feat: add security response headers middleware (closes #98)

Add SecurityHeaders() to internal/middleware and register it in the
global middleware stack so every response - dashboard, embedded static
assets, healthchecks, JSON API, and metrics - carries the six response
headers required by REPO_POLICIES.md before tagging 1.0:

  Strict-Transport-Security: max-age=31536000; includeSubDomains
  Content-Security-Policy:   default-src 'self'; script-src 'none';
                             style-src 'self'; img-src 'self';
                             font-src 'none'; connect-src 'none';
                             object-src 'none'; base-uri 'none';
                             form-action 'none'; frame-ancestors 'none'
  X-Frame-Options:           DENY
  X-Content-Type-Options:    nosniff
  Referrer-Policy:           no-referrer
  Permissions-Policy:        unused browser features denied

The dashboard template ships no JavaScript, no inline styles, no inline
event handlers and no images, and its only subresource is the embedded
stylesheet at /s/css/tailwind.min.css, so the policy needs neither
unsafe-inline nor unsafe-eval. frame-ancestors 'none' is the primary
anti-framing control with X-Frame-Options as the legacy fallback.

HSTS is emitted unconditionally rather than gated on r.TLS, because the
service runs behind a TLS-terminating proxy and the browser must still
enforce HTTPS end to end.

The headers are set before the request reaches the next handler, so
they are present on error responses too, including recovered panics and
request timeouts.

Tests cover each header's exact value, the CSP's required and forbidden
directives, presence on a 500 response, and a render of the real
dashboard through the middleware confirming the page still references
its stylesheet.

Dockerfile

@@ -5,7 +5,7 @@ FROM golang@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4
 RUN apk add --no-cache git make gcc musl-dev binutils-gold
 
 # golangci-lint v2.12.2, 2026-08-07
-RUN go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.12.2
+RUN go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@c0d3ddc9cf3faa61a4e378e879ece580256d76e5
 # goimports v0.42.0
 RUN go install golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0
 

README.md

@@ -17,6 +17,26 @@ without requiring an external database.
 
 ---
 
+## No DNS mocking. Ever.
+
+**DNS is never mocked in this project — not in tests, not anywhere else.**
+No mock resolvers, no fake DNS servers, no stubbed lookups.
+
+dnswatcher's entire purpose is correct behavior against the real DNS.
+Tests exercise real iterative resolution against live nameservers by
+design; a test suite that passes against a mock proves nothing about the
+one thing this program exists to do.
+
+When live tests are flaky, that is a robustness problem, and it gets
+fixed with robustness: retries with backoff, querying multiple
+independent nameservers, longer timeouts — or explicit opt-in gating
+decided by the project owner. Never with mocks.
+
+Contributions that introduce mocked, faked, or stubbed DNS will be
+rejected.
+
+---
+
 ## Features
 
 ### DNS Domain Monitoring (Apex Domains)
@@ -162,6 +182,46 @@ dnswatcher exposes a lightweight HTTP API for operational visibility:
 | `GET /api/v1/status`                  | Current monitoring state       |
 | `GET /metrics`                        | Prometheus metrics (optional)  |
 
+### Security Headers
+
+Every response — the dashboard, the static assets under `/s/...`, the
+healthchecks, the JSON API, and `/metrics` — carries the following
+headers, set by a global middleware:
+
+| Header                      | Value                                 |
+|-----------------------------|---------------------------------------|
+| `Strict-Transport-Security` | `max-age=31536000; includeSubDomains` |
+| `Content-Security-Policy`   | see below                             |
+| `X-Frame-Options`           | `DENY`                                |
+| `X-Content-Type-Options`    | `nosniff`                             |
+| `Referrer-Policy`           | `no-referrer`                         |
+| `Permissions-Policy`        | all unused browser features denied    |
+
+The content security policy is:
+
+```
+default-src 'self'; script-src 'none'; style-src 'self'; img-src 'self';
+font-src 'none'; connect-src 'none'; object-src 'none'; base-uri 'none';
+form-action 'none'; frame-ancestors 'none'
+```
+
+The dashboard ships no JavaScript (the 30-second refresh is a
+`<meta http-equiv="refresh">`), no inline styles, no inline event
+handlers, and no images; its only subresource is the embedded stylesheet
+at `/s/css/tailwind.min.css`, which `style-src 'self'` permits. The
+policy therefore needs neither `unsafe-inline` nor `unsafe-eval`.
+`frame-ancestors 'none'` is the primary anti-framing control, with
+`X-Frame-Options: DENY` retained as the legacy fallback.
+
+HSTS is emitted unconditionally, including over plain HTTP. dnswatcher is
+expected to run behind a TLS-terminating reverse proxy, and the browser
+must still be told to enforce HTTPS end to end, so the header is never
+gated on whether the request itself arrived over TLS.
+
+`Referrer-Policy: no-referrer` is stricter than the
+`strict-origin-when-cross-origin` baseline: the dashboard has no
+cross-origin navigation needs, and its URL may name internal hosts.
+
 ---
 
