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jupyter-server: Stored XSS in nbconvert HTTP handlers fixed

Fixed stored XSS vulnerability in nbconvert HTTP handlers where user-authored notebook HTML was rendered without a sandbox directive in Content-Security-Policy,

19 Jun 2026Read 1 minSeverity: act now

What changed

Fixed stored XSS vulnerability in nbconvert HTTP handlers where user-authored notebook HTML was rendered without a sandbox directive in Content-Security-Policy, allowing token exfiltration and kernel RCE.

Who it affects

All users of jupyter_server prior to v2.20.0 who serve untrusted notebooks via /nbconvert/html/.

What to do today

Upgrade to jupyter_server v2.20.0 or apply the provided workaround in jupyter_server_config.py.

The trail
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