The vulnerability stems from unsanitized href attributes in RSS feed display. The RSS Displayer block's view function would be responsible for rendering feed item links. Concrete CMS 9.1.0 patched this by adding sanitization, indicating the rendering logic in the view template/controller was the vulnerable component. While exact code isn't available, the pattern matches common XSS vulnerabilities where user-controlled attributes (href) are output without escaping in template rendering functions.