Summary

Although .ipynb previews are sanitized on the server side via /-/api/sanitize_ipynb, the inserted content is re-rendered on the client side without sanitization using marked() on elements with the .nb-markdown-cell class. During this process, links containing schemes such as javascript: can be regenerated.

As a result, when a victim views an attacker-crafted .ipynb file and clicks the link, arbitrary JavaScript is executed in the Gogs origin, leading to a click-based Stored XSS.

Details

After the rendered output of a .ipynb file is sanitized via /-/api/sanitize_ipynb and inserted into the DOM, only the Markdown cell portions are re-rendered using marked() and overwritten in the DOM. During this process, links with the javascript: scheme can be regenerated.

templates/repo/view_file.tmpl:42–71

{{else if .IsIPythonNotebook}}
  <script>
    $.getJSON("{{.RawFileLink}}", null, function(notebook_json) {
      var notebook = nb.parse(notebook_json);
      var rendered = notebook.render();
      $.ajax({
        type: "POST",
        url: '{{AppSubURL}}/-/api/sanitize_ipynb',
        data: rendered.outerHTML,
        processData: false,
        contentType: false,
      }).done(function(data) {
        $("#ipython-notebook").append(data);
        $("#ipython-notebook code").each(function(i, block) {
          $(block).addClass("py").addClass("python");
          hljs.highlightBlock(block);
        });

        // Overwrite image method to append proper prefix to the source URL
        var renderer = new marked.Renderer();
        var context = '{{.RawFileLink}}';
        context = context.substring(0, context.lastIndexOf("/"));
        renderer.image = function (href, title, text) {
          return `<img src="${context}/${href}"`
        };
        $("#ipython-notebook .nb-markdown-cell").each(function(i, markdown) {
          $(markdown).html(marked($(markdown).html(), {renderer: renderer}));
        });
      });
    });
  </script>

While regular HTML pages (including .ipynb preview pages) are served , CSP headers are applied .