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.

Affected: PyPI: black

### Summary

The GitHub Action bundled with Black can read the version from `tool.black.required-version` if `use_pyproject` is set to True. This version is not validated and therefore can be used to execute untrusted code in the context of a victim's repository.

### Details

This is similar to the previous CVE-2026-31900. The value in `tool.black.required-version` is accepted as the version to be installed by the action without validation.

### Proof of concept

Reproduction requires a victim to use Black's GitHub Action with `use_pyproject: true`. The attacker would then issue a PR to the victim's repository with `pyproject.toml` edited so that Black points to a malicious repository.

### Impact

This is a remote code execution vulnerability impacting users using the GitHub Action bundled with psf/black and using the `use_pyproject: true` setting.