 ## Architecture
@@ -174,7 +234,8 @@ internal/
   globals/globals.go           Build-time variables (version)
   logger/logger.go             slog structured logging (TTY detection)
   healthcheck/healthcheck.go   Health check service
-  middleware/middleware.go      HTTP middleware (logging, CORS, metrics auth)
+  middleware/middleware.go      HTTP middleware (logging, CORS, security
+                               headers, metrics auth)
   handlers/handlers.go         HTTP request handlers
   server/
     server.go                  HTTP server lifecycle

TODO.md

@@ -25,10 +25,24 @@ confirm make check still passes.
 
 # Completed Steps
 
-- 2026-08-07: golangci-lint bumped to v2.12.2 (pins in `Dockerfile` and
-  `script/bootstrap`), `.golangci.yml` replaced with the canonical v2
-  config (settings moved under `linters.settings` so thresholds now
-  apply); fixed all resulting `goconst`, `dupl`, and `lll` findings
+- 2026-08-09: security response headers middleware
+  (`SecurityHeaders()` in `internal/middleware/middleware.go`)
+  registered globally in `internal/server/routes.go`, so HSTS, CSP,
+  `X-Frame-Options`, `X-Content-Type-Options`, `Referrer-Policy`, and
+  `Permissions-Policy` are set on every response including `/s/...` and
+  `/metrics`; the CSP needs no `unsafe-inline`/`unsafe-eval` because the
+  dashboard ships no JavaScript and no inline styles; HSTS is emitted
+  unconditionally per policy (TLS-terminating proxy in front). Remaining
+  1.0 hardening items — `http.Server` timeouts, request body limits,
+  rate limiting, CORS scoping — are tracked separately
+- 2026-08-07: golangci-lint bumped to v2.12.2 (commit-pinned installs
+  in `Dockerfile` and `script/bootstrap`); `.golangci.yml` set to the
+  org-standard v2-schema config used across the org's repos
+  (owner-authorized; same file is being landed as canonical via prompts
+  PR #24), with settings under `linters.settings` so the
+  lll/funlen/cyclop/dupl thresholds apply; fixed the resulting
+  `goconst`, `dupl`, and `lll` findings; the informational `gomodguard`
+  deprecation warning under this config is accepted
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints,
   Makefile shims, README Entrypoints section
 - 2026-02-20: iterative DNS resolver implemented; tests made hermetic

internal/middleware/middleware.go

@@ -21,6 +21,60 @@ import (
 // corsMaxAge is the maximum age for CORS preflight responses.
 const corsMaxAge = 300
 
