CVE-2026-6658
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 · uneditedA vulnerability in jupyter/nbconvert versions <= 7.17.0 allows for Cross-site Scripting (XSS) via unsanitized `text/vnd.mermaid` output in HTML exports. The `data_mermaid` block in `share/templates/lab/base.html.j2` renders `text/vnd.mermaid` cell output directly into HTML without escaping, enabling attackers to inject arbitrary HTML/JavaScript by breaking out of the `<pre>` tag. This vulnerability impacts any server using nbconvert to render notebooks as HTML, allowing attackers to execute arbitrary JavaScript in the context of users viewing the HTML export.
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 vulnerability exists in nbconvert's HTML export template where Mermaid diagram content (text/vnd.mermaid mime type) is rendered directly into HTML without escaping. The data_mermaid block in share/templates/lab/base.html.j2 inserts content into a <pre> tag without sanitization, allowing attackers to break out of the tag and inject arbitrary JavaScript that executes when users view the exported HTML.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
CVSS 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
- Low
- User interaction
- Required
- Scope
- Changed
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.0/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
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 nbconvert installation and versionRun `pip show nbconvert` or check your package manager for the installed nbconvert versionAffected if The installed version is 7.17.0 or lower (vulnerable); versions above 7.17.0 are not affected
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Verify Mermaid rendering is in useInspect your notebook export workflow: check if notebooks contain text/vnd.mermaid mime type cells and are being converted to HTML using nbconvertAffected if Your exported HTML contains Mermaid diagram content from text/vnd.mermaid cells without additional sanitization
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Examine the vulnerable template fileLocate the file share/templates/lab/base.html.j2 in your nbconvert installation (typically in site-packages/nbconvert) and inspect the data_mermaid blockAffected if The template contains <pre>{{ content }}</pre> or similar without HTML escaping (e.g., missing the |e filter or |escape filter) and is processing text/vnd.mermaid content
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Test for unsanitized output in exported HTMLExport a notebook with Mermaid content to HTML using nbconvert and examine the raw HTML: search for the <pre> tag containing Mermaid content and verify whether special HTML characters are escapedAffected if The Mermaid content in the exported HTML <pre> tag is not HTML-escaped (you can see unescaped <, >, or & characters from the Mermaid syntax)
You are affected if nbconvert version 7.17.0 or lower is installed AND you export notebooks containing text/vnd.mermaid content to HTML without additional sanitization.
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.
From vendor dataUpgrade nbconvert to version >7.17.0 where proper HTML escaping is applied to text/vnd.mermaid output, or implement output filtering to sanitize Mermaid content before HTML conversion.
- Consultation1.0 h
- Implementation2.0 h
- Testing2.0 h
- Review / QA1.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2026-6658 — 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-2026-6658 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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- Version or environment caveats, and links to real fixes
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