The Strong Testimonials plugin for WordPress is vulnerable to Stored Cross-Site Scripting via 'lightbox_class' Shortcode Attribute in all versions up to, and including, 3.3.8 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. Exploitation requires that the targeted testimonial view has at least one published testimonial with a featured image and the lightbox wrapper enabled, as the vulnerable code path is only reached when a thumbnail is rendered.