render: do not leak JSON/XML encode errors

responder.go

@@ -95,7 +95,7 @@ func JSON(w http.ResponseWriter, r *http.Request, v interface{}) {
 	enc := json.NewEncoder(buf)
 	enc.SetEscapeHTML(true)
 	if err := enc.Encode(v); err != nil {
-		http.Error(w, err.Error(), http.StatusInternalServerError)
+		http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
 		return
 	}
 
@@ -112,7 +112,7 @@ func JSON(w http.ResponseWriter, r *http.Request, v interface{}) {
 func XML(w http.ResponseWriter, r *http.Request, v interface{}) {
 	b, err := xml.Marshal(v)
 	if err != nil {
-		http.Error(w, err.Error(), http.StatusInternalServerError)
+		http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
 		return
 	}
 
@@ -184,7 +184,7 @@ func channelEventStream(w http.ResponseWriter, r *http.Request, v interface{}) {
 
 			bytes, err := json.Marshal(v)
 			if err != nil {
-				w.Write([]byte(fmt.Sprintf("event: error\ndata: {\"error\":\"%v\"}\n\n", err))) //nolint:errcheck
+				w.Write([]byte("event: error\ndata: {\"error\":\"Internal Server Error\"}\n\n")) //nolint:errcheck
 				if f, ok := w.(http.Flusher); ok {
 					f.Flush()
 				}

responder_test.go

@@ -0,0 +1,43 @@
+package render
+
+import (
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+type leakyJSON struct{}
+
+func (leakyJSON) MarshalJSON() ([]byte, error) {
+	return nil, errors.New("secret token xyz")
+}
+
+func TestJSONDoesNotLeakEncodeError(t *testing.T) {
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/", nil)
+	JSON(rec, req, leakyJSON{})
+	if rec.Code != http.StatusInternalServerError {
+		t.Fatalf("status = %d, want 500", rec.Code)
+	}
+	body := rec.Body.String()
+	if strings.Contains(body, "secret token") {
+		t.Fatalf("leaked encode error: %q", body)
+	}
+	if !strings.Contains(body, http.StatusText(http.StatusInternalServerError)) {
+		t.Fatalf("body = %q, want Internal Server Error", body)
+	}
+}
+
+func TestJSONOK(t *testing.T) {
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/", nil)
+	JSON(rec, req, M{"ok": true})
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d", rec.Code)
+	}
+	if got := rec.Body.String(); !strings.Contains(got, `"ok":true`) {
+		t.Fatalf("body = %q", got)
+	}
+}