1 Introduction to the Spring Security REST plugin

The default behaviour of Spring Security is to store the authenticated principal in the HTTP session. However, in a RESTful scenario, we need to make sure our server is stateless.

If you are writing an API that will be used by other programs, you can use OAuth for this. But if you are exposing your API for a front-end Javascript client to implement a Single Page Interface, OAuth is not an option, specially if you want to authentication end users against your own user backend (eg: LDAP). In this case, a token-based authentication may be a more suitable implementation, like the following:

1. The client application requests and endpoint that requires authentication, so the server responds with a 401 response.
2. The client redirects the user to the login form.
3. The user enter credentials, and the client sends a request to the authentication endpoint. The server validates credentials, and if valid, generates, stores and sends back a token to the client.
4. The client then stores the token internally. It will be sent on every API method request.
5. The client sends again a request to the protected resource, passing the token as an HTTP header.
6. The server validates the token, and if valid, executes the actual operation requested.

As per the REST definition, the client is transferring its state on every request so the server is truly stateless. The approach to store tokens on the server is just an alternative to use HTTP basic authentication (see FAQ) (so credentials are not passed on every request). It also helps to perform the validation step (#5 in the diagram) faster, because the tokens, and the associated principal information may be cached. Finally, storing tokens gives you the chance to decide about expiration strategies.

More information about this strategy can be found on this post by James Ward.

This plugin helps you to wire your existing Spring Security authentication mechanism, provides you with ready-to-use token generation strategies and comes prepackaged with Memcached and GORM support for token storage.

Release History

• 17 February 2014
• 1.2.5
• 10 February 2014
• 1.2.4
• 4 February 2014
• 1.2.3
• 31 January 2014
• 1.2.2
• 15 January 2014
• 1.2.0
• 14 January 2014
• 1.1.0
• 13 January 2014
• 1.0.1
• 12 January 2014
• 1.0.0
• 10 January 2014
• 1.0.0.RC2
• 31 December 2013
• Initial 1.0.0.RC1 release.

2 Authentication Endpoint

If the authentication is successful, a token generator is used to generate a token, and a token storage implementation is used to store the token.

Finally, the JSON response sent back to the client is rendered by a restAuthenticationTokenJsonRenderer bean. The plugin offers you a default implementation that renders a response like this:

{
    "username": "john.doe",
    "token": "1a2b3c4d",
    "roles": [
        "ADMIN",
        "USER"
    ]
}

If you want your own, simply create a class implementing RestAuthenticationTokenJsonRenderer and wire it up in resources.groovy with name restAuthenticationTokenJsonRenderer.

The principal object stored in the security context, and passed to the JSON renderer, is coming from the configured authentication providers. In most cases, this will be a UserDetails object retrieved using the userDetailsService bean. If you want to render additional information in your JSON response, you have to:

1. Configure an alternative userDetailsService bean that retrieves the additional information you want, and put it in a principal object.
2. Configure an alternative restAuthenticationTokenJsonRenderer that reads that information from the restAuthenticationToken.principal object.

The following are the Config.groovy properties available:

Extracting credentials from the request

The plugin supports 2 ways of extracting the username and password: using request parameters, and using a JSON payload. For backwards compatibility, request parameters is the default option.

From request parameters

From a JSON request

To enable it:

The default implementation expects a request like this:

{
    "username": "john.doe",
    "password": "dontTellAnybody"
}

If your JSON request format is different, you can plug your own implementation by defining a class which extends AbstractJsonPayloadCredentialsExtractor. The default implementation looks like this:

@Log4j
class DefaultJsonPayloadCredentialsExtractor extends AbstractJsonPayloadCredentialsExtractor {
    UsernamePasswordAuthenticationToken extractCredentials(HttpServletRequest httpServletRequest) {
        def jsonBody = getJsonBody(httpServletRequest)
        log.debug "Extracted credentials from request params. Username: ${jsonBody.username}, password: ${jsonBody.password?.size()?'[PROTECTED]':'[MISSING]'}"
        new UsernamePasswordAuthenticationToken(jsonBody.username, jsonBody.password)
    }
}

Once you are done, register it in resources.groovy with the name credentialsExtractor.

2.1 Logout Endpoint

You can configure it in Config.groovy using this properties:

3 Token Generation

• Using java.security.SecureRandom.
• Using java.util.UUID.

The strategy used is configurable in Config.groovy:

Both of them generate tokens of 32 alphanumeric characters.

That should be enough for most of the human beings. But if you still want to provide your own implementation, simply write a class implementing TokenGenerator and wire it up in resources.groovy as tokenGenerator.

4 Token Storage

4.1 Memcached

For development, if you have Memcached installed locally with the default settings, just define grails.plugin.springsecurity.rest.token.storage.useMemcached = true. It should work.

4.2 GORM

The GORM support is still experimental.

