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Contributing New Providers

NOTE: The primary audience for this documentation are contributors to the main Backstage project that want to add support for new authentication providers. While you can follow it to implement your own custom providers it is much more advanced than using our built-in providers.

How Does Authentication Work?

The Backstage application can use various external authentication providers for authentication. An external provider is wrapped using an AuthProviderRouteHandlers interface for handling authentication. This interface consists of four methods. Each of these methods is hosted at an endpoint (by default) /api/auth/[provider]/method, where method performs a certain operation as follows:

  /auth/[provider]/start -> Initiate a login from the web page
  /auth/[provider]/handler/frame -> Handle a finished authentication operation
  /auth/[provider]/refresh -> Refresh the validity of a login
  /auth/[provider]/logout -> Log out a logged-in user

The flow is as follows:

  1. A user attempts to sign in.
  2. A popup window is opened, pointing to the auth endpoint. That endpoint does initial preparations and then re-directs the user to an external authenticator, still inside the popup.
  3. The authenticator validates the user and returns the result of the validation (success OR failure), to the wrapper's endpoint (handler/frame).
  4. The handler/frame rendered webpage will issue the appropriate response to the webpage that opened the popup window, and the popup is closed.
  5. The user signs out by clicking on a UI interface and the webpage makes a request to logout the user.

Implementing Your Own Auth Wrapper

The core interface of any auth wrapper is the AuthProviderRouteHandlers interface. This interface has four methods corresponding to the API described in the initial section. Any auth wrapper will have to implement this interface.

When initiating a login, a pop-up window is created by the frontend, to allow the user to initiate a login. This login request is done to the /start endpoint which is handled by the start method.

The start method re-directs to the external auth provider who authenticates the request and re-directs the request to the /handler/frame endpoint, which is handled by the frameHandler method.

The frameHandler returns an HTML response, containing a script that does a postMessage to the frontend window, containing the result of the request. The WebMessageResponse type is the message sent by the postMessage to the frontend.

A postMessageResponse utility function wraps the logic of generating a postMessage response that ensures that CORS is successfully handled. This function takes an express.Response, a WebMessageResponse and the URL of the frontend (appOrigin) as parameters and return an HTML page with the script and the message.

There is a helper class for OAuth2 based authentication providers, OAuthAdapter. This class implements the AuthProviderRouteHandlers interface for you, and instead requires you to implement OAuthHandlers, which is significantly easier.

Auth Environment Separation

The concept of an env is core to the way the auth backend works. It uses an env query parameter to identify the environment in which the application is running (development, staging, production, etc). Each runtime can simultaneously support multiple environments at the same time and the right handler for each request is identified and dispatched to, based on the env parameter.

OAuthEnvironmentHandler is a utility wrapper for an OAuthHandlers that implements the AuthProviderRouteHandlers interface while supporting multiple envs.

To instantiate OAuth providers (the same but for different environments), use OAuthEnvironmentHandler.mapConfig. It's a helper to iterate over a configuration object that is a map of environments to configurations. See one of the existing OAuth providers for an example of how it is used.

Given the following configuration:

development:
  clientId: abc
  clientSecret: secret
production:
  clientId: xyz
  clientSecret: supersecret

The OAuthEnvironmentHandler.mapConfig(config, envConfig => ...) call will split the config by the top level development and production keys, and pass on each block as envConfig.

For convenience, the AuthProviderFactory is a factory function that has to be implemented which can then generate a AuthProviderRouteHandlers for a given provider.

All of the supported providers provide an AuthProviderFactory that returns an OAuthEnvironmentHandler, capable of handling authentication for multiple environments.

Passport

We chose Passport as our authentication platform due to its comprehensive set of supported authentication strategies.

How to add a new strategy provider

Quick guide

1. Install the passport-js based provider package.

2. Create a new folder structure for the provider.

3. Implement the provider, extending the suitable framework if needed.

4. Add the provider to the backend.

Installing the dependencies:

cd plugins/auth-backend
yarn add passport-provider-a
yarn add @types/passport-provider-a

Create implementation

Make a new folder with the name of the provider following the below file structure:

plugins/auth-backend/src/providers/providerA
├── index.ts
└── provider.ts

plugins/auth-backend/src/providers/providerA/provider.ts defines the provider class which implements a handler for the chosen framework.

Adding an OAuth based provider

If we're adding an OAuth based provider we would implement the OAuthHandlers interface. By implementing this interface we can use the OAuthProvider class provided by lib/oauth, meaning we don't need to implement the full AuthProviderRouteHandlers interface that providers otherwise need to implement.

