app: add security headers middleware

X-Frame-Options: DENY and frame-ancestors 'none' stop clickjacking
of OIDC, register-confirm, and debug HTML pages. nosniff and no-referrer
are cheap defence-in-depth for the same surfaces.

Updates #3157

hscontrol/app.go

@@ -488,6 +488,20 @@ func (h *Headscale) ensureUnixSocketIsAbsent() error {
 	return os.Remove(h.cfg.UnixSocket)
 }
 
+// securityHeaders sets baseline response headers on every HTTP response:
+// deny framing (clickjacking), forbid MIME-type sniffing, drop the Referer
+// header on outbound navigation. Cheap defense-in-depth for HTML surfaces.
+func securityHeaders(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		h := w.Header()
+		h.Set("X-Frame-Options", "DENY")
+		h.Set("Content-Security-Policy", "frame-ancestors 'none'")
+		h.Set("X-Content-Type-Options", "nosniff")
+		h.Set("Referrer-Policy", "no-referrer")
+		next.ServeHTTP(w, r)
+	})
+}
+
 func (h *Headscale) createRouter(grpcMux *grpcRuntime.ServeMux) *chi.Mux {
 	r := chi.NewRouter()
 	r.Use(metrics.Collector(metrics.CollectorOpts{
@@ -501,6 +515,7 @@ func (h *Headscale) createRouter(grpcMux *grpcRuntime.ServeMux) *chi.Mux {
 	r.Use(middleware.RealIP)
 	r.Use(middleware.RequestLogger(&zerologRequestLogger{}))
 	r.Use(middleware.Recoverer)
+	r.Use(securityHeaders)
 
 	r.Post(ts2021UpgradePath, h.NoiseUpgradeHandler)
 

hscontrol/app_test.go

@@ -0,0 +1,26 @@
+package hscontrol
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSecurityHeaders(t *testing.T) {
+	handler := securityHeaders(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(context.Background(), http.MethodGet, "/", nil)
+	handler.ServeHTTP(rec, req)
+
+	h := rec.Result().Header
+	assert.Equal(t, "DENY", h.Get("X-Frame-Options"))
+	assert.Equal(t, "frame-ancestors 'none'", h.Get("Content-Security-Policy"))
+	assert.Equal(t, "nosniff", h.Get("X-Content-Type-Options"))
+	assert.Equal(t, "no-referrer", h.Get("Referrer-Policy"))
+}

hscontrol/debug.go

@@ -379,7 +379,7 @@ func (h *Headscale) debugHTTPServer() *http.Server {
 
 	debugHTTPServer := &http.Server{
 		Addr:         h.cfg.MetricsAddr,
-		Handler:      debugMux,
+		Handler:      securityHeaders(debugMux),
 		ReadTimeout:  types.HTTPTimeout,
 		WriteTimeout: 0,
 	}