public class PasswordBoxBindingBehavior : Behavior<PasswordBox>
    {
        protected override void OnAttached()
        {
            AssociatedObject.PasswordChanged += OnPasswordBoxValueChanged;
        }

        public SecureString Password
        {
            get { return (SecureString)GetValue(PasswordProperty); }
            set { SetValue(PasswordProperty, value); }
        }

        public static readonly DependencyProperty PasswordProperty =
            DependencyProperty.Register("Password", typeof(SecureString), typeof(PasswordBoxBindingBehavior), new PropertyMetadata(null));


        private void OnPasswordBoxValueChanged(object sender, RoutedEventArgs e)
        {
            Password = AssociatedObject.SecurePassword;
        }
    }


// usage  xaml
                    <PasswordBox>
                        <i:Interaction.Behaviors>
                            <behavior:PasswordBoxBindingBehavior
                                Password="{Binding Password, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
                        </i:Interaction.Behaviors>
                    </PasswordBox>


// usage VM
        public SecureString Password
        {
            get { return _password.Value; }
            set { this.SetProperty(_password, value); }
        }