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VulnerabilityPublished Updated

CVE-2026-31900: psf black · Improper Input Validation

Affects psf blackSource CVE Database V5

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Not known to be exploited · CVSS 8.7 High : upgrade to 26.3.0 soon.

Data as of 30 Oct 2026, 06:59 UTC

CVSS 4.0 base score
8.7High
How severe the flaw is on the 0–10 CVSS scale.
EPSS
0.4% chance of exploitation in 30 days – higher than 41% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch
Available – fixed in 26.3.0
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

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 11 Mar 2026

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponized: evidence found.

    Public exploit code: GitHub proofs of concept

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), GitHub proofs of concept, the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

What to do

Upgrade psf black to 26.3.0.

Affected software

ProductAffectedFixed in
psfblackAffected
  • <26.3.0
Fixed in
  • 26.3.0
PyPIblackpkg:pypi/blackOther threatsAffected
  • <26.3.0
Fixed in
  • 26.3.0

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

To mitigate this vulnerability, organizations should immediately upgrade Black to version 26.3.0 or later in all environments where it is used, particularly in GitHub Actions workflows. Review and restrict the use of the 'use_pyproject: true' option to trusted repositories only, or disable it if not essential. Implement strict branch protection rules and pull request review policies to prevent untrusted contributors from modifying pyproject.toml files. Use GitHub Actions secrets and environment protection rules to limit the scope of accessible credentials during workflows. Employ dependency scanning and code analysis tools to detect unauthorized changes to configuration files. Additionally, consider isolating CI environments and limiting permissions granted to GitHub Actions to the minimum necessary. Monitor CI/CD logs for unusual activity and audit repository changes regularly. Educate development teams about the risks of supply chain attacks and the importance of validating third-party code sources.

Description

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.

References

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