# CVE-2026-101003: Cesanta Mongoose stack-based buffer overflow in MQTT broker

Source: https://junglewise.ai/threats/cve-2026-101003-a-weakness-has-been-identified-in-cesanta-mongoose-up-to-7-21  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-28. Updated: 2026-09-28  
License: CC BY 4.0 with a link back

CVE-2026-101003 · Severity: medium · CVSS 5.3 · Published 2026-09-28

Cesanta Mongoose is an embedded web server and networking library used in IoT and embedded applications. A stack-based buffer overflow in the MQTT broker component allows remote attackers to crash the application or potentially execute arbitrary code without requiring authentication.

Technologies: [Cesanta Mongoose](https://junglewise.ai/threats/technologies/cesanta-mongoose). Vendors: [Cesanta](https://junglewise.ai/threats/vendors/cesanta).

## Quick answers

**Is CVE-2026-101003 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-101003 being exploited in the wild.

**How severe is CVE-2026-101003?** Medium severity, with a CVSS score of 5.3, published 28 September 2026.

**Which products and versions are affected?** Cesanta Mongoose up to 7.21.

**Is there a fix for CVE-2026-101003?** A patch was published on 2026-06-23: Patch released in version 7.22 (commit a9df523f76f43a38bd53b4232b9cfd4c16869e71) Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-101003 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-101003?** Check whether you run Cesanta Mongoose at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A stack-based buffer overflow vulnerability exists in the MQTT broker component of Mongoose, specifically in the fn function within tutorials/mqtt/mqtt-server/main.c. The vulnerability can be exploited remotely without authentication by sending a crafted MQTT message. A patch was released in version 7.22, committed as a9df523f76f43a38bd53b4232b9cfd4c16869e71.

## Affected products

- Cesanta Mongoose up to 7.21

## Timeline

- 2026-09-28: disclosed: Vulnerability disclosed and CVSS assessed at 5.3
- 2026-06-23: patched: Patch released in version 7.22 (commit a9df523f76f43a38bd53b4232b9cfd4c16869e71)

## References

- [https://github.com/cesanta/mongoose/](https://github.com/cesanta/mongoose/)
- [https://github.com/cesanta/mongoose/commit/a9df523f76f43a38bd53b4232b9cfd4c16869e71](https://github.com/cesanta/mongoose/commit/a9df523f76f43a38bd53b4232b9cfd4c16869e71)

## Related threats

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- [Mongoose TLS certificate parser bounds check in DER parsing](https://junglewise.ai/threats/cve-2026-73260-mongoose-is-an-embedded-web-server-and-network-library-prior-to-7)
- [Mongoose TLS short-record out-of-bounds read](https://junglewise.ai/threats/cve-2026-73252-mongoose-is-an-embedded-web-server-and-network-library-prior-to-7)
- [Mongoose IPv6CP length field bounds check buffer overflow](https://junglewise.ai/threats/cve-2026-63626-mongoose-is-an-embedded-web-server-and-network-library-prior-to-7)