Remote Code Execution via stored Cross-Site Scripting in nbconvert handlers

Summary

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. A flaw was found in Jupyter Server. The nbconvert HTTP handlers in Jupyter Server render user-authored notebook HTML without a sandbox directive in their Content-Security-Policy. This, combined with nbconvert.HTMLExporter's default non-sanitizing behavior, allows a notebook containing an HTML payload to trigger a stored Cross-Site Scripting (XSS) vulnerability. A remote attacker could exploit this to gain cookie access, full API authority, and achieve kernel Remote Code Execution (RCE). Red Hat severity: Important — CVSS 9 (CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:H). Weakness: CWE-79. No fixing RHSA erratum has published yet; monitor the Red Hat CVE page and patch when it ships.