The provider class takes the provider's options as a class parameter. It also imports the Strategy from the passport package.

import { Strategy as ProviderAStrategy } from 'passport-provider-a';

export type ProviderAProviderOptions = OAuthProviderOptions & {
  // extra options here
}

export class ProviderAAuthProvider implements OAuthHandlers {
  private readonly _strategy: ProviderAStrategy;

  constructor(options: ProviderAProviderOptions) {
    this._strategy = new ProviderAStrategy(
      {
        clientID: options.clientId,
        clientSecret: options.clientSecret,
        callbackURL: options.callbackUrl,
        passReqToCallback: false,
        response_type: 'code',
        /// ... etc
      }
      verifyFunction, // See the "Verify Callback" section
    );
  }

  async start() {}
  async handler() {}
}

Adding an non-OAuth based provider

An non-OAuth based provider could implement AuthProviderRouteHandlers instead.

type ProviderAOptions = {
  // ...
};

export class ProviderAAuthProvider implements AuthProviderRouteHandlers {
  private readonly _strategy: ProviderAStrategy;

  constructor(options: ProviderAOptions) {
    this._strategy = new ProviderAStrategy(
      {
        // ...
      },
      verifyFunction, // See the "Verify Callback" section
    );
  }

  async start() {}
  async frameHandler() {}
  async logout() {}
  async refresh() {} // If supported
}

Integration Wrapper

Each provider exports an object that provides a way to create new instances of the provider, along with related utilities like predefined sign-in resolvers.

The object is created using createAuthProviderIntegration, with the most important part being the create method that acts as the factory function for our provider.

The factory should return an implementation of AuthProviderFactory, which passes in a object with utilities for configuration, logging, token issuing, etc. The factory should return an implementation of AuthProviderRouteHandlers.

The factory is what decides the mapping from static configuration to the creation of auth providers. For example, OAuth providers use OAuthEnvironmentHandler to allow for multiple different configurations, one for each environment, which looks like this;

export const okta = createAuthProviderIntegration({
  create(options?: {
    /**
     * The profile transformation function used to verify and convert the auth response
     * into the profile that will be presented to the user.
     */
    authHandler?: AuthHandler<OAuthResult>;

    /**
     * Configure sign-in for this provider, without it the provider can not be used to sign users in.
     */
    signIn?: {
      /**
       * Maps an auth result to a Backstage identity for the user.
       */
      resolver: SignInResolver<OAuthResult>;
    };
  }) {
    return ({ providerId, globalConfig, config, resolverContext }) =>
      OAuthEnvironmentHandler.mapConfig(config, envConfig => {
        // read options from config
        const clientId = envConfig.getString('clientId');
        const clientSecret = envConfig.getString('clientSecret');

        // Use provided auth handler, or create a default one
        const authHandler: AuthHandler<OAuthResult> = options?.authHandler
          ? options.authHandler
          : async ({ fullProfile, params }) => ({
              profile: makeProfileInfo(fullProfile, params.id_token),
            });

        // instantiate our OAuthHandlers implementation
        const provider = new OktaAuthProvider({
          audience,
          clientId,
          clientSecret,
          callbackUrl,
          authHandler,
          signInResolver: options?.signIn?.resolver,
          resolverContext,
        });

        // Wrap the OAuthHandlers with OAuthProvider, which implements AuthProviderRouteHandlers
        return OAuthProvider.fromConfig(globalConfig, provider, {
          providerId,
          tokenIssuer,
        });
      });
  },
  resolvers: {
    /**
     * Looks up the user by matching their email local part to the entity name.
     */
    emailLocalPartMatchingUserEntityName: () => commonByEmailLocalPartResolver,

    // ... additional predefined resolvers
  },
});

The purpose of the different environments is to allow for a single auth-backend to serve as the authentication service for multiple different frontend environments, such as local development, staging, and production.

Verify Callback

Strategies require what is known as a verify callback. The purpose of a verify callback is to find the user that possesses a set of credentials. When Passport authenticates a request, it parses the credentials contained in the request. It then invokes the verify callback with those credentials as arguments [...]. If the credentials are valid, the verify callback invokes done to supply Passport with the user that authenticated.

If the credentials are not valid (for example, if the password is incorrect), done should be invoked with false instead of a user to indicate an authentication failure.

http://www.passportjs.org/docs/configure/

plugins/auth-backend/src/providers/providerA/index.ts is simply re-exporting the factory function to be used for hooking the provider up to the backend.

export { createProviderAProvider } from './provider';

Hook it up to the backend

plugins/auth-backend/src/providers/factories.ts When the auth-backend starts it sets up routing for all the available providers by calling the factory function of each provider. You need to import the factory function from the provider and add it to the factory:

import { createProviderAProvider } from './providerA';

const factories: { [providerId: string]: AuthProviderFactory } = {
  providerA: createProviderAProvider,
};

By doing this auth-backend automatically adds these endpoints:

router.get('/auth/providerA/start');
router.get('/auth/providerA/handler/frame');
router.post('/auth/providerA/handler/frame');
router.post('/auth/providerA/logout');
router.get('/auth/providerA/refresh'); // if supported
router.post('/auth/providerA/refresh'); // if supported

As you can see each endpoint is prefixed with both /auth and its provider name.

Test the new provider

You can curl -i localhost:7007/api/auth/providerA/start and which should provide a 302 redirect with a Location header. Paste the URL from that header into a web browser and you should be able to trigger the authorization flow.

Last updated on 1/29/2023
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