# CVE-2021-20235: Memory corruption in zeromq

> A remote attacker can trigger a buffer overflow in ZeroMQ via crafted requests if authentication is disabled, potentially leading to code execution or denial of service.

- **Severity:** High 8.1 (`CVSS:3.1/AV:N/AC:H/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:** 1 Apr 2021
- **Updated:** 3 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-20235, GHSA-fc3w-qxf5-7hp6, GHSA-hrfc-mjc4-cv8f, JLSEC-2026-515, UBUNTU-CVE-2021-20235

## What is CVE-2021-20235?

CVE-2021-20235 is a high-severity memory corruption vulnerability in zeromq, published on 1 Apr 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

There's a flaw in the zeromq server in versions before 4.3.3 in src/decoder_allocators.hpp. The decoder static allocator could have its sized changed, but the buffer would remain the same as it is a static buffer. A remote, unauthenticated attacker who sends a crafted request to the zeromq server could trigger a buffer overflow WRITE of arbitrary data if CURVE/ZAP authentication is not enabled. The greatest impact of this flaw is to application availability, data integrity, and confidentiality.

Weakness: CWE-787 (Out-of-bounds Write), CWE-120 (Buffer Copy without Checking Size of Input)

## Which versions are affected by CVE-2021-20235?

It affects 1 product.


Products: Zeromq Libzmq.

## How do I fix CVE-2021-20235?

Upgrade to a fixed version.

## Is CVE-2021-20235 being exploited?

There is no known exploitation of CVE-2021-20235 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-2021-20237](https://insights.sbomify.com/CVE-2021-20237/): Denial of service in zeromq (high 7.5)
- [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-2021-20235 and fix it if it is affected.

What it is: CVE-2021-20235 is a high-severity memory corruption vulnerability in zeromq, published on 1 Apr 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-2021-20235 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-2021-20235/ (Markdown: https://insights.sbomify.com/CVE-2021-20235/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-20235
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-20235
- OSV: https://osv.dev/vulnerability/CVE-2021-20235

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-20235/
Summaries are generated and may contain mistakes; the official description above is the record.