API requests include ext_nonce (short-lived HMAC token) and x-extension-session-id header; 403 responses trigger a nonce refresh and single retry
file:// URLs are filtered by extension (.html, .htm, .xhtml, .xht, .xhtm) via hasSupportedLocalFileExtension (centralized in utils.js) before any content script injection
Audio proxy (fetchProxyAudio): validate protocol (https: only), hostname (ezfurigana.com allowlist), and port (default/443 only) at both the background.js message layer and inside handleFetchProxyAudio in bg-api.js — two-layer defense-in-depth against SSRF