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Netty MQTT: Resource exhaustion in MqttDecoder

Moderate severity GitHub Reviewed Published May 5, 2026 in netty/netty • Updated May 14, 2026

Package

maven io.netty:netty-codec-mqtt (Maven)

Affected versions

>= 4.2.0.Alpha1, <= 4.2.12.Final
<= 4.1.132.Final

Patched versions

4.2.13.Final
4.1.133.Final

Description

Impact

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.

Resources

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

References

@chrisvest chrisvest published to netty/netty May 5, 2026
Published to the GitHub Advisory Database May 7, 2026
Reviewed May 7, 2026
Published by the National Vulnerability Database May 13, 2026
Last updated May 14, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
Low

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(5th percentile)

Weaknesses

Uncontrolled Resource Consumption

The product does not properly control the allocation and maintenance of a limited resource. Learn more on MITRE.

CVE ID

CVE-2026-44248

GHSA ID

GHSA-jfg9-48mv-9qgx

Source code

Credits

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