# CVE-2021-33641: Use after free in byacc

> A use-after-free vulnerability in byacc allows attackers to potentially execute arbitrary code or cause a denial of service via crafted input files.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 20 Jan 2023
- **Updated:** 3 Apr 2025
- **Also known as:** CLSA-2026-1782221814, GHSA-56c6-2pfc-9rvh

## What is CVE-2021-33641?

CVE-2021-33641 is a high-severity use after free vulnerability in byacc, published on 20 Jan 2023. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.

### Official description

When processing files, malloc stores the data of the current line. When processing comments, malloc incorrectly accesses the released memory (use after free).

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2021-33641?

It affects 1 product.


Products: Openeuler Byacc.

## How do I fix CVE-2021-33641?

Apply the vendor's patch.

## Is CVE-2021-33641 being exploited?

There is no known exploitation of CVE-2021-33641 in the wild and no public exploit code.

## Other vulnerabilities in byacc

- [CVE-2021-33642](https://insights.sbomify.com/CVE-2021-33642/): Denial of service in byacc (medium 5.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-33641 and fix it if it is affected.

What it is: CVE-2021-33641 is a high-severity use after free vulnerability in byacc, published on 20 Jan 2023. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2021-33641 in the wild and no public exploit code.

Affected product: byacc. 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-2021-33641/ (Markdown: https://insights.sbomify.com/CVE-2021-33641/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-33641
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-33641
- OSV: https://osv.dev/vulnerability/CVE-2021-33641

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-33641/
Summaries are generated and may contain mistakes; the official description above is the record.