A Second-Order Cross-Site Scripting (XSS) vulnerability exists in Textpattern CMS version 4.9.0 due to improper sanitization and contextual encoding of user-supplied input embedded within Atom feed XML elements. User-controlled parameters (e.g., category) are reflected into Atom fields such as <id> and <link href> without proper XML escaping. While the payload may not execute directly in modern browsers in raw XML context, it can execute when the feed is consumed by HTML-based feed readers, admin dashboards, or CMS aggregators that insert the feed content into the DOM using unsafe methods (e.g., innerHTML), resulting in JavaScript execution in a trusted context.