fix: harden payment resume and wxpay webhook routing

backend/internal/handler/payment_webhook_handler.go

 package handler
 
 import (
+	"context"
+	"fmt"
 	"io"
 	"log/slog"
 	"net/http"
@@ -77,9 +79,13 @@ func (h *PaymentWebhookHandler) handleNotify(c *gin.Context, providerKey string)
 	// This is needed when multiple instances of the same provider exist (e.g. multiple EasyPay accounts).
 	outTradeNo := extractOutTradeNo(rawBody, providerKey)
 
-	provider, err := h.paymentService.GetWebhookProvider(c.Request.Context(), providerKey, outTradeNo)
+	providers, err := h.paymentService.GetWebhookProviders(c.Request.Context(), providerKey, outTradeNo)
 	if err != nil {
 		slog.Warn("[Payment Webhook] provider not found", "provider", providerKey, "outTradeNo", outTradeNo, "error", err)
+		if providerKey == payment.TypeWxpay {
+			c.String(http.StatusBadRequest, "verify failed")
+			return
+		}
 		writeSuccessResponse(c, providerKey)
 		return
 	}
@@ -89,7 +95,7 @@ func (h *PaymentWebhookHandler) handleNotify(c *gin.Context, providerKey string)
 		headers[strings.ToLower(k)] = c.GetHeader(k)
 	}
 
-	notification, err := provider.VerifyNotification(c.Request.Context(), rawBody, headers)
+	resolvedProviderKey, notification, err := verifyNotificationWithProviders(c.Request.Context(), providers, rawBody, headers)
 	if err != nil {
 		truncatedBody := rawBody
 		if len(truncatedBody) > webhookLogTruncateLen {
@@ -103,17 +109,17 @@ func (h *PaymentWebhookHandler) handleNotify(c *gin.Context, providerKey string)
 
 	// nil notification means irrelevant event (e.g. Stripe non-payment event); return success.
 	if notification == nil {
-		writeSuccessResponse(c, providerKey)
+		writeSuccessResponse(c, resolvedProviderKey)
 		return
 	}
 
-	if err := h.paymentService.HandlePaymentNotification(c.Request.Context(), notification, providerKey); err != nil {
-		slog.Error("[Payment Webhook] handle notification failed", "provider", providerKey, "error", err)
+	if err := h.paymentService.HandlePaymentNotification(c.Request.Context(), notification, resolvedProviderKey); err != nil {
+		slog.Error("[Payment Webhook] handle notification failed", "provider", resolvedProviderKey, "error", err)
 		c.String(http.StatusInternalServerError, "handle failed")
 		return
 	}
 
-	writeSuccessResponse(c, providerKey)
+	writeSuccessResponse(c, resolvedProviderKey)
 }
 
 // extractOutTradeNo parses the webhook body to find the out_trade_no.
@@ -131,6 +137,25 @@ func extractOutTradeNo(rawBody, providerKey string) string {
 	return ""
 }
 
+func verifyNotificationWithProviders(ctx context.Context, providers []payment.Provider, rawBody string, headers map[string]string) (string, *payment.PaymentNotification, error) {
+	var lastErr error
+	for _, provider := range providers {
+		if provider == nil {
+			continue
+		}
+		notification, err := provider.VerifyNotification(ctx, rawBody, headers)
+		if err != nil {
+			lastErr = err
+			continue
+		}
+		return provider.ProviderKey(), notification, nil
+	}
+	if lastErr != nil {
+		return "", nil, lastErr
+	}
+	return "", nil, fmt.Errorf("no webhook provider could verify notification")
+}
+
 // wxpaySuccessResponse is the JSON response expected by WeChat Pay webhook.
 type wxpaySuccessResponse struct {
 	Code    string `json:"code"`

backend/internal/handler/payment_webhook_handler_test.go

@@ -3,11 +3,14 @@
 package handler
 
 import (
+	"context"
 	"encoding/json"
+	"errors"
 	"net/http"
 	"net/http/httptest"
 	"testing"
 