+// Security response header values applied to every response.
+//
+// The CSP is as strict as the dashboard allows: the template ships no
+// JavaScript, no inline styles, no inline event handlers and no images,
+// and its only subresource is the embedded stylesheet at
+// /s/css/tailwind.min.css, which style-src 'self' permits. Neither
+// unsafe-inline nor unsafe-eval is used. frame-ancestors 'none' is the
+// primary anti-framing control; X-Frame-Options is the legacy fallback.
+const (
+	// hstsValue is emitted unconditionally, including over plain HTTP,
+	// because the service runs behind a TLS-terminating proxy and the
+	// browser must still enforce HTTPS end to end.
+	hstsValue = "max-age=31536000; includeSubDomains"
+
+	cspValue = "default-src 'self'; " +
+		"script-src 'none'; " +
+		"style-src 'self'; " +
+		"img-src 'self'; " +
+		"font-src 'none'; " +
+		"connect-src 'none'; " +
+		"object-src 'none'; " +
+		"base-uri 'none'; " +
+		"form-action 'none'; " +
+		"frame-ancestors 'none'"
+
+	frameOptionsValue = "DENY"
+
+	contentTypeOptionsValue = "nosniff"
+
+	// referrerPolicyValue is stricter than the policy minimum of
+	// strict-origin-when-cross-origin: the dashboard has no
+	// cross-origin navigation needs and its URL may name internal
+	// hosts.
+	referrerPolicyValue = "no-referrer"
+
+	permissionsPolicyValue = "accelerometer=(), " +
+		"autoplay=(), " +
+		"camera=(), " +
+		"display-capture=(), " +
+		"encrypted-media=(), " +
+		"fullscreen=(), " +
+		"geolocation=(), " +
+		"gyroscope=(), " +
+		"magnetometer=(), " +
+		"microphone=(), " +
+		"midi=(), " +
+		"payment=(), " +
+		"picture-in-picture=(), " +
+		"publickey-credentials-get=(), " +
+		"screen-wake-lock=(), " +
+		"usb=(), " +
+		"xr-spatial-tracking=()"
+)
+
 // Params contains dependencies for Middleware.
 type Params struct {
 	fx.In
@@ -186,6 +240,37 @@ func (m *Middleware) CORS() func(http.Handler) http.Handler {
 	})
 }
 
+// SecurityHeaders returns middleware that sets the security response
+// headers required for production internet exposure on every response.
+//
+// The headers are set before the request reaches the next handler so
+// that they are present on every response, including panics recovered
+// by chi's Recoverer and timeouts produced by chi's Timeout.
+func (m *Middleware) SecurityHeaders() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(
+			writer http.ResponseWriter,
+			request *http.Request,
+		) {
+			header := writer.Header()
+			header.Set("Strict-Transport-Security", hstsValue)
+			header.Set("Content-Security-Policy", cspValue)
+			header.Set("X-Frame-Options", frameOptionsValue)
+			header.Set(
+				"X-Content-Type-Options",
+				contentTypeOptionsValue,
+			)
+			header.Set("Referrer-Policy", referrerPolicyValue)
+			header.Set(
+				"Permissions-Policy",
+				permissionsPolicyValue,
+			)
+
+			next.ServeHTTP(writer, request)
+		})
+	}
+}
+
 // MetricsAuth returns basic auth middleware for /metrics.
 func (m *Middleware) MetricsAuth() func(http.Handler) http.Handler {
 	if m.params.Config.MetricsUsername == "" {

