---
id: "CVE-2026-115417"
kind: "vulnerability"
aliases: ["CVE-2026-115417"]
title: "AWS aws-c-mqtt: oversized allocation in MQTT 3.1.1 adapter lets remote peer crash client"
severity: "high"
cvss: 8.2
epss: 0.0035
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["aws", "aws-c-mqtt", "aws-iot", "c", "iot-devices", "lab-disclosure", "mqtt", "package"]
not_affected_if: ["fixed-version", "native-mqtt5-only"]
published_at: "2026-10-22T00:00:00+00:00"
first_seen_at: "2026-10-23T21:57:32+00:00"
updated_at: "2026-10-25T03:03:47+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-115417", "first_seen_at": "2026-10-23T21:57:32+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-115417", "first_seen_at": "2026-10-24T07:15:51+00:00"}, {"source": "lab", "url": "https://aws.amazon.com/security/security-bulletins/rss/2026-147-aws/", "first_seen_at": "2026-10-24T20:05:36+00:00"}]
---

# AWS aws-c-mqtt: oversized allocation in MQTT 3.1.1 adapter lets remote peer crash client

aws-c-mqtt, the C MQTT library in the AWS Common Runtime, has a memory allocation with an excessive size value in its MQTT 3.1.1 compatibility adapter. A remote actor can cause a denial of service of the client process.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the aws-c-mqtt version in your build (for example the vendored copy or the lockfile); versions `>= 0.9.1` and `< 0.16.2` are affected.
- Check whether the AWS IoT Device SDK for C++ v2 is bundled, since it ships aws-c-mqtt.
- Check whether the application uses the MQTT 3.1.1 compatibility adapter rather than a native MQTT5 or MQTT 3.1.1 client path.

## Fix

- Upgrade aws-c-mqtt to `v0.16.2` or later.
- Upgrade the AWS IoT Device SDK for C++ v2 build that bundles it, and patch any forked or derivative copies.
- Until upgraded, connect only to trusted brokers such as AWS IoT Core and use TLS.

## Mitigated for you by

- Team (workaround): covers Client code no longer uses the MQTT 3.1.1 compatibility adapter, so the vulnerable code path is not exercised Limits: none stated Still: Switch to a native MQTT5 or MQTT 3.1.1 client where feasible, instead of the MQTT 3.1.1 compatibility adapter https://aws.amazon.com/security/security-bulletins/rss/2026-147-aws/

## Timeline

- 2026-10-23T21:57:32+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-115417
- 2026-10-24T07:15:51+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-115417
- 2026-10-24T20:05:36+00:00 lab https://aws.amazon.com/security/security-bulletins/rss/2026-147-aws/ — AWS: CVE-2026-115417 - Memory allocation with excessive size value in the MQTT 3.1.1 compatibility adapter in aws-c-mqtt

## Documents

- [advisory] AWS: CVE-2026-115417 - Memory allocation with excessive size value in the MQTT 3.1.1 compatibility adapter in aws-c-mqtt https://aws.amazon.com/security/security-bulletins/rss/2026-147-aws/

## From the sources

CVE-2026-115417 - Memory allocation with excessive size value in the MQTT 3.1.1 compatibility adapter in aws-c-mqtt

### CVE-2026-115417 - Memory allocation with excessive size value in the MQTT 3.1.1 compatibility adapter in aws-c-mqtt

Bulletin ID: 2026-147-AWS Scope: AWS Content Type: Important (requires attention) Publication Date: 10/22/2026 13:30 PM PDT

Description:

aws-c-mqtt is a C99 implementation of the MQTT 3.1.1 and MQTT5 specifications, distributed as part of the AWS Common Runtime (CRT) and bundled in the AWS IoT Device SDK for C++ v2. We identified CVE-2026-115417, a memory allocation with excessive size value (CWE-789) in the library's MQTT 3.1.1 compatibility adapter. Under certain conditions, a remote actor could cause a denial of service of the client process.

Impacted versions: >= 0.9.1 AND < 0.16.2

Resolution:

This issue has been addressed in aws-c-mqtt version v0.16.2. We recommend upgrading to the latest version and ensuring any forked or derivative code is patched to incorporate the new fixes.

Workarounds:

Connect only to trusted MQTT brokers (for example AWS IoT Core, which enforces the negotiated maximum packet size) and use TLS to prevent on-path tampering of broker responses. Where feasible, avoid the MQTT 3.1.1 compatibility adapter in favor of a native MQTT5 or MQTT 3.1.1 client. Upgrading to a fixed version is the complete remediation.

References:

- CVE-2026-115417
- GHSA-g3x5-86mr-65p5

Acknowledgement:

We would like to thank Eunho Kim for collaborating on this issue through the coordinated vulnerability disclosure process.

 Please email aws-security@amazon.com with any security questions or concerns.

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