diff --git a/cmd/server/config.go b/cmd/server/config.go
index d5f5f569..0c1a296c 100644
--- a/cmd/server/config.go
+++ b/cmd/server/config.go
@@ -70,6 +70,11 @@ type Config struct {
 
 	Timestamps *TimestampConfig `json:"timestamps,omitempty"`
 
+	// CORSAllowedOrigins is the list of origins permitted to make cross-origin
+	// requests. When empty (default), no Access-Control-* headers are sent,
+	// so browsers enforce same-origin policy. Set to ["*"] to allow all origins.
+	CORSAllowedOrigins []string `json:"corsAllowedOrigins,omitempty"`
+
 	DebugAffinity bool `json:"debugAffinity,omitempty"`
 
 	// ObserverBlacklist is a list of observer public keys to exclude from API
diff --git a/cmd/server/cors.go b/cmd/server/cors.go
new file mode 100644
index 00000000..9edc1ed6
--- /dev/null
+++ b/cmd/server/cors.go
@@ -0,0 +1,66 @@
+package main
+
+import "net/http"
+
+// corsMiddleware returns a middleware that sets CORS headers based on the
+// configured allowed origins. When CORSAllowedOrigins is empty (default),
+// no Access-Control-* headers are added, preserving browser same-origin policy.
+func (s *Server) corsMiddleware(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		origins := s.cfg.CORSAllowedOrigins
+		if len(origins) == 0 {
+			next.ServeHTTP(w, r)
+			return
+		}
+
+		reqOrigin := r.Header.Get("Origin")
+		if reqOrigin == "" {
+			next.ServeHTTP(w, r)
+			return
+		}
+
+		// Check if origin is allowed
+		allowed := false
+		wildcard := false
+		for _, o := range origins {
+			if o == "*" {
+				allowed = true
+				wildcard = true
+				break
+			}
+			if o == reqOrigin {
+				allowed = true
+				break
+			}
+		}
+
+		if !allowed {
+			// Origin not in allowlist — don't add CORS headers
+			if r.Method == http.MethodOptions {
+				// Still reject preflight with 403
+				w.WriteHeader(http.StatusForbidden)
+				return
+			}
+			next.ServeHTTP(w, r)
+			return
+		}
+
+		// Set CORS headers
+		if wildcard {
+			w.Header().Set("Access-Control-Allow-Origin", "*")
+		} else {
+			w.Header().Set("Access-Control-Allow-Origin", reqOrigin)
+			w.Header().Set("Vary", "Origin")
+		}
+		w.Header().Set("Access-Control-Allow-Methods", "GET, POST, OPTIONS")
+		w.Header().Set("Access-Control-Allow-Headers", "Content-Type, X-API-Key")
+
+		// Handle preflight
+		if r.Method == http.MethodOptions {
+			w.WriteHeader(http.StatusNoContent)
+			return
+		}
+
+		next.ServeHTTP(w, r)
+	})
+}
diff --git a/cmd/server/cors_test.go b/cmd/server/cors_test.go
new file mode 100644
index 00000000..acafaaee
--- /dev/null
+++ b/cmd/server/cors_test.go
@@ -0,0 +1,149 @@
+package main
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// newTestServerWithCORS creates a minimal Server with the given CORS config.
+func newTestServerWithCORS(origins []string) *Server {
+	cfg := &Config{CORSAllowedOrigins: origins}
+	srv := &Server{cfg: cfg}
+	return srv
+}
+
+// dummyHandler is a simple handler that writes 200 OK.
+var dummyHandler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	w.WriteHeader(http.StatusOK)
+	w.Write([]byte("ok"))
+})
+
+func TestCORS_DefaultNoHeaders(t *testing.T) {
+	srv := newTestServerWithCORS(nil)
+	handler := srv.corsMiddleware(dummyHandler)
+
+	req := httptest.NewRequest("GET", "/api/health", nil)
+	req.Header.Set("Origin", "https://evil.example")
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != 200 {
+		t.Fatalf("expected 200, got %d", rr.Code)
+	}
+	if v := rr.Header().Get("Access-Control-Allow-Origin"); v != "" {
+		t.Fatalf("expected no ACAO header, got %q", v)
+	}
+}
+
+func TestCORS_AllowlistMatch(t *testing.T) {
+	srv := newTestServerWithCORS([]string{"https://good.example"})
+	handler := srv.corsMiddleware(dummyHandler)
+
+	req := httptest.NewRequest("GET", "/api/health", nil)
