The default implementation for javax.security.auth.login.Configuration is dependent on system property java.security.auth.login.config. see http://docs.oracle.com/javase/6/docs/jre/api/security/jaas/spec/com/sun/security/auth/login/ConfigFile.html

AuthUtils:GetConfiguration() is trying to the storm config to obtain login Configuration object. The last setProperty() is try to bring system property to previous state.