+	"github.com/Wei-Shaw/sub2api/internal/payment"
 	"github.com/gin-gonic/gin"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -131,3 +134,70 @@ func TestExtractOutTradeNo(t *testing.T) {
 		})
 	}
 }
+
+func TestVerifyNotificationWithProvidersReturnsMatchedProvider(t *testing.T) {
+	firstErr := errors.New("wrong provider")
+	providers := []payment.Provider{
+		webhookHandlerProviderStub{
+			key:       payment.TypeWxpay,
+			verifyErr: firstErr,
+		},
+		webhookHandlerProviderStub{
+			key: payment.TypeWxpay,
+			notification: &payment.PaymentNotification{
+				OrderID: "sub2_42",
+				TradeNo: "trade-42",
+				Status:  payment.NotificationStatusSuccess,
+			},
+		},
+	}
+
+	providerKey, notification, err := verifyNotificationWithProviders(context.Background(), providers, "{}", map[string]string{"wechatpay-signature": "sig"})
+	require.NoError(t, err)
+	require.Equal(t, payment.TypeWxpay, providerKey)
+	require.NotNil(t, notification)
+	require.Equal(t, "sub2_42", notification.OrderID)
+}
+
+func TestVerifyNotificationWithProvidersFailsWhenAllProvidersReject(t *testing.T) {
+	providers := []payment.Provider{
+		webhookHandlerProviderStub{
+			key:       payment.TypeWxpay,
+			verifyErr: errors.New("verify failed a"),
+		},
+		webhookHandlerProviderStub{
+			key:       payment.TypeWxpay,
+			verifyErr: errors.New("verify failed b"),
+		},
+	}
+
+	_, _, err := verifyNotificationWithProviders(context.Background(), providers, "{}", nil)
+	require.Error(t, err)
+}
+
+type webhookHandlerProviderStub struct {
+	key          string
+	notification *payment.PaymentNotification
+	verifyErr    error
+}
+
+func (p webhookHandlerProviderStub) Name() string { return p.key }
+func (p webhookHandlerProviderStub) ProviderKey() string { return p.key }
+func (p webhookHandlerProviderStub) SupportedTypes() []payment.PaymentType {
+	return []payment.PaymentType{payment.PaymentType(p.key)}
+}
+func (p webhookHandlerProviderStub) CreatePayment(context.Context, payment.CreatePaymentRequest) (*payment.CreatePaymentResponse, error) {
+	panic("unexpected call")
+}
+func (p webhookHandlerProviderStub) QueryOrder(context.Context, string) (*payment.QueryOrderResponse, error) {
+	panic("unexpected call")
+}
+func (p webhookHandlerProviderStub) VerifyNotification(context.Context, string, map[string]string) (*payment.PaymentNotification, error) {
+	if p.verifyErr != nil {
+		return nil, p.verifyErr
+	}
+	return p.notification, nil
+}
+func (p webhookHandlerProviderStub) Refund(context.Context, payment.RefundRequest) (*payment.RefundResponse, error) {
+	panic("unexpected call")
+}

backend/internal/payment/load_balancer.go

@@ -45,11 +45,31 @@ type DefaultLoadBalancer struct {
 	counter       atomic.Uint64
 }
 
+type contextKey string
+
+const wxpayJSAPIAppIDContextKey contextKey = "payment.wxpay.jsapi_app_id"
+
 // NewDefaultLoadBalancer creates a new load balancer.
 func NewDefaultLoadBalancer(db *dbent.Client, encryptionKey []byte) *DefaultLoadBalancer {
 	return &DefaultLoadBalancer{db: db, encryptionKey: encryptionKey}
 }
 
+func WithWxpayJSAPIAppID(ctx context.Context, appID string) context.Context {
+	appID = strings.TrimSpace(appID)
+	if appID == "" {
+		return ctx
+	}
+	return context.WithValue(ctx, wxpayJSAPIAppIDContextKey, appID)
+}
+
+func wxpayJSAPIAppIDFromContext(ctx context.Context) string {
+	if ctx == nil {
+		return ""
+	}
+	appID, _ := ctx.Value(wxpayJSAPIAppIDContextKey).(string)
+	return strings.TrimSpace(appID)
+}
+
 // instanceCandidate pairs an instance with its pre-fetched daily usage.
 type instanceCandidate struct {
 	inst      *dbent.PaymentProviderInstance
@@ -116,6 +136,7 @@ func (lb *DefaultLoadBalancer) queryEnabledInstances(
 	}
 
 	var matched []*dbent.PaymentProviderInstance
+	expectedWxpayJSAPIAppID := wxpayJSAPIAppIDFromContext(ctx)
 	for _, inst := range instances {
 		// Stripe: match by provider_key because supported_types lists sub-types (card,link,alipay,wxpay),
 		// not "stripe" itself. The checkout page aggregates all sub-types under "stripe".
@@ -124,6 +145,16 @@ func (lb *DefaultLoadBalancer) queryEnabledInstances(
 				matched = append(matched, inst)
 			}
 		} else if InstanceSupportsType(inst.SupportedTypes, paymentType) {
+			if expectedWxpayJSAPIAppID != "" && normalizeVisibleMethodSupportType(paymentType) == TypeWxpay && inst.ProviderKey == TypeWxpay {
+				config, cfgErr := lb.decryptConfig(inst.Config)
+				if cfgErr != nil {
+					slog.Warn("skip wxpay instance with unreadable config during jsapi filtering", "instance_id", inst.ID, "error", cfgErr)
+					continue
+				}
+				if resolveWxpayJSAPIAppID(config) != expectedWxpayJSAPIAppID {
+					continue
+				}
+			}
 			matched = append(matched, inst)
 		}
 	}
@@ -358,6 +389,13 @@ func legacyVisibleMethodAlias(paymentType PaymentType) PaymentType {
 	}
 }
 
+func resolveWxpayJSAPIAppID(config map[string]string) string {
+	if appID := strings.TrimSpace(config["mpAppId"]); appID != "" {
+		return appID
+	}
+	return strings.TrimSpace(config["appId"])
+}
+
 // GetInstanceConfig decrypts and returns the configuration for a provider instance by ID.
 func (lb *DefaultLoadBalancer) GetInstanceConfig(ctx context.Context, instanceID int64) (map[string]string, error) {
 	inst, err := lb.db.PaymentProviderInstance.Get(ctx, instanceID)

backend/internal/service/payment_order.go

@@ -216,7 +216,11 @@ func (s *PaymentService) checkDailyLimit(ctx context.Context, tx *dbent.Tx, user
 }
 
 func (s *PaymentService) selectCreateOrderInstance(ctx context.Context, req CreateOrderRequest, cfg *PaymentConfig, payAmount float64) (*payment.InstanceSelection, error) {
-	sel, err := s.loadBalancer.SelectInstance(ctx, "", req.PaymentType, payment.Strategy(cfg.LoadBalanceStrategy), payAmount)
+	selectCtx, err := s.prepareCreateOrderSelectionContext(ctx, req)
+	if err != nil {
+		return nil, err
+	}
+	sel, err := s.loadBalancer.SelectInstance(selectCtx, "", req.PaymentType, payment.Strategy(cfg.LoadBalanceStrategy), payAmount)
 	if err != nil {
 		return nil, infraerrors.ServiceUnavailable("PAYMENT_GATEWAY_ERROR", fmt.Sprintf("payment method (%s) is not configured", req.PaymentType))
 	}
@@ -226,6 +230,44 @@ func (s *PaymentService) selectCreateOrderInstance(ctx context.Context, req Crea
 	return sel, nil
 }
 
+func (s *PaymentService) prepareCreateOrderSelectionContext(ctx context.Context, req CreateOrderRequest) (context.Context, error) {
+	if !requestNeedsWeChatJSAPICompatibility(req) {
+		return ctx, nil
+	}
+	if !s.usesOfficialWxpayVisibleMethod(ctx) {
+		return ctx, nil
+	}
+	expectedAppID, _, err := s.getWeChatPaymentOAuthCredential(ctx)
+	if err != nil {
+		return nil, err
+	}
+	return payment.WithWxpayJSAPIAppID(ctx, expectedAppID), nil
+}
+
+func requestNeedsWeChatJSAPICompatibility(req CreateOrderRequest) bool {
+	if payment.GetBasePaymentType(req.PaymentType) != payment.TypeWxpay {
+		return false
+	}
+	return req.IsWeChatBrowser || strings.TrimSpace(req.OpenID) != ""
+}
+
+func (s *PaymentService) usesOfficialWxpayVisibleMethod(ctx context.Context) bool {
+	if s == nil || s.configService == nil || s.configService.settingRepo == nil {
+		return false
+	}
+	vals, err := s.configService.settingRepo.GetMultiple(ctx, []string{
+		SettingPaymentVisibleMethodWxpayEnabled,
+		SettingPaymentVisibleMethodWxpaySource,
+	})
+	if err != nil {
+		return false
+	}
+	if vals[SettingPaymentVisibleMethodWxpayEnabled] != "true" {
+		return false
+	}
+	return NormalizeVisibleMethodSource(payment.TypeWxpay, vals[SettingPaymentVisibleMethodWxpaySource]) == VisibleMethodSourceOfficialWechat
+}
+
 func (s *PaymentService) invokeProvider(ctx context.Context, order *dbent.PaymentOrder, req CreateOrderRequest, cfg *PaymentConfig, limitAmount float64, payAmountStr string, payAmount float64, plan *dbent.SubscriptionPlan, sel *payment.InstanceSelection) (*CreateOrderResponse, error) {
 	prov, err := provider.CreateProvider(sel.ProviderKey, sel.InstanceID, sel.Config)
 	if err != nil {
@@ -239,6 +281,7 @@ func (s *PaymentService) invokeProvider(ctx context.Context, order *dbent.Paymen
 	}
 	resumeToken := ""
 	if resume := s.paymentResume(); resume != nil {
+		if resume.isSigningConfigured() {
 			resumeToken, err = resume.CreateToken(ResumeTokenClaims{
 				OrderID:            order.ID,
 				UserID:             order.UserID,
@@ -251,6 +294,7 @@ func (s *PaymentService) invokeProvider(ctx context.Context, order *dbent.Paymen
 				return nil, fmt.Errorf("create payment resume token: %w", err)
 			}
 		}
+	}
 	providerReturnURL, err := buildPaymentReturnURL(canonicalReturnURL, order.ID, resumeToken)
 	if err != nil {
 		return nil, err

backend/internal/service/payment_order_jsapi_test.go

+package service
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/payment"
+)
+
+const jsapiTestEncryptionKey = "0123456789abcdef0123456789abcdef"
+
+func TestSelectCreateOrderInstancePrefersJSAPICompatibleWxpayInstance(t *testing.T) {
+	ctx := context.Background()
+	client := newPaymentConfigServiceTestClient(t)
+
+	compatibleConfig := mustEncryptJSAPITestConfig(t, map[string]string{
+		"appId":       "wx-merchant-app",
+		"mpAppId":     "wx-mp-app",
+		"mchId":       "mch-compatible",
+		"privateKey":  "private-key",
+		"apiV3Key":    jsapiTestEncryptionKey,
+		"publicKey":   "public-key",
+		"publicKeyId": "key-compatible",
+		"certSerial":  "serial-compatible",
+	})
+	incompatibleConfig := mustEncryptJSAPITestConfig(t, map[string]string{
+		"appId":       "wx-merchant-other",
+		"mpAppId":     "wx-mp-other",
+		"mchId":       "mch-incompatible",
+		"privateKey":  "private-key",
+		"apiV3Key":    jsapiTestEncryptionKey,
+		"publicKey":   "public-key",
+		"publicKeyId": "key-incompatible",
+		"certSerial":  "serial-incompatible",
+	})
+
+	compatible, err := client.PaymentProviderInstance.Create().
+		SetProviderKey(payment.TypeWxpay).
+		SetName("wxpay-compatible").
+		SetConfig(compatibleConfig).
+		SetSupportedTypes("wxpay").
+		SetEnabled(true).
+		SetSortOrder(1).
+		Save(ctx)
+	if err != nil {
+		t.Fatalf("create compatible wxpay instance: %v", err)
+	}
+	_, err = client.PaymentProviderInstance.Create().
+		SetProviderKey(payment.TypeWxpay).
+		SetName("wxpay-incompatible").
+		SetConfig(incompatibleConfig).
+		SetSupportedTypes("wxpay").
+		SetEnabled(true).
+		SetSortOrder(2).
+		Save(ctx)
+	if err != nil {
+		t.Fatalf("create incompatible wxpay instance: %v", err)
+	}
+
+	configService := &PaymentConfigService{
+		entClient:     client,
+		settingRepo: &paymentConfigSettingRepoStub{values: map[string]string{
+			SettingPaymentVisibleMethodWxpayEnabled: "true",
+			SettingPaymentVisibleMethodWxpaySource:  VisibleMethodSourceOfficialWechat,
+		}},
+		encryptionKey: []byte(jsapiTestEncryptionKey),
+	}
+	loadBalancer := newVisibleMethodLoadBalancer(
+		payment.NewDefaultLoadBalancer(client, []byte(jsapiTestEncryptionKey)),
+		configService,
+	)
+	svc := &PaymentService{
+		entClient:     client,
+		loadBalancer:  loadBalancer,
+		configService: configService,
+	}
+
+	t.Setenv("WECHAT_OAUTH_MP_APP_ID", "wx-mp-app")
+	t.Setenv("WECHAT_OAUTH_MP_APP_SECRET", "wechat-secret")
+
+	sel, err := svc.selectCreateOrderInstance(ctx, CreateOrderRequest{
+		PaymentType:     payment.TypeWxpay,
+		OpenID:          "openid-123",
+		IsWeChatBrowser: true,
+	}, &PaymentConfig{LoadBalanceStrategy: string(payment.StrategyRoundRobin)}, 12.5)
+	if err != nil {
+		t.Fatalf("selectCreateOrderInstance returned error: %v", err)
+	}
+	if sel == nil {
+		t.Fatal("expected selected instance, got nil")
+	}
+	expectedInstanceID := fmt.Sprintf("%d", compatible.ID)
+	if sel.InstanceID != expectedInstanceID {
+		t.Fatalf("selected instance id = %q, want %q", sel.InstanceID, expectedInstanceID)
+	}
+}
+
+func mustEncryptJSAPITestConfig(t *testing.T, config map[string]string) string {
+	t.Helper()
+
+	data, err := json.Marshal(config)
+	if err != nil {
+		t.Fatalf("marshal config: %v", err)
+	}
+	encrypted, err := payment.Encrypt(string(data), []byte(jsapiTestEncryptionKey))
+	if err != nil {
+		t.Fatalf("encrypt config: %v", err)
+	}
+	return encrypted
+}

