GHSA-xrcr-gmf5-2r8j

### Summary A Stored Cross-site Scripting (XSS) vulnerability exists in the comment and issue description functionality. The application's HTML sanitizer explicitly allows `data:` URI schemes, enabling authenticated users to inject arbitrary JavaScript execution via malicious links. ### Details The vulnerability is located in `internal/markup/sanitizer.go`. The application uses the `bluemonday` HTML sanitizer but explicitly weakens the security policy by allowing the `data` URL scheme: ```go // internal/markup/sanitizer.go func NewSanitizer() { sanitizer.init.Do(func() { // ... // Data URLs sanitizer.policy.AllowURLSchemes("data") // ... }) } ``` While the Markdown renderer rewrites relative links (mitigating standard Markdown `[link](data:...)` attacks), Gogs supports **Raw HTML** input. Raw HTML anchor tags bypass the Markdown parser's link rewriting and are processed directly by the sanitizer. Since the sanitizer is configured to allow `data:` URIs, payloads like `<a href="data:text/html...">` are rendered as-is. ### PoC 1. Create a file named `exploit.md` in a repository. 2. Add the following content (Raw HTML): ```html <a href="data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4=">Click me for XSS</a> ``` 3. Commit and push the file. 4. Navigate to the file in the Gogs web interface. 5. Click the "Click me for XSS" link. 6. **Result:** An alert box with "XSS" appears, executing the JavaScript payload. ### Impact This is a **Stored XSS** vulnerability. Any user who views the malicious comment and clicks the link will execute the attacker-supplied JavaScript in their browser context. This allows attackers to: * Steal authentication cookies and session tokens. * Perform arbitrary actions on behalf of the victim (e.g., modifying repositories, adding collaborators). * Redirect users to malicious sites.