What Is CORS? The Same-Origin Policy (SOP) is a browser security rule: JavaScript on app.example.com cannot read responses from api.example.com — different subdomain, different origin. Without this, any website could use a visitor’s authenticated session to silently call other sites on their behalf. CORS (Cross-Origin Resource Sharing) is the mechanism by which a server explicitly relaxes SOP for trusted origins. When a browser sees a cross-origin request, it checks whether the server has granted permission before allowing JavaScript to read the response.