Black GitHub Action CVE-2026-114134

Exploitation requires the victim to use Black's GitHub Action with the non-default setting use_pyproject: true, and a CI workflow that runs that action against pull-request-supplied content; the attacker then submits a PR that rewrites tool.black.required-version in pyproject.toml to point at a malicious package (the pre-patch code accepted direct references such as URLs without validation). Limiting factors that reduce realistic risk: use_pyproject is opt-in and off by default; the action must be invoked in a workflow that executes untrusted PR changes; and GitHub by default requires a maintainer to approve workflow runs for first-time contributors, so unattended execution is most likely for returning or authorized contributors. The complete fix is upgrading to Black 26.10.1, which restricts required-version to released version specifiers.

No CVSS vector, EPSS score, or SSVC framework data was supplied, so quantitative exploitation-probability signals are unavailable and risk must be reasoned from the mechanism and decision label. This is not CISA KEV-listed and no active exploitation is reported, so it is 'no public exploit identified at time of analysis' in the strict sense - however, the GHSA advisory includes a step-by-step reproduction (a public PoC reference exists; it is not verified as working exploit code), lowering the barrier to weaponization. The real-world risk is gated by a non-default configuration: it only affects repositories using the Black GitHub Action with use_pyproject: true, and the attack requires a pull_request-triggered workflow that runs the action against untrusted PR content. Where those conditions hold, the impact is full remote code execution in the victim's CI runner (CWE-829, inclusion of functionality from an untrusted control sphere), which can expose CI secrets and poison builds - a genuine supply-chain concern. The vendor 'this_week' priority is consistent with a serious but conditionally-exploitable flaw rather than an act_now emergency; it is a real priority for CI-heavy Black Action users, but a high-severity-yet-config-limited issue for everyone else. Note the gap: severity cannot be cross-checked against a published CVSS because the input lists it as N/A.

A victim project uses Black's GitHub Action with use_pyproject: true in a pull_request-triggered CI workflow. An attacker opens a PR that edits pyproject.toml so tool.black.required-version is a direct reference (e.g. a URL or git reference) pointing to an attacker-controlled package; when the action runs it installs that package unvalidated and executes attacker code in the runner. A public reproduction description exists in the GHSA advisory, though it is not verified as working exploit code.