CVE-2026-44248

Resources

ANT-2026-09608
https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901027

The MQTT 5 header Properties section is parsed and buffered before any message size limit is applied.

Specifically, in MqttDecoder, the decodeVariableHeader() method is called before the bytesRemainingBeforeVariableHeader > maxBytesInMessage check. The decodeVariableHeader() can call other methods which will call decodeProperties(). Effectively, Netty does not apply any limits to the size of the properties being decoded.

Additionally, because MqttDecoder extends ReplayingDecoder, Netty will repeatedly re-parse the enormous Properties sections and buffer the bytes in memory, until the entire thing parses to completion.

This can cause high resource usage in both CPU and memory.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2026-44248 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.2.13.Final, 4.1.133.Final); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-44248? CVE-2026-44248 is a medium-severity uncontrolled resource consumption vulnerability in io.netty:netty-codec-mqtt (maven), affecting versions >= 4.2.0.Alpha1, <= 4.2.12.Final. It is fixed in 4.2.13.Final, 4.1.133.Final. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2026-44248? CVE-2026-44248 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of io.netty:netty-codec-mqtt are affected by CVE-2026-44248? io.netty:netty-codec-mqtt (maven) versions >= 4.2.0.Alpha1, <= 4.2.12.Final is affected.
4. Is there a fix for CVE-2026-44248? Yes. CVE-2026-44248 is fixed in 4.2.13.Final, 4.1.133.Final. Upgrade to this version or later.
5. Is CVE-2026-44248 exploitable, and should I be worried? Whether CVE-2026-44248 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-44248 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-44248?
   • Upgrade io.netty:netty-codec-mqtt to 4.2.13.Final or later
   • Upgrade io.netty:netty-codec-mqtt to 4.1.133.Final or later