# CVE-2026-11404: Cesanta Mongoose Out-of-Bounds Read in MG_TLS_BUILTIN ClientHello Session ID Parsing

> A remote attacker can crash HTTPS, MQTTS, or WSS services by sending a crafted TLS ClientHello with an oversized session ID length.

- **Severity:** High 8.7 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 9 Jul 2026
- **Updated:** 29 Aug 2026
- **Also known as:** DEBIAN-CVE-2026-11404, GHSA-cvgj-gr8r-q762, JLSEC-2026-1257, UBUNTU-CVE-2026-11404

## What is CVE-2026-11404?

CVE-2026-11404 is a high-severity denial of service vulnerability in Cesanta Mongoose, published on 9 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Cesanta Mongoose before 7.22 contains an out-of-bounds read in the built-in TLS server function mg_tls_server_recv_hello(), which uses an attacker-controlled session_id_len byte from a TLS ClientHello as a buffer index without validating it against the length of received data. A remote, unauthenticated attacker can send a single crafted ClientHello with an oversized session id length to read past the receive buffer, crashing any HTTPS, MQTTS, or WSS service built on MG_TLS_BUILTIN.

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2026-11404?


## How do I fix CVE-2026-11404?

Upgrade to a fixed version.

## Is CVE-2026-11404 being exploited?

There is no known exploitation of CVE-2026-11404 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-11404 and fix it if it is affected.

What it is: CVE-2026-11404 is a high-severity denial of service vulnerability in Cesanta Mongoose, published on 9 Jul 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-2026-11404 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-2026-11404/ (Markdown: https://insights.sbomify.com/CVE-2026-11404/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-11404
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-11404
- OSV: https://osv.dev/vulnerability/CVE-2026-11404

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-11404/
Summaries are generated and may contain mistakes; the official description above is the record.