Vulnerability in psf/black (CVE-2026-31900)
Summary
Vulnerability in psf/black (CVE-2026-31900). Risk of unauthorized operations or information disclosure. Exploitable via pyproject.toml. Mitigation: upgrade to 26.3.0 or later.
Original description
The description as published by the source.
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.
Patches
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.
Workarounds
Do not use the use_pyproject: true option in the psf/black GitHub Action.
Source: OSV.dev (Open Source Vulnerabilities) NVD GitHub Advisory OSV
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-31900 has been found in psf/black.
Attackers can target a specific entry point like pyproject.toml over the network to misuse the product.
Risk of unauthorized operations or information disclosure.
What to do: upgrade psf/black to 26.3.0 or later.
If unsure, ask your IT team or search "psf/black CVE-2026-31900" on the vendor's site.
CVE-2026-31900 (psf/black) — CWE-20
Attack surface: pyproject.toml / version
Patched: 26.3.0 — apply immediately
Workaround: Do not use the use_pyproject: true option in the psf/black GitHub Action.
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-20) exists in psf/black. Attackers reach the vulnerable code path via pyproject.toml.
Check the vendor advisory for the affected versions of psf/black. Attack surface: pyproject.toml / version.
Severity: ?. Risk of unauthorized operations or information disclosure.
Update to 26.3.0.
Workaround: Do not use the use_pyproject: true option in the psf/black GitHub Action.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'psf/black' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
grep -r 'psf/black' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
psf/blackを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply temporary workaround mitigate
Do not use the `use_pyproject: true` option in the psf/black GitHub Action.パッチが適用されるまでの応急処置として、Do not use the
use_pyproject: trueoption in the psf/black GitHub Action. を実施。回避策の副作用 (機能低下) を確認した上で。 -
3Apply patch patch
Upgrade psf/black to 26.3.0ステージング環境で 26.3.0 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
4Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
Fix status by distribution
Fixed packages published by each distribution for this CVE
- Debian DEBIAN-CVE-2026-31900 black
- Ubuntu UBUNTU-CVE-2026-31900 black
References
- advisory nvd.nist.gov
- package github.com
- web github.com
- web github.com