CVE-2026-44727: Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSP
(updated )
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
- access.redhat.com/errata/RHSA-2026:43038
- access.redhat.com/errata/RHSA-2026:60520
- access.redhat.com/security/cve/CVE-2026-44727
- bugzilla.redhat.com/show_bug.cgi?id=2491516
- github.com/advisories/GHSA-fcw5-x6j4-ccmp
- github.com/jupyter-server/jupyter_server
- github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd
- github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp
- github.com/pypa/advisory-database/tree/main/vulns/jupyter-server/PYSEC-2026-366.yaml
- nvd.nist.gov/vuln/detail/CVE-2026-44727
- pypi.org/project/jupyter-server
- security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-44727.json
Code Behaviors & Features
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