Critical 9.3Published Jun 22, 2026

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

Updated under an hour ago (Oct 26, 2026, 7:10 PM ET)

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It affects jupyter-server jupyter_server and 3 other products. Add what you run to find out whether that includes you.

No exploitation reported and no EPSS estimate yet.

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Every CVE for jupyter-server jupyter_server

Exploited in the last 30 days

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.

CVSS: 9.3 CRITICAL (v4.0, CNA)CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:A/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H
Weakness (CWE): CWE-79, CWE-1021
Known exploited: Not in CISA's KEV catalog
EPSS (exploitation in the next 30 days): Not loaded for this CVE
Assigned by: GitHub_M
NVD analysis: Modified
Record last updated: Aug 28, 2026
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

jupyter-server jupyter_server jupyter_server:jupyter_server (from the name)	before 2.20	CNA record
Red Hat Red Hat Migration Toolkit for Applications 8.2 redhat:migration_toolkit_applications	from 1784109883 not affected	CISA ADP
Red Hat Red Hat OpenShift AI 3.4 redhat:openshift_ai	from 1787076778 not affected	CISA ADP
jupyter jupyter_server jupyter:jupyter_server	before 2.20.0	NVD

References

References

https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp	x-refsource-confirm, mitigation, patch, vendor-advisory
https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd	x-refsource-misc, patch
https://access.redhat.com/errata/RHSA-2026:43038
https://access.redhat.com/errata/RHSA-2026:60520
https://access.redhat.com/security/cve/CVE-2026-44727
https://bugzilla.redhat.com/show_bug.cgi?id=2491516
https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-44727.json

The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.