BlackApplication · Python
CVE-2026-31900
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedBlack 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceThe Black GitHub action's `use_pyproject: true` option reads the Black version from the repository's pyproject.toml file. A malicious actor can modify pyproject.toml to include a direct URL reference pointing to a malicious repository, causing the action to fetch and execute arbitrary code within the CI/CD environment, thereby exposing secrets and permissions available to the GitHub Action runner.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
BlackApplication
< 26.3.0CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify Black GitHub action usage in workflowsSearch your .github/workflows/ directory for workflow files containing 'uses: black/*' or 'black-action' to confirm the Black GitHub action is in use.Affected if The Black GitHub action is present in CI/CD workflows.
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Check for use_pyproject configurationIn the identified workflow files, look for 'use_pyproject: true' under the Black action configuration.Affected if use_pyproject is set to true, enabling the vulnerable code path.
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Inspect pyproject.toml for Black configurationLocate and review the pyproject.toml file in the repository root. Look for a [tool.black] section and check if it contains a 'url' key with a value pointing to an external repository URL.Affected if pyproject.toml contains [tool.black] with a 'url' key pointing to a non-local or external URL (not just a local file path or version string).
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Verify Black version in useIf possible, check the installed or resolved Black version by examining action logs or lock files. Compare against the affected range: versions prior to 26.3.0 are vulnerable.Affected if Black version is below 26.3.0 and the use_pyproject option is enabled.
A user is affected if they use the Black GitHub action with use_pyproject: true and their pyproject.toml contains an external URL reference in the [tool.black] section, allowing potential remote code execution.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped26.3.0
Upgrade the Black GitHub action to version 26.3.0 or later to remediate this vulnerability. Additionally, audit recent pull requests that modified pyproject.toml files during the vulnerable period and rotate any potentially exposed secrets.
Black 26.3.0
- Upgrade Black to version 26.3.0 or later using pip: pip install black>=26.3.0
- Verify the upgrade was successful by running: black --version
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Black
Scoped from the published advisory
- Consultation3.0 h
- Implementation2.0 h
- Testing2.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,592.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-31900 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-31900 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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