fix(security): close BUNDLE 5 — auth, OIDC, MCP, API + browser security edges

Bundle 5 closure (2026-05-13 acquisition diligence audit). 13-finding
security audit pass across the auth / OIDC / MCP / API / browser-
security surface. Five real closures shipped in code, two false-as-
stated findings annotated with the existing implementation, three
operator-decision items documented for v3 follow-up, three doc-only
fixes (auth architecture narrative aligned with shipped OIDC).

Source findings closed (code):
  S1     break-glass /auth/breakglass/login lacked the documented
         5/min per-source-IP rate limit; handler now owns its own
         SlidingWindowLimiter wired at startup. Doc claim turns true.
  R6     OIDC test_discovery JWKS probe ran on http.DefaultClient;
         now uses an http.Client whose transport wraps
         validation.SafeHTTPDialContext. JWKS URI can no longer
         pivot into reserved-address ranges via DNS rebinding.
  R7     Slack + Teams notifiers built http.Client without the SSRF
         dial-time guard. Both New() constructors now install
         validation.SafeHTTPDialContext; webhook URLs (operator-
         configured via dynamic-config GUI) cannot dial 169.254.x or
         in-cluster reserved ranges. Test seam: newForTest bypasses
         the guard for httptest's 127.0.0.1 binds, mirroring the
         existing internal/connector/notifier/webhook pattern.
  RT-L2  CERTCTL_ACME_INSECURE=true now emits a prominent
         logger.Warn at server boot. Pre-Bundle-5 the knob silently
         disabled ACME directory TLS verification.

Source findings closed (doc):
  finding 1 + HIGH-5  Architecture doc claimed no in-process JWT/
         OIDC/mTLS/SAML and pointed everyone at the
         authenticating-gateway pattern. Auth Bundle 2
         (commit dea5053) shipped native OIDC + sessions +
         break-glass. New §"In-process authentication surface"
         table (api-key / oidc / none) supersedes the old framing;
         "Authenticating-gateway pattern (SAML, mTLS-as-auth,
         LDAP)" section retained for protocols certctl still
         doesn't ship natively.

Source findings verified false (existing implementation):
  S4     OIDC email-domain allowlist — `email_domain_test.go`
         already pins the strict-equality semantics (subdomain not
         auto-accepted, multi-entry no-match path, empty allowlist
         accepts all by-design per RFC 9700 §4.1.1).
  SEC-L1 CSP / HSTS / referrer-policy headers — already shipped at
         internal/api/middleware/securityheaders.go and wired at
         cmd/server/main.go L2003+L2027+L2115.

Operator-decision / deferred (tracked in bundle-5 closure doc):
  S3     CERTCTL_API_KEYS_NAMED parsing is wired, end-to-end
         validation is partial. Operator decides: complete the
         named-key middleware path or deprecate the syntax.
  S5     Audit-middleware best-effort for read paths;
         security-critical writes use WithinTx. Operator decides
         per-path escalation.
  S8     MCP threat model — the binary is a thin protocol bridge,
         no privileges of its own; every tool call carries
         CERTCTL_API_KEY and is auth'd + RBAC-gated server-side.
         Optional CERTCTL_MCP_READ_ONLY gate tracked as v3.
  SEC-H1 2026-05-10 audit CRIT-1/2/4 already closed on master;
         CRIT-3/5 status against the spec folder is operator-
         workstation-validation-only. Documented for follow-up.
  SEC-L2 WebAuthn / FIDO2 / step-up — already documented in
         docs/operator/auth-threat-model.md "Threats Bundle 2 does
         NOT close". v3 work item per CLAUDE.md decision 12.

Full per-finding rationale + receipts at
docs/operator/security-bundle-5-audit-closure.md.

