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.