Writing a Custom OAuth 2.0 Grant Type

OAuth 2.0 authorization servers provide support for four main grant types according to the OAuth 2.0 specification. It also has the flexibility to support any custom grant types. This topic provides instructions on how to implement a custom grant type for OAuth 2.0 authorization server and how to extend the behavior of default grant types.

The WSO2 Identity Server is used as the OAuth 2.0 authorization server implementation, which is an open source implementation.

Implementing a new grant type

When using the WSO2 Identity Server, you must do the following to implement a new grant type.

1. Implement the following two extensions.
   • GrantTypeHandler - This is the implementation of the grant type. Here you can implement the way, it must be validated and how token must be issued. You can write the new implementation by implementing the “AuthorizationGrantHandler” interface or by extending “AbstractAuthorizationGrantHandler”. In most cases, it is enough to extend the “AbstractAuthorizationGrantHandler” in the WSO2 OAuth component.
   • GrantTypeValidator - This is used to validate the grant request that is sent to the /token endpoint. You can define what parameters must be in the request and define the validation of them. You can write the new implementation by extending the “AbstractValidator” in Apache Amber component.
2. When implementation is done, package your class as a .jar file and place it in the <IS_HOME>/repository/component/lib directory.

3. To register the custom grant type, configure the <IS_HOME>/repository/conf/identity/identity.xml file by adding a new entry, in a manner similar to the following example, under the <OAuth><SupportedGrantTypes> element. Add a unique identifier between the <GrantTypeName> tags as seen in the code block below.

   <SupportedGrantType>
   	<GrantTypeName>grant type identifier </GrantTypeName>
   	<GrantTypeHandlerImplClass>full qualified class name of grant handler</GrantTypeHandlerImplClass>
   	<GrantTypeValidatorImplClass>full qualified class name of grant validator</GrantTypeValidatorImplClass>
   	<IdTokenAllowed>true</IdTokenAllowed>
   </SupportedGrantType>

   By making <IdTokenAllowed> 'true', it allows the flexibility to control IDtoken issuing for each grant and OIDC scope validator validates the grant types which should support with "openid" scope.

   To test this out, follow the instructions below to implement a custom-grant type sample. 

Using the grant type sample

The sample demonstrated here defines a new sample grant type called the "mobile" grant type. It is similar to the password grant type and the only difference is that a mobile number will be passed through instead of a password. The request to the /token API must contain the following two request parameters.

• grant_type=mobile
• mobileNumber=044322433

You can access the new grant type project sample from here. The grant handler and validator class is inside the org.wso2.sample.identity.oauth2.grant.mobile package. This can be modified as required.

Resources

The following Maven buildable source is compatible with WSO2 Identity Server 5.1.0 onwards. The attached .jar file can be directly used as well.

1. To generate the .jar file you must run the following Apache Maven command in the sample's location using the command line.

   mvn clean install

2. Copy the .jar file in target directory into the <IS_HOME>/repository/component/lib directory. You can also modify the project and build it using Apache Maven 3.

3. Configure the following in the <IS_HOME>/repository/conf/identity/identity.xml file under the <OAuth><SupportedGrantTypes> element.
   

   <SupportedGrantType>
   	<GrantTypeName>mobile</GrantTypeName>
   	<GrantTypeHandlerImplClass>org.wso2.sample.identity.oauth2.grant.mobile.MobileGrant</GrantTypeHandlerImplClass>
   	<GrantTypeValidatorImplClass>org.wso2.sample.identity.oauth2.grant.mobile.MobileGrantValidator</GrantTypeValidatorImplClass>
   	<IdTokenAllowed>true</IdTokenAllowed>
   </SupportedGrantType>

4. Restart the server.
5. Configure the new OAuth grant type.
   
   1. Sign in to the WSO2 Identity Server. Enter your username and password to log on to the Management Console. 
   2. Navigate to the Main menu to access the Identity menu. Click Add under Service Providers.
   3. Fill in the Service Provider Name and provide a brief Description of the service provider. See Adding a Service Provider for more information.
   4. Expand the OAuth/OpenID Connect Configuration and click Configure.
   5. Enter a callback URL. For example, http://localhost:8080/playground2/oauth2client. 
   6. Click Add.
      
   7. The OAuth Client Key and OAuth Client Secret will now be visible.
6. Send the grant request to the /token API using a cURL command.

   1. The HTTP POST body must contain the following two parameters: grant_type=mobile and mobileNumber.

      grant_type=mobile&mobileNumber=0333444

   2. Replace clientid:clientsecret with the OAuth Client Key and OAuth Client Secret respectively and run the following sample cURL command in a new terminal window. 

      curl --user clientid:clientsecret -k -d "grant_type=mobile&mobileNumber=0333444" -H "Content-Type: application/x-www-form-urlencoded" https://localhost:9443/oauth2/token

   3. You will receive the following JSON response with the access token.

      {"token_type":"bearer","expires_in":2823,"refresh_token":"26e1ebf16cfa4e67c3bf39d72d5c276","access_token":"d9ef87802a22cf7682c2e77df72c735"}

Customizing an existing grant type

As an alternative to creating a custom OAuth grant type, you can customize one of the existing grant types. The following two classes are sample implementations of customizing the password grant type in particular but any other grant type can be customized as well.

