Cap request body size and add baseline security headers

Part of a security-hardening pass (see wiki for the full backlog). decodeJSON had no size limit at all, so every JSON endpoint -- including the two unauthenticated ones (bootstrap, the Alertmanager webhook) -- would buffer an attacker-supplied body of unbounded size before it was even validated. decodeJSON now wraps the body in http.MaxBytesReader at a 1 MiB default; the webhook gets its own 8 MiB cap via decodeJSONLimit, since a real Alertmanager batch can be bigger than an ordinary API body. Also adds a securityHeaders middleware, applied globally: nosniff on every response (previously only the static site got it), and HSTS (180-day max-age, conservative on purpose) whenever cookieSecure's signal says the browser is on HTTPS. Checked the chart/gateway config first -- neither sets HSTS anywhere, so this was a real gap. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
2026-10-07 21:57:00 +02:00
parent df83adfe47
commit 7cd6fbf571
5 changed files with 162 additions and 1 deletions
@@ -94,10 +94,16 @@ func handleIntegrationWebhook(db *sql.DB, notify NotifyConfig) http.HandlerFunc
 	}
 }

+// maxWebhookBodyBytes is larger than maxBodyBytes: a real Alertmanager batch
+// can carry many alerts, each with several labels and annotations, and the
+// sender is a trusted piece of infrastructure rather than an arbitrary
+// caller.
+const maxWebhookBodyBytes = 8 << 20
+
 func receiveWebhook(w http.ResponseWriter, r *http.Request, db *sql.DB, notify NotifyConfig, src alertSource) {
 	teamID := src.teamID
 	var payload amPayload
-	if err := decodeJSON(r, &payload); err != nil {
+	if err := decodeJSONLimit(r, &payload, maxWebhookBodyBytes); err != nil {
 		respond(w, http.StatusBadRequest, errResp("invalid payload"))
 		return
 	}
@@ -72,8 +72,25 @@ func respond(w http.ResponseWriter, status int, v any) {
 	json.NewEncoder(w).Encode(v)
 }

+// maxBodyBytes caps an ordinary JSON request body. 1 MiB is far more than any
+// endpoint below needs — it exists so an unauthenticated caller (signup,
+// login, bootstrap) can't make the server buffer an arbitrarily large body
+// before the request is even validated.
+const maxBodyBytes = 1 << 20
+
 func decodeJSON(r *http.Request, v any) error {
+	return decodeJSONLimit(r, v, maxBodyBytes)
+}
+
+// decodeJSONLimit is decodeJSON with an explicit cap, for the one endpoint
+// (the Alertmanager webhook, see maxWebhookBodyBytes) whose real payloads can
+// legitimately be larger than maxBodyBytes.
+func decodeJSONLimit(r *http.Request, v any, limit int64) error {
 	defer r.Body.Close()
+	// w is nil: there is no ResponseWriter here to disable keep-alive with,
+	// which net/http documents as fine — the limit is still enforced, the
+	// connection just isn't closed early on a request that blows past it.
+	r.Body = http.MaxBytesReader(nil, r.Body, limit)
 	return json.NewDecoder(r.Body).Decode(v)
 }

@@ -79,6 +79,28 @@ func AuthMiddleware(db *sql.DB) func(http.Handler) http.Handler {
 	}
 }

+// securityHeaders sets headers that cost nothing to send on every response,
+// API or static site alike. nosniff is unconditional; HSTS only fires once
+// cookieSecure's signal says the browser is actually looking at this server
+// over HTTPS — TLS terminates at the gateway, which (as of this writing) sets
+// neither header itself.
+//
+// max-age is 180 days rather than the usual year-plus: short enough that if
+// HTTPS here ever broke for real, the header would age out of a browser's
+// cache well within a release cycle instead of locking anyone out of a
+// working server. Raise it once this has run clean for a while.
+func securityHeaders(publicURL string) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Header().Set("X-Content-Type-Options", "nosniff")
+			if cookieSecure(publicURL, r) {
+				w.Header().Set("Strict-Transport-Security", "max-age=15552000; includeSubDomains")
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}
+
 // AdminOnly rejects a caller who is not a system administrator. It runs inside
 // AuthMiddleware's group, so by the time it sees a request the caller is known.
 //
@@ -30,6 +30,7 @@ func NewRouter(db *sql.DB, notify NotifyConfig, cfg config.Config, version strin
 	r := chi.NewRouter()
 	r.Use(middleware.Logger)
 	r.Use(middleware.Recoverer)
+	r.Use(securityHeaders(notify.PublicURL))

