feat(security): bodyLimit on noAuth + security headers + encryption-key validation (H-1 master)

Closes three 2026-04-24 audit findings (all P2):
  - cat-s5-4936a1cf0118: noAuthHandler chain accepted arbitrary-size
    bodies (EST simpleenroll, SCEP, PKI CRL/OCSP, /health, /ready).
    Memory exhaustion vector without HTTP-layer auth gatekeeping.
  - cat-s11-missing_security_headers: zero security headers on any
    response. Clickjacking, MIME-sniffing, untrusted-origin resource
    loads against the dashboard and API.
  - cat-r-encryption_key_no_length_validation: CERTCTL_CONFIG_ENCRYPTION_KEY
    accepted with any non-empty value including a single character.
    PBKDF2-SHA256 (100k rounds) does not compensate for low-entropy
    passphrases at scale (CWE-916, CWE-329).

Changes:
- cmd/server/main.go::noAuthHandler chain — added bodyLimitMiddleware
  + securityHeadersMiddleware. Same default cap as authed surface
  (1MB via CERTCTL_MAX_BODY_SIZE), same 413 on overflow.
- cmd/server/main.go::middlewareStack (authed) — added
  securityHeadersMiddleware before corsMiddleware.
- internal/api/middleware/securityheaders.go (new) — SecurityHeaders
  middleware + SecurityHeadersDefaults() with conservative defaults:
  HSTS 1y+includeSubDomains, X-Frame-Options DENY, X-Content-Type-
  Options nosniff, Referrer-Policy no-referrer-when-downgrade, CSP
  default-src 'self' + img/data + style 'unsafe-inline' (Tailwind/Vite
  needs it; scripts still 'self' only) + connect 'self' + frame-
  ancestors 'none'. Operators behind a customising reverse proxy can
  disable any header by setting its config field to empty.
- internal/config/config.go::Validate() — enforce minEncryptionKeyLength
  = 32 bytes when CERTCTL_CONFIG_ENCRYPTION_KEY is set. Empty stays
  accepted (downstream fail-closed sentinel handles it). Structured
  error names the env var, the actual length, the required minimum,
  and the canonical generation command (`openssl rand -base64 32`).

Tests:
- internal/api/middleware/securityheaders_test.go (new) — 4 cases
  (defaults present, empty value disables single header, override
  applied, headers on 4xx/5xx).
- internal/config/config_test.go — 5 new cases for the encryption-key
  length check (empty accepted, 1-byte rejected, 31-byte rejected at
  boundary, 32-byte accepted, 44-byte realistic operator key accepted).

Documentation:
- CHANGELOG.md — H-1 section above D-2 under [unreleased] with
  Breaking-change callout (operators with low-entropy keys must rotate
  before upgrade).
- coverage-gap-audit-2026-04-24-v5/unified-audit.md — Live Tracker
  25/47 → 33/47, P1 14/14 (zero remaining), P2 11/27 → 16/27. Three
  H-1 findings flipped + closed-bundle row added.

Verification:
- go build ./... — clean
- go vet ./... — clean
- golangci-lint v2.11.4 run ./... — 0 issues
- go test ./internal/api/middleware/... — pass (incl. 4 new
  SecurityHeaders cases)
- go test ./internal/config/... — pass (incl. 5 new EncryptionKey
  cases)
- tsc --noEmit (frontend) — clean
- All sibling guardrails (S-1 / G-3 / D-1 / D-2 / B-1 / L-1) still pass

Audit findings closed:
- cat-s5-4936a1cf0118 (P2)
- cat-s11-missing_security_headers (P2)
- cat-r-encryption_key_no_length_validation (P2)

Breaking change:
- Operators with CERTCTL_CONFIG_ENCRYPTION_KEY shorter than 32 bytes
  must rotate before upgrade. Generate via `openssl rand -base64 32`.