internal/middleware/middleware_test.go

@@ -0,0 +1,333 @@
+package middleware_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/go-chi/chi/v5"
+
+	"sneak.berlin/go/dnswatcher/internal/config"
+	"sneak.berlin/go/dnswatcher/internal/globals"
+	"sneak.berlin/go/dnswatcher/internal/handlers"
+	"sneak.berlin/go/dnswatcher/internal/logger"
+	"sneak.berlin/go/dnswatcher/internal/middleware"
+	"sneak.berlin/go/dnswatcher/internal/notify"
+	"sneak.berlin/go/dnswatcher/internal/state"
+)
+
+// Expected security header values, spelled out literally so that any
+// change to the middleware has to be made deliberately here as well.
+const (
+	wantHSTS = "max-age=31536000; includeSubDomains"
+
+	wantCSP = "default-src 'self'; " +
+		"script-src 'none'; " +
+		"style-src 'self'; " +
+		"img-src 'self'; " +
+		"font-src 'none'; " +
+		"connect-src 'none'; " +
+		"object-src 'none'; " +
+		"base-uri 'none'; " +
+		"form-action 'none'; " +
+		"frame-ancestors 'none'"
+
+	wantFrameOptions = "DENY"
+
+	wantContentTypeOptions = "nosniff"
+
+	wantReferrerPolicy = "no-referrer"
+
+	wantPermissionsPolicy = "accelerometer=(), " +
+		"autoplay=(), " +
+		"camera=(), " +
+		"display-capture=(), " +
+		"encrypted-media=(), " +
+		"fullscreen=(), " +
+		"geolocation=(), " +
+		"gyroscope=(), " +
+		"magnetometer=(), " +
+		"microphone=(), " +
+		"midi=(), " +
+		"payment=(), " +
+		"picture-in-picture=(), " +
+		"publickey-credentials-get=(), " +
+		"screen-wake-lock=(), " +
+		"usb=(), " +
+		"xr-spatial-tracking=()"
+)
+
+// stylesheetPath is the only subresource the dashboard loads.
+const stylesheetPath = "/s/css/tailwind.min.css"
+
+// newTestLogger builds a logger for direct component construction.
+func newTestLogger(t *testing.T) *logger.Logger {
+	t.Helper()
+
+	glob, err := globals.New(nil)
+	if err != nil {
+		t.Fatalf("globals.New: %v", err)
+	}
+
+	log, err := logger.New(nil, logger.Params{Globals: glob})
+	if err != nil {
+		t.Fatalf("logger.New: %v", err)
+	}
+
+	return log
+}
+
+// newTestMiddleware builds a Middleware without an fx application.
+func newTestMiddleware(t *testing.T) *middleware.Middleware {
+	t.Helper()
+
+	glob, err := globals.New(nil)
+	if err != nil {
+		t.Fatalf("globals.New: %v", err)
+	}
+
+	mw, err := middleware.New(nil, middleware.Params{
+		Logger:  newTestLogger(t),
+		Globals: glob,
+		Config:  &config.Config{},
+	})
+	if err != nil {
+		t.Fatalf("middleware.New: %v", err)
+	}
+
+	return mw
+}
+
+// serveWithSecurityHeaders runs a GET through SecurityHeaders and
+// returns the recorded response.
+func serveWithSecurityHeaders(
+	t *testing.T,
+	target string,
+	handler http.Handler,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	mw := newTestMiddleware(t)
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, target, nil,
+	)
+
+	mw.SecurityHeaders()(handler).ServeHTTP(rec, req)
+
+	return rec
+}
+
+// okHandler writes a trivial 200 response.
+func okHandler() http.Handler {
+	return http.HandlerFunc(func(
+		writer http.ResponseWriter,
+		_ *http.Request,
+	) {
+		writer.WriteHeader(http.StatusOK)
+	})
+}
+
+func TestSecurityHeaders(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name   string
+		header string
+		want   string
+	}{
+		{
+			"hsts",
+			"Strict-Transport-Security",
+			wantHSTS,
+		},
+		{
+			"csp",
+			"Content-Security-Policy",
+			wantCSP,
+		},
+		{
+			"frame options",
+			"X-Frame-Options",
+			wantFrameOptions,
+		},
+		{
+			"content type options",
+			"X-Content-Type-Options",
+			wantContentTypeOptions,
+		},
+		{
+			"referrer policy",
+			"Referrer-Policy",
+			wantReferrerPolicy,
+		},
+		{
+			"permissions policy",
+			"Permissions-Policy",
+			wantPermissionsPolicy,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			rec := serveWithSecurityHeaders(t, "/", okHandler())
+
+			got := rec.Header().Get(tt.header)
+			if got != tt.want {
+				t.Errorf(
+					"%s = %q, want %q",
+					tt.header, got, tt.want,
+				)
+			}
+		})
+	}
+}
+
+// TestSecurityHeadersCSPDirectives guards the properties the repo
+// policy requires of the content security policy itself.
+func TestSecurityHeadersCSPDirectives(t *testing.T) {
+	t.Parallel()
+
+	rec := serveWithSecurityHeaders(t, "/", okHandler())
+	csp := rec.Header().Get("Content-Security-Policy")
+
