CVE-2025-23205: nbgrader: Clickjacking exposes formgrader via IFrame

### 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

On a corporate ML research platform or university AI course environment: Attacker Alice, a regular JupyterHub user without formgrader access, creates a malicious HTML page hosted on the same JupyterHub origin. The page silently embeds the formgrader URL in a hidden IFrame and runs JavaScript that polls the IFrame's contentDocument for loaded DOM content. Alice crafts a convincing pretext (e.g., 'check this notebook I shared with you') and sends the link to Bob, a TA or instructor with formgrader access. When Bob clicks the link, his authenticated session transparently loads formgrader inside the IFrame. Because Alice's page and the IFrame share the same origin, her JavaScript reads the full formgrader DOM—extracting student names, grades, submitted ML code, and dataset references—and POSTs everything to an attacker-controlled endpoint. Bob sees nothing unusual; the malicious page can display a decoy notebook to reduce suspicion.