# CVE-2021-42343: Workers for local Dask clusters mistakenly listened on public interfaces

> Dask LocalCluster workers incorrectly listen on external interfaces, allowing remote attackers to achieve code execution on exposed ports.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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:** 26 Oct 2021
- **Updated:** 4 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-42343, GHSA-hwqr-f3v9-hwxr, GHSA-j8fq-86c5-5v2r, PYSEC-2021-387, PYSEC-2021-871, UBUNTU-CVE-2021-42343, openSUSE-SU-2024:11766-1, openSUSE-SU-2024:13920-1, openSUSE-SU-2026:11043-1

## What is CVE-2021-42343?

CVE-2021-42343 is a critical vulnerability in n/a, published on 26 Oct 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

An issue was discovered in the Dask distributed package before 2021.10.0 for Python. Single machine Dask clusters started with dask.distributed.LocalCluster or dask.distributed.Client (which defaults to using LocalCluster) would mistakenly configure their respective Dask workers to listen on external interfaces (typically with a randomly selected high port) rather than only on localhost. A Dask cluster created using this method (when running on a machine that has an applicable port exposed) could be used by a sophisticated attacker to achieve remote code execution.

## Which versions are affected by CVE-2021-42343?

6 open source packages ship an affected version, including dask and distributed. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `dask` | PyPI | All versions | 2021.10.0 |
| `dask` | PyPI | All versions | 2021.10.0 |
| `dask` | PyPI | All versions | 2021.10.0 |
| `distributed` | PyPI | All versions | 2021.10.0 |
| `distributed` | PyPI | All versions | 2021.10.0 |
| `distributed` | PyPI | All versions | 2021.10.0 |
| `python-distributed` | openSUSE:Tumbleweed | All versions | 2022.1.0-1.1 |
| `python-distributed` | openSUSE:Tumbleweed | All versions | 2024.4.2-1.1 |
| `python-distributed` | openSUSE:Tumbleweed | All versions | 2026.6.0-1.1 |

Products: Anaconda Dask.

## How do I fix CVE-2021-42343?

Upgrade dask to 2021.10.0 or later.

## Is CVE-2021-42343 being exploited?

There is no known exploitation of CVE-2021-42343 in the wild and no public exploit code.

## Other vulnerabilities in n/a

- [CVE-2026-23528](https://insights.sbomify.com/CVE-2026-23528/): Dask distributed Vulnerable to Remote Code Execution via Jupyter Proxy and Dashboard (medium 5.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-42343 and fix it if it is affected.

What it is: CVE-2021-42343 is a critical vulnerability in n/a, published on 26 Oct 2021. 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-2021-42343 in the wild and no public exploit code.

Affected packages and fixed versions:
- dask (PyPI): affected all versions; fixed in 2021.10.0
- dask (PyPI): affected all versions; fixed in 2021.10.0
- dask (PyPI): affected all versions; fixed in 2021.10.0
- distributed (PyPI): affected all versions; fixed in 2021.10.0
- distributed (PyPI): affected all versions; fixed in 2021.10.0
- distributed (PyPI): affected all versions; fixed in 2021.10.0

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-2021-42343/ (Markdown: https://insights.sbomify.com/CVE-2021-42343/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-42343
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-42343
- OSV: https://osv.dev/vulnerability/CVE-2021-42343

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-42343/
Summaries are generated and may contain mistakes; the official description above is the record.