id: PYSEC-2026-366
published: "2026-06-29T11:50:52.185377Z"
modified: "2026-07-01T20:22:54.921081Z"
aliases:
- CVE-2026-44727
- GHSA-fcw5-x6j4-ccmp
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/<path>` 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```"
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
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
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