GHSA-52c5-vh7f-26fx

### Impact

The prosemirror_to_html gem is vulnerable to Cross-Site Scripting (XSS) attacks through malicious HTML attribute values. While tag content is properly escaped, attribute values are not, allowing attackers to inject arbitrary JavaScript code.

**Who is impacted:** - Any application using prosemirror_to_html to convert ProseMirror documents to HTML - Applications that process user-generated ProseMirror content are at highest risk - End users viewing the rendered HTML output could have malicious JavaScript executed in their browsers

**Attack vectors include:** - `href` attributes with `javascript:` protocol: `<a href="javascript:alert(document.cookie)">` - Event handlers: `<div onclick="maliciousCode()">` - `onerror` attributes on images: `<img src=x onerror="alert('XSS')">` - Other HTML attributes that can execute JavaScript

### Patches

A fix is currently in development. Users should upgrade to version **0.2.1** or later once released.

The patch escapes all HTML attribute values using `CGI.escapeHTML` to prevent injection attacks.

### Workarounds

Until a patched version is available, users can implement one or more of these mitigations:

1. **Sanitize output**: Pass the HTML output through a sanitization library like [Sanitize](https://github.com/rgrove/sanitize) or [Loofah](https://github.com/flavorjones/loofah): ```ruby html = ProsemirrorToHtml.render(document) safe_html = Sanitize.fragment(html, Sanitize::Config::RELAXED) ```

2. **Implement Content Security Policy (CSP)**: Add strict CSP headers to prevent inline JavaScript execution: ``` Content-Security-Policy: default-src 'self'; script-src 'self' ```

3. **Input validation**: If possible, validate and sanitize ProseMirror documents before conversion to prevent malicious content from entering the system.

### References

- Vulnerable code: https://github.com/etaminstudio/prosemirror_to_html/blob/ea8beb32f6c37f29f042ba4155ccf18504da716e/lib/prosemirror_to_html.rb#L249 - [OWASP XSS Prevention Cheat Sheet](https://cheatsheetseries.owasp.org/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.html) - [CWE-79: Improper Neutralization of Input During Web Page Generation](https://cwe.mitre.org/data/definitions/79.html)