fix(security): 修复密码重置链接 Host Header 注入漏洞 (P0-07)

ForgotPassword 原来从 c.Request.Host 构建重置链接基础 URL，攻击者
可伪造 Host 头将重置链接指向恶意域名窃取 token。

修复方案：
- ServerConfig 新增 frontend_url 配置项
- auth_handler 改为从配置读取前端 URL，未配置时拒绝请求
- Validate() 校验 frontend_url 必须为绝对 HTTP(S) URL
- 新增 TestValidateServerFrontendURL 单元测试
- config.example.yaml 添加配置说明

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/config/config.go

@@ -148,6 +148,7 @@ type ServerConfig struct {
 	Host               string    `mapstructure:"host"`
 	Port               int       `mapstructure:"port"`
 	Mode               string    `mapstructure:"mode"`                  // debug/release
+	FrontendURL        string    `mapstructure:"frontend_url"`          // 前端基础 URL，用于生成邮件中的外部链接
 	ReadHeaderTimeout  int       `mapstructure:"read_header_timeout"`   // 读取请求头超时（秒）
 	IdleTimeout        int       `mapstructure:"idle_timeout"`          // 空闲连接超时（秒）
 	TrustedProxies     []string  `mapstructure:"trusted_proxies"`       // 可信代理列表（CIDR/IP）
@@ -641,6 +642,7 @@ func Load() (*Config, error) {
 	if cfg.Server.Mode == "" {
 		cfg.Server.Mode = "debug"
 	}
+	cfg.Server.FrontendURL = strings.TrimSpace(cfg.Server.FrontendURL)
 	cfg.JWT.Secret = strings.TrimSpace(cfg.JWT.Secret)
 	cfg.LinuxDo.ClientID = strings.TrimSpace(cfg.LinuxDo.ClientID)
 	cfg.LinuxDo.ClientSecret = strings.TrimSpace(cfg.LinuxDo.ClientSecret)
@@ -714,6 +716,7 @@ func setDefaults() {
 	viper.SetDefault("server.host", "0.0.0.0")
 	viper.SetDefault("server.port", 8080)
 	viper.SetDefault("server.mode", "debug")
+	viper.SetDefault("server.frontend_url", "")
 	viper.SetDefault("server.read_header_timeout", 30) // 30秒读取请求头
 	viper.SetDefault("server.idle_timeout", 120)       // 120秒空闲超时
 	viper.SetDefault("server.trusted_proxies", []string{})
@@ -950,6 +953,12 @@ func setDefaults() {
 }
 
 func (c *Config) Validate() error {
+	if strings.TrimSpace(c.Server.FrontendURL) != "" {
+		if err := ValidateAbsoluteHTTPURL(c.Server.FrontendURL); err != nil {
+			return fmt.Errorf("server.frontend_url invalid: %w", err)
+		}
+		warnIfInsecureURL("server.frontend_url", c.Server.FrontendURL)
+	}
 	if c.JWT.ExpireHour <= 0 {
 		return fmt.Errorf("jwt.expire_hour must be positive")
 	}

backend/internal/config/config_test.go

@@ -424,6 +424,25 @@ func TestValidateAbsoluteHTTPURL(t *testing.T) {
 	}
 }
 
+func TestValidateServerFrontendURL(t *testing.T) {
+	viper.Reset()
+
+	cfg, err := Load()
+	if err != nil {
+		t.Fatalf("Load() error: %v", err)
+	}
+
+	cfg.Server.FrontendURL = "https://example.com"
+	if err := cfg.Validate(); err != nil {
+		t.Fatalf("Validate() frontend_url valid error: %v", err)
+	}
+
+	cfg.Server.FrontendURL = "/relative"
+	if err := cfg.Validate(); err == nil {
+		t.Fatalf("Validate() should reject relative server.frontend_url")
+	}
+}
+
 func TestValidateFrontendRedirectURL(t *testing.T) {
 	if err := ValidateFrontendRedirectURL("/auth/callback"); err != nil {
 		t.Fatalf("ValidateFrontendRedirectURL relative error: %v", err)

backend/internal/handler/auth_handler.go

@@ -2,6 +2,7 @@ package handler
 
 import (
 	"log/slog"
+	"strings"
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
 	"github.com/Wei-Shaw/sub2api/internal/handler/dto"
@@ -448,17 +449,12 @@ func (h *AuthHandler) ForgotPassword(c *gin.Context) {
 		return
 	}
 
-	// Build frontend base URL from request
-	scheme := "https"
-	if c.Request.TLS == nil {
-		// Check X-Forwarded-Proto header (common in reverse proxy setups)
-		if proto := c.GetHeader("X-Forwarded-Proto"); proto != "" {
-			scheme = proto
-		} else {
-			scheme = "http"
-		}
+	frontendBaseURL := strings.TrimSpace(h.cfg.Server.FrontendURL)
+	if frontendBaseURL == "" {
+		slog.Error("server.frontend_url not configured; cannot build password reset link")
+		response.InternalError(c, "Password reset is not configured")
+		return
 	}
-	frontendBaseURL := scheme + "://" + c.Request.Host
 
 	// Request password reset (async)
 	// Note: This returns success even if email doesn't exist (to prevent enumeration)

deploy/config.example.yaml

@@ -20,6 +20,10 @@ server:
   # Mode: "debug" for development, "release" for production
   # 运行模式："debug" 用于开发，"release" 用于生产环境
   mode: "release"
+  # Frontend base URL used to generate external links in emails (e.g. password reset)
+  # 用于生成邮件中的外部链接（例如：重置密码链接）的前端基础地址
+  # Example: "https://example.com"
+  frontend_url: ""
   # Trusted proxies for X-Forwarded-For parsing (CIDR/IP). Empty disables trusted proxies.
   # 信任的代理地址（CIDR/IP 格式），用于解析 X-Forwarded-For 头。留空则禁用代理信任。
   trusted_proxies: []