CVE-2025-59946: Emqx NanoMQ Use-After-Free Vulnerability

CVE-2025-59946 Overview

CVE-2025-59946 is a use-after-free vulnerability in NanoMQ, an open-source edge messaging platform that implements the MQTT protocol. The flaw exists in NanoMQ versions prior to 0.24.2 and stems from a data race condition affecting the subscription information list. An attacker with low privileges and network access can trigger the race condition, causing heap memory to be referenced after it has been freed. Exploitation results in a crash of the broker process, with potential for further memory corruption impact. The issue is tracked under [CWE-416: Use After Free] and was patched in NanoMQ version 0.24.2.

Critical Impact

Successful exploitation can cause heap use-after-free crashes in the NanoMQ broker, disrupting MQTT messaging across connected edge and IoT devices.

Affected Products

• EMQX NanoMQ versions prior to 0.24.2
• Deployments using NanoMQ as an MQTT broker for edge and IoT messaging
• Systems relying on NanoMQ subscription management with concurrent client connections

Discovery Timeline

• 2025-12-27 - CVE-2025-59946 published to NVD
• 2026-01-30 - Last updated in NVD database

Technical Details for CVE-2025-59946

Vulnerability Analysis

The vulnerability is a classical data race condition affecting the subscription information list (sub info list) inside NanoMQ. NanoMQ handles MQTT subscriptions across multiple concurrent threads to serve high-throughput edge messaging workloads. When subscription state is modified concurrently without proper synchronization, one thread can free a heap-allocated subscription structure while another thread still holds a reference to it. Subsequent access to the freed memory results in a heap use-after-free condition that crashes the broker.

The attack requires network reachability to the MQTT broker and authenticated low-privilege access. Attack complexity is high because the attacker must reliably interleave subscription operations to win the race window. When successful, the impact extends to confidentiality, integrity, and availability of the broker process.

Root Cause

The root cause is missing or insufficient locking around concurrent reads and writes to the subscription information list. Multiple worker threads access the same subscription metadata without atomic operations or mutex protection. This allows one thread to deallocate a structure that another thread continues to dereference, producing the use-after-free condition described in [CWE-416].

Attack Vector

An attacker connects to the NanoMQ broker over the network as an authenticated MQTT client. The attacker rapidly issues subscribe and unsubscribe operations against overlapping topic filters from concurrent sessions. By repeatedly racing these operations, the attacker triggers the window in which one thread frees a subscription entry while another thread reads it. The dangling pointer dereference corrupts heap state and crashes the broker. See the GitHub Security Advisory GHSA-xg37-23w7-72p5 and the GitHub Issue Discussion for additional technical detail.

Detection Methods for CVE-2025-59946

Indicators of Compromise

• Unexpected crashes or restarts of the nanomq broker process accompanied by SIGSEGV or SIGABRT signals in system logs
• Heap corruption messages or backtraces referencing subscription management functions in core dumps
• Bursts of rapid MQTT SUBSCRIBE and UNSUBSCRIBE packets from a single client or small group of clients targeting overlapping topic filters

Detection Strategies

• Monitor NanoMQ process logs and systemd journal entries for abnormal termination events and automatic restart cycles
• Inspect network flows for MQTT clients issuing high-rate subscription churn patterns inconsistent with normal IoT device behavior
• Run NanoMQ with AddressSanitizer or similar runtime instrumentation in test environments to surface use-after-free conditions during pre-production validation

Monitoring Recommendations

• Capture broker uptime metrics and alert on repeated process restarts within short time windows
• Forward MQTT broker telemetry and host event logs into a centralized analytics platform for correlation with client connection metadata
• Track per-client subscription and unsubscription rates to identify anomalous patterns consistent with race condition exploitation attempts

How to Mitigate CVE-2025-59946

Immediate Actions Required

• Upgrade all NanoMQ deployments to version 0.24.2 or later, which contains the patch for the subscription list data race
• Inventory edge and IoT environments to identify any embedded or containerized NanoMQ instances running vulnerable versions
• Restrict broker access to trusted clients using authentication, ACLs, and network segmentation until patching is complete

Patch Information

The vulnerability is fixed in NanoMQ version 0.24.2. Upgrade instructions and the corresponding patch are referenced in the NanoMQ Security Advisory GHSA-xg37-23w7-72p5. The fix introduces proper synchronization on the subscription information list to eliminate the race window.

Workarounds

• Limit the number of authenticated clients permitted to issue subscription operations and apply rate limiting on SUBSCRIBE and UNSUBSCRIBE traffic
• Place the NanoMQ broker behind a network ACL or firewall that restricts MQTT access to known device IP ranges
• Configure broker supervisors such as systemd or container orchestrators to automatically restart NanoMQ on crash, reducing availability impact while the patch is being deployed

# Configuration example: upgrade NanoMQ to the patched release
git clone https://github.com/nanomq/nanomq.git
cd nanomq
git checkout 0.24.2
mkdir build && cd build
cmake ..
make -j$(nproc)
sudo make install
nanomq start --conf /etc/nanomq.conf