Deferred follow-ups:
- Weak-key dictionary check (reject password123, common ASCII patterns)
  — adds operational friction with low marginal entropy gain at the
  32-byte minimum.
- CSP 'unsafe-inline' for styles — required for Tailwind/Vite
  per-component <style> blocks; removing requires HTML report or
  component refactor outside H-1 scope.
- Permissions-Policy header — dashboard uses no advanced browser APIs
  (camera, mic, geolocation); deferred until a real consumer needs it.

internal/api/middleware/securityheaders.go

@@ -0,0 +1,96 @@
+package middleware
+
+import (
+	"net/http"
+	"strings"
+)
+
+// SecurityHeadersConfig configures the SecurityHeaders middleware.
+//
+// Each field is the literal value to send. An empty string means
+// "do not send this header" — operators behind a customising reverse
+// proxy can disable any header per-deployment without touching code.
+// Defaults are applied via SecurityHeadersDefaults() which encodes
+// the H-1 closure's recommended baseline for an HTTPS-only API+UI
+// host: HSTS, deny-frame, no-MIME-sniff, conservative CSP, and a
+// no-referrer-when-downgrade fallback.
+//
+// H-1 closure (cat-s11-missing_security_headers).
+type SecurityHeadersConfig struct {
+	HSTS                  string // Strict-Transport-Security
+	FrameOptions          string // X-Frame-Options
+	ContentTypeOptions    string // X-Content-Type-Options
+	ReferrerPolicy        string // Referrer-Policy
+	ContentSecurityPolicy string // Content-Security-Policy
+}
+
+// SecurityHeadersDefaults returns a recommended baseline.
+//
+// CSP: default-src 'self' confines fetches to the same origin.
+// img-src 'self' data: allows inline base64 images (used by the
+// dashboard's certctl-logo and a few status icons).
+// style-src 'self' 'unsafe-inline' is required because Tailwind
+// (via Vite) injects per-component <style> blocks at build time;
+// without 'unsafe-inline' the dashboard would render unstyled.
+// 'unsafe-inline' is intentionally NOT in script-src — the
+// front-end ships as a bundled JS file, no inline scripts.
+//
+// HSTS: 1-year max-age + includeSubDomains. No `preload` directive
+// because preload submission requires explicit operator action and
+// the deployment topology may not span all subdomains.
+//
+// X-Frame-Options: DENY — the dashboard does not need to be embedded
+// anywhere, and DENY is more conservative than SAMEORIGIN against
+// clickjacking via subdomain takeover.
+//
+// X-Content-Type-Options: nosniff — prevent MIME sniffing on
+// JSON/PEM responses that browsers might otherwise interpret as HTML.
+//
+// Referrer-Policy: no-referrer-when-downgrade — preserves Referer
+// for same-origin navigation (useful for support/diagnostics) but
+// strips it on HTTPS→HTTP transitions.
+func SecurityHeadersDefaults() SecurityHeadersConfig {
+	return SecurityHeadersConfig{
+		HSTS:                  "max-age=31536000; includeSubDomains",
+		FrameOptions:          "DENY",
+		ContentTypeOptions:    "nosniff",
+		ReferrerPolicy:        "no-referrer-when-downgrade",
+		ContentSecurityPolicy: "default-src 'self'; img-src 'self' data:; style-src 'self' 'unsafe-inline'; script-src 'self'; connect-src 'self'; frame-ancestors 'none'",
+	}
+}
+
+// SecurityHeaders returns a middleware that applies the configured
+// HTTP response headers on every response. Headers configured to the
+// empty string are omitted (operator opted out for that deployment).
+//
+// Apply BEFORE the audit middleware so headers reach 4xx/5xx responses
+// — which is where header omissions matter most for the security
+// posture (an attacker probing for misconfiguration sees the same
+// headers on a 401 as on a 200).
+func SecurityHeaders(cfg SecurityHeadersConfig) func(http.Handler) http.Handler {
+	// Pre-trim each value once; the per-request hot path stays a
+	// straight set of map writes.
+	type headerEntry struct{ name, value string }
+	entries := make([]headerEntry, 0, 5)
+	add := func(name, value string) {
+		v := strings.TrimSpace(value)
+		if v != "" {
+			entries = append(entries, headerEntry{name, v})
+		}
+	}
+	add("Strict-Transport-Security", cfg.HSTS)
+	add("X-Frame-Options", cfg.FrameOptions)
+	add("X-Content-Type-Options", cfg.ContentTypeOptions)
+	add("Referrer-Policy", cfg.ReferrerPolicy)
+	add("Content-Security-Policy", cfg.ContentSecurityPolicy)
+
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			h := w.Header()
+			for _, e := range entries {
+				h.Set(e.name, e.value)
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}

