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Tool Directory

CVE-2026-31900

HighPublished

Black's vulnerable version parsing leads to RCE in GitHub Action

Published: Mar 11, 2026
Updated: Mar 13, 2026
Assigning CNA: GitHub_M
Evidence observed: Aug 25, 2026

Primary CVSS

8.7/ 10High

nvd · CVSS 4.0

CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

EPSS probability estimate

0.5%

Low · next 30 days

Percentile: 39.0%
Model date: Sep 21, 2026

EPSS is a statistical estimate, not a certainty or a measure of impact. Combine it with CVSS, KEV status, exposure and your environment.

Summary

Black is the uncompromising Python code formatter. Black provides a GitHub action 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.

Sources

cve-2026-31900-labPoC
CVE-2026-31900 Vulnerable Lab - psf/black GitHub Action RCE

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