PYSEC-2026-366

Vulnerability from pysec - Published: 2026-06-29 11:50 - Updated: 2026-07-01 20:22
VLAI
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.

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.

Impact

An 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.

Patches

Fixed in v2.20.0, commit 6cbee8d

Workarounds

For 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:

import jupyter_server.nbconvert.handlers as _nb

def _csp(self):
    return super(type(self), self).content_security_policy + "; sandbox allow-scripts"

_nb.NbconvertFileHandler.content_security_policy = property(_csp)
_nb.NbconvertPostHandler.content_security_policy = property(_csp)
Severity
9.3 (Critical)
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
Impacted products
Name purl
jupyter-server pkg:pypi/jupyter-server
Aliases
To clipboard 

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 2.19.0"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "jupyter-server",
        "purl": "pkg:pypi/jupyter-server"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.20.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "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"
      ]
    }
  ],
  "aliases": [
    "CVE-2026-44727",
    "GHSA-fcw5-x6j4-ccmp"
  ],
  "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`\u0027s 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```",
  "id": "PYSEC-2026-366",
  "modified": "2026-07-01T20:22:54.921081Z",
  "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"
    }
  ],
  "severity": [
    {
      "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSP "
}


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Sightings

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Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
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