CVE-2019-11778 in Mosquitto
Summary
by MITRE
If an MQTT v5 client connects to Eclipse Mosquitto versions 1.6.0 to 1.6.4 inclusive, sets a last will and testament, sets a will delay interval, sets a session expiry interval, and the will delay interval is set longer than the session expiry interval, then a use after free error occurs, which has the potential to cause a crash in some situations.
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Analysis
by VulDB Data Team • 08/30/2020
The vulnerability CVE-2019-11778 represents a critical memory safety issue within Eclipse Mosquitto messaging broker versions 1.6.0 through 1.6.4. This flaw manifests when an mqtt v5 client establishes a connection with specific parameter configurations that create a temporal inconsistency in memory management. The vulnerability operates through a sophisticated interplay of client-side configuration parameters including last will and testament settings, will delay intervals, and session expiry intervals. When these parameters are configured in a particular relationship where the will delay interval exceeds the session expiry interval, the broker's memory management system encounters a use after free condition that can lead to system instability and potential crash scenarios.
The technical execution of this vulnerability stems from improper handling of memory allocation and deallocation sequences within the mosquitto broker's mqtt v5 protocol implementation. When a client connects with the specified parameters, the broker's internal state management creates a scenario where memory allocated for last will and testament data structures becomes freed prematurely while still being referenced by subsequent operations. This memory management failure directly maps to CWE-416, which describes the use after free vulnerability category where a pointer is used after the memory it points to has been freed. The flaw occurs in the broker's session management subsystem where the timing of memory cleanup operations does not properly account for the lifecycle of asynchronous will delay mechanisms that may still require access to freed memory structures.
The operational impact of this vulnerability extends beyond simple service disruption to potentially enable denial of service attacks against mqtt broker infrastructure. An attacker capable of establishing connections to affected mosquitto instances could systematically trigger the memory corruption by configuring the specific parameter relationships described in the vulnerability. The crash potential creates a reliable method for service disruption that could affect any system relying on mosquitto for mqtt messaging, particularly in industrial IoT deployments, smart city infrastructure, and enterprise messaging systems where broker availability is critical. The vulnerability affects the broker's ability to maintain stable connections and process subsequent client requests, potentially creating cascading failures in mqtt-based communication networks.
Mitigation strategies for CVE-2019-11778 require immediate patching of affected mosquitto versions to 1.6.5 or later, which contains the necessary memory management corrections. Organizations should also implement connection monitoring and rate limiting to reduce the attack surface, particularly in environments where untrusted clients might connect to mqtt brokers. The fix addresses the underlying memory management issue by ensuring proper synchronization between will delay interval processing and session expiry interval handling, preventing premature memory deallocation. Security teams should also consider implementing network segmentation and access controls to limit exposure of mqtt brokers to potentially malicious clients, while monitoring for anomalous connection patterns that might indicate exploitation attempts. The vulnerability highlights the importance of thorough testing of edge cases in protocol implementations, particularly those involving asynchronous operations and memory management in high-availability systems.