+	forbidden := []string{"unsafe-inline", "unsafe-eval"}
+	for _, directive := range forbidden {
+		if strings.Contains(csp, directive) {
+			t.Errorf("CSP must not contain %q: %q", directive, csp)
+		}
+	}
+
+	required := []string{
+		"default-src 'self'",
+		"script-src 'none'",
+		"style-src 'self'",
+		"frame-ancestors 'none'",
+	}
+	for _, directive := range required {
+		if !strings.Contains(csp, directive) {
+			t.Errorf("CSP must contain %q: %q", directive, csp)
+		}
+	}
+}
+
+// TestSecurityHeadersOnErrorResponse verifies the headers are emitted
+// even when the wrapped handler fails, since they are set before the
+// handler runs.
+func TestSecurityHeadersOnErrorResponse(t *testing.T) {
+	t.Parallel()
+
+	failing := http.HandlerFunc(func(
+		writer http.ResponseWriter,
+		_ *http.Request,
+	) {
+		http.Error(
+			writer,
+			"boom",
+			http.StatusInternalServerError,
+		)
+	})
+
+	rec := serveWithSecurityHeaders(t, "/api/v1/status", failing)
+
+	if rec.Code != http.StatusInternalServerError {
+		t.Fatalf("status = %d, want 500", rec.Code)
+	}
+
+	if got := rec.Header().Get(
+		"X-Content-Type-Options",
+	); got != wantContentTypeOptions {
+		t.Errorf(
+			"X-Content-Type-Options = %q, want %q",
+			got, wantContentTypeOptions,
+		)
+	}
+
+	if got := rec.Header().Get(
+		"Strict-Transport-Security",
+	); got != wantHSTS {
+		t.Errorf(
+			"Strict-Transport-Security = %q, want %q",
+			got, wantHSTS,
+		)
+	}
+}
+
+// newTestHandlers builds real Handlers with empty monitoring state.
+func newTestHandlers(t *testing.T) *handlers.Handlers {
+	t.Helper()
+
+	glob, err := globals.New(nil)
+	if err != nil {
+		t.Fatalf("globals.New: %v", err)
+	}
+
+	log := newTestLogger(t)
+
+	notifier, err := notify.New(nil, notify.Params{
+		Logger: log,
+		Config: &config.Config{},
+	})
+	if err != nil {
+		t.Fatalf("notify.New: %v", err)
+	}
+
+	hnd, err := handlers.New(nil, handlers.Params{
+		Logger:  log,
+		Globals: glob,
+		State:   state.NewForTest(),
+		Notify:  notifier,
+	})
+	if err != nil {
+		t.Fatalf("handlers.New: %v", err)
+	}
+
+	return hnd
+}
+
+// TestDashboardRendersWithSecurityHeaders renders the real dashboard
+// through the middleware and checks that the policy still permits the
+// one stylesheet the page loads.
+func TestDashboardRendersWithSecurityHeaders(t *testing.T) {
+	t.Parallel()
+
+	mw := newTestMiddleware(t)
+	hnd := newTestHandlers(t)
+
+	router := chi.NewRouter()
+	router.Use(mw.SecurityHeaders())
+	router.Get("/", hnd.HandleDashboard())
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/", nil,
+	)
+
+	router.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rec.Code)
+	}
+
+	body := rec.Body.String()
+	if !strings.Contains(body, stylesheetPath) {
+		t.Errorf("dashboard does not reference %q", stylesheetPath)
+	}
+
+	if !strings.Contains(body, "dnswatcher") {
+		t.Errorf("dashboard body looks empty: %d bytes", len(body))
+	}
+
+	csp := rec.Header().Get("Content-Security-Policy")
+	if csp != wantCSP {
+		t.Errorf("CSP = %q, want %q", csp, wantCSP)
+	}
+
+	// The stylesheet is same-origin, so style-src 'self' allows it.
+	if !strings.Contains(csp, "style-src 'self'") {
+		t.Errorf("CSP would block %q: %q", stylesheetPath, csp)
+	}
+}

internal/server/routes.go

@@ -21,6 +21,7 @@ func (s *Server) SetupRoutes() {
 	// Global middleware
 	s.router.Use(chimw.Recoverer)
 	s.router.Use(chimw.RequestID)
+	s.router.Use(s.mw.SecurityHeaders())
 	s.router.Use(s.mw.Logging())
 	s.router.Use(s.mw.CORS())
 	s.router.Use(chimw.Timeout(requestTimeout))

script/bootstrap

@@ -4,14 +4,14 @@
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
 # or apk (detected in that order); assumes nothing is present.
 # golangci-lint and goimports are installed via `go install` at the same
-# pinned refs the Dockerfile uses (never "latest").
+# pinned commits the Dockerfile uses (never "latest").
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 # Pinned versions, 2026-08-07 (same pins as the Dockerfile)
 # golangci-lint v2.12.2
-GOLANGCI_LINT_REF="github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.12.2"
+GOLANGCI_LINT_REF="github.com/golangci/golangci-lint/v2/cmd/golangci-lint@c0d3ddc9cf3faa61a4e378e879ece580256d76e5"
 # goimports v0.42.0
 GOIMPORTS_REF="golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0"