diff --git a/api/middleware/securityHeaders.go b/api/middleware/securityHeaders.go
new file mode 100644
index 0000000..6bc359b
--- /dev/null
+++ b/api/middleware/securityHeaders.go
@@ -0,0 +1,33 @@
+package middleware
+
+import "net/http"
+
+const contentSecurityPolicy = "default-src 'self'; " +
+	"script-src 'self'; " +
+	"style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; " +
+	"font-src 'self' https://fonts.gstatic.com; " +
+	"img-src 'self' data: blob: https:; " +
+	"connect-src 'self'; " +
+	"object-src 'none'; " +
+	"base-uri 'self'; " +
+	"form-action 'self'; " +
+	"frame-ancestors 'none'"
+
+const strictTransportSecurity = "max-age=31536000"
+
+func SecurityHeaders(hsts bool) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			h := w.Header()
+			h.Set("Content-Security-Policy", contentSecurityPolicy)
+			h.Set("X-Content-Type-Options", "nosniff")
+			h.Set("X-Frame-Options", "DENY")
+			h.Set("Referrer-Policy", "strict-origin-when-cross-origin")
+			h.Set("Cross-Origin-Opener-Policy", "same-origin")
+			if hsts {
+				h.Set("Strict-Transport-Security", strictTransportSecurity)
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}
diff --git a/api/router.go b/api/router.go
index 00e600a..32c9a09 100644
--- a/api/router.go
+++ b/api/router.go
@@ -280,6 +280,7 @@ func InitRoutes(container *UseCases, settings config.Settings) *chi.Mux {
 	frontend := InitFrontendRoutes(settings.Environment)
 
 	apiRouter := chi.NewRouter()
+	apiRouter.Use(middleware.SecurityHeaders(settings.IsProduction()))
 	enableCorsIfConfigured(apiRouter, settings.CorsAllowedOrigins)
 	apiRouter.Use(chimiddleware.GetHead)
 	apiRouter.Get("/healthz", GetHealth)
diff --git a/docs/deployment.md b/docs/deployment.md
index 248c3c8..71e3901 100644
--- a/docs/deployment.md
+++ b/docs/deployment.md
@@ -270,6 +270,12 @@ If RenewCMS runs on the same host as the proxy, consider firewalling port 8080.
 
 Either way, set `TRUSTED_PROXIES` (see [configuration.md](configuration.md)) to the proxy's own address as seen by RenewCMS, typically `127.0.0.1` when the proxy and RenewCMS share a host, so the login/password-reset rate limiter honors the forwarded client IP instead of the proxy's own address.
 
+### Security headers
+
+RenewCMS sends `Content-Security-Policy` (including `frame-ancestors 'none'`), `X-Content-Type-Options`, `X-Frame-Options`, `Referrer-Policy` and `Cross-Origin-Opener-Policy` itself, so the proxy doesn't need to add them. With `ENVIRONMENT=production` it also sends `Strict-Transport-Security: max-age=31536000` (browsers ignore it over plain HTTP, so serve the site over HTTPS only). Don't add the same headers in the proxy, or they will be duplicated.
+
+The CSP allows Google Fonts (`fonts.googleapis.com`, `fonts.gstatic.com`) and `https:` images, because the SPA loads its fonts from Google and articles may embed remote images. `style-src` keeps `'unsafe-inline'` for the article editor; `script-src` is `'self'` only.
+
 ## Backups
 
 Everything worth backing up is under the data directory. Take a consistent SQLite copy while the service is running:
diff --git a/test/security_headers_test.go b/test/security_headers_test.go
new file mode 100644
index 0000000..bb772a3
--- /dev/null
+++ b/test/security_headers_test.go
@@ -0,0 +1,62 @@
+package test
+
+import (
+	"RenewCMS/api/middleware"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func serveWithSecurityHeaders(hsts bool) http.Header {
+	handler := middleware.SecurityHeaders(hsts)(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/", nil))
+	return rec.Result().Header
+}
+
+func TestSecurityHeadersAlwaysSetBaseline(t *testing.T) {
+	headers := serveWithSecurityHeaders(false)
+
+	expected := map[string]string{
+		"X-Content-Type-Options":     "nosniff",
+		"X-Frame-Options":            "DENY",
+		"Referrer-Policy":            "strict-origin-when-cross-origin",
+		"Cross-Origin-Opener-Policy": "same-origin",
+	}
+	for name, want := range expected {
+		if got := headers.Get(name); got != want {
+			t.Errorf("expected %s %q, got %q", name, want, got)
+		}
+	}
+}
+
+func TestSecurityHeadersContentSecurityPolicy(t *testing.T) {
+	csp := serveWithSecurityHeaders(false).Get("Content-Security-Policy")
+
+	for _, directive := range []string{
+		"default-src 'self'",
+		"script-src 'self';",
+		"object-src 'none'",
+		"base-uri 'self'",
+		"frame-ancestors 'none'",
+	} {
+		if !strings.Contains(csp, directive) {
+			t.Errorf("expected CSP to contain %q, got %q", directive, csp)
+		}
+	}
+	if strings.Contains(csp, "script-src 'self' 'unsafe") {
+		t.Errorf("script-src must not allow unsafe sources, got %q", csp)
+	}
+}
+
+func TestSecurityHeadersHSTSOnlyWhenEnabled(t *testing.T) {
+	if got := serveWithSecurityHeaders(false).Get("Strict-Transport-Security"); got != "" {
+		t.Errorf("expected no HSTS header, got %q", got)
+	}
+	if got := serveWithSecurityHeaders(true).Get("Strict-Transport-Security"); got != "max-age=31536000" {
+		t.Errorf("expected HSTS max-age=31536000, got %q", got)
+	}
+}