Concrete CMS versions 9.5.2 and earlier contain a stored DOM-based Cross-site Scripting (XSS) vulnerability in the Gallery block's per-image Caption field. This occurs because the bundled Magnific Popup lightbox script improperly handles caption content by parsing it as HTML via jQuery's .append() method instead of inserting it as plain text. A user with page edit permissions can inject malicious scripts into image captions, which execute in the browsers of visitors who open the affected image lightbox.