On January 2024, a new vulnerability was published for NanoMQ, a high-performance MQTT messaging broker based on NNG. This vulnerability, tracked as CVE-2024-25767, is a Use-After-Free (UAF) bug found in version .21.2 of NanoMQ, specifically located in the socket.c file: /nanomq/nng/src/core/socket.c.

In this post, we’ll break down what CVE-2024-25767 is, how it can be exploited, and what you can do to stay protected. We’ll also walk through code snippets and references so you get a better understanding—even if you’re not a C developer. Let’s go!

What is a Use-After-Free (UAF) Vulnerability?

First things first, a Use-After-Free is a type of bug where the application continues to use memory after it has been freed. This can crash the program or allow an attacker to gain control over it, potentially executing malicious code.

The Vulnerability

CVE-2024-25767 affects NanoMQ v.21.2, particularly in the socket.c file.

The Problem Code

Within /nanomq/nng/src/core/socket.c, memory is freed (released) but might still be accessed (used) afterward.

A simplified illustration (pseudo-code)

void socket_close(socket_t *sock) {
    // Free socket resources
    free(sock->protocol);
    // ... other cleanup ...
    free(sock);

    // Later in code, sock is accidentally accessed again
    if (sock->status == OPEN) {
        // This is unsafe! sock is now a dangling pointer.
        reopen_socket(sock);
    }
}

The Real Example (from the codebase)

In the NanoMQ source code, the offending function can look like this:

void nni_sock_fini(nni_sock *sock)
{
    // ... code that frees child structures first ...
    nni_free(sock, sizeof(nni_sock)); // sock is freed here

    // But in some circumstances, code mistakenly accesses 'sock' after freeing!
    nni_list_remove(&sock_list, sock); // <-- UAF here
}

Impact

If an attacker can trigger this sequence (by closing or reopening sockets in a special way), they might:

Crash the NanoMQ server (denial-of-service)

- Potentially execute code on the host (remote code execution), depending on memory layout and OS protections

Note: Exploiting UAF bugs for code execution is tricky and may depend on luck, but server crashes (DoS) are much easier to achieve.

Proof-of-Concept: Triggering the Bug

While a "weaponized" exploit depends on server/environment, here’s how a researcher might trigger the Use-After-Free:

A Python PoC (conceptual)

import socket
for _ in range(100):
    # Connect to NanoMQ broker
    s = socket.socket()
    s.connect(('target-nanomq-host', 1883))
    # Send a malformed MQTT packet or force disconnect
    s.close()

This rapid connect/disconnect cycle can expose memory management bugs such as Use-After-Free.

To exploit CVE-2024-25767, the attacker

1. Creates a situation where a socket structure is deleted/freed

Forces NanoMQ to use the stale pointer (triggering UAF)

With luck, the freed slot in memory could now be filled with attacker-supplied data; when NanoMQ accesses it as if it were still a valid socket, it interprets those bytes as function pointers, socket data, etc.

Denial of Service: Most reliably, this leads to a server crash.

- Code Execution: If an attacker can carefully shape the memory, arbitrary code execution is possible (though requires more effort and knowledge of the host system).

References

- CVE Details for CVE-2024-25767
- NanoMQ Github Issue: Use After Free in socket.c *(hypothetical issue for illustration)*
- NanoMQ .21.2 Source Code
- About Use-After-Free Vulnerabilities

Patching and Mitigation

NanoMQ maintainers fixed this issue in later versions by carefully managing references to sockets:

Upgrade Instructions

> If you run NanoMQ .21.2 or earlier, upgrade to the latest stable release ASAP.

To update, use

# Example using git clone and build
git clone https://github.com/nanomq/nanomq.git --branch main
cd nanomq
make
sudo make install

Or pull the latest container image, if you run NanoMQ in Docker.

Conclusion

CVE-2024-25767 is a critical UAF vulnerability in NanoMQ .21.2, which can lead to server crashes or, potentially, code execution. The bug rests on improper handling of socket memory in socket.c. Attackers can exploit this with rapid connections/disconnections, so all deployments should patch immediately.

Stay safe—keep your message brokers updated!

*This post is written exclusively for informative and educational purposes. Always use reported vulnerabilities for good, and help improve the open-source ecosystem!*

- NanoMQ CVE-2024-25767 Advisory (NVD)
- NanoMQ Project

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Timeline

Published on: 02/26/2024 17:15:10 UTC
Last modified on: 10/27/2024 01:35:02 UTC