<PasswordBox x:Name="TboxPassword" Grid.Row="1" Grid.Column="0"
                controls:TextboxHelper.Watermark="Password ..."
                controls:TextboxHelper.ClearTextButton="True"
                Margin="10, 10, 0, 0">
    <i:Interaction.Behaviors>
        <misc:PasswordBoxBehavior Password="{Binding Path=Password, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True, NotifyOnValidationError=True}"/>
    </i:Interaction.Behaviors>
</PasswordBox>

public class PasswordBoxBehavior : Behavior<PasswordBox>
{
    #region Fields

    private readonly object _tryToExecuteActionSyncObject = new object();
    private bool _isUpdating;

    #endregion

    #region Properties

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

    public static readonly DependencyProperty PasswordProperty =
        DependencyProperty.Register("Password", typeof(string), typeof(PasswordBoxBehavior),
        new PropertyMetadata(string.Empty, OnPasswordPropertyChanged));

    #endregion

    #region Methods

    protected override void OnAttached()
    {
        base.OnAttached();

        AssociatedObject.PasswordChanged += OnAssociatedObjectPasswordChanged;
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();

        AssociatedObject.PasswordChanged -= OnAssociatedObjectPasswordChanged;
    }

    private void OnAssociatedObjectPasswordChanged(object sender, RoutedEventArgs e)
    {
        TryToExecuteAction(() => Password = AssociatedObject == null
            ? string.Empty
            : AssociatedObject.Password);
    }

    private static void OnPasswordPropertyChanged
        (DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        PasswordBoxBehavior passwordBoxBehavior;
        if (sender == null
            || (passwordBoxBehavior = sender as PasswordBoxBehavior) == null
            || passwordBoxBehavior.AssociatedObject == null)
        {
            return;
        }

        passwordBoxBehavior.TryToExecuteAction
            (() => passwordBoxBehavior.AssociatedObject.Password =
                    (e.NewValue == null
                        ? string.Empty
                        : (string) e.NewValue));
    }

    private void TryToExecuteAction(Action actionToExecute)
    {
        bool continueExecution;
        lock (_tryToExecuteActionSyncObject)
        {
            continueExecution = _isUpdating == false;
            _isUpdating = true;
        }

        if (continueExecution == false)
        {
            return;
        }

        try
        {
            if (actionToExecute != null)
            {
                actionToExecute();
            }
        }
        finally
        {
            lock (_tryToExecuteActionSyncObject)
            {
                _isUpdating = false;
            }
        }
    }

    #endregion
}

{"Cannot add instance of type 'PasswordBoxBehavior' to a collection of type 'BehaviorCollection'. Only items of type 'T' are allowed."}