The Simple SEO Slideshow plugin for WordPress is vulnerable to Stored Cross-Site Scripting via Shortcode Attributes in all versions up to, and including, 1.2.8 due to insufficient input sanitization and output escaping. 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 accesses an injected page. WordPress KSES does not strip malicious shortcode attribute values on post save, allowing contributor-level users to persist payloads that execute for any visitor, including administrators reviewing the post.
https://plugins.trac.wordpress.org/browser/simple-seo-slideshow/trunk/simpleslideshow.php#L356
https://plugins.trac.wordpress.org/browser/simple-seo-slideshow/trunk/simpleslideshow.php#L334
https://plugins.trac.wordpress.org/browser/simple-seo-slideshow/trunk/simpleslideshow.php#L235