To use GORM, those are the relevant configuration properties:

The domain class should look like this:

class AuthenticationToken {
    String tokenValue
    String username
}

In this case, instead of storing the whole UserDetails object, only the username is stored. This is because applications using this strategy will probably have the standard User and Role domain classes. Then, the username is passed to the default userDetailsService bean, which in the case of the default Spring Security Core GORM implementation will fetch the information from the mentioned domain classes.

5 Token Validation Filter

If the validation is successful, the principal object is stored in the security context. This allows you to use in your application @Secured, springSecurityService.principal and so on.

springSecurityService.currentUser expects a grails.plugin.springsecurity.userdetails.GrailsUser to perform a DB query. However, this plugins stores in the security context just a principal Object, because it does not assume you are using domain classes to store the users. Use springSecurityService.principal instead.

Validation Endpoint

There is also an endpoint available that you can call in case you want to know if a given token is valid. It looks for the token in a HTTP header as well, and if the token is still valid, it renders its JSON representation. If the token does not exist, it will render a grails.plugin.springsecurity.rest.login.failureStatusCode response (403 by default).

The relevant configuration properties are:

6 CORS support

The CORS plugin activates itself by default. If you don't want it for some environments, you can use cors.enabled = false within the appropriate environment block in your Config.groovy.

If you don't want CORS support at all, you can skip the plugin by excluding it when defining this plugin in your BuildConfig.groovy:

compile ':spring-security-rest:{{VERSION}}', {
    exclude 'cors
}

7 Delegating authentication to OAuth providers

However, there might be situations where you want to delegate the authentication against a third-party provider, like Google or Facebook. Unfortunately, your pure Javascript front-end application cannot request the providers directly using OAuth, because then the access keys will be made public.

So is this plugin's responsibility to provide endpoints so your Grails backend acts as a proxy for your front-end client.

The flow is something like the following:

1. The client application requests and endpoint that requires authentication, so the server responds with a 401 response (*).
2. The client redirects the user to the login form (*).
3. This time, instead of using username and password, the user clicks on "Login with Google" button.
4. Browser navigates to a Grails URL. Grails will generate a Google Login URL, giving Google a Grails callback URL.
5. Browser navigates to Google Login. User logs in, and Google redirects the browser to the Grails callback URL.
6. Browser navigates to that Grails callback URL. Then, Grails will use OAuth to fetch user information (like email) from Google. Based on that, will generate a REST API token and fetch and store principal information. The response from Grails will be a front-end URL where the token is a parameter.
7. The browser will navigate to that URL, and the Javascript logic will read the token from the URL and store it locally.
8. The client sends again a request to the protected resource, passing the token as an HTTP header (*).

The steps flagged with (*) remain unchanged from the normal flow.

To support OAuth, this plugin uses Profile & Authentication Client for Java. So you can use any OAuth (1.0 and 2.0) provider they support. This includes at the time of writing:

• Dropbox.
• Facebook.
• GitHub.
• Google.
• LinkedIn.
• Twitter.
• Windows Live.
• Wordpress.
• Yahoo.
• Paypal.

To start the OAuth authentication flow, from your frontend application, generate a link to <YOUR_GRAILS_APP>/oauth/authenticate/<provider>. The user clicking on that link represents step 4 in the previous diagram.

Note that you can define the frontend callback URL in Config.groovy under grails.plugin.springsecurity.rest.oauth.frontendCallbackUrl. You need to define a closure that will be called with the token value as parameter:

grails.plugin.springsecurity.rest.oauth.frontendCallbackUrl = { String tokenValue -> "http://my.frontend-app.com/welcome#token=${tokenValue}" }

You can also define the URL as a callback parameter in the original link, eg:

http://your-grails-api.com/oauth/authenticate/google?callback=http://your-frontend-app.com/auth-success.html?token=

In this case, the token will be concatenated to the end of the URL.

Upon successful OAuth authorisation (after step 6.1 in the above diagram), an OauthUser will be stored in the security context. This is done by a bean named oauthUserDetailsService. The default implementation delegates to the configured userDetailsService bean, passing the profile ID as the username:

class DefaultOauthUserDetailsService implements OauthUserDetailsService {
    @Delegate
    UserDetailsService userDetailsService
    OauthUser loadUserByUserProfile(UserProfile userProfile, Collection<GrantedAuthority> defaultRoles) {
        UserDetails userDetails
        OauthUser oauthUser
        try {
            userDetails = userDetailsService.loadUserByUsername userProfile.id
            userDetails.authorities.addAll defaultRoles
            oauthUser = new OauthUser(userDetails.username, userDetails.password, userDetails.authorities, userProfile)
        } catch (exception) {
            oauthUser = new OauthUser(userProfile.id, 'N/A', defaultRoles, userProfile)
        }
        return oauthUser
    }
}

If you want to provide your own implementation, define it in resources.groovy with bean name oauthUserDetailsService. Make sure you implements the interface OauthUserDetailsService

Below are some examples on how to configure it for Google, Facebook and Twitter.

