# CVE-2026-42009: Gnutls: gnutls: denial of service via dtls packet reordering vulnerability

> A remote attacker can cause a denial of service in GnuTLS by exploiting a flaw in DTLS packet reordering logic that mishandles duplicate sequence numbers.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 May 2026
- **Updated:** 26 Oct 2026
- **Also known as:** ALPINE-CVE-2026-42009, AZL-87110, BELL-CVE-2026-42009, DEBIAN-CVE-2026-42009, ECHO-b6c2-c0e5-0816, GHSA-9gx7-g5hv-xjjj

## What is CVE-2026-42009?

CVE-2026-42009 is a high-severity denial of service vulnerability in Gnu Gnutls, published on 18 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. A remote attacker could exploit an issue in the Datagram Transport Layer Security (DTLS) packet reordering logic. The comparator function, responsible for ordering DTLS packets by sequence numbers, did not correctly handle packets with duplicate sequence numbers. This could lead to unstable packet ordering or undefined behavior, resulting in a denial of service.

Weakness: CWE-475 (Undefined Behavior for Input to API)

## Which versions are affected by CVE-2026-42009?

It affects 14 products.


Products: Gnu Gnutls, Redhat Enterprise Linux, Redhat Enterprise Linux for Els, Redhat Enterprise Linux for Eus, Redhat Enterprise Linux for Ibm Z Systems, Redhat Enterprise Linux for Ibm Z Systems Els, Redhat Enterprise Linux for Ibm Z Systems Eus, Redhat Enterprise Linux for Power Little Endian, Redhat Enterprise Linux for Power Little Endian Els, Redhat Enterprise Linux for Power Little Endian Eus, Redhat Enterprise Linux for Update Services for Sap Solutions, Redhat Enterprise Linux Server for Power Little Endian Update Services for Sap Solutions, Redhat Hardened Images, Redhat Openshift Container Platform.

## How do I fix CVE-2026-42009?

Apply the vendor's patch.

## Is CVE-2026-42009 being exploited?

There is no known exploitation of CVE-2026-42009 in the wild and no public exploit code.

## Other vulnerabilities in Gnu Gnutls

- [CVE-2008-1948](https://insights.sbomify.com/CVE-2008-1948/): Memory corruption in n/a (critical 10.0)
- [CVE-2017-5334](https://insights.sbomify.com/CVE-2017-5334/): Use after free in n/a (critical 9.8)
- [CVE-2017-5336](https://insights.sbomify.com/CVE-2017-5336/): Memory corruption in n/a (critical 9.8)
- [CVE-2017-5337](https://insights.sbomify.com/CVE-2017-5337/): Memory corruption in n/a (critical 9.8)
- [CVE-2008-1949](https://insights.sbomify.com/CVE-2008-1949/): Null dereference in n/a (critical 9.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-42009 and fix it if it is affected.

What it is: CVE-2026-42009 is a high-severity denial of service vulnerability in Gnu Gnutls, published on 18 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-2026-42009 in the wild and no public exploit code.

Affected product: Gnu Gnutls. 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-2026-42009/ (Markdown: https://insights.sbomify.com/CVE-2026-42009/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-42009
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-42009
- OSV: https://osv.dev/vulnerability/CVE-2026-42009

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-42009/
Summaries are generated and may contain mistakes; the official description above is the record.