Craft is a content management system (CMS). The fix for CVE-2025-35939 in craftcms/cms introduced a strip_tags() call in src/web/User.php to sanitize return URLs before they are stored in the session. However, strip_tags() only removes HTML tags (angle brackets) -- it does not inspect or filter URL schemes. Payloads like javascript:alert(document.cookie) contain no HTML tags and pass through strip_tags() completely unmodified, enabling reflected XSS when the return URL is rendered in an href attribute. This vulnerability is fixed in 5.9.7 and 4.17.3.

Affected: Packagist: craftcms/cms

### Summary

The fix for CVE-2025-35939 in `craftcms/cms` introduced a `strip_tags()` call in `src/web/User.php` to sanitize return URLs before they are stored in the session. However, `strip_tags()` only removes HTML tags (angle brackets) -- it does not inspect or filter URL schemes. Payloads like `javascript:alert(document.cookie)` contain no HTML tags and pass through `strip_tags()` completely unmodified, enabling reflected XSS when the return URL is rendered in an `href` attribute.

### Details The patched code in is:

```php public function setReturnUrl($url): void { parent::setReturnUrl(strip_tags($url)); } ```

`strip_tags()` removes HTML tags (e.g., `<script>`, `<img>`) from a string, but it is **not** a URL sanitizer. When the sanitized return URL is subsequently rendered in an `href` attribute context (e.g., `<a href="{{ returnUrl }}">`), the following dangerous payloads survive `strip_tags()` completely unmodified:

1. **`javascript:` protocol URLs** -- `javascript:alert(document.cookie)` contains no HTML tags, so `strip_tags()` returns it verbatim. When placed in an `href`, clicking the link executes the JavaScript.

2. **`data:` URIs** -- `data:text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==` uses Base64 encoding and contains no tags at all, bypassing `strip_tags()` entirely.

3. **Protocol-relative URLs** -- `//evil.com/steal` contains no tags and is passed through unchanged. When rendered as an `href`, the browser resolves it relative to the current…