How to bind to a PasswordBox in MVVM

I have come across a problem with binding to a PasswordBox. It seems it's a security risk but I am using the MVVM pattern so I wish to bypass this. I found some interesting code here (has anyone used this or something similar?)

http://www.wpftutorial.net/PasswordBox.html

It technically looks great, but I am unsure of how to retrieve the password.

I basically have properties in my LoginViewModel for Username and Password. Username is fine and is working as it's a TextBox.

I used the code above as stated and entered this

<PasswordBox ff:PasswordHelper.Attach="True"
    ff:PasswordHelper.Password="{Binding Path=Password}" Width="130"/>

When I had the PasswordBox as a TextBox and Binding Path=Password then the property in my LoginViewModel was updated.

My code is very simple, basically I have a Command for my Button. When I press it CanLogin is called and if it returns true it calls Login.
You can see I check my property for Username here which works great.

In Login I send along to my service a Username and Password, Username contains data from my View but Password is Null|Empty

private DelegateCommand loginCommand;

public string Username { get; set; }
public string Password { get; set; }


public ICommand LoginCommand
{
    get
    {
        if (loginCommand == null)
        {
            loginCommand = new DelegateCommand(
                Login, CanLogin );
        }
        return loginCommand;
    }
}

private bool CanLogin()
{
    return !string.IsNullOrEmpty(Username);
}

private void Login()
{
    bool result = securityService.IsValidLogin(Username, Password);

    if (result) { }
    else { }
}

This is what I am doing

<TextBox Text="{Binding Path=Username, UpdateSourceTrigger=PropertyChanged}"
         MinWidth="180" />

<PasswordBox ff:PasswordHelper.Attach="True" 
             ff:PasswordHelper.Password="{Binding Path=Password}" Width="130"/>

I have my TextBox, this is no problem, but in my ViewModel the Password is empty.

Am I doing something wrong or missing a step?

I put a breakpoint and sure enough the code enter the static helper class but it never updates my Password in my ViewModel.

Solution

Sorry, but you're doing it wrong.

People should have the following security guideline tattooed on the inside of their eyelids:
Never keep plain text passwords in memory.

The reason the WPF/Silverlight PasswordBox doesn't expose a DP for the Password property is security related.
If WPF/Silverlight were to keep a DP for Password it would require the framework to keep the password itself unencrypted in memory. Which is considered quite a troublesome security attack vector. The PasswordBox uses encrypted memory (of sorts) and the only way to access the password is through the CLR property.

I would suggest that when accessing the PasswordBox.Password CLR property you'd refrain from placing it in any variable or as a value for any property.
Keeping your password in plain text on the client machine RAM is a security no-no.
So get rid of that public string Password { get; set; } you've got up there.

When accessing PasswordBox.Password, just get it out and ship it to the server ASAP. Don't keep the value of the password around and don't treat it as you would any other client machine text. Don't keep clear text passwords in memory.

I know this breaks the MVVM pattern, but you shouldn't ever bind to PasswordBox.Password Attached DP, store your password in the ViewModel or any other similar shenanigans.

If you're looking for an over-architected solution, here's one:
1. Create the IHavePassword interface with one method that returns the password clear text.
2. Have your UserControl implement a IHavePassword interface.
3. Register the UserControl instance with your IoC as implementing the IHavePassword interface.
4. When a server request requiring your password is taking place, call your IoC for the IHavePassword implementation and only than get the much coveted password.

Just my take on it.

-- Justin