The XSS vulnerability manifests when Markdown input containing HTML payloads (like <svg onload=alert(0)>) is processed and rendered. Radiant CMS's Markdown filter is responsible for converting Markdown to HTML. Since the exploit requires the 'Markdown' filter selection and results in unescaped HTML execution, the vulnerability likely resides in the MarkdownFilter's rendering logic where HTML sanitization is omitted. The absence of patched versions and the CWE-79 classification further support this conclusion.