Medium severity · published Jun 22, 2026
CVE-2026-44727
Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSP
5.4
CVSS
0.4%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
Required
Confidentiality
Low
Integrity
Low
Availability
None
Scope
Changed
Exploitation
Is it being used?
Exploit prediction (EPSS)0.4% · higher than 38% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
JU
jupyter_server · jupyter-server
< 2.20 · source: vendor
JS
Jupyter Server · Jupyter
before 2.20.0 · source: NVD
RH
Red Hat Migration Toolkit for Applications 8.2 · Red Hat
All versions, or not specified · source: CISA
RH
Red Hat OpenShift AI (RHOAI) · Red Hat
All versions, or not specified · source: CISA
RH
Red Hat OpenShift AI 3.4 · Red Hat
All versions, or not specified · source: CISA
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-44727
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-44727
Patch
https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd
Mitigation
https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp
access.redhat.com
https://access.redhat.com/errata/RHSA-2026:43038
access.redhat.com
https://access.redhat.com/errata/RHSA-2026:60520
access.redhat.com
https://access.redhat.com/security/cve/CVE-2026-44727
bugzilla.redhat.com
https://bugzilla.redhat.com/show_bug.cgi?id=2491516
More from jupyter_server
8.8
CVE-2026-114135
Jupyter Server: Gateway WebSocket handler bypasses kernel execute authorization
8.8
CVE-2026-35397
jupyter-server path traversal allows access to sibling directories sharing root_dir name prefix
7.3
CVE-2026-40110
jupyter-server CORS origin validation bypass via unanchored regex in allow_origin_pat
7.1
CVE-2026-86049
Jupyter Server: 5xx request logging leaks token-bearing Referer header values
6.8
CVE-2026-40934
jupyter-server authentication cookies remain valid after password reset due to static cookie secret