CVE-2026-44727

Summary

Jupyter Server before 2.20.0 has a stored cross-site scripting flaw (CVE-2026-44727). The `NbconvertFileHandler` and `NbconvertPostHandler` served converted notebook output without a sandboxing Content-Security-Policy, so malicious HTML or JavaScript embedded in a notebook executes in the victim's browser within the Jupyter origin. Opening or being shown a crafted notebook can hijack a user's Jupyter session.

How to fix it

• Upgrade `jupyter-server` to 2.20.0 or later, which restores the sandbox Content-Security-Policy on the nbconvert handlers.
• Until you upgrade, treat notebooks from untrusted sources as hostile and do not open or render them in a shared Jupyter instance.

How to avoid it in your code

• Render untrusted, user-controlled content under a strict sandboxing Content-Security-Policy, and serve downloadable HTML from a separate origin where possible.
• Sanitize or escape document and notebook output that can contain HTML or JavaScript before display.
• Keep notebook and data-science tooling patched; these tools routinely render untrusted input and are a recurring XSS surface.

References

• https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp
• https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd
• https://nvd.nist.gov/vuln/detail/CVE-2026-44727
• https://github.com/jupyter-server/jupyter_server