{"id":"PYSEC-2026-366","summary":"Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSP ","details":"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`. \n\nCombined 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.\n\n### Impact\n\nAn authenticated victim who navigates to `/nbconvert/html/\u003cpath\u003e` containing attacker-authored output can have their token exfiltrated to another domain because it is executed in the Jupyter origin.\n\n### Patches\n\nFixed in v2.20.0, commit [6cbee8d](https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd)\n \n\n### Workarounds\n\nFor deployments where editing the installed jupyter_server is impractical (containerized builds, read-only images), adding this to jupyter_server_config.py has the same effect as the patch above without touching source files:\n\n```\nimport jupyter_server.nbconvert.handlers as _nb\n\ndef _csp(self):\n    return super(type(self), self).content_security_policy + \"; sandbox allow-scripts\"\n\n_nb.NbconvertFileHandler.content_security_policy = property(_csp)\n_nb.NbconvertPostHandler.content_security_policy = property(_csp)\n```","aliases":["CVE-2026-44727","GHSA-fcw5-x6j4-ccmp"],"modified":"2026-06-29T12:15:23.602664022Z","published":"2026-06-29T11:50:52.185377Z","references":[{"type":"WEB","url":"https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp"},{"type":"WEB","url":"https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd"},{"type":"PACKAGE","url":"https://github.com/jupyter-server/jupyter_server"},{"type":"PACKAGE","url":"https://pypi.org/project/jupyter-server"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-fcw5-x6j4-ccmp"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-44727"}],"affected":[{"package":{"name":"jupyter-server","ecosystem":"PyPI","purl":"pkg:pypi/jupyter-server"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.20.0"}]}],"versions":["0.0.0","0.0.1","0.0.2","0.0.3","0.0.4","0.0.5","0.1.0","0.1.1","0.2.0","0.2.1","0.3.0","1.0.0","1.0.0rc0","1.0.0rc1","1.0.0rc10","1.0.0rc11","1.0.0rc12","1.0.0rc13","1.0.0rc14","1.0.0rc15","1.0.0rc16","1.0.0rc2","1.0.0rc3","1.0.0rc4","1.0.0rc5","1.0.0rc6","1.0.0rc7","1.0.0rc8","1.0.0rc9","1.0.1","1.0.10","1.0.11","1.0.2","1.0.3","1.0.4","1.0.5","1.0.6","1.0.7","1.0.8","1.0.9","1.1.0","1.1.1","1.1.2","1.1.3","1.1.4","1.10.0","1.10.1","1.10.2","1.11.0","1.11.1","1.11.2","1.12.0","1.12.1","1.13.0","1.13.1","1.13.2","1.13.3","1.13.4","1.13.5","1.15.0","1.15.1","1.15.2","1.15.3","1.15.4","1.15.5","1.15.6","1.16.0","1.17.0","1.17.1","1.18.0","1.18.1","1.19.0","1.19.1","1.2.0","1.2.1","1.2.2","1.2.3","1.21.0","1.23.0","1.23.1","1.23.2","1.23.3","1.23.4","1.23.5","1.23.6","1.24.0","1.3.0","1.4.0","1.4.1","1.5.0","1.5.1","1.6.0","1.6.1","1.6.2","1.6.3","1.6.4","1.7.0","1.7.0a1","1.7.0a2","1.8.0","1.9.0","2.0.0","2.0.0a0","2.0.0a1","2.0.0a2","2.0.0b0","2.0.0b1","2.0.0rc0","2.0.0rc1","2.0.0rc2","2.0.0rc3","2.0.0rc4","2.0.0rc5","2.0.0rc6","2.0.0rc7","2.0.0rc8","2.0.1","2.0.2","2.0.3","2.0.4","2.0.5","2.0.6","2.0.7","2.1.0","2.10.0","2.10.1","2.11.0","2.11.1","2.11.2","2.12.0","2.12.1","2.12.2","2.12.3","2.12.4","2.12.5","2.13.0","2.14.0","2.14.1","2.14.2","2.15.0","2.16.0","2.17.0","2.18.0","2.18.1","2.18.2","2.19.0","2.2.0","2.2.1","2.3.0","2.4.0","2.5.0","2.6.0","2.7.0","2.7.1","2.7.2","2.7.3","2.8.0","2.9.0","2.9.1"],"database_specific":{"last_known_affected_version_range":"\u003c= 2.19.0","source":"https://github.com/pypa/advisory-database/blob/main/vulns/jupyter-server/PYSEC-2026-366.yaml"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V4","score":"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"}]}