# CVE-2021-20237: Denial of service in zeromq

> Remote attackers can cause denial of service via memory exhaustion in ZeroMQ when authentication is disabled.

- **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:** 28 May 2021
- **Updated:** 3 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-20237, GHSA-4p5v-h92w-6wxw, GHSA-98wx-fq4r-jfm2, JLSEC-2026-517, UBUNTU-CVE-2021-20237

## What is CVE-2021-20237?

CVE-2021-20237 is a high-severity denial of service vulnerability in zeromq, published on 28 May 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

An uncontrolled resource consumption (memory leak) flaw was found in ZeroMQ's src/xpub.cpp in versions before 4.3.3. This flaw allows a remote unauthenticated attacker to send crafted PUB messages that consume excessive memory if the CURVE/ZAP authentication is disabled on the server, causing a denial of service. 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-2021-20237?

It affects 1 product.


Products: Zeromq Libzmq.

## How do I fix CVE-2021-20237?

Upgrade to a fixed version.

## Is CVE-2021-20237 being exploited?

There is no known exploitation of CVE-2021-20237 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-2021-20234](https://insights.sbomify.com/CVE-2021-20234/): Denial of service in zeromq (medium 6.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-20237 and fix it if it is affected.

What it is: CVE-2021-20237 is a high-severity denial of service vulnerability in zeromq, published on 28 May 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-2021-20237 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-2021-20237/ (Markdown: https://insights.sbomify.com/CVE-2021-20237/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-20237
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-20237
- OSV: https://osv.dev/vulnerability/CVE-2021-20237

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-20237/
Summaries are generated and may contain mistakes; the official description above is the record.