nbgrader's `frame-ancestors: self` grants all users access to formgrader

Impact

Enabling frame-ancestors: 'self' grants any JupyterHub user the ability to extract formgrader content by sending malicious links to users with access to formgrader, at least when using the default JupyterHub configuration of enable_subdomains = False.

#1915 disables a protection which would allow user Alice to craft a page embedding formgrader in an IFrame. If Bob visits that page, his credentials will be sent and the formgrader page loaded. Because Alice's page is on the same Origin as the formgrader iframe, Javasript on Alice's page has full access to the contents of the page served by formgrader using Bob's credentials.

Workarounds

• Disable frame-ancestors: self, or
• enable per-user and per-service subdomains with JupyterHub.enable_subdomains = True (then even if embedding in an IFrame is allowed, the host page does not have access to the contents of the frame).

References

JupyterHub documentation on why and when frame-ancestors: self is insecure, and why it was disabled by default: https://jupyterhub.readthedocs.io/en/stable/explanation/websecurity.html#:~:text=frame-ancestors