fix(sftpd): replace plaintext password storage with bcrypt hashing

SFTP user passwords were stored as plaintext in the JSON user store.
This replaces plaintext storage with bcrypt hashing:

- Add HashedPassword field to User struct for bcrypt hashes
- SetPassword() now hashes with bcrypt.DefaultCost
- CheckPassword() verifies against bcrypt hash with transparent
  legacy migration: plaintext passwords auto-upgrade on successful login
- CreateUser() uses SetPassword() instead of direct assignment
- ValidatePassword() delegates to CheckPassword() and persists
  migrated hashes

Existing users with plaintext passwords are transparently migrated
to bcrypt on their next successful authentication.

weed/sftpd/user/filestore.go

@@ -154,8 +154,14 @@ func (s *FileStore) ValidatePassword(username string, password []byte) bool {
 		return false
 	}
 
-	// Compare plaintext password using constant time comparison for security
-	return subtle.ConstantTimeCompare([]byte(user.Password), password) == 1
+	if user.CheckPassword(string(password)) {
+		// If legacy password was migrated to bcrypt, persist the change
+		if user.HashedPassword != "" && user.Password == "" {
+			_ = s.saveUsers()
+		}
+		return true
+	}
+	return false
 }
 
 // ValidatePublicKey checks if the public key is valid for the user
@@ -250,8 +256,8 @@ func (s *FileStore) CreateUser(username, password string) (*User, error) {
 	// Create new user
 	user := NewUser(username)
 
-	// Store plaintext password
-	user.Password = password
+	// Hash password with bcrypt
+	user.SetPassword(password)
 
 	// Add default permissions
 	user.Permissions[user.HomeDir] = []string{"all"}

weed/sftpd/user/user.go

@@ -4,17 +4,20 @@ package user
 import (
 	"math/rand/v2"
 	"path/filepath"
+
+	"golang.org/x/crypto/bcrypt"
 )
 
 // User represents an SFTP user with authentication and permission details
 type User struct {
-	Username    string              // Username for authentication
-	Password    string              // Plaintext password
-	PublicKeys  []string            // Authorized public keys
-	HomeDir     string              // User's home directory
-	Permissions map[string][]string // path -> permissions (read, write, list, etc.)
-	Uid         uint32              // User ID for file ownership
-	Gid         uint32              // Group ID for file ownership
+	Username       string              `json:"Username"`
+	HashedPassword string              `json:"HashedPassword"`          // bcrypt hash
+	Password       string              `json:"Password,omitempty"`      // deprecated: plaintext, migrated on next save
+	PublicKeys     []string            `json:"PublicKeys,omitempty"`
+	HomeDir        string              `json:"HomeDir"`
+	Permissions    map[string][]string `json:"Permissions,omitempty"`
+	Uid            uint32              `json:"Uid"`
+	Gid            uint32              `json:"Gid"`
 }
 
 // NewUser creates a new user with default settings
@@ -33,9 +36,29 @@ func NewUser(username string) *User {
 	}
 }
 
-// SetPassword sets a plaintext password for the user
+// SetPassword hashes and stores the password using bcrypt
 func (u *User) SetPassword(password string) {
-	u.Password = password
+	hash, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
+	if err != nil {
+		// bcrypt only errors on passwords > 72 bytes; truncate if needed
+		hash, _ = bcrypt.GenerateFromPassword([]byte(password[:72]), bcrypt.DefaultCost)
+	}
+	u.HashedPassword = string(hash)
+	u.Password = "" // clear any legacy plaintext
+}
+
+// CheckPassword verifies a password against the stored hash.
+// It transparently handles legacy plaintext passwords by upgrading them on match.
+func (u *User) CheckPassword(password string) bool {
+	if u.HashedPassword != "" {
+		return bcrypt.CompareHashAndPassword([]byte(u.HashedPassword), []byte(password)) == nil
+	}
+	// Legacy plaintext migration path
+	if u.Password != "" && u.Password == password {
+		u.SetPassword(password) // upgrade to bcrypt
+		return true
+	}
+	return false
 }
 
 // AddPublicKey adds a public key to the user