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:

// 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 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 `` are rendered as-is.

PoC

1. Create a file named exploit.md in a repository.
2. Add the following content (Raw HTML):

   Click me for XSS
   

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.