Is jupyter-server safe?

GHSA-fcw5-x6j4-ccmp

The nbconvert HTTP handlers in jupyterserver 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 displaydata output triggers stored XSS with cookie access, full /api/ authority, and kernel RCE.

An authenticated victim who navigates to /nbconvert/html/ containing attacker-authored output can have their token exfiltrated to another domain because it is executed in the Jupyter origin.

Fixed in v2.20.0, commit 6cbee8d

For deployments where editing the installed jupyterserver is impractical (containerized builds, read-only images), adding this to jupyterserverconfig.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)