GHSA-jq8v-rmf6-65jw

# 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`

```html {{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`

```go 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):

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

2. 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" />

3. 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.