7.1 Google

grails {
    plugin {
        springsecurity {
            rest {
                oauth {
                    frontendCallbackUrl = { String tokenValue -> "http://my.frontend-app.com/welcome#token=${tokenValue}" }
                    google {
                        client = org.pac4j.oauth.client.Google2Client
                        key = 'xxxx.apps.googleusercontent.com'
                        secret = 'xxx'
                        scope = org.pac4j.oauth.client.Google2Client.Google2Scope.EMAIL
                        defaultRoles = ['ROLE_USER', 'ROLE_GOOGLE']
                    }
                }
            }
        }
    }
}

The scope can be from any value of the enum org.pac4j.oauth.client.Google2Client.Google2Scope

7.2 Facebook

grails {
    plugin {
        springsecurity {
            rest {
                oauth {
                    frontendCallbackUrl = { String tokenValue -> "http://my.frontend-app.com/welcome#token=${tokenValue}" }
                    facebook {
                        client = org.pac4j.oauth.client.FacebookClient
                        key = 'xxx'
                        secret = 'yyy'
                        scope = 'email,user_location'
                        fields = 'id,name,first_name,middle_name,last_name,username'
                        defaultRoles = ['ROLE_USER', 'ROLE_FACEBOOK']
                    }
                }
            }
        }
    }
}

The scope is a comma-separated list, without blanks, of Facebook permissions. See the Facebook documentation for more details.

fields may contain a comma-separated list, without blanks, of user fields.

Both scope and fields are optional, but it's highly recommendable to fine tune those lists so you don't ask for information you don't need.

7.3 Twitter

grails {
    plugin {
        springsecurity {
            rest {
                oauth {
                    frontendCallbackUrl = { String tokenValue -> "http://my.frontend-app.com/welcome#token=${tokenValue}" }
                    twitter {
                        client = org.pac4j.oauth.client.TwitterClient
                        key = 'xxx'
                        secret = 'yyy'
                        defaultRoles = ['ROLE_USER', 'ROLE_TWITTER']
                    }
                }
            }
        }
    }
}

There is no additional configuration for Twitter.

8 Debugging

log4j = {
    ...
    debug  'com.odobo',
           'grails.app.controllers.com.odobo',
           'grails.app.services.com.odobo',
           'org.pac4j',
           'org.springframework.security'
    …
}

9 Frequently Asked Questions

Why this token-based implementation? Can't I use HTTP basic authentication?

In theory you can. The only restriction to be truly stateless is to not use HTTP sessions at all. So if you go with basic authentication, you need to transfer the credentials back and forth every time.

Let's think about that. Keep in mind that your frontend is a pure HTML/Javascript application, consuming a REST API from the Grails side. So the first time, the Javascript application will make an API query and will receive a 403 response indicating that authentication is required. Then you present the user a form to enter credentials, you grab them, encode them with Base64 and in the next request, you send an HTTP header like Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==.

Now remember you are doing RESTful application, so the session state is maintained in the client. That means that you would need to store that Base64 encoded string somewhere: cookies? HTML5 local storage? In any case, they are accessible using browser tools. And that's the point: there is a huge security risk because Base64 it's not encryption, just encoding. And it can be easily decoded.

You could argue that someone can access the token in the browser. Yes, but having the token will not allow him to obtain user's credentials. The tokens are just not decodable. And they can be revoked if necessary.

This is more or less the strategy used by OAuth: they use basically tokens in headers. More on this on the next question.

There is also more reasons to be in favour of tokens:

• With basic auth, every single API call would have to check credentials. In the token-based implementation, specially if you use Memcached, the authentication results are cached.
• With basic auth, you are sending the credentials all the time. Ok, you can use SSL, but still I think it's more elegant to use tokens.

Moreover, if you use tokens, you have the chance to implement expiration policies.

A couple of link with further explanations on the token-based flow:

• http://www.jamesward.com/2013/05/13/securing-single-page-apps-and-rest-services
• http://blog.brunoscopelliti.com/authentication-to-a-restful-web-service-in-an-angularjs-web-app

Why can't the API be secured with OAuth

Because to do so, you would need to store consumer key and consumer secret in the browser. Seriously, you don't want to do that. The problem with OAuth is that it's designed for when the consumer is a server-side application. And it just does not work well with pure Javascript front-ends. In this scenario, your frontend would be the OAuth consumer and your Grails backend the OAuth provider

A different story is to delegate the actual authentication to other OAuth providers: this scenario is possible, and actually this plugin supports it. In this case, the consumer will be the Grails application, and it's absolutely fine to store consumer keys on the server, as they are never exposed to the browser.

Why you didn't use any of the existing OAuth plugins? Why pac4j?

I'm aware of plugins like OAuth and Spring Security OAuth, but all of them rely on Spring Security Core's way of using HTTP sessions. So not acceptable.

I chose pac4j because:

1. They support major OAuth 2.0 providers out-of-the-box, whereas Scribe does not.
2. It's deadly simple and works just fine.

I'm also aware of a pac4j-spring-security module. See my previous response on HTTP sessions.

Dude, this is awesome. How can I compensate you?

I doubt you can :). You may try giving me free beers the next time you see me in a conference. Or you can just express your gratitude via Twitter