Vulnerability-Lookup

CVE-2026-44727 (GCVE-0-2026-44727)

Vulnerability from cvelistv5 – Published: 2026-06-22 19:56 – Updated: 2026-06-23 12:15

Title

Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSP

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.

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

SSVC

Exploitation: none Automatable: no Technical Impact: total

CISA Coordinator (v2.0.3)

CWE

CWE-79 - Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
CWE-1021 - Improper Restriction of Rendered UI Layers or Frames

Assigner

GitHub_M

References

2 references

URL	Tags
https://github.com/jupyter-server/jupyter_server/…	x_refsource_CONFIRM
https://github.com/jupyter-server/jupyter_server/…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version
jupyter-server	jupyter_server	Affected: < 2.20

Show details on NVD website

JSON

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                "id": "CVE-2026-44727",
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                  },
                  {
                    "Technical Impact": "total"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-06-23T12:15:20.487628Z",
                "version": "2.0.3"
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        }
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      "metrics": [
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          "name": "https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp",
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          ],
          "url": "https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp"
        },
        {
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          "tags": [
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      "source": {
        "advisory": "GHSA-fcw5-x6j4-ccmp",
        "discovery": "UNKNOWN"
      },
      "title": "Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSP"
    }
  },
  "cveMetadata": {
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    "assignerShortName": "GitHub_M",
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    "datePublished": "2026-06-22T19:56:56.776Z",
    "dateReserved": "2026-05-07T18:04:17.309Z",
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    },
    "vulnrichment": {
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GHSA-FCW5-X6J4-CCMP

Vulnerability from github – Published: 2026-06-18 15:04 – Updated: 2026-06-18 15:04

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.

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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 2.19.0"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "jupyter-server"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.20.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-44727"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1021",
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-18T15:04:07Z",
    "nvd_published_at": null,
    "severity": "CRITICAL"
  },
  "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": "GHSA-fcw5-x6j4-ccmp",
  "modified": "2026-06-18T15:04:07Z",
  "published": "2026-06-18T15:04:07Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "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 "
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

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