CVE-2026-44727: Jupyter Server

Medium severity, CVSS 5.4. EPSS: 0.4% chance of exploitation in the next 30 days.

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 products

• Jupyter Jupyter Server: before 2.20.0 (fixed in 2.20.0)

Published 2026-06-22. Last modified 2026-08-28.