CVE-2026-44727: Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSP

June 18, 2026

The nbconvert HTTP handlers in jupyter_server render user-authored notebook HTML under the Jupyter origin without a sandbox directive in their Content-Security-Policy.

Combined with nbconvert.HTMLExporter’s default non-sanitizing behavior, a notebook carrying an HTML payload in a display_data output triggers stored XSS with cookie access, full /api/* authority, and kernel RCE.

References

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

Code Behaviors & Features

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