Validate an Oauth token before executing an Nifi Processor

[#1]

I am a newbie to Nifi, and so some help would be very much appreciated.
I wish to validate an Oauth token for a valid role, before executing a Processor, and return a 403 if it is not valid.
Could you please advise me on the best way to do this.
Thanks very much in advance
pxm

[#2]

Usually when authenticating with kerberos, kerberos provides a valid tgt. Until expiration, the service  renews by itself from the domain policy. However you can force a renewal through 

#kdestroy

then as the service user kinit with the service/server keytab

#kinit -kt /etc/security/keytabs/nifi.headless.keytab