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SAML Authenticator Web 8.5 with ADFS

This post continues the setup of Single Sign-On for Web 8.5 with ADFS presented in an earlier blog post.

This SAML Authenticator is a .NET HTTP Module that is configured to intercept all requests going into the SDL Web website (the CME) and do the following:
  • if request is post-back from ADFS
    • decrypt SAML token
    • extract user name
    • set cookie with user name
    • set user name in request SSO header
  • else
    • if cookie exists
      • extract user name from cookie
      • set user name in request SSO header
    • else
      • redirect browser to ADFS form-login
The cookie is encrypted, in order to prevent the user name from being spoofed.

In more detail, the code is as follows:

public void BeginRequest()
{
    string user = GetUserData();
    if (user == null)
    {
        Request.Headers.Remove(Configuration.HeaderUser);
    }
    else
    {
        Request.Headers[Configuration.HeaderUser] = user;
    }
}

The method executes when a request is intercepted. The configuration HeaderUser is the name of the SSO header that the SDL Web SsoAgentHttpModule is expecting to be set, in order to impersonate that user.

The method GetUserData:

private string GetUserData()
{
    string result = null;
    SamlResponse samlResponse;

    if ((samlResponse = new SamlResponse()).IsValid)
    {
        UserData user = samlResponse.ExtractUser();

        if (user == null)
        {
            new SamlRequest().ForceLogin();
        }
        else
        {
            Response.SetCookie(new HttpCookie(Configuration.CookieName, Util.RsaEncrypt(user.UserId)));

            if (string.IsNullOrEmpty(user.OriginalUrl))
            {
                result = user.UserId;
            }
            else
            {
                Response.Redirect(user.OriginalUrl);
            }
        }
    }
    else
    {
        HttpCookie cookie = Request.Cookies[Configuration.CookieName];

        if (cookie == null)
        {
            new SamlRequest().ForceLogin();
        }
        else
        {
            result = Util.RsaDecrypt(cookie.Value);
            if (result == null)
            {
                new SamlRequest().ForceLogin();
            }
        }
    }

    return result;
}

The classes SamlRequest and SamlResponse handle the request redirect to ADFS login, and response token handling respectively.

SamleResponse.ExtractUser method is presented below:

public UserData ExtractUser()
{
    UserData user = new UserData();

    AdfsClient adfsClient = new AdfsClient();
    Saml2SecurityToken token = adfsClient.DecryptSaml2Token(SamlResponseXml);
    IPrincipal principal = adfsClient.GetPrincipal(token);

    ExtractClaims(principal, user);

    return user;
}

The code above makes use of the custom AdfsClient class, which handles communication and token handling with the ADFS server.

The code first decodes the SAML encrypted XML and converts the string representation of the token into a .NET System.IdentityModel.Tokens.Sam2SecurityToken object.

public Saml2SecurityToken DecryptSaml2Token(string tokenXml)
{
    using (StringReader stringReader = new StringReader(tokenXml))
    using (XmlReader reader = XmlReader.Create(stringReader))
    {
        reader.ReadToFollowing("EncryptedAssertion", SAML2_ASSERTION);
        return (Saml2SecurityToken)SecurityTokenHandlers.ReadToken(reader);
    }
}

public ClaimsPrincipal GetPrincipal(SecurityToken decryptedToken)
{
    ReadOnlyCollection<ClaimsIdentity> identities = SecurityTokenHandlers.ValidateToken(decryptedToken);
    return new ClaimsPrincipal(identities[0]);
}

The latter method, GetPrincipal, validates the decrypted token and by doing so, it creates a collection of ClaimsIdentity objects contained in that token. We then return a new ClaimsPrincipal instance based on the first identity in the token.

Lastly, the SamlRequest.ForceLogin method simply outputs an HTML document that automatically performs a JavaScript POST to the ADFS passive-authentication login-form. It also passes in the Relying Party identifier for this application (without such an identifier, the ADFS won't show the form-login screen).

We must also configure the HTTP Module in file web.config of the SDL Web website, located in folder [SDLWebHome]\web.


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