CVE-2021-33176 in MQTT Broker
Summary
by MITRE • 06/08/2021
VerneMQ MQTT Broker versions prior to 1.12.0 are vulnerable to a denial of service attack as a result of excessive memory consumption due to the handling of untrusted inputs. These inputs cause the message broker to consume large amounts of memory, resulting in the application being terminated by the operating system.
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Analysis
by VulDB Data Team • 06/11/2021
The CVE-2021-33176 vulnerability affects VerneMQ MQTT brokers version 1.11.0 and earlier, representing a critical denial of service condition that stems from improper input validation mechanisms. This vulnerability manifests when the broker processes untrusted data inputs through its message handling pipeline, specifically targeting the memory allocation and management subsystems. The flaw enables attackers to craft malicious payloads that trigger excessive memory consumption patterns, ultimately leading to system resource exhaustion and application termination by the operating system's memory management subsystem. The vulnerability directly impacts the broker's ability to maintain stable operations and can result in complete service disruption for legitimate users.
The technical root cause of this vulnerability resides in the broker's insufficient validation of incoming message data, particularly concerning message size and structure parameters. When processing untrusted inputs, the system fails to implement adequate bounds checking and resource allocation limits, allowing malicious actors to submit crafted messages that cause the broker to allocate memory in an uncontrolled manner. This memory consumption pattern typically involves iterative allocation of large buffers or recursive data structures that grow exponentially with each malicious input processed. The vulnerability is classified under CWE-400 as an Uncontrolled Resource Consumption weakness, which aligns with the operational characteristics of denial of service attacks that exhaust system resources. The attack vector specifically targets the message processing pipeline where unvalidated data enters the system, making it particularly dangerous for production environments where the broker handles high volumes of messages from potentially untrusted sources.
The operational impact of CVE-2021-33176 extends beyond simple service interruption, creating cascading effects that can compromise entire IoT infrastructure deployments. Organizations relying on VerneMQ for critical messaging services face potential business disruption, especially in scenarios where the broker serves as a central messaging hub for industrial control systems, smart city applications, or connected vehicle communications. The vulnerability can be exploited through various message types including topic names, message payloads, and client connection parameters, making it particularly challenging to defend against through traditional network filtering approaches. Attackers can leverage this vulnerability to perform sustained denial of service attacks that may require system restarts or manual intervention to restore normal operations. The memory exhaustion behavior can also trigger system-level alerts and monitoring systems, potentially masking the actual attack vector while causing operational confusion. This vulnerability directly maps to ATT&CK technique T1499.004 for network denial of service, specifically targeting the availability aspect of the CIA triad.
Mitigation strategies for CVE-2021-33176 must address both immediate operational needs and long-term architectural improvements. The primary and most effective mitigation involves upgrading to VerneMQ version 1.12.0 or later, which includes proper input validation and memory consumption limits. Organizations should also implement network-level controls including rate limiting and connection throttling to prevent rapid memory exhaustion attacks. Additionally, deploying monitoring solutions that track memory usage patterns and trigger alerts when abnormal consumption spikes occur can provide early detection capabilities. System administrators should configure appropriate operating system limits using ulimit or similar mechanisms to prevent individual processes from consuming excessive memory resources. The implementation of input sanitization at the application layer, including message size limits and structural validation, provides additional defense-in-depth measures. Organizations should also consider implementing network segmentation and access controls to limit exposure of vulnerable broker instances to untrusted networks, reducing the attack surface and potential exploitation vectors for this particular vulnerability.