# CVE-2026-42013: Gnutls: gnutls: certificate validation bypass due to oversized subject alternative name

> Remote attackers can bypass certificate validation via oversized Subject Alternative Names, enabling spoofing and man-in-the-middle attacks.

- **Severity:** High 8.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 26 May 2026
- **Updated:** 29 Oct 2026
- **Also known as:** ALPINE-CVE-2026-42013, AZL-89178, BELL-CVE-2026-42013, DEBIAN-CVE-2026-42013, ECHO-0a73-78d8-34d4, GHSA-2pw7-g86g-76q7

## What is CVE-2026-42013?

CVE-2026-42013 is a high-severity vulnerability in Cert Manager support for Red Hat OpenShift release 1.17, published on 26 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

A flaw was found in gnutls. When validating certificates, an oversized Subject Alternative Name (SAN) could cause the validation process to incorrectly fall back to checking the Common Name (CN) field. This could allow a remote attacker to bypass proper certificate validation, potentially leading to spoofing or man-in-the-middle attacks.

Weakness: CWE-295 (Improper Certificate Validation)

## Which versions are affected by CVE-2026-42013?


## How do I fix CVE-2026-42013?

Apply the vendor's patch.

## Is CVE-2026-42013 being exploited?

There is no known exploitation of CVE-2026-42013 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-42013 and fix it if it is affected.

What it is: CVE-2026-42013 is a high-severity vulnerability in Cert Manager support for Red Hat OpenShift release 1.17, published on 26 May 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-2026-42013 in the wild and no public exploit code.

Affected product: Cert Manager support for Red Hat OpenShift release 1.17. 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-2026-42013/ (Markdown: https://insights.sbomify.com/CVE-2026-42013/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-42013
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-42013
- OSV: https://osv.dev/vulnerability/CVE-2026-42013

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-42013/
Summaries are generated and may contain mistakes; the official description above is the record.