+	req.Header.Set("Origin", "https://good.example")
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != 200 {
+		t.Fatalf("expected 200, got %d", rr.Code)
+	}
+	if v := rr.Header().Get("Access-Control-Allow-Origin"); v != "https://good.example" {
+		t.Fatalf("expected origin echo, got %q", v)
+	}
+	if v := rr.Header().Get("Access-Control-Allow-Methods"); v != "GET, POST, OPTIONS" {
+		t.Fatalf("expected methods header, got %q", v)
+	}
+	if v := rr.Header().Get("Access-Control-Allow-Headers"); v != "Content-Type, X-API-Key" {
+		t.Fatalf("expected headers header, got %q", v)
+	}
+	if v := rr.Header().Get("Vary"); v != "Origin" {
+		t.Fatalf("expected Vary: Origin, got %q", v)
+	}
+}
+
+func TestCORS_AllowlistNoMatch(t *testing.T) {
+	srv := newTestServerWithCORS([]string{"https://good.example"})
+	handler := srv.corsMiddleware(dummyHandler)
+
+	req := httptest.NewRequest("GET", "/api/health", nil)
+	req.Header.Set("Origin", "https://evil.example")
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != 200 {
+		t.Fatalf("expected 200, got %d", rr.Code)
+	}
+	if v := rr.Header().Get("Access-Control-Allow-Origin"); v != "" {
+		t.Fatalf("expected no ACAO header for non-matching origin, got %q", v)
+	}
+}
+
+func TestCORS_PreflightAllowed(t *testing.T) {
+	srv := newTestServerWithCORS([]string{"https://good.example"})
+	handler := srv.corsMiddleware(dummyHandler)
+
+	req := httptest.NewRequest("OPTIONS", "/api/health", nil)
+	req.Header.Set("Origin", "https://good.example")
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusNoContent {
+		t.Fatalf("expected 204, got %d", rr.Code)
+	}
+	if v := rr.Header().Get("Access-Control-Allow-Origin"); v != "https://good.example" {
+		t.Fatalf("expected origin echo, got %q", v)
+	}
+}
+
+func TestCORS_PreflightRejected(t *testing.T) {
+	srv := newTestServerWithCORS([]string{"https://good.example"})
+	handler := srv.corsMiddleware(dummyHandler)
+
+	req := httptest.NewRequest("OPTIONS", "/api/health", nil)
+	req.Header.Set("Origin", "https://evil.example")
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusForbidden {
+		t.Fatalf("expected 403, got %d", rr.Code)
+	}
+}
+
+func TestCORS_Wildcard(t *testing.T) {
+	srv := newTestServerWithCORS([]string{"*"})
+	handler := srv.corsMiddleware(dummyHandler)
+
+	req := httptest.NewRequest("GET", "/api/health", nil)
+	req.Header.Set("Origin", "https://anything.example")
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != 200 {
+		t.Fatalf("expected 200, got %d", rr.Code)
+	}
+	if v := rr.Header().Get("Access-Control-Allow-Origin"); v != "*" {
+		t.Fatalf("expected *, got %q", v)
+	}
+	// Wildcard should NOT set Vary: Origin
+	if v := rr.Header().Get("Vary"); v == "Origin" {
+		t.Fatalf("wildcard should not set Vary: Origin")
+	}
+}
+
+func TestCORS_NoOriginHeader(t *testing.T) {
+	srv := newTestServerWithCORS([]string{"https://good.example"})
+	handler := srv.corsMiddleware(dummyHandler)
+
+	req := httptest.NewRequest("GET", "/api/health", nil)
+	// No Origin header
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != 200 {
+		t.Fatalf("expected 200, got %d", rr.Code)
+	}
+	if v := rr.Header().Get("Access-Control-Allow-Origin"); v != "" {
+		t.Fatalf("expected no ACAO without Origin header, got %q", v)
+	}
+}
diff --git a/cmd/server/routes.go b/cmd/server/routes.go
index 70cfbbd0..34084304 100644
--- a/cmd/server/routes.go
+++ b/cmd/server/routes.go
@@ -104,6 +104,9 @@ func (s *Server) getMemStats() runtime.MemStats {
 // RegisterRoutes sets up all HTTP routes on the given router.
 func (s *Server) RegisterRoutes(r *mux.Router) {
 	s.router = r
+	// CORS middleware (must run before route handlers)
+	r.Use(s.corsMiddleware)
+
 	// Performance instrumentation middleware
 	r.Use(s.perfMiddleware)