CVE-2024-6564

Executive Summary

CVE-2024-6564 is a buffer overflow vulnerability in the Renesas rcar_dev_init function, which is part of the secure boot firmware for Renesas R-Car SoCs. The flaw arises because the function uses an untrusted user-supplied value (rcar_image_number) as a loop counter before verifying it against the maximum allowed image count (RCAR_MAX_BL3X_IMAGE). This allows an attacker with local privileged access to overwrite critical memory regions, potentially bypassing the entire secure boot chain and executing arbitrary code at the highest privilege level (EL3/secure monitor). The vulnerability has a CVSS score of 6.7 (Medium) and currently has no known public exploit or patch. The EPSS score is 0.0%, indicating low active exploitation in the wild, but the impact on embedded systems security is severe.

CVSS Breakdown & Threat Level

• CVSS Score: 6.7 (Medium)
• Vector: CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H
• Attack Vector: Local (requires physical or local shell access to the device)
• Attack Complexity: Low (no special conditions needed)
• Privileges Required: High (attacker must already have root or kernel-level access)
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: High (full memory read)
• Integrity Impact: High (arbitrary memory write)
• Availability Impact: High (can crash or corrupt secure firmware)

Threat Level: High for embedded/IoT devices using Renesas R-Car SoCs in automotive, industrial, or critical infrastructure. The vulnerability allows a complete bypass of secure boot, enabling persistent, undetectable firmware implants.

Deep Dive & Mechanics

The vulnerability resides in the rcar_dev_init function within the Renesas secure boot firmware (BL3x stage). The function iterates over a list of boot images using a loop counter derived from an untrusted input parameter rcar_image_number. The code does not validate that rcar_image_number is less than RCAR_MAX_BL3X_IMAGE before using it as an index into a fixed-size array.

Vulnerable Code Flow:

1. The function receives rcar_image_number from an untrusted source (e.g., a storage partition or boot parameter).
2. It uses this value as a loop counter without bounds checking.
3. If rcar_image_number exceeds the array size, the loop writes beyond the allocated buffer, corrupting adjacent memory (e.g., stack, heap, or other firmware structures).
4. An attacker can craft a malicious boot image that sets rcar_image_number to a large value, causing the overflow to overwrite critical pointers or code, ultimately hijacking the secure boot flow.

Impact: Full bypass of secure boot chain. The attacker can load arbitrary unsigned firmware, disable security features, or install persistent backdoors at the firmware level.

Vulnerable vs. Patched Code

Vulnerable Code (pseudocode):

void rcar_dev_init(uint32_t rcar_image_number) {
    // No bounds check
    for (uint32_t i = 0; i < rcar_image_number; i++) {
        // Write to buffer without verifying index
        bl3x_images[i] = load_image(i);
    }
}

Patched Code (expected fix):

void rcar_dev_init(uint32_t rcar_image_number) {
    // Add bounds check
    if (rcar_image_number > RCAR_MAX_BL3X_IMAGE) {
        // Handle error, abort boot
        secure_boot_abort();
        return;
    }
    for (uint32_t i = 0; i < rcar_image_number; i++) {
        bl3x_images[i] = load_image(i);
    }
}

The patch adds a validation check at the beginning of the function to ensure rcar_image_number does not exceed the maximum allowed image count. Without this check, the loop can overflow the buffer.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• Physical or local shell access to a Renesas R-Car device (e.g., R-Car H3, M3, E3).
• Ability to modify boot parameters or storage partitions (e.g., via JTAG, U-Boot, or compromised OS).

Exploitation Steps:

1. Identify the target device and gain local privileged access (e.g., root shell via UART or SSH).
2. Locate the boot parameter that controls rcar_image_number (often stored in a configuration partition or passed via U-Boot environment).
3. Craft a malicious boot image that sets rcar_image_number to a value greater than RCAR_MAX_BL3X_IMAGE (e.g., 0xFFFFFFFF).
4. Trigger the boot process (reboot or power cycle). The vulnerable function will overflow the buffer, overwriting adjacent memory.
5. Control the overflow to overwrite a function pointer or return address, redirecting execution to attacker-controlled shellcode.
6. Achieve secure boot bypass – the attacker’s unsigned firmware is loaded and executed at EL3.

Example Payload (conceptual):

// Overflow to overwrite return address with shellcode address
uint32_t overflow_data[] = {
    0x41414141, // padding
    0x41414141,
    0x41414141,
    0xdeadbeef  // new return address pointing to shellcode
};

Blue Team (Defense & Detection)

Detection:

• Monitor boot logs for anomalies (e.g., unexpected number of images loaded).
• Use hardware security modules (HSMs) to verify boot image integrity.
• Implement runtime bounds checking in firmware (if possible).

Mitigation:

• Apply firmware patch from Renesas when available.
• Restrict physical access to devices.
• Enable secure boot chain with hardware root of trust (e.g., OTP fuses).
• Use code signing and verify all boot images before loading.

Indicators of Compromise (IOCs)

• File Hashes: (None publicly available; custom firmware images may have unique hashes)
• Network Signatures: (None; vulnerability is local)
• Process/Service Names: rcar_dev_init (in firmware)
• Memory Artifacts: Corrupted stack/heap in secure firmware region
• Boot Logs: Unexpected number of images loaded (e.g., > RCAR_MAX_BL3X_IMAGE)
• Registry/Config Changes: Modified boot parameters (e.g., rcar_image_number set to large value)

Detection Engineering

YARA Rule

rule CVE_2024_6564_Renesas_BufferOverflow {
    meta:
        description = "Detects buffer overflow in Renesas rcar_dev_init function"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2024-6564"
    strings:
        $s1 = "rcar_dev_init" ascii wide nocase
        $s2 = "rcar_image_number" ascii wide nocase
        $s3 = "RCAR_MAX_BL3X_IMAGE" ascii wide nocase
    condition:
        all of ($s*) and #s1 > 0
}

Sigma Rule

title: CVE-2024-6564 Renesas Secure Boot Bypass
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects exploitation attempts of buffer overflow in Renesas rcar_dev_init
author: Valters IT
date: 2025-04-10
logsource:
    product: linux
    service: boot
detection:
    selection:
        EventID: 1
        ImagePath|contains: 'rcar_dev_init'
        CommandLine|contains: 'rcar_image_number'
    condition: selection
falsepositives:
    - Legitimate firmware updates
level: high

Suricata Rule

alert ip any any -> any any (msg:"CVE-2024-6564 Renesas rcar_dev_init Buffer Overflow Attempt"; content:"rcar_image_number"; depth:100; flow:to_server; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Apply firmware patch from Renesas when available (check Renesas security advisories).
2. Restrict physical access to devices to prevent local exploitation.
3. Enable secure boot with hardware root of trust (e.g., OTP fuses) to prevent unsigned firmware loading.
4. Monitor boot logs for anomalies (e.g., unexpected number of images).

Long-term Measures:

• Implement runtime bounds checking in all firmware functions.
• Use code signing and verify all boot images before loading.
• Conduct regular firmware security audits.
• Deploy hardware security modules (HSMs) for critical systems.

Workaround: If patching is not possible, disable the rcar_dev_init function or hardcode rcar_image_number to a safe value (e.g., 0) in the boot configuration.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert