diff --git a/backend/internal/service/auth_service.go b/backend/internal/service/auth_service.go
index 5a5ca03d..6e685869 100644
--- a/backend/internal/service/auth_service.go
+++ b/backend/internal/service/auth_service.go
@@ -75,8 +75,8 @@ func (s *AuthService) Register(ctx context.Context, email, password string) (str
 
 // RegisterWithVerification 用户注册（支持邮件验证），返回token和用户
 func (s *AuthService) RegisterWithVerification(ctx context.Context, email, password, verifyCode string) (string, *User, error) {
-	// 检查是否开放注册
-	if s.settingService != nil && !s.settingService.IsRegistrationEnabled(ctx) {
+	// 检查是否开放注册（默认关闭：settingService 未配置时不允许注册）
+	if s.settingService == nil || !s.settingService.IsRegistrationEnabled(ctx) {
 		return "", nil, ErrRegDisabled
 	}
 
@@ -132,6 +132,10 @@ func (s *AuthService) RegisterWithVerification(ctx context.Context, email, passw
 	}
 
 	if err := s.userRepo.Create(ctx, user); err != nil {
+		// 优先检查邮箱冲突错误（竞态条件下可能发生）
+		if errors.Is(err, ErrEmailExists) {
+			return "", nil, ErrEmailExists
+		}
 		log.Printf("[Auth] Database error creating user: %v", err)
 		return "", nil, ErrServiceUnavailable
 	}
@@ -152,8 +156,8 @@ type SendVerifyCodeResult struct {
 
 // SendVerifyCode 发送邮箱验证码（同步方式）
 func (s *AuthService) SendVerifyCode(ctx context.Context, email string) error {
-	// 检查是否开放注册
-	if s.settingService != nil && !s.settingService.IsRegistrationEnabled(ctx) {
+	// 检查是否开放注册（默认关闭）
+	if s.settingService == nil || !s.settingService.IsRegistrationEnabled(ctx) {
 		return ErrRegDisabled
 	}
 
@@ -185,8 +189,8 @@ func (s *AuthService) SendVerifyCode(ctx context.Context, email string) error {
 func (s *AuthService) SendVerifyCodeAsync(ctx context.Context, email string) (*SendVerifyCodeResult, error) {
 	log.Printf("[Auth] SendVerifyCodeAsync called for email: %s", email)
 
-	// 检查是否开放注册
-	if s.settingService != nil && !s.settingService.IsRegistrationEnabled(ctx) {
+	// 检查是否开放注册（默认关闭）
+	if s.settingService == nil || !s.settingService.IsRegistrationEnabled(ctx) {
 		log.Println("[Auth] Registration is disabled")
 		return nil, ErrRegDisabled
 	}
@@ -270,7 +274,7 @@ func (s *AuthService) IsTurnstileEnabled(ctx context.Context) bool {
 // IsRegistrationEnabled 检查是否开放注册
 func (s *AuthService) IsRegistrationEnabled(ctx context.Context) bool {
 	if s.settingService == nil {
-		return true
+		return false // 安全默认：settingService 未配置时关闭注册
 	}
 	return s.settingService.IsRegistrationEnabled(ctx)
 }
diff --git a/backend/internal/service/auth_service_register_test.go b/backend/internal/service/auth_service_register_test.go
index a31267ab..bfd504a3 100644
--- a/backend/internal/service/auth_service_register_test.go
+++ b/backend/internal/service/auth_service_register_test.go
@@ -113,6 +113,15 @@ func TestAuthService_Register_Disabled(t *testing.T) {
 	require.ErrorIs(t, err, ErrRegDisabled)
 }
 
+func TestAuthService_Register_DisabledByDefault(t *testing.T) {
+	// 当 settings 为 nil（设置项不存在）时，注册应该默认关闭
+	repo := &userRepoStub{}
+	service := newAuthService(repo, nil, nil)
+
+	_, _, err := service.Register(context.Background(), "user@test.com", "password")
+	require.ErrorIs(t, err, ErrRegDisabled)
+}
+
 func TestAuthService_Register_EmailVerifyEnabledButServiceNotConfigured(t *testing.T) {
 	repo := &userRepoStub{}
 	// 邮件验证开启但 emailCache 为 nil（emailService 未配置）
@@ -155,7 +164,9 @@ func TestAuthService_Register_EmailVerifyInvalid(t *testing.T) {
 
 func TestAuthService_Register_EmailExists(t *testing.T) {
 	repo := &userRepoStub{exists: true}
-	service := newAuthService(repo, nil, nil)
+	service := newAuthService(repo, map[string]string{
+		SettingKeyRegistrationEnabled: "true",
+	}, nil)
 
 	_, _, err := service.Register(context.Background(), "user@test.com", "password")
 	require.ErrorIs(t, err, ErrEmailExists)
@@ -163,7 +174,9 @@ func TestAuthService_Register_EmailExists(t *testing.T) {
 
 func TestAuthService_Register_CheckEmailError(t *testing.T) {
 	repo := &userRepoStub{existsErr: errors.New("db down")}
-	service := newAuthService(repo, nil, nil)
+	service := newAuthService(repo, map[string]string{
+		SettingKeyRegistrationEnabled: "true",
+	}, nil)
 
 	_, _, err := service.Register(context.Background(), "user@test.com", "password")
 	require.ErrorIs(t, err, ErrServiceUnavailable)
@@ -171,15 +184,30 @@ func TestAuthService_Register_CheckEmailError(t *testing.T) {
 
 func TestAuthService_Register_CreateError(t *testing.T) {
 	repo := &userRepoStub{createErr: errors.New("create failed")}
-	service := newAuthService(repo, nil, nil)
+	service := newAuthService(repo, map[string]string{
+		SettingKeyRegistrationEnabled: "true",
+	}, nil)
 
 	_, _, err := service.Register(context.Background(), "user@test.com", "password")
 	require.ErrorIs(t, err, ErrServiceUnavailable)
 }
 
+func TestAuthService_Register_CreateEmailExistsRace(t *testing.T) {
+	// 模拟竞态条件：ExistsByEmail 返回 false，但 Create 时因唯一约束失败
+	repo := &userRepoStub{createErr: ErrEmailExists}
+	service := newAuthService(repo, map[string]string{
+		SettingKeyRegistrationEnabled: "true",
+	}, nil)
+
+	_, _, err := service.Register(context.Background(), "user@test.com", "password")
+	require.ErrorIs(t, err, ErrEmailExists)
+}
+
 func TestAuthService_Register_Success(t *testing.T) {
 	repo := &userRepoStub{nextID: 5}
-	service := newAuthService(repo, nil, nil)
+	service := newAuthService(repo, map[string]string{
+		SettingKeyRegistrationEnabled: "true",
+	}, nil)
 
 	token, user, err := service.Register(context.Background(), "user@test.com", "password")
 	require.NoError(t, err)
diff --git a/backend/internal/service/email_service.go b/backend/internal/service/email_service.go
index afd8907c..55e137d6 100644
--- a/backend/internal/service/email_service.go
+++ b/backend/internal/service/email_service.go
@@ -5,6 +5,7 @@ import (
 	"crypto/rand"
 	"crypto/tls"
 	"fmt"
+	"log"
 	"math/big"
 	"net/smtp"
 	"strconv"
@@ -256,7 +257,9 @@ func (s *EmailService) VerifyCode(ctx context.Context, email, code string) error
 	// 验证码不匹配
 	if data.Code != code {
 		data.Attempts++
-		_ = s.cache.SetVerificationCode(ctx, email, data, verifyCodeTTL)
+		if err := s.cache.SetVerificationCode(ctx, email, data, verifyCodeTTL); err != nil {
+			log.Printf("[Email] Failed to update verification attempt count: %v", err)
+		}
 		if data.Attempts >= maxVerifyCodeAttempts {
 			return ErrVerifyCodeMaxAttempts
 		}
@@ -264,7 +267,9 @@ func (s *EmailService) VerifyCode(ctx context.Context, email, code string) error
 	}
 
 	// 验证成功，删除验证码
-	_ = s.cache.DeleteVerificationCode(ctx, email)
+	if err := s.cache.DeleteVerificationCode(ctx, email); err != nil {
+		log.Printf("[Email] Failed to delete verification code after success: %v", err)
+	}
 	return nil
 }
 
diff --git a/backend/internal/service/setting_service.go b/backend/internal/service/setting_service.go
index 6ce8ba2b..965253cf 100644
--- a/backend/internal/service/setting_service.go
+++ b/backend/internal/service/setting_service.go
@@ -141,8 +141,8 @@ func (s *SettingService) UpdateSettings(ctx context.Context, settings *SystemSet
 func (s *SettingService) IsRegistrationEnabled(ctx context.Context) bool {
 	value, err := s.settingRepo.GetValue(ctx, SettingKeyRegistrationEnabled)
 	if err != nil {
-		// 默认开放注册
-		return true
+		// 安全默认：如果设置不存在或查询出错，默认关闭注册
+		return false
 	}
 	return value == "true"
 }