critical CVSS 9.8 EPSS <0.1%

CVE-2026-31900·Python vulnerability

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.

Severity
critical
Software
Python
Fixed in
26.3.0
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at <0.1% — higher than 25% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Read the two together. CVE-2026-31900 scores 9.8 on CVSS, so a successful attack would be serious — but at <0.1%, almost nobody is currently attempting it. That is a different situation from a flaw under mass exploitation, and it is usually the difference between patching on your next maintenance window and patching tonight.

Affected versions

Until: 26.3.0

Fixed in: 26.3.0

How to fix this CVE

Update Python to version 26.3.0 or later to patch the Black code formatter GitHub Action vulnerability. This fix prevents malicious pull requests from injecting arbitrary code through pyproject.toml configuration manipulation. Verify the patched version is installed across all systems that run Python-based CI/CD workflows.

sudo dnf update python3

Defensia detects this vulnerability

How to check if you are affected

  1. Run 'python3 --version' to check the installed Python version; version 26.3.0 or later indicates the fix is applied
  2. Verify Black is installed by running 'pip3 show black' and confirm the version is 26.3.0 or newer
  3. Review pyproject.toml files in repositories for direct URL references to external repositories (lines containing 'url =' with http/https pointing outside official repositories)
  4. Check GitHub Actions workflow logs for unexpected package installations or network connections to unfamiliar repositories during Black execution

FAQ

What is CVE-2026-31900?

This vulnerability affects Black's GitHub Action when use_pyproject: true is enabled, allowing attackers to inject malicious code via pyproject.toml by specifying a direct URL to a compromised Black repository, leading to arbitrary code execution within the CI/CD environment.

Is CVE-2026-31900 being actively exploited?

No, this vulnerability is not currently listed on the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits are known to be available.

What versions of Python are affected by CVE-2026-31900?

Black versions prior to 26.3.0 are affected. All Python 3.x versions using vulnerable Black releases are at risk when GitHub Actions with use_pyproject: true are configured.

How do I check if my server is vulnerable to CVE-2026-31900?

Run 'pip3 show black' and check the version number. If it is earlier than 26.3.0, your system is vulnerable. Additionally, audit any GitHub Actions workflows using 'use_pyproject: true' for potential exploitation vectors.

Does Defensia detect CVE-2026-31900?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Python and Black are installed on a monitored server, CVE-2026-31900 will appear in your dashboard with remediation steps.

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References

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