fix: 密码长度上限校验 + 请求体大小限制防 DoS

- 新增 validatePassword 校验函数（6-72 字节，bcrypt 硬上限），统一应用于 ChangePassword/CreateUser/UpdateUser，超长密码返回清晰提示而非迷惑性的"密码加密失败"
- 新增 BodyLimit 中间件（http.MaxBytesReader），对所有 /api JSON 端点限制 1 MiB 请求体，防止内存耗尽型 DoS；/ws 走连接劫持不受影响

internal/handler/auth.go

@@ -1,6 +1,7 @@
 package handler
 
 import (
+	"errors"
 	"net/http"
 	"time"
 
@@ -13,6 +14,22 @@ import (
 	"golang.org/x/crypto/bcrypt"
 )
 
+const (
+	minPasswordLen = 6
+	maxPasswordLen = 72 // bcrypt 硬上限：超过 72 字节会被 bcrypt 截断或报错
+)
+
+func validatePassword(pw string) error {
+	n := len(pw)
+	if n < minPasswordLen {
+		return errors.New("密码长度不能少于6位")
+	}
+	if n > maxPasswordLen {
+		return errors.New("密码长度不能超过72字节")
+	}
+	return nil
+}
+
 type loginRequest struct {
 	Username string `json:"username" binding:"required"`
 	Password string `json:"password" binding:"required"`
@@ -93,8 +110,8 @@ func ChangePassword(c *gin.Context) {
 		return
 	}
 
-	if len(req.NewPassword) < 6 {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "新密码长度不能少于6位"})
+	if err := validatePassword(req.NewPassword); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
 		return
 	}
 

internal/handler/user.go

@@ -82,6 +82,11 @@ func CreateUser(c *gin.Context) {
 		return
 	}
 
+	if err := validatePassword(req.Password); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+
 	hash, err := bcrypt.GenerateFromPassword([]byte(req.Password), bcrypt.DefaultCost)
 	if err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": "密码加密失败"})
@@ -162,6 +167,10 @@ func UpdateUser(c *gin.Context) {
 	}
 
 	if req.Password != "" {
+		if err := validatePassword(req.Password); err != nil {
+			c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+			return
+		}
 		hash, err := bcrypt.GenerateFromPassword([]byte(req.Password), bcrypt.DefaultCost)
 		if err != nil {
 			c.JSON(http.StatusInternalServerError, gin.H{"error": "密码加密失败"})

internal/middleware/ratelimit.go

@@ -84,3 +84,12 @@ func LoginRateLimit() gin.HandlerFunc {
 		c.Next()
 	}
 }
+
+// BodyLimit 限制请求体大小，防止内存耗尽型 DoS。
+// 对所有 /api JSON 端点生效；WebSocket 走连接劫持，握手阶段不受影响。
+func BodyLimit(max int64) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		c.Request.Body = http.MaxBytesReader(c.Writer, c.Request.Body, max)
+		c.Next()
+	}
+}

internal/router/router.go

@@ -14,11 +14,13 @@ import (
 func Setup(r *gin.Engine) {
 	r.GET("/ws", vpn.HandleWS)
 
-	r.POST("/api/login", middleware.LoginRateLimit(), handler.Login)
+	bodyLimit := middleware.BodyLimit(1 << 20) // 1 MiB
+
+	r.POST("/api/login", bodyLimit, middleware.LoginRateLimit(), handler.Login)
 	r.GET("/api/version", handler.GetVersion)
 
 	auth := r.Group("/api")
-	auth.Use(middleware.AuthMiddleware())
+	auth.Use(middleware.AuthMiddleware(), bodyLimit)
 	{
 		auth.GET("/me", handler.Me)
 		auth.PUT("/me/password", handler.ChangePassword)
@@ -27,7 +29,7 @@ func Setup(r *gin.Engine) {
 	}
 
 	admin := r.Group("/api/admin")
-	admin.Use(middleware.AuthMiddleware(), middleware.AdminMiddleware())
+	admin.Use(middleware.AuthMiddleware(), middleware.AdminMiddleware(), bodyLimit)
 	{
 		admin.GET("/stats", handler.GetAdminStats)
 		admin.GET("/users/count", handler.GetUserCount)