CVE-2021-45937 in wolfMQTT
Summary
by MITRE • 01/01/2022
wolfSSL wolfMQTT 1.9 has a heap-based buffer overflow in MqttClient_DecodePacket (called from MqttClient_WaitType and MqttClient_Connect).
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Analysis
by VulDB Data Team • 01/05/2022
The vulnerability identified as CVE-2021-45937 represents a critical heap-based buffer overflow within the wolfSSL wolfMQTT library version 1.9. This flaw resides in the MqttClient_DecodePacket function which serves as a core component for parsing MQTT protocol messages received by the client. The vulnerability manifests when the MQTT client processes incoming packets, specifically during the decoding phase where it fails to properly validate the length of incoming data before attempting to copy it into heap-allocated buffers. The function is invoked from two primary locations within the MQTT client implementation: MqttClient_WaitType and MqttClient_Connect, indicating that the overflow can occur during both the initial connection establishment and subsequent message waiting operations. This architectural positioning makes the vulnerability particularly dangerous as it can be triggered during normal operational flows when the client receives malformed MQTT packets from network sources.
The technical nature of this buffer overflow stems from inadequate input validation mechanisms within the MQTT packet parsing logic. When the MqttClient_DecodePacket function processes incoming MQTT messages, it appears to assume certain fixed sizes for packet components without performing adequate bounds checking against the actual received data length. This assumption leads to situations where a maliciously crafted MQTT packet could cause the function to write data beyond the allocated buffer boundaries, potentially overwriting adjacent heap memory regions. The heap-based nature of the overflow means that the memory corruption occurs in dynamically allocated memory areas rather than on the stack, which can lead to more subtle and potentially exploitable conditions. The vulnerability specifically affects the wolfMQTT library's ability to safely handle variable-length MQTT packet structures, particularly those involving topic names, user properties, or other extensible fields that can vary in size according to the MQTT protocol specification.
The operational impact of this vulnerability extends beyond simple memory corruption, as it creates potential pathways for remote code execution and system compromise when the affected wolfMQTT library is integrated into networked applications. Systems utilizing the vulnerable library in IoT devices, industrial control systems, or any MQTT-based communication infrastructure could be at risk of exploitation by attackers who can send specially crafted MQTT packets to the target system. The vulnerability's trigger points in both MqttClient_WaitType and MqttClient_Connect suggest that exploitation could occur at multiple stages of the MQTT client lifecycle, making it difficult to fully mitigate without addressing the core parsing logic. From an attack perspective, this vulnerability aligns with the attack pattern described in the MITRE ATT&CK framework under the T1203 technique for exploitation of remote services, while the buffer overflow itself maps to CWE-121 which specifically addresses stack-based and heap-based buffer overflow conditions. The potential for memory corruption in heap-allocated regions can lead to denial of service conditions, data corruption, or more severe exploitation outcomes depending on the specific memory layout and application context.
Mitigation strategies for CVE-2021-45937 must focus on both immediate remediation and architectural improvements to prevent similar vulnerabilities. The most direct approach involves upgrading to a patched version of the wolfSSL wolfMQTT library where the buffer overflow has been addressed through proper input validation and bounds checking mechanisms. Organizations should also implement network-level filtering to prevent malformed MQTT packets from reaching systems running vulnerable versions of the library, particularly in environments where the MQTT client communicates with untrusted network sources. Additional defensive measures include implementing memory corruption detection mechanisms such as address sanitizer or heap integrity checking, though these may introduce performance overhead. The vulnerability highlights the importance of following secure coding practices as outlined in the CWE guidelines, specifically emphasizing the need for proper input validation, bounds checking, and memory management in network protocol implementations. Security monitoring should be enhanced to detect unusual patterns of MQTT traffic that might indicate exploitation attempts, and regular security assessments should be conducted to identify other potential vulnerabilities in MQTT implementations and similar network protocols.