 	r.Get("/healthz", func(w http.ResponseWriter, r *http.Request) {
 		respond(w, http.StatusOK, map[string]string{"status": "ok"})
@@ -0,0 +1,115 @@
+package api_test
+
+import (
+	"bytes"
+	"net/http"
+	"strings"
+	"testing"
+
+	"git.ryuvia.com/niklas/terdut-server/internal/api"
+)
+
+// ---------------------------------------------------------------------------
+// Security headers
+// ---------------------------------------------------------------------------
+
+func TestSecurityHeaders_NosniffAlwaysSet(t *testing.T) {
+	s := newTS(t) // no PublicURL: the HTTPS signal is off
+	resp := s.req(t, http.MethodGet, "/api/me", nil)
+	defer resp.Body.Close()
+
+	if got := resp.Header.Get("X-Content-Type-Options"); got != "nosniff" {
+		t.Errorf("X-Content-Type-Options = %q, want nosniff", got)
+	}
+	if got := resp.Header.Get("Strict-Transport-Security"); got != "" {
+		t.Errorf("Strict-Transport-Security = %q, want unset without an https PublicURL", got)
+	}
+}
+
+func TestSecurityHeaders_HSTSWhenPublicURLIsHTTPS(t *testing.T) {
+	s := newTS(t, api.NotifyConfig{PublicURL: "https://terdut.example.com"})
+	resp := s.req(t, http.MethodGet, "/api/me", nil)
+	defer resp.Body.Close()
+
+	got := resp.Header.Get("Strict-Transport-Security")
+	if !strings.HasPrefix(got, "max-age=") || !strings.Contains(got, "includeSubDomains") {
+		t.Errorf("Strict-Transport-Security = %q, want a max-age with includeSubDomains", got)
+	}
+}
+
+// ---------------------------------------------------------------------------
+// Request body size limits
+// ---------------------------------------------------------------------------
+
+// TestBodySizeLimit_OrdinaryEndpointRejectsOversizedBody confirms an
+// unauthenticated endpoint can't be made to buffer an arbitrarily large body:
+// past maxBodyBytes, decodeJSON fails exactly as it would on any other
+// malformed body, rather than the server reading the whole thing first.
+func TestBodySizeLimit_OrdinaryEndpointRejectsOversizedBody(t *testing.T) {
+	s := newTS(t)
+
+	huge := bytes.Repeat([]byte("a"), 2<<20) // 2 MiB, past the 1 MiB default
+	body := []byte(`{"username":"` + string(huge) + `","password":"x"}`)
+
+	resp, err := http.Post(s.URL+"/api/login", "application/json", bytes.NewReader(body))
+	if err != nil {
+		t.Fatalf("POST /api/login: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusBadRequest {
+		t.Errorf("status = %d, want %d (oversized body treated as invalid)", resp.StatusCode, http.StatusBadRequest)
+	}
+}
+
+// TestBodySizeLimit_WebhookAllowsLargerBodyThanDefault confirms the
+// Alertmanager webhook's separate, larger cap actually takes effect: a body
+// bigger than the ordinary default but within maxWebhookBodyBytes is still
+// accepted, not rejected by the smaller limit every other endpoint gets.
+func TestBodySizeLimit_WebhookAllowsLargerBodyThanDefault(t *testing.T) {
+	s := newTS(t)
+
+	// Padding kept inside one alert's annotation, comfortably past the 1 MiB
+	// default and still well under the webhook's 8 MiB cap.
+	padding := strings.Repeat("a", 3<<20) // 3 MiB
+	payload := `{"version":"4","status":"firing","groupKey":"big-group",` +
+		`"groupLabels":{"alertname":"BigAlert"},"alerts":[{"status":"firing",` +
+		`"labels":{"alertname":"BigAlert"},"annotations":{"note":"` + padding + `"},` +
+		`"startsAt":"2026-05-20T10:00:00Z","endsAt":"0001-01-01T00:00:00Z",` +
+		`"fingerprint":"fp-big"}]}`
+
+	resp, err := http.Post(s.URL+"/api/integrations/"+s.ingestKey+"/alertmanager",
+		"application/json", strings.NewReader(payload))
+	if err != nil {
+		t.Fatalf("POST webhook: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Errorf("status = %d, want %d (body under the webhook's own cap)", resp.StatusCode, http.StatusOK)
+	}
+}
+
+// TestBodySizeLimit_WebhookRejectsPastItsOwnCap confirms the webhook's larger
+// cap is still a cap, not an exemption from one.
+func TestBodySizeLimit_WebhookRejectsPastItsOwnCap(t *testing.T) {
+	s := newTS(t)
+
+	huge := strings.Repeat("a", 9<<20) // 9 MiB, past the 8 MiB webhook cap
+	payload := `{"version":"4","status":"firing","groupKey":"huge-group",` +
+		`"groupLabels":{"alertname":"HugeAlert"},"alerts":[{"status":"firing",` +
+		`"labels":{"alertname":"HugeAlert"},"annotations":{"note":"` + huge + `"},` +
+		`"startsAt":"2026-05-20T10:00:00Z","endsAt":"0001-01-01T00:00:00Z",` +
+		`"fingerprint":"fp-huge"}]}`
+
+	resp, err := http.Post(s.URL+"/api/integrations/"+s.ingestKey+"/alertmanager",
+		"application/json", strings.NewReader(payload))
+	if err != nil {
+		t.Fatalf("POST webhook: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusBadRequest {
+		t.Errorf("status = %d, want %d (body past the webhook's own cap)", resp.StatusCode, http.StatusBadRequest)
+	}
+}