Monday, September 11, 2017

Integrating JSON Web Tokens with Kerberos using Apache Kerby

JSON Web Tokens (JWTs) are a standard way of encapsulating a number of claims about a particular subject. Kerberos is a long-established and widely-deployed SSO protocol, used extensively in the Big-Data space in recent years. An interesting question is to examine how a JWT could be used as part of the Kerberos protocol. In this post we will consider one possible use-case, where a JWT is used to convey additional authorization information to the kerberized service provider.

This use-case is based on a document available at HADOOP-10959, called "A Complement and Short Term Solution to TokenAuth Based on
Kerberos Pre-Authentication Framework", written by Kai Zheng and Weihua Jiang of Intel (also see here).

1) The test-case

To show how to integrate JWTs with Kerberos we will use a concrete test-case available in my github repo here:
  • cxf-kerberos-kerby: This project contains a number of tests that show how to use Kerberos with Apache CXF, where the KDC used in the tests is based on Apache Kerby
The test-case relevant to this blog entry is the JWTJAXRSAuthenticationTest. Here we have a trivial "double it" JAX-RS service implemented using Apache CXF, which is secured using Kerberos. An Apache Kerby-based KDC is launched which the client code uses to obtain a service ticket using JAAS (all done transparently by CXF), which is sent to the service code as part of the Authorization header when making the invocation.

So far this is just a fairly typical example of a kerberized web-service request. What is different is that the service configuration requires a level of authorization above and beyond the kerberos ticket, by insisting that the user must have a particular role to access the web service. This is done by inserting the CXF SimpleAuthorizingInterceptor into the service interceptor chain. An authenticated user must have the "boss" role to access this service. 

So we need somehow to convey the role of the user as part of the kerberized request. We can do this using a JWT as will be explained in the next few sections.

2) High-level overview of JWT use-case with Kerberos
As stated above, we need to convey some additional claims about the user to the service. This can be done by including a JWT containing those claims in the Kerberos service ticket. Let's assume that the user is in possession of a JWT that is issued by an IdP that contains a number of claims relating to that user (including the "role" as required by the service in our test-case). The token must be sent to the KDC when obtaining a service ticket.

The KDC must validate the token (checking the signature is correct, and that the signing identity is trusted, etc.). The KDC must then extract some relevant information from the token and insert it somehow into the service ticket. The kerberos spec defines a structure that can be used for this purposes called the AuthorizationData, which consists of a "type" along with some data to be interpreted according to the "type". We can use this structure to insert the encoded JWT as part of the data.  

On the receiving side, the service can extract the AuthorizationData structure from the received ticket and parse it accordingly to retrieve the JWT, and obtain whatever claims are desired from this token accordingly.

3) Sending a JWT Token to the KDC

Let's take a look at how the test-case works in more detail, starting with the client. The test code retrieves a JWT for "alice" by invoking on the JAX-RS interface of the Apache CXF STS. The token contains the claim that "alice" has the "boss" role, which is required to invoke on the "double it" service. Now we need to send this token to the KDC to retrieve a service ticket for the "double it" service, with the JWT encoded in the ticket.

This cannot be done by the built-in Java GSS implementation. Instead we will use Apache Kerby. Apache Kerby has been covered extensively on this blog (see for example here). As well as providing the implementation for the KDC used in our test-case, Apache Kerby provides a complete GSS implementation that supports tokens in the forthcoming 1.1.0 release. To use the Kerby GSS implementation we need to register the KerbyGssProvider as a Java security provider.

To actually pass the JWT we got from the STS to the Kerby GSS layer, we need to use a custom version of the CXF HttpAuthSupplier interface. The KerbyHttpAuthSupplier implementation takes the JWT String, and creates a Kerby KrbToken class using it. This class is added to the private credential list of the current JAAS Subject. This way it will be available to the Kerby GSS layer, which will send the token to the KDC using Kerberos pre-authentication as defined in the document which is linked at the start of this post.

4) Processing the received token in the KDC

The Apache Kerby-based KDC extracts the JWT token from the pre-authentication data entry and verifies that it is signed and that the issuer is trusted. The KDC is configured in the test-case with a certificate to use for this purpose, and also with an issuer String against which the issuer of the JWT must match. If there is an audience claim in the token, then it must match the principal of the service for which we are requesting a ticket. 

If the verification of the received JWT passes, then it is inserted into the AuthorizationData structure in the issued service ticket. The type that is used is a custom value defined here, as this behaviour is not yet standardized. The JWT is serialized and added to the data part of the token. Note that this behaviour is fully customizable.

5) Processing the AuthorizationData structure on the service end

After the service successfully authenticates the client, we have to access the AuthorizationData part of the ticket to extract the JWT. This can all be done using the Java APIs, Kerby is not required on the receiving side. The standard CXF interceptor for Kerberos is subclassed in the tests, to set up a custom CXF SecurityContext using the GssContext. By casting it to a ExtendedGSSContext, we can access the AuthorizationData and hence the JWT. The role claim is then extracted from the JWT and used to enforce the standard "isUserInRole" method of the CXF SecurityContext. 

If you are interested in exploring this topic further, please get involved with the Apache Kerby project, and help us to further improve and expand this integration between JWT and Kerberos.

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