# CVE-2026-73261: Mongoose TCP option parser bounds check vulnerability

Source: https://junglewise.ai/threats/cve-2026-73261-mongoose-is-an-embedded-web-server-and-network-library-prior-to-7  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-14. Updated: 2026-10-16  
License: CC BY 4.0 with a link back

CVE-2026-73261 · Severity: high · CVSS 7.5 · Published 2026-10-14

Mongoose is an embedded web server and networking library used in IoT and embedded systems. A flaw in its built-in TCP/IP stack allows an unauthenticated attacker on the network to send malformed TCP options that corrupt the maximum segment size (MSS) value for a connection. This causes the application to send truncated or empty data packets while the connection appears healthy, leading to data loss or application malfunction.

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

## Quick answers

**Is CVE-2026-73261 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-73261 being exploited in the wild.

**How severe is CVE-2026-73261?** High severity, with a CVSS score of 7.5, published 14 October 2026.

**Which products and versions are affected?** Cesanta Mongoose before 7.23.

**Is there a fix for CVE-2026-73261?** Yes: fixed in 7.23. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-73261 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-73261?** 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.

## How to fix

- Fixed in 7.23

## Technical details

The vulnerability exists in the handle_opt() function in src/net_builtin.c, where a malformed TCP option with a declared length exceeding remaining option bytes causes out-of-bounds memory reads. Specifically, a NOP followed by a truncated MSS option causes opts[3] to be read outside the options area and used as the low byte of the MSS (dmss) value. An unauthenticated remote peer can trigger this via crafted TCP packets on the network, corrupting the connection's MSS configuration without authentication or user interaction.

## Affected products

- Cesanta Mongoose before 7.23

## Timeline

- 2026-10-14: advisory: CVE-2026-73261 published

## References

- [https://github.com/cesanta/mongoose/commit/cc7c6d2d18c28fcc6d085633dddd88402946140a](https://github.com/cesanta/mongoose/commit/cc7c6d2d18c28fcc6d085633dddd88402946140a)
- [https://github.com/cesanta/mongoose/releases/tag/7.23](https://github.com/cesanta/mongoose/releases/tag/7.23)

## Related threats

- [Mongoose heap out-of-bounds read in path validation](https://junglewise.ai/threats/cve-2026-115473-mongoose-is-an-embedded-web-server-and-network-library-prior-to)
- [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)
- [Cesanta Mongoose stack-based buffer overflow in MQTT broker](https://junglewise.ai/threats/cve-2026-101003-a-weakness-has-been-identified-in-cesanta-mongoose-up-to-7-21)