The Stape Conversion Tracking WordPress plugin before 2.3.11 does not validate the sub-path appended to its same-origin proxy's upstream endpoint, allowing unauthenticated visitors to redirect the server-side request to an arbitrary host and have the response body and its content type returned from the site's own origin with security response headers stripped, leading to Server-Side Request Forgery, including loopback and internal hosts, and Cross-Site Scripting.