CVE-2024-3935 is a vulnerability classified under CWE-415 (Double Free) affecting the Eclipse Foundation's Mosquitto MQTT broker versions 2.0.0 through 2.0.18. The issue arises specifically when the broker is configured to create an outgoing bridge connection that includes an incoming topic with topic remapping enabled. Under these conditions, if the remote MQTT client sends a crafted PUBLISH packet, the broker's memory management mishandles the packet, leading to a double free of allocated memory. This double free results in a crash of the Mosquitto broker, effectively causing a denial of service (DoS). The vulnerability requires the attacker to have low-level privileges (PR:L) and authentication to the broker but does not require user interaction or elevated privileges. The CVSS 4.0 score is 6.0 (medium severity), reflecting the network attack vector, low attack complexity, and significant impact on availability. No known exploits have been reported in the wild as of the publication date. The flaw does not impact confidentiality or integrity directly but can disrupt service availability, which is critical in MQTT-dependent environments such as IoT, smart grids, and industrial control systems. The vulnerability is particularly relevant for deployments using bridge connections with topic remapping, a common configuration in complex MQTT topologies.

The primary impact of CVE-2024-3935 is denial of service due to broker crashes, which can disrupt MQTT messaging infrastructure. For European organizations, this can affect IoT deployments, smart city applications, industrial automation, and other critical systems relying on MQTT for real-time data exchange. Disruption of MQTT brokers can lead to loss of telemetry data, delayed or failed command execution, and potential cascading failures in dependent systems. In sectors such as manufacturing, energy, transportation, and healthcare, where MQTT is often used for device communication, this can result in operational downtime and safety risks. The requirement for authentication limits exploitation to insiders or compromised clients, but the low complexity and network vector mean attackers within the network or with valid credentials can cause outages. The lack of confidentiality or integrity impact reduces the risk of data breaches but does not diminish the operational risk posed by service interruptions. European organizations with large-scale IoT or industrial deployments using Mosquitto should consider this vulnerability a significant operational risk.

1. Monitor for updates from the Eclipse Foundation and apply patches or upgrades to Mosquitto versions beyond 2.0.18 as soon as they become available. 2. Temporarily disable or restrict outgoing bridge connections that use topic remapping until patched. 3. Implement strict access controls and authentication mechanisms to limit which clients can establish bridge connections or publish to sensitive topics. 4. Use network segmentation to isolate MQTT brokers from untrusted networks and reduce exposure to potentially malicious clients. 5. Deploy MQTT traffic anomaly detection to identify unusual PUBLISH packets or patterns indicative of exploitation attempts. 6. Regularly audit broker configurations to ensure topic remapping is only used where necessary and securely configured. 7. Consider fallback or redundancy mechanisms for MQTT brokers to maintain service availability during potential crashes. 8. Educate operational teams about the vulnerability and signs of broker instability to enable rapid response.