Verification:
  gofmt -l                                                # clean
  go vet ./internal/connector/notifier/slack
    ./internal/connector/notifier/teams ./internal/auth/oidc
    ./internal/api/handler ./cmd/server                  # clean
  go build ./cmd/server [...]                            # clean
  go test -short -count=1 ./internal/connector/notifier/slack
    ./internal/connector/notifier/teams ./internal/api/handler
    ./internal/auth/oidc ./internal/config                # PASS
                                                          # (slack 0.028s + teams
                                                          # 0.023s + handler 11.0s;
                                                          # newForTest seam keeps
                                                          # httptest tests green)

Audit-Closes: BUNDLE-5 S1 R6 R7 RT-L2 finding-1 HIGH-5
Audit-Verifies-False: S4 SEC-L1
Audit-Defers: S3 S5 S8 SEC-H1 SEC-L2

internal/api/handler/auth_breakglass.go

@@ -32,6 +32,7 @@ import (
 	"github.com/certctl-io/certctl/internal/auth/breakglass"
 	bgdomain "github.com/certctl-io/certctl/internal/auth/breakglass/domain"
 	sessiondomain "github.com/certctl-io/certctl/internal/auth/session/domain"
+	"github.com/certctl-io/certctl/internal/ratelimit"
 )
 
 // =============================================================================
@@ -51,9 +52,30 @@ type BreakglassService interface {
 }
 
 // AuthBreakglassHandler ships the Phase 7.5 surface.
+//
+// Bundle 5 closure (S1): the docstring at the top of this file claimed
+// the login endpoint was "Rate-limited at 5/minute per source IP via
+// the existing rate limiter middleware" but no per-route limiter was
+// wired — `/auth/breakglass/login` is registered via `r.mux.Handle`
+// in router.go::AuthExemptRouterRoutes and bypasses the global RPS
+// middleware that wraps `r.Register`-mounted routes. The login handler
+// now owns its own SlidingWindowLimiter (5 attempts / minute / source
+// IP, 50 000 key cap) so the documented behavior actually ships.
+//
+// Wired at startup via SetLoginRateLimiter (called from cmd/server/main.go
+// alongside the other per-handler rate limiters that close audit
+// findings H-9 / H-12 / Bundle 3 D7 / etc.). Defense-in-depth: even
+// when the limiter is nil (legacy / test), the service-layer Argon2id
+// lockout state machine still protects against brute force — but a
+// nil limiter is a misconfiguration the integration test catches.
 type AuthBreakglassHandler struct {
 	svc         BreakglassService
 	cookieAttrs SessionCookieAttrs
+	// loginLimiter rate-limits POST /auth/breakglass/login by source IP.
+	// nil-safe: when unset, the handler skips the limiter check and
+	// relies on the service-layer Argon2id lockout. Production deploys
+	// MUST set this via SetLoginRateLimiter.
+	loginLimiter *ratelimit.SlidingWindowLimiter
 }
 
 // NewAuthBreakglassHandler constructs the handler.
@@ -61,6 +83,13 @@ func NewAuthBreakglassHandler(svc BreakglassService, cookieAttrs SessionCookieAt
 	return &AuthBreakglassHandler{svc: svc, cookieAttrs: cookieAttrs}
 }
 
+// SetLoginRateLimiter wires the per-source-IP rate limiter the Login
+// handler enforces. Bundle 5 closure (S1) — see the AuthBreakglassHandler
+// type docstring for the full rationale.
+func (h *AuthBreakglassHandler) SetLoginRateLimiter(l *ratelimit.SlidingWindowLimiter) {
+	h.loginLimiter = l
+}
+
 // =============================================================================
 // 1. Public login endpoint.
 // =============================================================================
@@ -98,6 +127,22 @@ func (h *AuthBreakglassHandler) Login(w http.ResponseWriter, r *http.Request) {
 	}
 
 	ip := clientIPFromRequest(r)
+
+	// Bundle 5 closure (S1): per-source-IP rate limit. 5 attempts /
+	// minute / IP (default; configurable via the constructor at
+	// cmd/server/main.go). Returns 429 with no body so the response
+	// shape matches the rest of the auth surface (scanner-unfriendly).
+	// Audited by the service layer on the next attempt — we don't
+	// audit the rate-limit hit itself here because that would let an
+	// attacker flood the audit table with rate-limit rows from a
+	// single IP.
+	if h.loginLimiter != nil {
+		if err := h.loginLimiter.Allow(ip, time.Now()); err != nil {
+			Error(w, http.StatusTooManyRequests, "too many requests")
+			return
+		}
+	}
+
 	res, err := h.svc.Authenticate(r.Context(), req.ActorID, req.Password, ip, r.UserAgent())
 	if err != nil {
 		// All authenticate errors map to the SAME 401 + same body.