Gogs EUVD-2026-39066

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 without a Content Security Policy (CSP), CSP headers are applied only to attachment delivery routes.

internal/cmd/web.go:323

c.Header().Set("Content-Security-Policy", "default-src 'none'; style-src 'unsafe-inline'; sandbox")

Steps to Reproduce

1. As the attacker, add and push/commit a .ipynb file containing a javascript: link in a Markdown cell to a repository.

• Example (PoC):

     {
       "nbformat": 4,
       "nbformat_minor": 2,
       "metadata": {},
       "cells": [
         {
           "cell_type": "markdown",
           "metadata": {},
           "source": [
             "[poc](javascript:alert(document.domain))"
           ]
         }
       ]
     }

1. The victim opens the file on Gogs (e.g., /<user>/<repo>/src/<branch>/poc.ipynb).

<img width="2386" height="1218" alt="image" src="https://github.com/user-attachments/assets/b0d93fd8-c5ca-4058-8af0-98dee590d3ad" />

1. When the victim clicks the poc link displayed in the preview, alert(document.domain) is executed in the same Gogs origin.

<img width="2390" height="1388" alt="image" src="https://github.com/user-attachments/assets/0eb6ebe8-632c-4a41-8a11-46471514b4c4" />

Minimum Required Privileges

• Attacker: Ability to place a .ipynb file as a regular (non-admin) user
• For example: a general user who can create a public repository and add files.
• Or: write access (collaborator, etc.) to an existing repository that the victim will view.
• Victim: Permission to view the repository (a click is required).

Impact

• Unauthorized actions performed with the victim’s account privileges (e.g., repository settings changes, Issue operations,誘導 to token creation).
• Theft of information accessible to the victim (repository/Issue/Wiki contents, tokens exposed in page context).
• If the victim is an administrator, the impact may escalate to instance-wide configuration changes and user management.