CVE-2025-66023: NanoMQ MQTT Broker UAF Vulnerability

CVE-2025-66023 Overview

CVE-2025-66023 is a Heap-Use-After-Free (UAF) vulnerability in NanoMQ MQTT Broker, an all-around Edge Messaging Platform. The vulnerability exists within the MQTT bridge client component, implemented via the underlying NanoNNG library. When NanoMQ acts as a bridge connecting to a remote MQTT broker, a malicious remote broker can trigger a crash resulting in Denial of Service (DoS) or potential memory corruption by accepting the connection and immediately sending a malformed packet sequence.

Critical Impact

A malicious remote MQTT broker can cause service crashes or memory corruption in NanoMQ bridge deployments by sending malformed packet sequences immediately after connection establishment.

Affected Products

• NanoMQ MQTT Broker versions prior to 0.24.5
• NanoNNG library (underlying implementation)
• Systems using NanoMQ as an MQTT bridge to remote brokers

Discovery Timeline

• 2026-01-01 - CVE CVE-2025-66023 published to NVD
• 2026-01-02 - Last updated in NVD database

Technical Details for CVE-2025-66023

Vulnerability Analysis

This Use After Free vulnerability (CWE-416) occurs in the MQTT bridge client component of NanoMQ when it connects to a remote MQTT broker. The flaw stems from improper state management during the MQTT connection handshake process. According to the MQTT protocol specification, the CONNACK packet must always be the first packet received from the broker after a connection is established. However, NanoMQ versions prior to 0.24.5 did not enforce this protocol requirement strictly.

When a malicious broker accepts a connection and immediately sends a malformed or out-of-sequence packet instead of the expected CONNACK, the NanoMQ client enters an inconsistent state. This state confusion leads to memory being freed prematurely while still being referenced elsewhere in the code, creating the classic Use After Free condition.

Root Cause

The root cause is insufficient protocol adherence validation in the MQTT client SDK embedded in NanoMQ. The client implementation failed to verify that CONNACK is always the first packet processed after connection establishment. This oversight allowed a malicious server to exploit the state machine by sending unexpected packets, causing the client to access memory that had already been deallocated.

Attack Vector

The attack requires network access and involves a malicious MQTT broker that NanoMQ is configured to bridge with. The attack sequence proceeds as follows:

1. NanoMQ initiates a bridge connection to the malicious broker
2. The malicious broker accepts the TCP connection
3. Instead of sending the expected CONNACK packet, the broker immediately sends a malformed packet sequence
4. NanoMQ's state machine becomes confused, leading to premature memory deallocation
5. Subsequent operations attempt to access the freed memory, causing a crash or potential memory corruption

The vulnerability can be exploited to achieve Denial of Service through application crashes. While the primary impact is availability-related, the memory corruption aspect could potentially lead to more severe consequences in certain scenarios.

Detection Methods for CVE-2025-66023

Indicators of Compromise

• Unexpected NanoMQ process crashes or restarts when bridge connections are active
• Core dumps showing heap corruption or UAF-related errors in NanoNNG library functions
• Abnormal connection patterns from remote MQTT brokers with immediate disconnections
• Log entries indicating malformed packet processing errors during bridge handshake

Detection Strategies

• Monitor NanoMQ service stability and correlate crashes with bridge connection events
• Implement network traffic analysis to detect unusual MQTT packet sequences during connection establishment
• Deploy application-level monitoring to detect memory corruption signatures
• Review NanoMQ logs for protocol violation warnings related to packet ordering

Monitoring Recommendations

• Set up alerts for NanoMQ process crashes with automatic restart monitoring
• Enable verbose logging for MQTT bridge connections to capture packet sequence anomalies
• Monitor memory usage patterns in NanoMQ processes for signs of heap corruption
• Track connection establishment success/failure rates with remote brokers

How to Mitigate CVE-2025-66023

Immediate Actions Required

• Upgrade NanoMQ to version 0.24.5 or later immediately
• Audit all configured MQTT bridge connections to ensure they connect only to trusted brokers
• Implement network segmentation to restrict which brokers can be bridged
• Consider temporarily disabling non-essential bridge connections until patching is complete

Patch Information

The fix is available in NanoMQ version 0.24.5. The patch enforces stricter protocol adherence in the MQTT client SDK embedded in NanoMQ. Specifically, it ensures that CONNACK is always the first packet processed in the line, preventing the state confusion that led to the Heap-Use-After-Free when a malicious server sent a malformed packet sequence immediately after connection establishment.

For detailed patch information, refer to the GitHub Pull Request and the GitHub Security Advisory.

Workarounds

• Validate the remote broker before establishing bridge connections to ensure it is trusted and legitimate
• Implement allow-listing for broker endpoints that NanoMQ is permitted to bridge with
• Deploy network-level controls to prevent connections to untrusted MQTT brokers
• Consider using a reverse proxy or gateway to validate MQTT traffic before it reaches NanoMQ

If immediate patching is not possible, organizations should carefully review their bridge configurations and ensure that all connected remote brokers are trusted entities. Network-level filtering can provide an additional layer of protection by restricting which external endpoints NanoMQ can communicate with.