### 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.

CWE-79 Cross-site Scripting (XSS)