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Java 8 Kerberos MS-SFU Demonstration Code

This small Maven Java project demonstrates how to use Kerberos SFU extension implemented in OpenJDK since version 8.

Background

Microsoft has implemented Kerberos extension known as Kerberos constrained delegation (MS-SFU) in its ActiveDirectory product to allow impersonation and propose a more secure alternative to Kerberos V5 delegation.

Reference documentation: https://msdn.microsoft.com/en-us/library/cc246071.aspx

The protocol overview compares MS-SFU delegation with Kerberos V5 delegation. MS-SFU grants confidentiality for user's TGT credentials.

In Java 8, MS-SFU support has been implemented in JGSS API: https://docs.oracle.com/javase/8/docs/technotes/guides/security/jgss/jgss-features.html

Deployment example

Suppose your Java code has to access a webservice (REST or SOAP, so over HTTP) which requires SPNEGO authentication - care to not mismatch with Kerberos V5 HTTP authentication.

The webservice expects the user to authenticate itself with his Kerberos credentials thanks to a TGS generated from his TGT.

Suppose your Java code has authenticated a visitor thanks to any other authentication mechanism than Kerberos/SPNEGO, for instance with SAML assertions, preventing the use of standard Kerberos delegation. The only way for your code to generate a TGS targetting the webservice on behalf of a visitor is to use SFU Kerberos extensions, also known as a impersonation and Kerberos constrained delegation.

Principle

From the visitor's login name, the Java code trusted as a service in ActiveDirectory uses S4U2self message to get a service ticket (TGS) for the visitor. This process is also known as protocol transition.

In Java, JGSS API method impersonate is used to create specific GSS credentials in that purpose. At TGS generation, ActiveDirectory checks for Kerberos constrained delegation configuration set on the Java service account.

Thanks to this service ticket, code uses S4U2proxy message to generate a TGS ticket for the target webservice SPN on behalf of the visitor.

Use case

Suppose a webservice is available at http://webservice-host.domain.ltd and requires SPNEGO authentication with SPN HTTP/webservice-host.domain.ltd in realm DOMAIN.LTD.

Accord to Kerberos RFC, based on URI, a client (a browser) has to do a DNS lookup from hostname and a reverse-DNS lookup to create SPN using a FQDN.

Here, the SPN HTTP/webservice-host.domain.ltd is supposed to be canonicalized.

If a DNS alias and virtual host is defined for your service, you should apply hostname canonicalization, this demonstration code does not.

Deployment

Here is the procedure to create a service account for your Java code:

  • Create a standard user account javaservice with no password expiration and user cannot change password options

  • Generate its keytab with command

ktpass -princ HTTP/[email protected] -mapuser DOMAIN\javaservice
 -pass <password> -crypto all -ptype KRB5_NT_PRINCIPAL
 -out C:\Temp\javaservice.keytab

  • In javaservice account properties, grant constrained Kerberos delegation to webservice canonicalized SPN HTTP/webservice-host.domain.ltd by looking its corresponding service account.

  • Copy keytab on your system and edit template java.login.config

service {
  com.sun.security.auth.module.Krb5LoginModule required
  useKeyTab=true
  storeKey=true
  doNotPrompt=true
  keyTab="/path/to/javaservice.keytab"
  principal="HTTP/[email protected]";
};
  • Edit POM file to set target service SPN and a user login to test with
  • Simply run mvn test

Required system properties to run the code are visible in source code documentation and in Maven pom.xml file. Java 8 with unlimited JCE may be required if AES-256 is selected at Kerberos JAAS login.

Credits

This code has been designed from OpenJDK JGSS test cases: http://cr.openjdk.java.net/~weijun/6355584/webrev.00/test/sun/security/krb5/auto/

Many thanks to Achim Grolms for his good introduction to Kerberos concepts.