CVE-2026-76816 in Netty
Summary
by MITRE • 08/24/2026
Netty is an asynchronous, event-driven network application framework. Prior to versions 4.1.137.Final and 4.2.17.Final, MqttEncoder does not validate client identifiers, will topics, usernames, and PUBLISH topic names before encoding, allowing prohibited null bytes in MQTT UTF-8 string fields and potentially causing routing, access-control, or identity mismatches in downstream brokers. The vulnerability is exploitable when an application uses Netty's MQTT encoder to construct messages from user-controlled input. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final.
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Analysis
by VulDB Data Team • 08/24/2026
Netty serves as a foundational asynchronous, event-driven network application framework widely utilized for building high-performance client-server applications across various protocols including MQTT. Within this ecosystem, the MqttEncoder component is responsible for serializing MQTT protocol messages into byte streams suitable for transmission over networks. A critical security deficiency was identified in versions prior to 4.1.137.Final and 4.2.17.Final where the encoder failed to perform adequate input validation on specific fields within MQTT packets. Specifically, the component did not check client identifiers, will topics, usernames, or PUBLISH topic names for prohibited null bytes before encoding these values into UTF-8 string formats mandated by the MQTT specification. This oversight creates a significant gap in data integrity enforcement at the serialization layer of the application stack.
The technical flaw stems from the absence of strict boundary checks on user-controlled input during the message construction phase. In standard MQTT implementations, certain fields are defined as UTF-8 strings which must not contain null bytes because such characters can terminate string processing prematurely or be interpreted differently by downstream systems depending on their implementation details. By allowing null bytes to pass through into the encoded packet, Netty enables attackers to craft malformed messages that appear valid at the transport layer but carry hidden control sequences when processed by MQTT brokers. This behavior aligns with CWE-20 Improper Input Validation and CWE-749 Exposed Dangerous Method or Function as it involves failing to sanitize input before use in a security-sensitive context.
The operational impact of this vulnerability is substantial, particularly in environments where Netty-based applications act as intermediaries between clients and MQTT brokers. An attacker who can inject null bytes into these specific fields may cause routing errors within the broker infrastructure, leading to message delivery failures or misdirection. More critically, the presence of null bytes can facilitate identity mismatches by allowing a client to impersonate another user if the authentication logic relies on string comparison that terminates at the first null character. This could result in unauthorized access control bypasses where an attacker gains privileges associated with a different account or topic subscription level. Such scenarios are particularly dangerous in IoT deployments and enterprise messaging systems where strict identity verification is paramount for secure operation.
From a threat modeling perspective, this vulnerability facilitates attacks categorized under MITRE ATT&CK technique T1078 Valid Accounts as it allows for potential authentication bypass through input manipulation. It also relates to injection-type vulnerabilities where malicious data alters the intended execution or interpretation of downstream systems. The risk is exacerbated in scenarios involving legacy brokers that do not strictly enforce MQTT protocol constraints, making them susceptible to parsing errors or unexpected behavior when encountering null-terminated strings embedded within UTF-8 fields.
To mitigate this vulnerability, organizations must upgrade their Netty dependencies immediately to version 4.1.137.Final or later for the 4.1.x branch and version 4.2.17.Final or later for the 4.2.x branch. These releases include patches that enforce strict validation rules on MQTT string fields, ensuring that prohibited characters like null bytes are rejected during encoding rather than passed through to downstream systems. Additionally, developers should implement defense-in-depth strategies by validating input at multiple layers of the application stack. Application-level sanitization routines should be employed to filter out non-printable or control characters before they reach network serialization components. Regular security audits and static code analysis tools configured with rules for CWE-20 can help identify similar validation gaps in other parts of the codebase, ensuring robust protection against input-based attacks across all MQTT interactions.