The vulnerability manifests in the modalRelatedFieldItemEdit.aspx endpoint where user-supplied input (TbItemName parameter) is directly reflected in the response without proper HTML encoding. The provided exploit demonstrates that submitting a crafted SVG payload in TbItemName parameter triggers XSS when rendered. This indicates the backend processing function (likely Page_Load() in ASP.NET Web Forms architecture) fails to sanitize user input before incorporating it into the page output. The combination of the vulnerable endpoint pattern and the demonstrated exploit mechanism strongly points to the field editor's form handling code as the vulnerable component.