backend/internal/service/payment_resume_service.go

@@ -33,6 +33,9 @@ const (
 	VisibleMethodSourceEasyPayWechat  = "easypay_wxpay"
 
 	wechatPaymentResumeTokenType = "wechat_payment_resume"
+
+	paymentResumeNotConfiguredCode    = "PAYMENT_RESUME_NOT_CONFIGURED"
+	paymentResumeNotConfiguredMessage = "payment resume tokens require a configured signing key"
 )
 
 type ResumeTokenClaims struct {
@@ -70,6 +73,17 @@ func NewPaymentResumeService(signingKey []byte) *PaymentResumeService {
 	return &PaymentResumeService{signingKey: signingKey}
 }
 
+func (s *PaymentResumeService) isSigningConfigured() bool {
+	return s != nil && len(s.signingKey) > 0
+}
+
+func (s *PaymentResumeService) ensureSigningKey() error {
+	if s.isSigningConfigured() {
+		return nil
+	}
+	return infraerrors.ServiceUnavailable(paymentResumeNotConfiguredCode, paymentResumeNotConfiguredMessage)
+}
+
 func NormalizeVisibleMethod(method string) string {
 	return payment.GetBasePaymentType(strings.TrimSpace(method))
 }
@@ -240,6 +254,9 @@ func buildPaymentReturnURL(base string, orderID int64, resumeToken string) (stri
 }
 
 func (s *PaymentResumeService) CreateToken(claims ResumeTokenClaims) (string, error) {
+	if err := s.ensureSigningKey(); err != nil {
+		return "", err
+	}
 	if claims.OrderID <= 0 {
 		return "", fmt.Errorf("resume token requires order id")
 	}
@@ -250,6 +267,9 @@ func (s *PaymentResumeService) CreateToken(claims ResumeTokenClaims) (string, er
 }
 
 func (s *PaymentResumeService) ParseToken(token string) (*ResumeTokenClaims, error) {
+	if err := s.ensureSigningKey(); err != nil {
+		return nil, err
+	}
 	var claims ResumeTokenClaims
 	if err := s.parseSignedToken(token, &claims); err != nil {
 		return nil, infraerrors.BadRequest("INVALID_RESUME_TOKEN", "resume token payload is invalid")
@@ -261,6 +281,9 @@ func (s *PaymentResumeService) ParseToken(token string) (*ResumeTokenClaims, err
 }
 
 func (s *PaymentResumeService) CreateWeChatPaymentResumeToken(claims WeChatPaymentResumeClaims) (string, error) {
+	if err := s.ensureSigningKey(); err != nil {
+		return "", err
+	}
 	claims.OpenID = strings.TrimSpace(claims.OpenID)
 	if claims.OpenID == "" {
 		return "", fmt.Errorf("wechat payment resume token requires openid")
@@ -282,6 +305,9 @@ func (s *PaymentResumeService) CreateWeChatPaymentResumeToken(claims WeChatPayme
 }
 
 func (s *PaymentResumeService) ParseWeChatPaymentResumeToken(token string) (*WeChatPaymentResumeClaims, error) {
+	if err := s.ensureSigningKey(); err != nil {
+		return nil, err
+	}
 	var claims WeChatPaymentResumeClaims
 	if err := s.parseSignedToken(token, &claims); err != nil {
 		return nil, infraerrors.BadRequest("INVALID_WECHAT_PAYMENT_RESUME_TOKEN", "wechat payment resume token payload is invalid")
@@ -330,11 +356,7 @@ func (s *PaymentResumeService) parseSignedToken(token string, dest any) error {
 }
 
