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Fencer Intel
critical 9.8

CVE-2026-31900 json

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

published 8mo ago modified 3mo ago also EUVD-2026-11315 , GHSA-v53h-f6m7-xcgm , PYSEC-2026-2120

cvss
9.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.5%
EPSS 0.5% chance of exploitation in 30 days, percentile 44%
public exploits
0
none known

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.

Patch / fix

  • github-actions psf/black 26.3.0
  • pypi black 26.3.0
  • psf black 26.3.0
  • python black 26.3.0
  • black Debian 14 26.3.1-1

debian advisories DEBIAN-CVE-2026-31900

  • python-black SLES for SAP applications 16.0, SLES 16.0 25.1.0-160000.3.1

suse advisories SUSE-SU-2026:20928-1

  • python-black openSUSE Leap 16.0 25.1.0-160000.3.1
  • python-black openSUSE Tumbleweed 26.3.1-1.1

opensuse advisories openSUSE-SU-2026:20417-1openSUSE-SU-2026:10372-1

Is CVE-2026-31900 in your dependencies? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

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CISA SSVC

Exploitation
none
Automatable
no
Technical impact
total
Assessed
7mo ago

Exploit prediction

0.52% chance of exploitation in the next 30 days, higher than 44% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

assigned by version score vector
CNA 4.0 8.7 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
CNA 4.0 8.7 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
NVD 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
OSV 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected packages

ecosystem package affected fixed in
github-actions psf/black
<26.3.0
26.3.0
pypi black
<26.3.0
26.3.0

Affected products

  • psf black

Classification

Weakness
  • CWE-20 Improper Input Validation

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.