• RoleBasedPasswordGrant - This does some RBAC validation apart from the authentication before granting access.
• ModifiedAccessTokenPasswordGrant - This implementation customized the access token value.

This section provides instructions on how to 

1. Copy the .jar file into the <IS_HOME>/repository/component/lib directory. You can also modify the project and build it using Apache Maven 3.

2. Configure the following in the <IS_HOME>/repository/conf/identity/identity.xml file under the <OAuth><SupportedGrantTypes> element.

   <SupportedGrantType>
   	<GrantTypeName>password</GrantTypeName>
   	<GrantTypeHandlerImplClass>org.wso2.sample.identity.oauth2.grant.password.ModifiedAccessTokenPasswordGrant</GrantTypeHandlerImplClass>
   </SupportedGrantType>

3. Restart the server.

4. Configure the OAuth grant type you customized.
   
   1. Sign in to the WSO2 Identity Server. Enter your username and password to log on to the Management Console. 
   2. Navigate to the Main menu to access the Identity menu. Click Add under Service Providers.
   3. Fill in the Service Provider Name and provide a brief Description of the service provider. See Adding a Service Provider for more information.
   4. Expand the OAuth/OpenID Connect Configuration and click Configure.
   5. Enter a callback URL. For example, http://localhost:8080/playground2/oauth2client. 
   6. Click Add.
   7. The OAuth Client Key and OAuth Client Secret will now be visible.

5. Send the password grant request to the /token API using a cURL command.

   1. Replace clientid:clientsecret with the OAuth Client Key and OAuth Client Secret respectively and run the following sample cURL command in a new terminal window. 

      curl --user clientid:clientsecret -k -d "grant_type=password&username=admin&password=admin" -H "Content-Type: application/x-www-form-urlencoded" https://localhost:9443/oauth2/token

   2. You will see the following json response and the modified access token with an email address.

      {"token_type":"bearer","expires_in":2955,"refresh_token":"6865c8d67b42c0c23e634a8fc5aa81f","access_token":"982f40d4-0bb6-41ce-ac5a-1da06a83e475asela@soasecurity.org"}

if(mobileNumber != null) {
    //validate mobile number
    authStatus =  isValidMobileNumber(mobileNumber);

    if(authStatus) {
        // if valid set authorized mobile number as grant user
        AuthenticatedUser mobileUser = new AuthenticatedUser();
        mobileUser.setUserName(mobileNumber);
        oAuthTokenReqMessageContext.setAuthorizedUser(mobileUser);
        oAuthTokenReqMessageContext.setScope(oAuthTokenReqMessageContext.getOauth2AccessTokenReqDTO().getScope());
    } else{
        ResponseHeader responseHeader = new ResponseHeader();
        responseHeader.setKey("SampleHeader-999");
        responseHeader.setValue("Provided Mobile Number is Invalid.");
        oAuthTokenReqMessageContext.addProperty("RESPONSE_HEADERS", new ResponseHeader[]{responseHeader});
    }

}

curl --user <Client_id>:<Client_secret> -k -d "grant_type=mobile&mobileNumber=0333444" -H "Content-Type: application/x-www-form-urlencoded" https://localhost:9443/oauth2/token

curl -v --user vSfeQ9jfNodY1tv9KLNNxLOw7kwa:CEUWu7fDNy_RYg5lO_mp8PLf7nQa -k -d "grant_type=mobile&mobileNumber=0363444" -H "Content-Type: application/x-www-form-urlencoded" https://localhost:9443/oauth2/token

> POST /oauth2/token HTTP/1.1
> Authorization: Basic dlNmZVE5amZOb2RZMXR2OUtMTk54TE93N2t3YTpDRVVXdTdmRE55X1JZZzVsT19tcDhQTGY3blFh
> User-Agent: curl/7.29.0
> Host: localhost:9443
> Accept: */*
> Content-Type: application/x-www-form-urlencoded
> Content-Length: 38
> 
* upload completely sent off: 38 out of 38 bytes
< HTTP/1.1 400 Bad Request
< Date: Wed, 13 Jan 2016 06:05:33 GMT
< SampleHeader-999: Provided Mobile Number is Invalid.
< Content-Type: application/json
< Content-Length: 87
< Connection: close
< Server: WSO2 Carbon Server
< 
* Closing connection 0
* SSLv3, TLS alert, Client hello (1):
{"error":"invalid_grant","error_description":"Provided Authorization Grant is invalid"}