 func (s *PaymentResumeService) sign(payload string) string {
-	key := s.signingKey
-	if len(key) == 0 {
-		key = []byte(paymentResumeFallbackSigningKey)
-	}
-	mac := hmac.New(sha256.New, key)
+	mac := hmac.New(sha256.New, s.signingKey)
 	_, _ = mac.Write([]byte(payload))
 	return base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
 }

backend/internal/service/payment_resume_service_test.go

@@ -4,6 +4,10 @@ package service
 
 import (
 	"context"
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
 	"net/url"
 	"strconv"
 	"testing"
@@ -150,6 +154,27 @@ func TestPaymentResumeTokenRoundTrip(t *testing.T) {
 	}
 }
 
+func TestCreateTokenRejectsMissingSigningKey(t *testing.T) {
+	t.Parallel()
+
+	svc := NewPaymentResumeService(nil)
+	_, err := svc.CreateToken(ResumeTokenClaims{OrderID: 42})
+	if err == nil {
+		t.Fatal("CreateToken should reject missing signing key")
+	}
+}
+
+func TestParseTokenRejectsFallbackSignedTokenWhenSigningKeyMissing(t *testing.T) {
+	t.Parallel()
+
+	token := mustCreateFallbackSignedToken(t, ResumeTokenClaims{OrderID: 42, UserID: 7})
+	svc := NewPaymentResumeService(nil)
+	_, err := svc.ParseToken(token)
+	if err == nil {
+		t.Fatal("ParseToken should reject tokens when signing key is missing")
+	}
+}
+
 func TestWeChatPaymentResumeTokenRoundTrip(t *testing.T) {
 	t.Parallel()
 
@@ -183,6 +208,31 @@ func TestWeChatPaymentResumeTokenRoundTrip(t *testing.T) {
 	}
 }
 
+func TestCreateWeChatPaymentResumeTokenRejectsMissingSigningKey(t *testing.T) {
+	t.Parallel()
+
+	svc := NewPaymentResumeService(nil)
+	_, err := svc.CreateWeChatPaymentResumeToken(WeChatPaymentResumeClaims{OpenID: "openid-123"})
+	if err == nil {
+		t.Fatal("CreateWeChatPaymentResumeToken should reject missing signing key")
+	}
+}
+
+func TestParseWeChatPaymentResumeTokenRejectsFallbackSignedTokenWhenSigningKeyMissing(t *testing.T) {
+	t.Parallel()
+
+	token := mustCreateFallbackSignedToken(t, WeChatPaymentResumeClaims{
+		TokenType:   wechatPaymentResumeTokenType,
+		OpenID:      "openid-123",
+		PaymentType: payment.TypeWxpay,
+	})
+	svc := NewPaymentResumeService(nil)
+	_, err := svc.ParseWeChatPaymentResumeToken(token)
+	if err == nil {
+		t.Fatal("ParseWeChatPaymentResumeToken should reject tokens when signing key is missing")
+	}
+}
+
 func TestNormalizeVisibleMethodSource(t *testing.T) {
 	t.Parallel()
 
@@ -315,3 +365,17 @@ func (c *captureLoadBalancer) SelectInstance(_ context.Context, providerKey stri
 	c.lastPaymentType = paymentType
 	return &payment.InstanceSelection{ProviderKey: providerKey, SupportedTypes: paymentType}, nil
 }
+
+func mustCreateFallbackSignedToken(t *testing.T, claims any) string {
+	t.Helper()
+
+	payload, err := json.Marshal(claims)
+	if err != nil {
+		t.Fatalf("marshal claims: %v", err)
+	}
+	encodedPayload := base64.RawURLEncoding.EncodeToString(payload)
+	mac := hmac.New(sha256.New, []byte(paymentResumeFallbackSigningKey))
+	_, _ = mac.Write([]byte(encodedPayload))
+	signature := base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
+	return encodedPayload + "." + signature
+}

