# CVE-2026-11404: Cesanta Mongoose out-of-bounds read in mg_tls_server_recv_hello

Source: https://junglewise.ai/threats/cve-2026-11404-cesanta-mongoose-out-of-bounds-read-in-mg-tls-server-recv-hello  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-09. Updated: 2026-07-09  
License: CC BY 4.0 with a link back

CVE-2026-11404 · Severity: high · CVSS 7.5 · Published 2026-07-09

Cesanta Mongoose is a popular embedded networking library used to provide web, MQTT, and WebSocket capabilities to hardware devices. A security flaw in its built-in encryption handling allows a remote attacker to crash the device's secure services by sending a specially crafted connection request. This can lead to a denial-of-service, disrupting the device's ability to communicate securely or perform its primary functions.

## Quick answers

**Is CVE-2026-11404 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-11404 being exploited in the wild.

**How severe is CVE-2026-11404?** High severity, with a CVSS score of 7.5, published 9 July 2026.

**Which products and versions are affected?** Cesanta Mongoose before 7.22.

**Is there a fix for CVE-2026-11404?** A patch was published on 2026-06-24: Version 7.22 released with fix Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-11404 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-11404?** Check whether anything you run is named in the advisory. 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

An out-of-bounds read vulnerability exists in the mg_tls_server_recv_hello() function within Cesanta Mongoose's built-in TLS implementation (MG_TLS_BUILTIN). The root cause is the use of an attacker-controlled session_id_len byte from a TLS ClientHello message as a buffer index without proper validation against the actual length of the received data. A remote, unauthenticated attacker can exploit this by sending a single crafted ClientHello packet with an oversized session ID length. This causes the server to read past the receive buffer, resulting in a crash and denial-of-service for any service utilizing the built-in TLS stack (such as HTTPS, MQTTS, or WSS). The issue is resolved in version 7.22.

## Affected products

- Cesanta Mongoose before 7.22

## Timeline

- 2026-06-24: patched: Version 7.22 released with fix
- 2026-07-09: disclosed: CVE published and advisory released

## References

- [https://github.com/cesanta/mongoose](https://github.com/cesanta/mongoose)
- [https://github.com/cesanta/mongoose/commit/c288ac1f38424ffdd4d2fd5e1893fd4962642db3](https://github.com/cesanta/mongoose/commit/c288ac1f38424ffdd4d2fd5e1893fd4962642db3)
- [https://github.com/cesanta/mongoose/releases/tag/7.22](https://github.com/cesanta/mongoose/releases/tag/7.22)
- [https://www.vulncheck.com/advisories/cesanta-mongoose-out-of-bounds-read-in-mg-tls-builtin-clienthello-session-id-parsing](https://www.vulncheck.com/advisories/cesanta-mongoose-out-of-bounds-read-in-mg-tls-builtin-clienthello-session-id-parsing)