NIST deprecates RSA-2048 from 2030. Find every one of them before attackers do.
🐍
🐍 PyPI
Not in CISA KEV
MEDIUM severity

Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSPGHSA-fcw5-x6j4-ccmp

MEDIUMFix: jupyter-server/jupyter_server@6cbee8d

GHSA-fcw5-x6j4-ccmp is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in jupyter-server. A fix is available for jupyter-server — see the affected versions and patch details below.

Also known asCVE-2026-44727PYSEC-2026-366
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • A successful exploit gives an attacker total control of the affected component, not partial access.
  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for GHSA-fcw5-x6j4-ccmp.

EPSS Exploitation Probability

via FIRST.org ↗
0.4%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs38th percentile — riskier than 38% of all scored CVEsHighest risk
0.00%0.31%0.63%0.94%0.2%0.4%Jul 26Oct 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected
🐍jupyter-server

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

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.

Impact

An authenticated victim who navigates to /nbconvert/html/<path> containing attacker-authored output can have their token exfiltrated to another domain because it is executed in the Jupyter origin.

Patches

Fixed in v2.20.0, commit 6cbee8d

Workarounds

For deployments where editing the installed jupyter_server is impractical (containerized builds, read-only images), adding this to jupyter_server_config.py has the same effect as the patch above without touching source files:

import jupyter_server.nbconvert.handlers as _nb

def _csp(self):
    return super(type(self), self).content_security_policy + "; sandbox allow-scripts"

_nb.NbconvertFileHandler.content_security_policy = property(_csp)
_nb.NbconvertPostHandler.content_security_policy = property(_csp)

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPIjupyter-serverall versions2.20.0pip install --upgrade 'jupyter-server==2.20.0'

Affected Products

1 product · 1 configurations
Application
jupyter serverjupyter
< 2.20.0
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for jupyter-server, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update jupyter-server to 2.20.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-fcw5-x6j4-ccmp is resolved across your whole dependency graph.

  3. Workarounds

    Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant
ProductFixed inAdvisory
Red Hat Migration Toolkit for Applications 8.2mta/mta-solution-server-rhel9:1784109883RHSA-2026:43038
Red Hat OpenShift AI 3.3rhoai/odh-workbench-jupyter-datascience-cpu-py312-rhel9:1790331087RHSA-2026:73987
Red Hat OpenShift AI 3.4rhoai/odh-th06-cpu-torch210-py312-rhel9:1787076778RHSA-2026:60520

Frequently Asked Questions

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. ### Impact An authenticated victim who navigates to `/nbconvert/html/<path>` containing attacker-authored output can have their token exfiltrated to another domain because it is executed in the Jupyter origin. #
O3 Security · Impact-Aware SCA

Is GHSA-fcw5-x6j4-ccmp in your dependencies?

Find it across PyPI, including transitive dependencies.

Scan my dependencies How O3 SCA works
Jupyter Server: Stored XSS in `NbconvertFileHandler` /…