# CVE-2026-56389: Arbitrary Command Execution in GNU Bison

> Attackers can execute arbitrary commands by providing a malicious grammar file to GNU Bison during HTML report generation.

- **Severity:** Medium 6.8 (`CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:A/VC:N/VI:H/VA:L/SC:L/SI:L/SA:L`)
- **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:** 29 Jul 2026
- **Updated:** 29 Jul 2026
- **Also known as:** AZL-94133, DEBIAN-CVE-2026-56389, ECHO-5a4d-f9a3-722e, GHSA-735p-4prj-f62v, JLSEC-2026-1345, UBUNTU-CVE-2026-56389, USN-8777-1

## What is CVE-2026-56389?

CVE-2026-56389 is a medium-severity command injection vulnerability in GNU Bison, published on 29 Jul 2026. 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

GNU Bison allows for an execution of an arbitrary program during HTML report generation due to improper handling of grammar-defined configuration variables. A grammar file can override the executable used for the XML‑to‑HTML transformation step via %define tool.xsltproc, which is accepted without restriction and passed directly to execvp().
 
When running bison --html on a attacker-provided grammar, this behavior allows execution of an arbitrary program with the privileges of the Bison process.

Maintainers of this project were notified about this vulnerability, and fixed the issue in commit 3169c1e7a2c6acc4c59dfcf8b089896d6881925b. However, they did not provide vulnerable version range. Version 3.8.2 was tested and confirmed as vulnerable, other versions were not tested but might also be vulnerable.

Weakness: CWE-78 (Improper Neutralization of Special Elements used in an OS Command)

## Which versions are affected by CVE-2026-56389?

It affects 1 product.


Products: Gnu Bison.

## How do I fix CVE-2026-56389?

Apply the vendor's patch.

## Is CVE-2026-56389 being exploited?

There is no known exploitation of CVE-2026-56389 in the wild and no public exploit code.

## Other vulnerabilities in GNU Bison

- [CVE-2020-24240](https://insights.sbomify.com/CVE-2020-24240/): Use after free in n/a (medium 5.5)
- [CVE-2020-14150](https://insights.sbomify.com/CVE-2020-14150/): Denial of service in n/a (medium 5.5)
- [CVE-2026-56390](https://insights.sbomify.com/CVE-2026-56390/): Arbitrary Output Location Change in GNU Bison (medium 4.6)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-56389 and fix it if it is affected.

What it is: CVE-2026-56389 is a medium-severity command injection vulnerability in GNU Bison, published on 29 Jul 2026. 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-2026-56389 in the wild and no public exploit code.

Affected product: GNU Bison. 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-2026-56389/ (Markdown: https://insights.sbomify.com/CVE-2026-56389/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-56389
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-56389
- OSV: https://osv.dev/vulnerability/CVE-2026-56389

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-56389/
Summaries are generated and may contain mistakes; the official description above is the record.