CVE-2026-44727

CRITICAL 9.0

Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSP

CVSS Score

9.0

EPSS Score

0.2%

EPSS Percentile

13th

Jupyter Server is the backend for Jupyter web applications. Prior to 2.20, 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. This vulnerability is fixed in 2.20.

CWE	CWE-79 CWE-1021
Vendor	jupyter-server
Product	jupyter_server
Published	Jun 22, 2026
Last Updated	Jul 22, 2026

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Affected Versions

jupyter-server / jupyter_server

< 2.20

References

NVD ↗ CVE.org ↗ EPSS Data ↗

github.com: https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp github.com: https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd access.redhat.com: https://access.redhat.com/security/cve/CVE-2026-44727 bugzilla.redhat.com: https://bugzilla.redhat.com/show_bug.cgi?id=2491516 security.access.redhat.com: https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-44727.json access.redhat.com: https://access.redhat.com/errata/RHSA-2026:43038