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

nbgrader is a system for assigning and grading notebooks. 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. This issue has been addressed in release 0.9.5 and all users are advised to upgrade. Users unable to upgrade may 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).

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将资源暴露给错误范围

nbgrader 安全漏洞

nbgrader是Project Jupyter开源的一个笔记本分配和评分系统。 nbgrader存在安全漏洞，该漏洞源于 nbgrader 默认允许其内容被嵌入 IFrame，而 JupyterHub 的默认配置（enable_subdomains = False）使得嵌入的 IFrame 与父页面共享相同的来源，从而导致恶意用户可以通过 JavaScript 访问和操作 formgrader 的内容，进而窃取用户凭据或数据。

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