backend/internal/service/payment_webhook_provider.go

@@ -16,25 +16,58 @@ import (
 // It resolves the original provider instance from the order whenever possible and
 // only falls back to a registry provider for legacy/single-instance scenarios.
 func (s *PaymentService) GetWebhookProvider(ctx context.Context, providerKey, outTradeNo string) (payment.Provider, error) {
+	providers, err := s.GetWebhookProviders(ctx, providerKey, outTradeNo)
+	if err != nil {
+		return nil, err
+	}
+	if len(providers) == 0 {
+		return nil, payment.ErrProviderNotFound
+	}
+	return providers[0], nil
+}
+
+// GetWebhookProviders returns provider candidates that can verify the webhook.
+// Official WeChat Pay may require multiple candidates because the callback body
+// cannot be bound to a merchant before decryption.
+func (s *PaymentService) GetWebhookProviders(ctx context.Context, providerKey, outTradeNo string) ([]payment.Provider, error) {
 	if outTradeNo != "" {
 		order, err := s.entClient.PaymentOrder.Query().Where(paymentorder.OutTradeNo(outTradeNo)).Only(ctx)
 		if err == nil {
 			if psHasPinnedProviderInstance(order) {
-				return s.getPinnedOrderProvider(ctx, order)
+				prov, err := s.getPinnedOrderProvider(ctx, order)
+				if err != nil {
+					return nil, err
+				}
+				return []payment.Provider{prov}, nil
 			}
 			inst, err := s.getOrderProviderInstance(ctx, order)
 			if err != nil {
 				return nil, fmt.Errorf("load order provider instance: %w", err)
 			}
 			if inst != nil {
-				return s.createProviderFromInstance(ctx, inst)
+				prov, err := s.createProviderFromInstance(ctx, inst)
+				if err != nil {
+					return nil, err
+				}
+				return []payment.Provider{prov}, nil
+			}
+			if strings.TrimSpace(providerKey) == payment.TypeWxpay {
+				return s.getEnabledWebhookProvidersByKey(ctx, providerKey)
 			}
 			if !s.webhookRegistryFallbackAllowed(ctx, providerKey) {
 				return nil, fmt.Errorf("webhook provider fallback is ambiguous for %s", providerKey)
 			}
 			s.EnsureProviders(ctx)
-			return s.registry.GetProviderByKey(providerKey)
+			prov, err := s.registry.GetProviderByKey(providerKey)
+			if err != nil {
+				return nil, err
 			}
+			return []payment.Provider{prov}, nil
+		}
+	}
+
+	if strings.TrimSpace(providerKey) == payment.TypeWxpay {
+		return s.getEnabledWebhookProvidersByKey(ctx, providerKey)
 	}
 
 	if !s.webhookRegistryFallbackAllowed(ctx, providerKey) {
@@ -42,7 +75,11 @@ func (s *PaymentService) GetWebhookProvider(ctx context.Context, providerKey, ou
 	}
 
 	s.EnsureProviders(ctx)
-	return s.registry.GetProviderByKey(providerKey)
+	prov, err := s.registry.GetProviderByKey(providerKey)
+	if err != nil {
+		return nil, err
+	}
+	return []payment.Provider{prov}, nil
 }
 
 func (s *PaymentService) getPinnedOrderProvider(ctx context.Context, o *dbent.PaymentOrder) (payment.Provider, error) {
@@ -78,3 +115,34 @@ func (s *PaymentService) webhookRegistryFallbackAllowed(ctx context.Context, pro
 func psHasPinnedProviderInstance(order *dbent.PaymentOrder) bool {
 	return order != nil && order.ProviderInstanceID != nil && strings.TrimSpace(*order.ProviderInstanceID) != ""
 }
+
+func (s *PaymentService) getEnabledWebhookProvidersByKey(ctx context.Context, providerKey string) ([]payment.Provider, error) {
+	providerKey = strings.TrimSpace(providerKey)
+	instances, err := s.entClient.PaymentProviderInstance.Query().
+		Where(
+			paymentproviderinstance.ProviderKeyEQ(providerKey),
+			paymentproviderinstance.EnabledEQ(true),
+		).
+		Order(dbent.Asc(paymentproviderinstance.FieldSortOrder)).
+		All(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("query webhook provider instances: %w", err)
+	}
+	if len(instances) == 0 {
+		return nil, payment.ErrProviderNotFound
+	}
+
+	providers := make([]payment.Provider, 0, len(instances))
+	for _, inst := range instances {
+		prov, provErr := s.createProviderFromInstance(ctx, inst)
+		if provErr != nil {
+			slog.Warn("skip webhook provider instance", "provider", providerKey, "instanceID", inst.ID, "error", provErr)
+			continue
+		}
+		providers = append(providers, prov)
+	}
+	if len(providers) == 0 {
+		return nil, payment.ErrProviderNotFound
+	}
+	return providers, nil
+}

