# CVE-2021-4209: Null dereference in GnuTLS

> An authenticated attacker can cause a denial of service in GnuTLS by providing zero-length input to hash update functions.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 24 Aug 2022
- **Updated:** 14 Oct 2026
- **Also known as:** DEBIAN-CVE-2021-4209, GHSA-xj78-h3gx-cm6v, JLSEC-2026-522, MGASA-2022-0098, OESA-2022-1889, SUSE-SU-2022:0677-1, SUSE-SU-2022:0678-1, SUSE-SU-2022:0717-1, UBUNTU-CVE-2021-4209, USN-5750-1, openSUSE-SU-2022:0717-1

## What is CVE-2021-4209?

CVE-2021-4209 is a medium-severity vulnerability in Gnu Gnutls, published on 24 Aug 2022. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

A NULL pointer dereference flaw was found in GnuTLS. As Nettle's hash update functions internally call memcpy, providing zero-length input may cause undefined behavior. This flaw leads to a denial of service after authentication in rare circumstances.

Weakness: CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2021-4209?

It affects 5 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `gnutls` | SUSE:Linux Enterprise Micro 5.0 | All versions | 3.6.7-14.16.1 |
| `gnutls` | SUSE:Linux Enterprise Micro 5.1 | All versions | 3.6.7-14.16.1 |
| `gnutls` | SUSE:Linux Enterprise Module for Basesystem 15 SP3 | All versions | 3.6.7-14.16.1 |
| `gnutls` | SUSE:Linux Enterprise Real Time 15 SP2 | All versions | 3.6.7-14.16.1 |
| `gnutls` | SUSE:Linux Enterprise Server 12 SP5 | All versions | 3.3.27-3.6.1 |
| `gnutls` | SUSE:Linux Enterprise Server 12 SP5 | All versions | 3.4.17-8.11.1 |
| `gnutls` | SUSE:Linux Enterprise Server for SAP Applications 12 SP5 | All versions | 3.3.27-3.6.1 |
| `gnutls` | SUSE:Linux Enterprise Server for SAP Applications 12 SP5 | All versions | 3.4.17-8.11.1 |
| `gnutls` | SUSE:Linux Enterprise Software Development Kit 12 SP5 | All versions | 3.3.27-3.6.1 |
| `gnutls` | openSUSE:Leap 15.3 | All versions | 3.6.7-14.16.1 |

Products: Gnu Gnutls, Netapp Active Iq Unified Manager, Netapp Hci Bootstrap Os, Netapp Solidfire \& Hci Management Node, Redhat Enterprise Linux.

## How do I fix CVE-2021-4209?

Apply the vendor's patch.

## Is CVE-2021-4209 being exploited?

There is no known exploitation of CVE-2021-4209 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-2021-4209 and fix it if it is affected.

What it is: CVE-2021-4209 is a medium-severity vulnerability in Gnu Gnutls, published on 24 Aug 2022. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-4209 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-2021-4209/ (Markdown: https://insights.sbomify.com/CVE-2021-4209/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-4209
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-4209
- OSV: https://osv.dev/vulnerability/CVE-2021-4209

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-4209/
Summaries are generated and may contain mistakes; the official description above is the record.