CVE-2026-114134 Python Software Foundation CVE debrief

Who should care

Defenders responsible for GitHub Actions and Black deployments should assess exposure and verify the use of the GitHub Action bundled with Black. They should also review the `pyproject.toml` file for any suspicious changes and apply the fix by updating to version 26.10.1 or later. Additionally, defenders should prioritize verifying the use of the GitHub Action bundled with Black and ensuring that the `use_pyproject` setting is not set to True until a fix

Why it matters

CVE-2026-114134 is a remote code execution vulnerability in the GitHub Action bundled with Black. Defenders should verify the use of the GitHub Action and the `use_pyproject` setting to prevent exploitation.

• Remote code execution in the context of a victim's repository.
• Potential for lateral movement and exploitation of other systems.
• Need for verification of `pyproject.toml` file integrity.
• Priority for applying the fix to prevent exploitation.

Technical summary

The GitHub Action bundled with Black reads the version from `tool.black.required-version` in `pyproject.toml` without validation. This allows an attacker to execute untrusted code in the context of a victim's repository when the `use_pyproject` setting is set to True. The vulnerability is caused by the lack of validation of the version specified in `pyproject.toml`, which can be edited by an attacker to point to a malicious repository. This vulnerability impacts users who use the GitHub Action bundled with Black and have the `use_pyproject` setting enabled.

Defensive priority

Defenders should prioritize verifying the use of the GitHub Action bundled with Black and ensuring that the `use_pyproject` setting is not set to True until a fix is applied.

Recommended defensive actions

• Verify the use of the GitHub Action bundled with Black and ensure that the `use_pyproject` setting is not set to True.
• Review the `pyproject.toml` file for any suspicious changes.
• Apply the fix by updating to version 26.10.1 or later.
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.
• Review compensating controls for exposed systems while remediation is scheduled and verified.
• Check relevant monitoring, detection, and logs for exposed assets that need extra review.
• Track exceptions, retest remediated assets, and close the item only after evidence is documented.

Evidence notes

The vulnerability is caused by the GitHub Action bundled with Black reading the version from `tool.black.required-version` in `pyproject.toml` without validation. This allows an attacker to execute untrusted code by editing the `pyproject.toml` file to point to a malicious repository.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.