The vulnerability stems from two key points: 1) The API endpoint accepting unsanitized user input in page content (shown in PoC request), and 2) The template rendering engine failing to escape stored content. While the exact file paths are inferred from standard MVC patterns in PHP CMS architectures, the confidence remains high for the API handler due to direct evidence in the PoC, and medium for the parser component based on typical XSS vulnerability patterns.