# CVE-2026-6986: Cesanta Mongoose GCM Authentication Tag tls_aes128.c mg_aes_gcm_decrypt signature verification

> Remote attackers can bypass GCM authentication tag verification in Cesanta Mongoose, potentially compromising TLS security.

- **Severity:** Medium 6.3 (`CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/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-2026-6986, GHSA-p5j9-fjff-qrmr, JLSEC-2026-373, UBUNTU-CVE-2026-6986

## What is CVE-2026-6986?

CVE-2026-6986 is a medium-severity 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 security vulnerability has been detected in Cesanta Mongoose up to 7.20. This issue affects the function mg_aes_gcm_decrypt of the file /src/tls_aes128.c of the component GCM Authentication Tag Handler. Such manipulation leads to improper verification of cryptographic signature. The attack may be performed from remote. A high complexity level is associated with this attack. The exploitability is assessed as difficult. The exploit has been disclosed publicly and may be used. Upgrading to version 7.21 is capable of addressing this issue. It is advisable to upgrade the affected component. VulDB has contacted the vendor early and they confirmed quickly, that this issue got fixed already.

Weakness: CWE-347 (Improper Verification of Cryptographic Signature), CWE-345 (Insufficient Verification of Data Authenticity)

## Which versions are affected by CVE-2026-6986?

It affects 1 product.


Products: Cesanta Mongoose.

## How do I fix CVE-2026-6986?

Upgrade to a fixed version.

## Is CVE-2026-6986 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-6986 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-2026-6986 and fix it if it is affected.

What it is: CVE-2026-6986 is a medium-severity 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-2026-6986 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-2026-6986/ (Markdown: https://insights.sbomify.com/CVE-2026-6986/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-6986
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-6986
- OSV: https://osv.dev/vulnerability/CVE-2026-6986

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-6986/
Summaries are generated and may contain mistakes; the official description above is the record.