# CVE-2021-20234: Denial of service in zeromq

> A memory leak in the ZeroMQ client allows malicious servers to crash connecting clients, impacting system availability.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 1 Apr 2021
- **Updated:** 3 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-20234, GHSA-47vr-832f-crm9, GHSA-wfr2-29gj-5w87, JLSEC-2026-514, UBUNTU-CVE-2021-20234

## What is CVE-2021-20234?

CVE-2021-20234 is a medium-severity denial of service vulnerability in zeromq, published on 1 Apr 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

An uncontrolled resource consumption (memory leak) flaw was found in the ZeroMQ client in versions before 4.3.3 in src/pipe.cpp. This issue causes a client that connects to multiple malicious or compromised servers to crash. The highest threat from this vulnerability is to system availability.

Weakness: CWE-401 (Missing Release of Memory after Effective Lifetime), CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2021-20234?

It affects 1 product.


Products: Zeromq Libzmq.

## How do I fix CVE-2021-20234?

Upgrade to a fixed version.

## Is CVE-2021-20234 being exploited?

There is no known exploitation of CVE-2021-20234 in the wild and no public exploit code.

## Other vulnerabilities in zeromq

- [CVE-2020-36400](https://insights.sbomify.com/CVE-2020-36400/): Memory corruption in n/a (critical 9.8)
- [CVE-2021-20235](https://insights.sbomify.com/CVE-2021-20235/): Memory corruption in zeromq (high 8.1)
- [CVE-2021-20237](https://insights.sbomify.com/CVE-2021-20237/): Denial of service in zeromq (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-20234 and fix it if it is affected.

What it is: CVE-2021-20234 is a medium-severity denial of service vulnerability in zeromq, published on 1 Apr 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2021-20234 in the wild and no public exploit code.

Affected product: zeromq. 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-2021-20234/ (Markdown: https://insights.sbomify.com/CVE-2021-20234/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-20234
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-20234
- OSV: https://osv.dev/vulnerability/CVE-2021-20234

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-20234/
Summaries are generated and may contain mistakes; the official description above is the record.