internal/api/middleware/securityheaders_test.go

@@ -0,0 +1,104 @@
+package middleware
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// TestSecurityHeaders_DefaultsAllPresent asserts every default header
+// arrives on a 200 response. H-1 closure (cat-s11-missing_security_headers).
+func TestSecurityHeaders_DefaultsAllPresent(t *testing.T) {
+	mw := SecurityHeaders(SecurityHeadersDefaults())
+	handler := mw(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte("ok"))
+	}))
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/test", nil)
+	handler.ServeHTTP(rec, req)
+
+	for _, h := range []string{
+		"Strict-Transport-Security",
+		"X-Frame-Options",
+		"X-Content-Type-Options",
+		"Referrer-Policy",
+		"Content-Security-Policy",
+	} {
+		if got := rec.Header().Get(h); got == "" {
+			t.Errorf("expected header %q to be set, got empty", h)
+		}
+	}
+	if got := rec.Header().Get("X-Content-Type-Options"); got != "nosniff" {
+		t.Errorf("X-Content-Type-Options: got %q, want %q", got, "nosniff")
+	}
+	if got := rec.Header().Get("X-Frame-Options"); got != "DENY" {
+		t.Errorf("X-Frame-Options: got %q, want %q", got, "DENY")
+	}
+}
+
+// TestSecurityHeaders_EmptyValueDisablesHeader asserts an operator can
+// disable a single header by setting its config field to empty without
+// affecting the others.
+func TestSecurityHeaders_EmptyValueDisablesHeader(t *testing.T) {
+	cfg := SecurityHeadersDefaults()
+	cfg.HSTS = "" // simulate operator override
+	mw := SecurityHeaders(cfg)
+	handler := mw(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/", nil))
+
+	if got := rec.Header().Get("Strict-Transport-Security"); got != "" {
+		t.Errorf("HSTS should be omitted when config value is empty; got %q", got)
+	}
+	// Other headers still present
+	if got := rec.Header().Get("X-Frame-Options"); got == "" {
+		t.Errorf("X-Frame-Options should still be present (empty HSTS only disables HSTS)")
+	}
+}
+
+// TestSecurityHeaders_OverrideValueApplied asserts a non-default value
+// makes it through.
+func TestSecurityHeaders_OverrideValueApplied(t *testing.T) {
+	cfg := SecurityHeadersDefaults()
+	cfg.FrameOptions = "SAMEORIGIN"
+	mw := SecurityHeaders(cfg)
+	handler := mw(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/", nil))
+
+	if got := rec.Header().Get("X-Frame-Options"); got != "SAMEORIGIN" {
+		t.Errorf("X-Frame-Options: got %q, want %q", got, "SAMEORIGIN")
+	}
+}
+
+// TestSecurityHeaders_AppliedOnErrorResponses asserts headers are
+// present on 4xx/5xx as well as 2xx — this is critical for the
+// security posture (an attacker probing for misconfiguration sees
+// the same headers on a 401 as on a 200).
+func TestSecurityHeaders_AppliedOnErrorResponses(t *testing.T) {
+	mw := SecurityHeaders(SecurityHeadersDefaults())
+	handler := mw(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+	}))
+
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/", nil))
+
+	if rec.Code != http.StatusUnauthorized {
+		t.Fatalf("status: got %d, want %d", rec.Code, http.StatusUnauthorized)
+	}
+	if got := rec.Header().Get("Strict-Transport-Security"); got == "" {
+		t.Errorf("HSTS missing on 401 response (must be on every response)")
+	}
+	if got := rec.Header().Get("Content-Security-Policy"); got == "" {
+		t.Errorf("CSP missing on 401 response")
+	}
+}