Forms authentication

Nathan Armstrong edited this page Apr 6, 2017 · 19 revisions

This document provides an overview on how to enable forms authentication in your Nancy application. For a full working sample, please refer to the Nancy.Demo.Authentication.Forms project in the Nancy solution.

To enable forms authentication in your application, you need to complete the following steps

  1. Install the Nancy.Authentication.Forms package
  2. Implement an IUserMapper
  3. Implement routes to handle login and logout
  4. Configure and enable Forms Authentication

The user mapper

The user mapper is responsible for mapping a user from an identifier. The identifier is the token that will be put in the authentication cookie which will be used to re-establish the identity of the user that is performing the request, so that you do not need to enter your credentials for each request.

We have chosen to use a GUID as the identifier. The reason for this is that using something like the username or id is a potential vulnerability, because, if the cookie encryption was somehow compromised, it would be easy to spoof the identity of another user by guessing their username or id.

The identifier should never be read by a human, so making it a GUID also discourages that it is ever displayed anywhere, and each GUID is both unique and non-sequential, making it a lot harder to spoof than the identity of someone else.

It is also important to know that the identifier should be treated as permanent for the user that it was generated for and will be reused across requests and application sessions.

The IUserMapper interface is defined in the following way

public interface IUserMapper
    /// <summary>
    /// Get the real username from an identifier
    /// </summary>
    /// <param name="identifier">User identifier</param>
    /// <param name="context">The current NancyFx context</param>
    /// <returns>Matching populated IUserIdentity object, or empty</returns>
    IUserIdentity GetUserFromIdentifier(Guid identifier, NancyContext context);

A single method whose sole responsibility is to take the GUID identifier and return the corresponding user. If the identifier could not be mapped to a user then null should be returned. For more information about the IUserIdentity interface, refer to the [Authentication overview](Authentication overview).

Modifying your application to handle forms authentication

Once you have implemented an IUserMapper, it is time to add the resources to your application that are responsible for handling the login and logout procedures. These are implemented using normal routes, and should be placed in a module that does not require the user to be authenticated to access.

Below is an example of the basic structure of such a module. Please note that the paths of the resources, and the name of the module, can be anything; it’s not necessary to follow the naming in this example:

public class LoginModule : NancyModule
    public LoginModule()
        Get["/login"] = parameters => {
            // Called when the user visits the login page or is redirected here because
            // an attempt was made to access a restricted resource. It should return
            // the view that contains the login form

        Get["/logout"] = parameters => {
            // Called when the user clicks the sign out button in the application. Should
            // perform one of the Logout actions (see below)

        Post["/login"] = parameters => {
            // Called when the user submits the contents of the login form. Should
            // validate the user based on the posted form data, and perform one of the
            // Login actions (see below)

When the user logs in or out, certain actions need to be taken, such as creating or removing the authentication cookie and redirecting the user to the appropriate location. To aid with these common tasks, there are several extension methods (located in the Nancy.Authentication.Forms namespace) that you can use

  • LoginAndRedirect - Logs the user in and redirects the user back to the url that they came from. You can specify a fallback url that will be used if no redirect url was available. If you're using a form for posting, be sure to use action="" as it will keep your returnUrl intact.
  • LoginWithoutRedirect - Logs the user in and returns a response with status code 200 (OK)
  • Login will call the IsAjaxRequest extension method on the current request and perform a LoginAndRedirect if it was not an Ajax call, otherwise it will perform LoginWithoutRedirect
  • LogoutAndRedirect - Logs the user out and redirects to the provided url
  • LogoutWithoutRedirect - Logs the user out and returns a response with status code 200 (OK)
  • Logout will call the IsAjaxRequest extension method on the current request and perform a LogoutAndRedirect if it was not an Ajax call, otherwise it will perform LogoutWithoutRedirect

Note1: The IsAjaxRequest extension method (found in Nancy.Extensions.RequestExtensions) will look for the X-Requested-With header and return true if it contains the value XMLHttpRequest

Note 2: Please make sure the routes you define for displaying the login page and taking login credentials are not secured behind a RequiresAuthentication call - you may laugh, but this is quite a common mistake :-)

Enabling forms authentication

The only thing left to do now is actually enable Forms Authentication.
Forms Authentication can be enabled for:

  • All modules (ie. application wide)
  • Per module (ie. explicitly on a module by module basis).

