CVE-2026-58191: Appium: Reflected XSS / arbitrary JS in @appium/base-driver /test/guinea-pig* routes

Appium’s base-driver mounts the built-in /test/guinea-pig, /test/guinea-pig-scrollable and /test/guinea-pig-app-banner routes unconditionally on every server. The handler reflects the throwError query param, the comments POST field, and the User-Agent request header into the returned HTML via compileLodashTemplate, which interpolates <%= expr %> as String(expr) with no HTML/JS escaping. This yields reflected XSS, and the throwError value is reflected inside a <script> block, giving arbitrary JavaScript execution on the server’s origin. No authentication, no session, no driver and no plugin are required, and the default bind address is 0.0.0.0.