# CVE-2026-6985: Cesanta Mongoose TCP Option net_builtin.c handle_opt infinite loop

> A remote attacker can cause a denial of service via an infinite loop in the TCP option handler of the Cesanta Mongoose library.

- **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:** 25 Apr 2026
- **Updated:** 27 Apr 2026
- **Also known as:** DEBIAN-CVE-2026-6985, GHSA-3ppf-435x-3p66, JLSEC-2026-372, UBUNTU-CVE-2026-6985

## What is CVE-2026-6985?

CVE-2026-6985 is a medium-severity denial of service vulnerability in Cesanta Mongoose, published on 25 Apr 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 7.20. This vulnerability affects the function handle_opt of the file /src/net_builtin.c of the component TCP Option Handler. This manipulation of the argument optlen causes infinite loop. The attack is possible to be carried out remotely. The exploit has been made available to the public and could be used for attacks. Upgrading to version 7.21 is able to resolve this issue. Upgrading the affected component is advised. VulDB has contacted the vendor early and they confirmed quickly, that this issue got fixed already.

Weakness: CWE-835 (Loop with Unreachable Exit Condition), CWE-404 (Improper Resource Shutdown or Release)

## Which versions are affected by CVE-2026-6985?

It affects 1 product.


Products: Cesanta Mongoose.

## How do I fix CVE-2026-6985?

Upgrade to a fixed version.

## Is CVE-2026-6985 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-6985 exists.

## Other vulnerabilities in Cesanta Mongoose

- [CVE-2022-25299](https://insights.sbomify.com/CVE-2022-25299/): Arbitrary File Write (critical 9.8)
- [CVE-2020-25756](https://insights.sbomify.com/CVE-2020-25756/): Memory corruption in n/a (critical 9.8)
- [CVE-2019-19307](https://insights.sbomify.com/CVE-2019-19307/): Integer overflow in n/a (critical 9.8)
- [CVE-2019-12951](https://insights.sbomify.com/CVE-2019-12951/): Memory corruption in n/a (critical 9.8)
- [CVE-2018-20353](https://insights.sbomify.com/CVE-2018-20353/): Use after free in n/a (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-6985 and fix it if it is affected.

What it is: CVE-2026-6985 is a medium-severity denial of service vulnerability in Cesanta Mongoose, published on 25 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-6985 exists.

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-2026-6985/ (Markdown: https://insights.sbomify.com/CVE-2026-6985/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-6985
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-6985
- OSV: https://osv.dev/vulnerability/CVE-2026-6985

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-6985/
Summaries are generated and may contain mistakes; the official description above is the record.