GHSA-fcw5-x6j4-ccmp Jupyter Server: Stored XSS in `NbconvertFileHandler` / `NbconvertPostHandler` via missing `sandbox` CSP

• PyPI jupyter-server: < 2.20.0 → fixed in 2.20.0

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/` 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) ```