Vulnerability · Published Jun 22, 2026 · Assigned by GitHub_M

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

Critical 9.0

In plain English

A critical-severity cross-site scripting in jupyter_server from jupyter-server. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 38% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)9.0Critical

Exploit odds (30 days)0.4%top 61.7% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

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.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	jupyter-server	no fix listed	DEBIAN-CVE-2026-44727
Debian:13	jupyter-server	no fix listed	DEBIAN-CVE-2026-44727
Debian:14	jupyter-server	2.20.0-1	DEBIAN-CVE-2026-44727
PyPI	jupyter-server	2.20.0	GHSA-fcw5-x6j4-ccmp
PyPI	jupyter-server	2.20.0	PYSEC-2026-366
Ubuntu:22.04:LTS	jupyter-server	no fix listed	UBUNTU-CVE-2026-44727
Ubuntu:24.04:LTS	jupyter-server	no fix listed	UBUNTU-CVE-2026-44727
Ubuntu:25.10	jupyter-server	no fix listed	UBUNTU-CVE-2026-44727
Ubuntu:26.04:LTS	jupyter-server	no fix listed	UBUNTU-CVE-2026-44727
Ubuntu:26.10	jupyter-server	no fix listed	UBUNTU-CVE-2026-44727

Affected products (as reported)

Vendor	Product	Versions
jupyter-server	jupyter_server	< 2.20

Weakness: CWE-79, CWE-1021 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:H

References

https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp x_refsource_CONFIRM
https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd x_refsource_MISC
https://access.redhat.com/security/cve/CVE-2026-44727 vdb-entry, x_refsource_REDHAT
RHBZ#2491516 issue-tracking, x_refsource_REDHAT
https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-44727.json x_sadp-csaf-vex
https://access.redhat.com/errata/RHSA-2026:43038 vendor-advisory, x_refsource_REDHAT
https://access.redhat.com/errata/RHSA-2026:60520 vendor-advisory, x_refsource_REDHAT