Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSP
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 software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | jupyter-server | < 2.20.0 | 2.20.0 |
References
- https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp
- https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd
- https://github.com/jupyter-server/jupyter_server
- https://pypi.org/project/jupyter-server
- https://github.com/advisories/GHSA-fcw5-x6j4-ccmp
- https://nvd.nist.gov/vuln/detail/CVE-2026-44727
- 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://github.com/pypa/advisory-database/tree/main/vulns/jupyter-server/PYSEC-2026-366.yaml
- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-44727.json
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N- Weakness (CWE)
CWE-1021, CWE-79- Platform
- Python (PyPI)
- Package
jupyter-server- Disclosed
- 18.06.2026
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-366 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-fcw5-x6j4-ccmp - CC BY 4.0
Vulnerability history: jupyter-server
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 14
- First → latest
- 11.2020 09.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 1
- High 7
- Medium 6
Disclosures per year
- 2020: 2
- 2021: 0
- 2022: 2
- 2023: 3
- 2024: 1
- 2025: 0
- 2026: 6
-
Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSP this entry
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