Impersonate using Forms Authentication

Question

I have an ASP.NET site that must use Forms Authentication and not Windows Authentication to access a ActiveDirectoryMembershipProvider. The site must use forms because they need a designed input form instead of the browser authentication popup that Windows authentication uses.

The site needs to impersonate the user logged in via Active Directory to access user specific files.

However, the WindowsIdentity.GetCurrent() is not the same as the HttpContext.Current.User.Identity although my web.config contains:

<authentication mode="Forms">
    <forms loginUrl="login.aspx" timeout="480"/>
</authentication>
<identity impersonate="true" />

I cannot use LoginUser() and the WindowsIdentity.Impersonate() because I need to impersonate as the AD user to get their specific permissions, and I don't know the user's password because Forms takes care of logging in.

Is it possible maybe from the login.aspx.cs, to take the System.Web.UI.WebControls.Login.Password, then save the LoginUser() token in a session variable for WindowsIdentity.Impersonate() later? Or maybe a much more secure method of Impersonating the right way?

I'm confused why Forms authentication can't automatically <identity impersonate="true" />

I've read this http://msdn.microsoft.com/en-us/library/ms998351.aspx but it uses Windows Authentication.

Answer 1

Impersonating a user using Forms Authentication can be done. The following code does work.

The Visual Studio Magazine article referred to by Robert is an excellent resource. There are a some issues with the example code in the article, so I've included some working code below.

Note: If you are using Visual Studio, make sure to launch it "Run as Administrator" to avoid problems with UAC blocking impersonation.

// in your login page (hook up to OnAuthenticate event)
protected void LoginControl_Authenticate(object sender, AuthenticateEventArgs e)
{
    int token;
    // replace "YOURDOMAIN" with your actual domain name
    e.Authenticated = LogonUser(LoginUser.UserName,"YOURDOMAIN",LoginUser.Password,8,0,out token);
    if (e.Authenticated) {
        Session.Add("principal", new WindowsPrincipal(new WindowsIdentity(new IntPtr(token))));
    }
}

[DllImport("advapi32.dll", SetLastError = true)]
public static extern bool LogonUser(string lpszUsername, string lpszDomain, string lpszPassword,
    int dwLogonType, int dwLogonProvider, out int TokenHandle);


// in global.asax.cs
void Application_PreRequestHandlerExecute(object send, EventArgs e)
{
    if (Thread.CurrentPrincipal.Identity.IsAuthenticated == true && HttpContext.Current.Session != null) {
        WindowsPrincipal windowsPrincipal = (WindowsPrincipal)Session["principal"];
        Session["principal"] = (GenericPrincipal)Thread.CurrentPrincipal;
        Thread.CurrentPrincipal = windowsPrincipal;
        HttpContext.Current.User = windowsPrincipal;
        HttpContext.Current.Items["identity"] = ((WindowsIdentity)windowsPrincipal.Identity).Impersonate();
    }
}

// in global.asax.cs
void Application_PostRequestHandlerExecute(object send, EventArgs e)
{
    if (HttpContext.Current.Session != null && Session["principal"] as GenericPrincipal != null) {
        GenericPrincipal genericPrincipal = (GenericPrincipal)Session["principal"];
        Session["principal"] = (WindowsPrincipal)Thread.CurrentPrincipal;
        Thread.CurrentPrincipal = genericPrincipal;
        HttpContext.Current.User = genericPrincipal;
        ((WindowsImpersonationContext)HttpContext.Current.Items["identity"]).Undo();
    }
}

// test that impersonation is working (add this and an Asp:Label to a test page)
protected void Page_Load(object sender, EventArgs e)
{
    try {
        // replace YOURSERVER and YOURDB with your actual server and database names
        string connstring = "data source=YOURSERVER;initial catalog=YOURDB;integrated security=True";
        using (SqlConnection conn = new SqlConnection(connstring)) {
            conn.Open();
            SqlCommand cmd = new SqlCommand("SELECT SUSER_NAME()", conn);
            using (SqlDataReader rdr = cmd.ExecuteReader()) {
                rdr.Read();
                Label1.Text = "SUSER_NAME() = " + rdr.GetString(0);
            }
        }
    }
    catch {
    }
}

Update:

You should also handle Application_EndRequest, because calls like Response.End() will bypass Application_PostRequestHandlerExecute.

Another issue is that the WindowsIdentity may get garbage collected, so you should create a new WindowsIdentity and WindowsPrincipal from the logon token on every request.

Update2:

I'm not sure why this is getting downvoted, because it works. I've added the pinvoke signature and some test code. Again, launch Visual Studio using "Run as Administrator". Google how to do that if you don't know how.

Answer 2

If your users are using IE then you can turn on integrated security for the website and your users will be authenticated silently (no login dialog, no login page). Your impersonation will then work. If you need to target other browsers then this may not work (the user will probably be presented with a login dialog).

Your current impersonation will never work because your users are logging in using an account other than their domain account. You can't expect the site to impersonate a user which hasn't supplied his credentials. That would go against basic security principals.

Answer 3

You may find this useful:

• how to create a login screen that allows Admin users to log in as another user in the user database

EDIT

On reading your question more closely, I am not sure if that approach would work with your scenario though; when you login using Forms Authentication and Impersonate Active Directory user

Answer 4

We got the same problem recently, the customer wanted their users can log in by AD account and then this credential must be used to access Analysis Service as well as all other databases. They wanted it that way because they implemented an auditing system and all access must be done by current logged in account.

We tried Forms authentication and Win32 LogonUser() API for impersonating part, it worked but it also asks us for user's password as plain text. Later, we decided to utilized Windows authentication, it saves us lot of time (no more AD authentication, impersonate manually). Of course, there was also no fancy login page.

Answer 5

For just in case, and a bit late, i found something that works for me and it's really simple but of course is only for testing purposes...

Just set a cookie with your username.

//Login button. You can give whatever input to the form
protected void Login_Click(object sender, EventArgs e)
{
    FormsAuthentication.SetAuthCookie("your_username", createPersistentCookie: true);
    Response.Redirect("~/");
}

Any comments accepted...

Answer 6

In Visual Studio install NuGet Open your solution

Then in the package console run Install-Package 51Degrees.mobi

It will then add 51Degrees to your website. You can then edit the 51Degrees.mobi.config in your App Data to remove the redirect section.

You will now have up to date browser capabilities