CVE-2026-73863 in NanoMQ
Summary
by MITRE • 09/18/2026
NanoMQ is an MQTT broker. Prior to 0.24.14, NanoMQ's broker-side MQTT v5 nmq_subinfo_decode() function in nng/src/sp/protocol/mqtt/mqtt_parser.c reuses len_of_varint from the outer Properties Length while parsing each SUBSCRIPTION_IDENTIFIER. A remote client can send a SUBSCRIBE packet with a multi-byte Properties Length and repeated subscription identifiers, causing get_var_integer() to begin at an incorrect offset and read beyond the heap message buffer. The flaw is reachable through the broker receive path and can crash the broker, while the separately reported topic-option off-by-one occurs later and is not this vulnerability. This issue is fixed in version 0.24.14.
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Analysis
by VulDB Data Team • 09/18/2026
NanoMQ serves as a high-performance MQTT broker designed for Internet of Things (IoT) deployments, facilitating message exchange between constrained devices and backend systems. The security flaw identified within versions prior to 0.24.14 resides in the protocol parsing logic, specifically within the nmq_subinfo_decode function located in the nng source code under the mqtt_parser module. This component is responsible for decoding subscription information contained within MQTT v5 SUBSCRIBE packets received from remote clients. The vulnerability stems from an incorrect handling of variable-length integers during the processing of property lengths and subscription identifiers.
The technical root cause involves a logic error where the parser incorrectly reuses the len_of_varint value derived from the outer Properties Length field when parsing individual SUBSCRIPTION_IDENTIFIER fields. In MQTT v5, properties are encoded with length prefixes to allow for efficient serialization. When a remote client constructs a maliciously crafted SUBSCRIBE packet featuring a multi-byte Properties Length and multiple repeated subscription identifiers, the parser fails to correctly adjust its internal offset pointer after processing each identifier. Instead of recalculating or advancing the read position based on the actual size of the processed data, it relies on the initial outer length value. This miscalculation causes the get_var_integer function to begin reading from an incorrect memory offset within the heap-allocated message buffer.
This misalignment leads directly to a out-of-bounds read condition, allowing the application to access memory regions beyond the intended boundaries of the input buffer. From a security impact perspective, this vulnerability is classified as CWE-125: Out-of-bounds Read. The immediate operational consequence is that an unauthenticated remote attacker can trigger a denial of service by causing the NanoMQ broker process to crash due to invalid memory access. While out-of-bounds reads are sometimes leveraged for information disclosure or code execution, in this specific context involving protocol parsing and buffer handling, the primary confirmed impact is system instability and service interruption. The vulnerability is reachable through the standard broker receive path, meaning it can be exploited by any client capable of establishing an MQTT connection to the affected broker instance.
The attack vector aligns with MITRE ATT&CK technique T1498: Network Denial of Service, specifically under sub-techniques involving resource exhaustion or service disruption via protocol abuse. It is important to distinguish this flaw from other reported issues in NanoMQ, such as a separate topic-option off-by-one error that occurs later in the processing pipeline; those are distinct vulnerabilities with different root causes and exploitation paths. The current issue is strictly limited to the initial parsing phase of subscription identifiers within MQTT v5 packets.
To mitigate this vulnerability, organizations running NanoMQ must upgrade to version 0.24.14 or any subsequent release where the parsing logic has been corrected to properly handle variable-length integer offsets for each subscription identifier independently. Until an upgrade is performed, network-level controls such as strict rate limiting on MQTT connections and deep packet inspection can help detect anomalous SUBSCRIBE packets with unusually large property lengths or repetitive structures that may indicate exploitation attempts. Additionally, deploying the broker in a containerized environment with resource limits can contain the impact of a potential crash, preventing it from affecting other services running on the same host infrastructure.