public class TextToPasswordCharConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var str = new string('*', value?.ToString().Length ?? 0);
        return str;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        //throw new NotImplementedException();
        return new object();
    }
}

public string PasswordField
    {
        get => passwordValue;
        set
        {
            passwordValue = value;
            OnPropertyChanged();
        }
    }

public abstract class NotificationObject : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    protected void SetField<T>(ref T field, T value, [CallerMemberName] string propertyName = null)
    {
        if (!EqualityComparer<T>.Default.Equals(field, value))
        {
            field = value;
            OnPropertyChanged(propertyName);
        }
    }
}

<TextBox Grid.Row="0" Grid.Column="0" TextWrapping="Wrap" Margin="140,174,30,10">
        <TextBox.Text>
            <Binding Path="PasswordField" ValidatesOnDataErrors="True" Converter="{StaticResource TextToPasswordCharConverter}" UpdateSourceTrigger="PropertyChanged">
                <Binding.ValidationRules>
                    <ExceptionValidationRule />
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>
    </TextBox>