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/auth/oidc/test_discovery.go

@@ -9,10 +9,20 @@ import (
 	"context"
 	"fmt"
 	"net/http"
+	"time"
 
 	gooidc "github.com/coreos/go-oidc/v3/oidc"
+
+	"github.com/certctl-io/certctl/internal/validation"
 )
 
+// oidcOutboundTimeout bounds every test-discovery HTTP call (discovery
+// document fetch + JWKS reachability probe + userinfo if configured).
+// Shared by the SSRF-safe transport dialer (Bundle 5 R6 closure) and
+// the http.Client so the dial budget and the read/write budget land
+// on the same wall-clock horizon.
+const oidcOutboundTimeout = 10 * time.Second
+
 // TestDiscoveryResult is the report TestDiscovery returns. The HTTP
 // layer marshals this verbatim. Each field is independently observable
 // so the GUI can render a per-check status row.
@@ -134,12 +144,35 @@ func (s *Service) TestDiscovery(ctx context.Context, issuerURL string) (*TestDis
 // Kept distinct from go-oidc's internal JWKS fetcher because we want
 // to surface the HTTP status to the operator without requiring a
 // token-verify round-trip.
+//
+// Bundle 5 closure (audit R6): the GET runs through an SSRF-safe
+// http.Client whose transport's DialContext is wrapped in
+// validation.SafeHTTPDialContext. Pre-Bundle-5 the discovery probe
+// used http.DefaultClient and could be pointed at reserved-address
+// ranges via DNS rebinding (operator pastes a JWKS URI from the
+// dynamic-config GUI; admin RBAC for OIDC providers is sensitive but
+// not a system-wide super-admin gate). Now the dial-time guard re-
+// resolves the target host and rejects loopback / link-local /
+// private + cloud-metadata before any HTTP byte goes out. The
+// 10-second timeout matches the package-wide oidcOutboundTimeout
+// budget so token endpoint + JWKS + userinfo probes share the same
+// wall-clock horizon.
 var jwksReachable = func(ctx context.Context, jwksURI string) (bool, error) {
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, jwksURI, nil)
 	if err != nil {
 		return false, err
 	}
-	resp, err := http.DefaultClient.Do(req)
+	client := &http.Client{
+		Timeout: oidcOutboundTimeout,
+		Transport: &http.Transport{
+			DialContext:           validation.SafeHTTPDialContext(oidcOutboundTimeout),
+			MaxIdleConns:          10,
+			IdleConnTimeout:       90 * time.Second,
+			TLSHandshakeTimeout:   10 * time.Second,
+			ExpectContinueTimeout: 1 * time.Second,
+		},
+	}
+	resp, err := client.Do(req)
 	if err != nil {
 		return false, err
 	}