Black: GitHub Action installs unvalidated required-version from pyproject.toml

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.