# CVE-2026-101003: Cesanta Mongoose MQTT Broker main.c fn stack-based overflow

> A stack-based buffer overflow in the Cesanta Mongoose MQTT Broker allows remote attackers to execute arbitrary code or cause denial of service.

- **Severity:** Medium 6.9 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:P`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 28 Sep 2026
- **Updated:** 1 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-101003, GHSA-g7gm-388f-77vf, UBUNTU-CVE-2026-101003

## What is CVE-2026-101003?

CVE-2026-101003 is a medium-severity memory corruption vulnerability in Cesanta Mongoose, published on 28 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

A weakness has been identified in Cesanta Mongoose up to 7.21. Affected by this vulnerability is the function fn of the file tutorials/mqtt/mqtt-server/main.c of the component MQTT Broker. Executing a manipulation can lead to stack-based buffer overflow. The attack can be launched remotely. The exploit has been made available to the public and could be used for attacks. Upgrading to version 7.22 addresses this issue. This patch is called a9df523f76f43a38bd53b4232b9cfd4c16869e71. Upgrading the affected component is advised.

Weakness: CWE-121 (Stack-based Buffer Overflow), CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer)

## Which versions are affected by CVE-2026-101003?


## How do I fix CVE-2026-101003?

Upgrade to a fixed version.

## Is CVE-2026-101003 being exploited?

There is no known exploitation of CVE-2026-101003 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-101003 and fix it if it is affected.

What it is: CVE-2026-101003 is a medium-severity memory corruption vulnerability in Cesanta Mongoose, published on 28 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-101003 in the wild and no public exploit code.

Affected product: Cesanta Mongoose. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-101003/ (Markdown: https://insights.sbomify.com/CVE-2026-101003/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-101003
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-101003
- OSV: https://osv.dev/vulnerability/CVE-2026-101003

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-101003/
Summaries are generated and may contain mistakes; the official description above is the record.