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

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.

Problem type

Affected products

jupyter-server

jupyter_server

< 2.20 - AFFECTED

References

https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp

https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp

x_refsource_CONFIRM

https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd

https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd

x_refsource_MISC

GitHub Security Advisories

GHSA-fcw5-x6j4-ccmp

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

https://github.com/advisories/GHSA-fcw5-x6j4-ccmp

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)

github.com

https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-fcw5-x6j4-ccmp

github.com

https://github.com/jupyter-server/jupyter_server/commit/6cbee8d65e71abac851c4492fea987ad080580bd

github.com

https://github.com/advisories/GHSA-fcw5-x6j4-ccmp

JSON source

https://cveawg.mitre.org/api/cve/CVE-2026-44727

Click to expand

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    "assignerShortName": "GitHub_M",
    "dateUpdated": "2026-06-22T19:56:56.776Z",
    "dateReserved": "2026-05-07T18:04:17.309Z",
    "datePublished": "2026-06-22T19:56:56.776Z",
    "state": "PUBLISHED"
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        "dateUpdated": "2026-06-22T19:56:56.776Z"
      },
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          "value": "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."
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