CVE-2026-44727
Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSP
- State
- Published
- Severity
- 9.3 Critical
- Published
- Jun 22, 2026
- Updated
- Aug 28, 2026
- Reserved
- May 7, 2026
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.
Scores
| CVSS | Score | Vector | Source |
|---|---|---|---|
| 4.0 | 9.3 Critical | 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 | GitHub_M |
| 3.1 | 9.0 Critical | CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:H | redhat-SADP |
Weaknesses
Affected Products
| Product | Versions |
|---|---|
jupyter_serverjupyter-server |
|
References
- https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp
- https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd
- https://access.redhat.com/security/cve/CVE-2026-44727vdb-entry
- RHBZ#2491516issue-tracking
- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-44727.json
- https://access.redhat.com/errata/RHSA-2026:43038vendor-advisory
- https://access.redhat.com/errata/RHSA-2026:60520vendor-advisory
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