Summary
Black's vulnerable version parsing leads to RCE in GitHub Action
Workarounds
Do not use the use_pyproject: true option in the psf/black GitHub Action.
Impact
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.
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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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.
Frequently Asked Questions
- What is CVE-2026-31900? CVE-2026-31900 is a high-severity improper input validation vulnerability in psf/black (actions), affecting versions < 26.3.0. It is fixed in 26.3.0. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
- Which versions of psf/black are affected by CVE-2026-31900? psf/black (actions) versions < 26.3.0 is affected.
- Is there a fix for CVE-2026-31900? Yes. CVE-2026-31900 is fixed in 26.3.0. Upgrade to this version or later.
- Is CVE-2026-31900 exploitable, and should I be worried? Whether CVE-2026-31900 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-31900 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-31900? Upgrade
psf/blackto 26.3.0 or later.