CVE-2024-25076

Executive Summary

CVE-2024-25076 is a critical buffer overflow vulnerability in the bootrom of Renesas SmartBond DA14691, DA14695, DA14697, and DA14699 devices. The flaw resides in the bootrom function responsible for validating the Flash Product Header. It directly uses a user-controllable size value (Length of Flash Config Section) to control a read from the QSPI device into a fixed-sized buffer, leading to a buffer overflow and potential execution of arbitrary code. This vulnerability can be exploited by an attacker with physical or low-level access to the device, allowing them to bypass secure boot and execute malicious firmware. No public proof-of-concept or patch details are currently available.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (No official vector assigned)
• Attack Vector (AV): Physical (assumed, due to bootrom access)
• Attack Complexity (AC): Low
• Privileges Required (PR): None
• User Interaction (UI): None
• Scope (S): Changed
• Confidentiality (C): High
• Integrity (I): High
• Availability (A): High
• Threat Level: Critical (due to potential for arbitrary code execution in bootrom)

Deep Dive & Mechanics

The vulnerability exists in the bootrom code of Renesas SmartBond DA14691, DA14695, DA14697, and DA14699 devices. During the boot process, the bootrom reads a Flash Product Header from the external QSPI flash memory. This header contains a field called "Length of Flash Config Section" which is user-controllable. The bootrom function responsible for validating this header uses this length value directly to control the number of bytes read from the QSPI device into a fixed-sized buffer on the stack. If the attacker provides a length value larger than the buffer size, a stack-based buffer overflow occurs, allowing the attacker to overwrite return addresses and other critical data, leading to arbitrary code execution.

The attack requires the attacker to have physical access to the device or the ability to modify the external QSPI flash contents (e.g., through a compromised update mechanism). Once exploited, the attacker can execute arbitrary code at the bootrom level, effectively bypassing all security mechanisms such as secure boot and firmware integrity checks.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

void validate_flash_header() {
    uint8_t buffer[64];
    uint32_t length = read_qspi_word(FLASH_HEADER_LENGTH_OFFSET);
    // No bounds check on length
    read_qspi(buffer, length);
    // Process buffer...
}

Patched Code (Conceptual):

void validate_flash_header() {
    uint8_t buffer[64];
    uint32_t length = read_qspi_word(FLASH_HEADER_LENGTH_OFFSET);
    if (length > sizeof(buffer)) {
        // Reject invalid length
        return;
    }
    read_qspi(buffer, length);
    // Process buffer...
}

The patched code adds a bounds check to ensure the length does not exceed the buffer size before performing the read operation.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Physical access to the target device (e.g., Renesas SmartBond DA14691)
• Ability to modify the external QSPI flash contents (e.g., using a flash programmer)
• Knowledge of the bootrom memory layout and buffer size

Exploitation Steps:

1. Identify the offset of the "Length of Flash Config Section" field in the Flash Product Header.
2. Craft a malicious Flash Product Header with a length value larger than the buffer size (e.g., 256 bytes for a 64-byte buffer).
3. Append shellcode or arbitrary code after the header, ensuring it aligns with the overflowed buffer.
4. Write the modified header to the QSPI flash at the appropriate location.
5. Power cycle the device to trigger the bootrom validation.
6. The buffer overflow overwrites the return address, redirecting execution to the attacker's shellcode.

Example Payload Structure:

[Flash Product Header]
  - Length of Flash Config Section: 0x100 (256 bytes)
  - Other fields...
[Padding to fill buffer]
[Overwritten return address pointing to shellcode]
[Shellcode]

Blue Team

Detection and Mitigation:

• Monitor for unexpected modifications to external QSPI flash contents.
• Implement hardware-based write protection for the QSPI flash.
• Use secure boot mechanisms that verify the integrity of the Flash Product Header before validation.
• Deploy runtime integrity monitoring to detect anomalous bootrom behavior.
• Conduct regular firmware audits to ensure no unauthorized modifications.

Indicators of Compromise (IOCs)

• File Hashes: None available (no public exploit code)
• Network Signatures: None (exploitation is local/physical)
• Behavioral IOCs:
  • Unexpected device behavior during boot (e.g., hangs, crashes)
  • Unauthorized firmware execution
  • Modification of external QSPI flash contents
• Registry/Config Changes: N/A (embedded device)

Detection Engineering

YARA Rule

rule CVE_2024_25076_Renesas_Bootrom_Overflow {
    meta:
        description = "Detects potential exploitation of CVE-2024-25076 via malicious Flash Product Header"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-25076"
    strings:
        $header_magic = { 48 65 61 64 } // Example magic bytes for Flash Product Header
        $overflow_length = { FF 00 00 00 } // Example large length value (256 bytes)
    condition:
        $header_magic at 0 and $overflow_length
}

Sigma Rule

title: CVE-2024-25076 Renesas SmartBond Bootrom Buffer Overflow
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects attempts to exploit CVE-2024-25076 via malicious Flash Product Header
author: Valters IT
date: 2025-04-10
logsource:
    product: embedded
    service: bootrom
detection:
    selection:
        EventID: 1001
        HeaderLength: 256
    condition: selection
falsepositives:
    - Legitimate firmware updates with large config sections
level: critical

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2024-25076 Renesas SmartBond Bootrom Overflow Attempt"; content:"|48 65 61 64|"; offset:0; depth:4; content:"|FF 00 00 00|"; within:100; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

• Vendor Patch: No official patch available. Contact Renesas for updates.
• Workarounds:
  • Implement hardware-based write protection for QSPI flash.
  • Use secure boot with cryptographic verification of the Flash Product Header.
  • Restrict physical access to devices.
  • Monitor for unauthorized flash modifications.
• Long-term: Replace affected devices with patched versions if available.

Valters IT
Securing Digital Frontiers