Black provides a [GitHub action](https://black.readthedocs.io/en/stable/integrations/github actions.html) for formatting code. This action supports an option, use pyproject: true, for reading the version of Black to use from the repository pyproject.toml. A malicious pull request could edit pyproject.toml to use a direct URL reference to a malicious repository. This could lead to arbitrary code execution in the context of the GitHub Action. Attackers could then gain access to secrets or permissions available in the context of the action.

Version 26.3.0 fixes this vulnerability by tightening the validation of the version field. Users who use the GitHub Action as psf/black@stable will automatically pick up this update.

Do not use the use pyproject: true option in the psf/black GitHub Action.