backend/internal/service/payment_webhook_provider_test.go

@@ -208,10 +208,28 @@ func TestGetOrderProviderInstanceLeavesProviderKeyMatchUnresolvedWhenTypeNotSupp
 func TestGetWebhookProviderRejectsAmbiguousRegistryFallback(t *testing.T) {
 	ctx := context.Background()
 	client := newPaymentConfigServiceTestClient(t)
+	wxpayConfigA := encryptWebhookProviderConfig(t, map[string]string{
+		"appId":       "wx-app-a",
+		"mchId":       "mch-a",
+		"privateKey":  "private-key-a",
+		"apiV3Key":    webhookProviderTestEncryptionKey,
+		"publicKey":   "public-key-a",
+		"publicKeyId": "public-key-id-a",
+		"certSerial":  "cert-serial-a",
+	})
+	wxpayConfigB := encryptWebhookProviderConfig(t, map[string]string{
+		"appId":       "wx-app-b",
+		"mchId":       "mch-b",
+		"privateKey":  "private-key-b",
+		"apiV3Key":    webhookProviderTestEncryptionKey,
+		"publicKey":   "public-key-b",
+		"publicKeyId": "public-key-id-b",
+		"certSerial":  "cert-serial-b",
+	})
 	_, err := client.PaymentProviderInstance.Create().
 		SetProviderKey(payment.TypeWxpay).
 		SetName("wxpay-a").
-		SetConfig("{}").
+		SetConfig(wxpayConfigA).
 		SetSupportedTypes("wxpay").
 		SetEnabled(true).
 		Save(ctx)
@@ -219,19 +237,51 @@ func TestGetWebhookProviderRejectsAmbiguousRegistryFallback(t *testing.T) {
 	_, err = client.PaymentProviderInstance.Create().
 		SetProviderKey(payment.TypeWxpay).
 		SetName("wxpay-b").
-		SetConfig("{}").
+		SetConfig(wxpayConfigB).
 		SetSupportedTypes("wxpay").
 		SetEnabled(true).
 		Save(ctx)
 	require.NoError(t, err)
 
+	svc := &PaymentService{
+		entClient:       client,
+		loadBalancer:    newWebhookProviderTestLoadBalancer(client),
+		registry:        payment.NewRegistry(),
+		providersLoaded: true,
+	}
+
+	providers, err := svc.GetWebhookProviders(ctx, payment.TypeWxpay, "")
+	require.NoError(t, err)
+	require.Len(t, providers, 2)
+}
+
+func TestGetWebhookProvidersRejectAmbiguousFallbackForNonWxpay(t *testing.T) {
+	ctx := context.Background()
+	client := newPaymentConfigServiceTestClient(t)
+	_, err := client.PaymentProviderInstance.Create().
+		SetProviderKey(payment.TypeAlipay).
+		SetName("alipay-a").
+		SetConfig("{}").
+		SetSupportedTypes("alipay").
+		SetEnabled(true).
+		Save(ctx)
+	require.NoError(t, err)
+	_, err = client.PaymentProviderInstance.Create().
+		SetProviderKey(payment.TypeAlipay).
+		SetName("alipay-b").
+		SetConfig("{}").
+		SetSupportedTypes("alipay").
+		SetEnabled(true).
+		Save(ctx)
+	require.NoError(t, err)
+
 	svc := &PaymentService{
 		entClient:       client,
 		registry:        payment.NewRegistry(),
 		providersLoaded: true,
 	}
 
-	_, err = svc.GetWebhookProvider(ctx, payment.TypeWxpay, "")
+	_, err = svc.GetWebhookProviders(ctx, payment.TypeAlipay, "")
 	require.Error(t, err)
 	require.Contains(t, err.Error(), "ambiguous")
 }
@@ -260,8 +310,10 @@ func TestGetWebhookProviderAllowsSingleInstanceRegistryFallback(t *testing.T) {
 		providersLoaded: true,
 	}
 
-	prov, err := svc.GetWebhookProvider(ctx, payment.TypeStripe, "")
+	providers, err := svc.GetWebhookProviders(ctx, payment.TypeStripe, "")
 	require.NoError(t, err)
+	require.Len(t, providers, 1)
+	prov := providers[0]
 	require.Equal(t, payment.TypeStripe, prov.ProviderKey())
 }
 
@@ -308,7 +360,7 @@ func TestGetWebhookProviderRejectsRegistryFallbackForPinnedOrder(t *testing.T) {
 		providersLoaded: true,
 	}
 
-	_, err = svc.GetWebhookProvider(ctx, payment.TypeWxpay, "sub2_test_pinned_order")
+	_, err = svc.GetWebhookProviders(ctx, payment.TypeWxpay, "sub2_test_pinned_order")
 	require.Error(t, err)
 	require.Contains(t, err.Error(), "provider instance")
 }