diff --git a/internal/server/server.go b/internal/server/server.go
index 6866f83..f791276 100644
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -154,10 +154,20 @@ func (s *Server) Handler() http.Handler {
 	}
 	uiSrv.Register(mux)
 
-	// Apply global middleware: request logging.
+	// Apply global middleware: request logging and security headers.
 	// Rate limiting is applied per-route above (login, token/validate).
 	var root http.Handler = mux
 	root = middleware.RequestLogger(s.logger)(root)
+
+	// Security (SEC-04): apply baseline security headers to ALL responses
+	// (both API and UI). These headers are safe for every content type:
+	//   - X-Content-Type-Options prevents MIME-sniffing attacks.
+	//   - Strict-Transport-Security enforces HTTPS for 2 years.
+	//   - Cache-Control prevents caching of authenticated responses.
+	// The UI sub-mux already sets these plus CSP/X-Frame-Options/Referrer-Policy
+	// which will override where needed (last Set wins before WriteHeader).
+	root = globalSecurityHeaders(root)
+
 	return root
 }
 
@@ -1309,6 +1319,20 @@ func extractBearerFromRequest(r *http.Request) (string, error) {
 // docsSecurityHeaders adds the same defensive HTTP headers as the UI sub-mux
 // to the /docs and /docs/openapi.yaml endpoints.
 //
+// globalSecurityHeaders sets baseline security headers on every response.
+// Security (SEC-04): API responses previously lacked X-Content-Type-Options,
+// HSTS, and Cache-Control. These three headers are safe for all content types
+// and do not interfere with JSON API clients or the HTMX UI.
+func globalSecurityHeaders(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		h := w.Header()
+		h.Set("X-Content-Type-Options", "nosniff")
+		h.Set("Strict-Transport-Security", "max-age=63072000; includeSubDomains")
+		h.Set("Cache-Control", "no-store")
+		next.ServeHTTP(w, r)
+	})
+}
+
 // Security (DEF-09): without these headers the Swagger UI HTML page is
 // served without CSP, X-Frame-Options, or HSTS, leaving it susceptible
 // to clickjacking and MIME-type confusion in browsers.
diff --git a/internal/server/server_test.go b/internal/server/server_test.go
index 82037d1..8d44cca 100644
--- a/internal/server/server_test.go
+++ b/internal/server/server_test.go
@@ -612,3 +612,47 @@ func TestOversizedJSONBodyRejected(t *testing.T) {
 		t.Errorf("expected 400 for oversized body, got %d", rr.Code)
 	}
 }
+
+// TestSecurityHeadersOnAPIResponses verifies that the global security-headers
+// middleware (SEC-04) sets X-Content-Type-Options, Strict-Transport-Security,
+// and Cache-Control on all API responses, not just the UI.
+func TestSecurityHeadersOnAPIResponses(t *testing.T) {
+	srv, _, _, _ := newTestServer(t)
+	handler := srv.Handler()
+
+	wantHeaders := map[string]string{
+		"X-Content-Type-Options":    "nosniff",
+		"Strict-Transport-Security": "max-age=63072000; includeSubDomains",
+		"Cache-Control":             "no-store",
+	}
+
+	t.Run("GET /v1/health", func(t *testing.T) {
+		rr := doRequest(t, handler, "GET", "/v1/health", nil, "")
+		if rr.Code != http.StatusOK {
+			t.Fatalf("status = %d, want 200", rr.Code)
+		}
+		for header, want := range wantHeaders {
+			got := rr.Header().Get(header)
+			if got != want {
+				t.Errorf("%s = %q, want %q", header, got, want)
+			}
+		}
+	})
+
+	t.Run("POST /v1/auth/login", func(t *testing.T) {
+		createTestHumanAccount(t, srv, "sec04-user")
+		rr := doRequest(t, handler, "POST", "/v1/auth/login", map[string]string{
+			"username": "sec04-user",
+			"password": "testpass123",
+		}, "")
+		if rr.Code != http.StatusOK {
+			t.Fatalf("status = %d, want 200; body: %s", rr.Code, rr.Body.String())
+		}
+		for header, want := range wantHeaders {
+			got := rr.Header().Get(header)
+			if got != want {
+				t.Errorf("%s = %q, want %q", header, got, want)
+			}
+		}
+	})
+}