Most people usually enable it for all modules (or not at all). So why would you enable it on a per module basis only? If you've got some routes that might be using [Stateless Authentication](Stateless Authentication). This means part of your website might be returning json results (so half of the site is an API) where the other half is like a normal website with views returned, etc. Having a hybrid authentication site means you wouldn't want to have forms authentication applied to those API routes, and vice versa.

All modules (ie application wide)

FormsAuthentication.Enable(pipelines, formsAuthConfiguration);

Per module

FormsAuthentication.Enable(module, formsAuthConfiguration);

For the all modules way, It can either be added to the ApplicationStartup or RequestStartup in your Bootstrapper class. Which you should choose often depends on your implementation of the IUserMapper, that is, should the user mapper have an application or request lifetime.

The formsAuthConfiguration variable that is passed into the FormsAuthentication.Enable method, is an instance of the FormsAuthenticationConfiguration type, which enables you to customize the behavior of the forms authentication provider.

For instance, here is a basic configuration

var formsAuthConfiguration =
    new FormsAuthenticationConfiguration()
        RedirectUrl = "~/login",
        UserMapper = container.Resolve<IUserMapper>(),

It configures the Forms Authentication to redirect the user to ~/login when an unauthorised user attempts to gain access to a protected resource and it tells it which user mapper it should use.

Depending on if this is placed in ApplicationStartup or RequestStartup the mapper will have different lifetimes. Also, please note that the method for resolving the mapper, from the container, will vary depending on which bootstrapper you are using.

The following configurations can be made

  • RedirectingQuerystringKey - The name of the querystring parameter that will contain the URL of the secure resource that was being accessed while being unauthorized. This is used to redirect the user back once the login was successful. The default name is returnUrl
  • RedirectingUrl - The (relative) url that the user should be redirected to when trying to access a secure resource when not being authenticated. This is usually your login page and should be in the format ~/login
  • UserMapper - The IUserMapper that should be used during authentication
  • RequiresSSL - Determines if the forms authentication cookie can only be accessed over a secure request or not. The default is false
  • DisableRedirect - Determines whether to redirect to the login page during an unauthorized access
  • CryptographyConfiguration - The CryptographyConfiguration.Default that should be used when working with the forms authentication cookie. If this is not specified then CryptographyConfiguration.Default will be used as the default.

A word on cryptography

If you do not specify a value for the CryptographyConfiguration property on the FormsAuthenticationConfiguration object, then it will default to using CryptographyConfiguration.Default.

The default behavior uses the RandomKeyGenerator which means that a new crypto key is generated each time the application starts, so if your application is restarted, or the appdomain recycled, then user’s authentication cookies will become invalid; this will also be the case between machines in a load balanced environment. Don’t panic though! Changing this configuration is simple to do, for more in depth information read about the [Cryptographic configurations](The cryptography helpers).

For this reason you should always use your own CryptographyConfiguration, below is an example of what that could look like when used together with forms authentication.

var cryptographyConfiguration = new CryptographyConfiguration(
    new RijndaelEncryptionProvider(new PassphraseKeyGenerator("SuperSecretPass", new byte[] { 1, 2, 3, 4, 5, 6, 7, 8 })),
    new DefaultHmacProvider(new PassphraseKeyGenerator("UberSuperSecure", new byte[] { 1, 2, 3, 4, 5, 6, 7, 8 })));

var config = 
    new FormsAuthenticationConfiguration()
        CryptographyConfiguration = cryptographyConfiguration,
        RedirectUrl = "/login",
        UserMapper = container.Resolve<IUserMapper>(),

More Info

Forms authentication with nancyfx

Multiple forms authentication sections

« Part 22. Stateless Authentication  —  Documentation overview  —  Part 22. Basic Authentication »

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