Gogs has Stored XSS in `.ipynb` Preview

# 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}}
  

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., ///src//poc.ipynb).

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

# 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.