GHSA-3w28-36p9-w929: Gogs's Unauthenticated Jupyter Notebook (ipynb) Sanitizer allows arbitrary data: URIs leading to XSS

Summary The Jupyter Notebook (ipynb) sanitizer endpoint at POST /-/api/sanitize_ipynb allows arbitrary data: URIs without proper restrictions, potentially leading to Cross-Site Scripting (XSS). The endpoint uses bluemonday.UGCPolicy() with p.AllowURLSchemes("data") which permits all data URI schemes including data:text/html, enabling attackers to inject malicious HTML/JavaScript. Additionally, the endpoint has no authentication middleware, allowing any registered user to exploit this vulnerability. Severity High Affected Versions All versions using the vulnerable endpoint Vulnerability Details - CVE ID: (To be assigned) - Entry Point: POST /-/api/sanitize_ipynb - Attack Vector: Network - Authentication Required: No (only needs a registered user account) Impact An attacker with a registered user account can: - Send malicious HTML containing data:text/html URIs to the sanitization endpoint - Receive sanitized but attacker-controlled HTML in the response - Execute arbitrary JavaScript in the attacker's browser context through XSS - Potentially exploit other users if the sanitized output is rendered in their context The vulnerability has higher severity because: 1. No authentication required (only needs a registered user account) 2. Unlike the safer pattern in internal/markup/sanitizer.go:39 which uses isSafeDataURI to only allow safe image MIME types, this endpoint allows ALL data URIs including HTML 3. The returned HTML can be used to craft XSS attacks Proof of Concept Attacker sends a POST request to the sanitization endpoint: POST /-/api/sanitize_ipynb HTTP/1.1 Host: target.gogs.instance Content-Type: text/plain <a href="data:text/html,<script>alert(document.cookie)</script>">click</a> The server returns the sanitized HTML with the data URI preserved: <a href="data:text/html,<script>alert(document.cookie)</script>">click</a> When this HTML is rendered in a browser, the JavaScript within the data URI will execute, leading to XSS. Affected Componen