CVE-2026-6658: jupyter/nbconvert allows Cross-site Scripting (XSS) via unsanitized `text/vnd.mermaid` output in HTML exports

June 26, 2026 (updated November 6, 2026)

A 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.

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Code Behaviors & Features

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