    return ERROR;
}
uint32_t end_addr = base_addr + image_size;

The patched version ensures that the addition does not overflow and that the image size is within the allowed range before performing arithmetic.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Access to a device running vulnerable Renesas ATF (e.g., R-Car, RZ/G series)
• Ability to send a crafted firmware image (e.g., via network update or JTAG)
• Knowledge of the target’s memory layout

Exploitation Steps:

1. Craft a malicious firmware image with a manipulated header where the image_size field is set to a value that causes underflow (e.g., 0xFFFFFFFF minus a small offset).
2. Send the image to the target device during the boot or update process.
3. The vulnerable range check underflows, allowing the image to be loaded into a protected memory region (e.g., secure RAM at address 0x00000000).
4. The loaded code executes with elevated privileges, bypassing secure boot.

Example Payload (Conceptual):

# Craft malicious image header
import struct

# Target: load to address 0x00000000 (secure RAM)
# image_size = 0xFFFFFFFF - (MAX_ALLOWED_ADDR - 0x1000) to underflow
image_size = 0xFFFFFFFF - (0x80000000 - 0x1000)  # Example underflow
header = struct.pack('<II', 0x00000000, image_size)  # base_addr, size
payload = header + b'\x90' * 0x1000  # NOP sled + shellcode

Blue Team

Detection:

• Monitor for anomalous firmware update requests with unusual size fields.
• Implement integrity checks on image headers before loading.
• Use hardware memory protection units (MPUs) to enforce strict region permissions.

Hunting:

• Search logs for failed range check errors or unexpected memory accesses.
• Analyze network traffic for firmware images with size fields that cause underflow (e.g., size > 0x80000000).

Mitigation:

• Apply vendor patch when available.
• If no patch, implement a software workaround by adding explicit bounds checks before arithmetic operations.
• Restrict network access to firmware update interfaces.

Indicators of Compromise (IOCs)

• Network: Firmware update packets with image size field > 0x80000000 (indicating potential underflow)
• Memory: Unexpected writes to secure memory regions (e.g., address 0x00000000-0x0000FFFF)
• Logs: Error messages related to image range check failures or memory access violations
• File Hashes: Malicious firmware images with manipulated headers (SHA256: TBD)

Detection Engineering

YARA Rule

rule CVE_2024_6285_Integer_Underflow {
    meta:
        description = "Detects malicious firmware images exploiting CVE-2024-6285 integer underflow"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-6285"
    strings:
        $header_magic = { 41 54 46 00 }  // Example ATF magic bytes
        $large_size = /[\x80-\xff]{4}/   // Size field > 0x80000000
    condition:
        $header_magic at 0 and $large_size at 4
}

Sigma Rule

title: CVE-2024-6285 Firmware Image Integer Underflow
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects attempts to load firmware images with size fields indicating integer underflow
author: Valters IT
logsource:
    product: linux
    service: syslog
detection:
    selection:
        - "image size check failed"
        - "address range violation"
        - "unexpected memory write"
    condition: selection
falsepositives:
    - Legitimate firmware updates with large images (rare)
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2024-6285 Firmware Image Integer Underflow"; content:"|41 54 46 00|"; offset:0; depth:4; content:"|ff ff ff ff|"; within:4; distance:4; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected devices from untrusted networks to prevent remote exploitation.
2. Monitor vendor advisories from Renesas for official patches.
3. Implement network segmentation for firmware update interfaces.

Long-Term Fix:

• Apply the vendor patch when released.
• If no patch is available, modify the ATF source code to add bounds checks before arithmetic operations (see patched code example).
• Use hardware security features (e.g., TrustZone, MPU) to enforce memory protections even if firmware loading is compromised.

Workaround:

• Disable remote firmware update capabilities if not essential.
• Use signed firmware images with cryptographic verification to prevent tampering.

Valters IT – Securing embedded systems through proactive threat intelligence.