The WP Carousel Free plugin for WordPress is vulnerable to Stored Cross-Site Scripting via crafted fancybox `data-caption` attributes in all versions up to, and including, 2.7.10. This is due to the `fancybox-config.js` script reading the carousel container's `id` attribute directly from the DOM to construct a jQuery selector without sanitization. When a Contributor crafts an HTML block with a malformed carousel container ID (containing characters invalid for jQuery selectors), the custom fancybox configuration throws a JavaScript error and fails to initialize. This causes the bundled fancybox library (v3.5.7) to fall back to its default caption handling, which renders the `data-caption` attribute content as raw HTML. Since WordPress allows `data-*` attributes through `wp_kses_post()`, this makes it possible for authenticated attackers, with Contributor-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user clicks an image in the crafted carousel lightbox.