XSS in Jupyter Server
CVE-2026-44727
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.
Vulnerability class: XSS (Cross-Site Scripting)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N.
EPSS exploit prediction
EPSS: 0.004 (35.6th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 36th percentile — 35.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.004 · change: 0.000
Affected products
- Jupyter Server
- Jupyter-Server Jupyter Server — versions < 2.20
Weakness classification (CWE)
References
- github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp (Patch, Mitigation, Vendor Advisory)
- github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987a… (Patch)
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
Frequently asked questions
- What is CVE-2026-44727?
- CVE-2026-44727 is a medium-severity vulnerability in Jupyter Server, classified under Cross-site Scripting. CVSS score: 5.4/10. Published 2026-06-22.
- How severe is CVE-2026-44727?
- Medium severity. CVSS v3 base score is 5.4 out of 10.