An Expression Blend behavior for WPF that enables binding to the Password property of the PasswordBox control.

PasswordBoxBehavior.cs

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Interactivity;

namespace Balioglu
{
    // USAGE:
    //
    //<PasswordBox>
    //  <blend:Interaction.Behaviors>
    //      <l:PasswordBoxBehavior UpdateSourceTrigger="PropertyChanged"
    //                             Binding="{Binding Source={StaticResource MyViewModel},Path=Password}" />
    //  </blend:Interaction.Behaviors>
    //</PasswordBox>
    //

    /// <summary>
    /// Enables binding to the <see cref="Password" /> property of the <see cref="PasswordBox" /> control.
    /// </summary>
    /// <remarks>
    /// Note that storing plain text passwords in memory is a security risk. You should avoid using this solution
    /// unless it is absolutely necessary to use the MVVM pattern.
    /// </remarks>
    public sealed class PasswordBoxBehavior : Behavior<PasswordBox>
    {
        public static readonly DependencyProperty BindingProperty =
            DependencyProperty.Register("Binding", typeof(String), typeof(PasswordBoxBehavior),
                new FrameworkPropertyMetadata(OnBindingChanged) { BindsTwoWayByDefault = true });

        public static readonly DependencyProperty UpdateSourceTriggerProperty =
            DependencyProperty.Register("UpdateSourceTrigger", typeof(UpdateSourceTrigger), typeof(PasswordBoxBehavior),
                new PropertyMetadata(UpdateSourceTrigger.LostFocus), ValidateUpdateSourceTrigger);

        /// <summary>
        /// Gets or sets the password.
        /// </summary>
        public String Binding
        {
            get
            {
                return (String) GetValue(BindingProperty);
            }
            set
            {
                SetValue(BindingProperty, value);
            }
        }

        /// <summary>
        /// Gets or sets the timing of binding source updates.
        /// </summary>
        public UpdateSourceTrigger UpdateSourceTrigger
        {
            get
            {
                return (UpdateSourceTrigger) GetValue(UpdateSourceTriggerProperty);
            }
            set
            {
                SetValue(UpdateSourceTriggerProperty, value);
            }
        }

        private static void OnBindingChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            PasswordBoxBehavior behavior = d as PasswordBoxBehavior;
            if (behavior != null)
            {
                behavior.SetTargetPassword((String) e.NewValue);
            }
        }

        private static Boolean ValidateUpdateSourceTrigger(Object value)
        {
            UpdateSourceTrigger trigger = (UpdateSourceTrigger) value;

            return
                trigger == UpdateSourceTrigger.Default ||
                trigger == UpdateSourceTrigger.LostFocus ||
                trigger == UpdateSourceTrigger.PropertyChanged;
        }

        private Boolean m_passwordBoxChanged;
        private Boolean m_isUpdatingBinding;

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

            PasswordBox passwordBox = AssociatedObject;

            passwordBox.PasswordChanged += OnPasswordBoxChanged;
            passwordBox.LostFocus += OnPasswordBoxLostFocus;

            passwordBox.SetBinding(Validation.ValidationAdornerSiteForProperty, new Binding { Source = this });
        }

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

            PasswordBox passwordBox = AssociatedObject;

            passwordBox.PasswordChanged -= OnPasswordBoxChanged;
            passwordBox.LostFocus -= OnPasswordBoxLostFocus;

            BindingOperations.ClearBinding(passwordBox, Validation.ValidationAdornerSiteForProperty);
        }

        private void SetTargetPassword(String password)
        {
            if (m_isUpdatingBinding)
            {
                return;
            }

            PasswordBox passwordBox = AssociatedObject;

            passwordBox.PasswordChanged -= OnPasswordBoxChanged;
            passwordBox.Password = password;
            passwordBox.PasswordChanged += OnPasswordBoxChanged;
        }

        private void OnPasswordBoxChanged(Object sender, RoutedEventArgs e)
        {
            if (UpdateSourceTrigger == UpdateSourceTrigger.PropertyChanged)
            {
                UpdateBinding();
            }
            else
            {
                m_passwordBoxChanged = true;
            }
        }

        private void OnPasswordBoxLostFocus(Object sender, RoutedEventArgs e)
        {
            if (UpdateSourceTrigger == UpdateSourceTrigger.PropertyChanged || !m_passwordBoxChanged)
            {
                return;
            }

            UpdateBinding();

            m_passwordBoxChanged = false;
        }

        private void UpdateBinding()
        {
            m_isUpdatingBinding = true;

            Binding = AssociatedObject.Password;

            m_isUpdatingBinding = false;
        }
    }
}