diff --git a/internal/server/routes.go b/internal/server/routes.go
index 9cc0103..a35cd67 100644
--- a/internal/server/routes.go
+++ b/internal/server/routes.go
@@ -16,6 +16,11 @@ import (
 
 const routeTimeout = 60 * time.Second
 
+// cspHeader is the Content-Security-Policy applied to the embedded web SPA.
+// The SPA loads external scripts and stylesheets from the same origin only;
+// all API communication uses same-origin fetch (no WebSockets).
+const cspHeader = "default-src 'self'; script-src 'self'; style-src 'self'"
+
 // SetupRoutes configures the HTTP routes and middleware.
 func (srv *Server) SetupRoutes() {
 	srv.router = chi.NewRouter()
@@ -133,6 +138,11 @@ func (srv *Server) setupSPA() {
 		writer http.ResponseWriter,
 		request *http.Request,
 	) {
+		writer.Header().Set(
+			"Content-Security-Policy",
+			cspHeader,
+		)
+
 		readFS, ok := distFS.(fs.ReadFileFS)
 		if !ok {
 			fileServer.ServeHTTP(writer, request)