krb5: Sending a request containing PA-ENCRYPTED-CHALLENGE padata element without using FAST could result in NULL dereference in KDC which leads to DoS

A flaw was found in krb5. This flaw allows an unauthenticated attacker to cause a NULL dereference in the KDC by sending a request containing a PA-ENCRYPTED-CHALLENGE padata element without using FAST. The highest threat from this vulnerability is to system availability...